HOME AGLIBRARY CATALOG
Sustainable Soil Management
Soil System Guide
Appropriate Technology Transfer for Rural Areas
Prepared by Preston Sullivan
ATTRA Technical Specialist
The ATTRA Project is operated by the National Center for AppropriateTechnology under a grant from the Rural Business - Cooperative Service, U.S. Departmentof Agriculture. These organizations do not recommend or endorse products, companies,or individuals. ATTRA is located in the Ozark Mountains on the University of Arkansascampus in Fayetteville, at PO Box 3657, Fayetteville, Arkansas, 72702. ATTRA staffprefer to receive requests for information about sustainable agriculture via thetoll-free number 800-346-9140.
Part I: Principles and Characteristics of Sustainable Soils
The Living Soil: Texture and Structure
The Living Soil: Importance of Soil Organisms
Organic Matter, Humus, and the Soil Foodweb
Soil Tilth and Organic Matter
Tillage, Organic Matter, and Plant Productivity
Effect of Nitrogen on Organic Matter
Fertilizer Amendments and Biologically Active Soil
Topsoil –Your Farms's Capital
Summary of Part I
Part II: Management Steps to Build Soil Quality
1. Assess Soil Health and Biological Activity on Your Farm
2. Utilize Tools and Techniques to Build Soils
3. Continue to Monitor with Your Tools
Part III: Examples of Successful Soil Builders (Farmer Profiles)
Additional Information Resources
Books and Periodicals
Soil Web Sites
Abstract: This publication covers basic soil properties andmanagement steps toward building and maintaining healthy soils. The publication isdivided into three distinct sections, each with its own purpose. Section 1 dealswith basic soil principles and provides a understanding of living soils and how theywork. In section 1 you will find answers to why soil organisms and organic matterare important. Section 2 covers management steps to build soil quality on your farm.The last section covers farmer stories of people who have successfully built up theirsoil. A large resource section of other available information concludes the publication.
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What are some features of good soil? Any farmer will tell you that a good soil:
All these criteria indicate a soil that functions effectively today and will continueto produce long into the future. Creating soils with these characteristics canbe accomplished by utilizing management practices that optimize the processes foundin native soils.
Sustainable: the ability to keep in existence; maintain or prolong; to provide sustenance for.
How does soil in its native condition function? How do forests and native grasslandsproduce plants and animals in the complete absence of fertilizer and tillage? Whatare the principles by which these soils function? The answers to these questionsassure that the soil will be productive and profitable now and for future generations.A good thing happens when the soil's natural productivity is managed in a sustainableway; the reliance on purchased inputs declines, while land value and income generationincreases year by year. Some of the things we spend money on can be done by the naturalprocess itself for little or nothing. It's an upward spiral of continued success.To understand this better, let's start with the basics.
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Soils are made up of four components: minerals, air, water, and organic matter.In most soils minerals represent around 45% of the total volume, water and air about25% each, and organic matter from 2% to 5%. The mineral portion consists of threedistinct particle sizes classified as sand, silt or clay. Sand is the largest sizeparticle that can be considered soil. Sand is largely the mineral quartz, thoughother minerals are also present. Since quartz contains no plant nutrients, sand isthe lowest contributor to soil fertility of the three soil particle sizes. Furthermore,sand cannot hold nutrients–they leach out easily with rainfall. That is why sandysoils are not as productive as loams and need to be spoon-fed fertilizer. Silt particlesare much smaller than sand but, like sand, silt is mostly quartz.
The smallest of all the soil particles is clay. Clays are quite different fromsand or silt and contain appreciable amounts of plant nutrients. Clay has a largesurface area resulting from the plate-like shape of the individual particles. Thetextural designation of a soil is derived from the relative portions of sand, silt,and clay. A sandy loam, for example, has much more sand and much less clay than doesa clay loam. A loam soil is a mixture of sand, silt and clay. Most soils are sometype of loam. They are more accurately described by the words the preface the wordloam, such as: sandy loam or clay loam. The texture designations are found in Table1.
|Table 1. Soil textures|
|Coarse Textured |
Fine sandy loam
Silty clay loam
Another soil characteristic–soil structure–is different from soil texture. Structurerefers to the combination or "aggregation" of sand, silt and clay particlesinto larger secondary clusters. If you grab a handful of soil, good structure isapparent when the sand, silt, and clay particles are aggregated into granules orcrumbs. Both texture and structure determine pore space for air and water circulation,erosion resistance, looseness, ease of tillage, and root penetration. However, whiletexture is an innate property of the native soil and does not change with agriculturalactivities, structure can be improved or destroyed readily through our choice andtiming of farm practices.
The organic soil component contains all the living creatures in the soil and thedead ones in various stages of decomposition. An acre of living soil can contain900 pounds of earthworms, 2400 pounds of fungi, 1500 pounds of bacteria, 133 poundsof protozoa, 890 pounds of arthropods and algae, and even small mammals in some cases(2). In fact, the soil could be viewed as a living entity, ratherthan an inert body.
The soil's organic matter also contains dead organisms, plant matter and otherorganic materials in various phases of decomposition. Humus, the dark-colored organicmaterial in the final stages of decomposition, is relatively stable. Both organicmatter and humus serve as a reservoir of plant nutrients; they also help to buildsoil structure and provide other benefits.
The type of healthy living soil required to support humans now and far into thefuture will be balanced in nutrients and high in humus with a high diversity of soilorganisms. It will produce healthy plants with minimal weed, disease and insect pressure.To accomplish this we work with the natural processes and optimize their functionsto sustain our farms.
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Figure 1 The soil is teaming with organisms which cycle nutrients from soil
to plant and back again
If you look out at a landscape you might wonder how native prairies and forestsfunction in the complete absence of tillage and fertilizers? These soils are tilledby soil organisms, not by machinery. They are fertilized too, but the fertility isused again and again and never leaves the site. Native soils are covered with a layerof plant litter and/or growing plants throughout the year. Beneath the surface litterlayer, a rich complexity of soil organisms decompose plant residue and dead roots,then release their stored nutrients slowly over time. In fact, topsoil is the mostbiologically diverse part of the earth (3). Soil-dwelling organismsrelease bound-up minerals converting them into plant-available forms that are thentaken up by the plants growing on the site. The organisms recycle nutrients againand again from the death and decay of each new generation of plants growing on thesite.
There are many different types of creatures that live on or in the soil. Each hasa role to play. These organisms will work for the farmer's benefit if we simply managefor their survival. Consequently we may refer to them as soil livestock. While thereis a great variety of organisms that contribute to soil fertility, earthworms, arthropods,and the various microorganisms merit particular attention.
Earthworms: Earthworm burrows enhance water infiltration andsoil aeration. Earthworm tunneling can increase the rate of water entry into theground 4 to 10 times higher than fields that lack worm tunnels (4).This reduces water runoff, recharges groundwater, and helps store more soil waterfor dry spells. Vertical earthworm burrows pipe air deeper into the soil, stimulatingmicrobial nutrient cycling at those deeper levels. Tillage done by earthworms canreplace some expensive tillage work done by machinery.
Worms eat dead plant material left on top of the soil and redistribute the organicmatter and nutrients throughout the topsoil layer. Nutrient-rich organic compoundsline the tunnels that may remain in place for years if not disturbed. During droughtsthese tunnels allow for deep plant root penetration into subsoil regions of highermoisture content. In addition to organic matter, worms also consume soil and soilmicrobes as they move through the soil. The soil clusters they expel from their digestivetracts is known as a worm cast or casting. Each worm cast is separate from othercasts and ranges in size from that of a mustard seed to a sorghum seed dependingon the size of the worm. The soluble nutrient content of worm casts is considerablyhigher than those of the original soil (see Table 2). A good population of earthwormscan process 20,000 pounds of topsoil per year, with turnover rates as high as 200tons per acre having been reported in some exceptional cases (5).
Earthworms also secrete a plant growth stimulant. Reported increases in plantgrowth due to earthworm activity may be attributed to this substance, not just toimproved soil quality.
|Table 2. Nutrients in worm casts compared to the surrounding soil from which they came.|
|From Graff, O. 1971. (6) Soil had 4% organic matter.|
Earthworms thrive where there is no-tillage–generally, the less tillage, the better,and the shallower the tillage, the better. Worm numbers can be reduced by as muchas 90% by deep and frequent tillage (7). Tillage reduces earthwormpopulations by drying the soil, burying the plant residue they feed on, and makingthe soil easier to freeze. Tillage also destroys their vertical burrows and can killand cut up the worms themselves. Emergence times for young worms are spring and fall–theirmost active periods just when most farmers are interested in tillage. Worms are dormantin the hot part of the summer and the cold of winter. Table 3 shows the effect oftillage and cropping practices on earthworm numbers.
|Table 3. Effect of crop management on earthworm populations.|
|From Kladivko, 1995. (8)|
As a rule, earthworm numbers can be increased by reducing or eliminating tillage(especially fall tillage), never using the moldboard plow, reducing residue particlesize (using a straw chopper on the combine), adding animal manure, and growing greenmanure crops. It is beneficial to leave as much surface residue as possible yearround. Cropping systems that typically have the most earthworms are in descendingorder: perennial cool-season grass grazed rotationally, then warm-season perennialgrass grazed rotationally, then annual croplands using no-till. Ridge-till and striptillage will generally have more earthworms than clean tillage involving plowingand disking.
Earthworms prefer a near neutral soil pH, moist soil conditions, and plenty ofplant residue on the soil surface. They are sensitive to certain pesticides and someincorporated fertilizers. Carbamate insecticides including Furadan, Sevin, and Temik,are harmful to earthworms, notes worm biologist Clive Edwards of Ohio State University(4). Some insecticides in the organophosphate family are mildlytoxic to earthworms while synthetic pyrethroids are harmless to them (4).Most herbicides have little effect on worms except for the triazines, such as Atrazine,which are moderately toxic. Also, anhydrous ammonia kills earthworms in the injectionzone.
As a rule, earthworm numbers can be increased by reducing or eliminating tillage.
For more information on managing earthworms, order The Farmer's EarthwormHandbook: Managing Your Underground Moneymakers, by David Ernst. Ernst's bookcontains details on what earthworms need to live, increasing worm numbers, the effectsof tillage, manure and livestock management effects on earthworms, how 193 chemicalsaffect earthworms, and more. See the Additional InformationResources section of this publication for ordering information. Alsovisit the earthworm websites listed under this section.
Arthropods: In addition to earthworms, there are many other speciesof soil organisms that can be seen by the naked eye. Among them are sowbugs, millipedes,centipedes, slugs, snails and springtails. These are the primary decomposers. Theirrole is to eat and shred the large particles of plant and animal residues. Some buryresidue, bringing it into contact with other soil organisms that further decomposeit. Some members of this group prey on smaller soil organisms. The springtails area small insect, which eat mostly fungi. Their waste is rich in plant nutrients thatare released after other fungi and bacteria decompose it. Also of interest are dungbeetles, which play a valuable role in recycling manure and reducing livestock intestinalparasites and flies.
Bacteria: Most numerous among soil organisms are the bacteria;every gram of soil contains at least a million of these tiny one-celled organisms.There are many different species of bacteria, each with its own role in the soilenvironment. One of the major benefits bacteria provide for plants is in helpingthem take up nutrients. Some species release nitrogen, sulfur, phosphorus, and traceelements from organic matter. Others break down soil minerals and release potassium,phosphorus, magnesium, calcium and iron. Still other species make and release naturalplant growth hormones, which stimulate root growth.
A few species of bacteria fix nitrogen in the roots of legumes while others fixnitrogen independently of plant association. Bacteria are responsible for convertingnitrogen from ammonium to nitrate and back again depending on certain soil conditions.Other benefits to plants provided by various species of bacteria include increasingthe solubility of nutrients, improving soil structure, fighting root diseases, anddetoxifying soil.
Fungi: Fungi come in many different species, sizes and shapesin soil. Some species appear as thread-like colonies, while others are one-celledyeasts. Slime molds and mushrooms are also fungi. Many fungi aid plants by breakingdown organic matter or by releasing nutrients from soil minerals. Fungi are generallyearly to colonize larger pieces of organic matter and begin the decomposition process.Some fungi produce plant hormones, while others produce antibiotics including penicillin.There are even species of fungi that trap harmful plant-parasitic nematodes.
The mycorrhizae (my-cor-ry-'zee) group of fungi lives either on or in plant rootsand act to extend the reach of root hairs into the soil. Mycorrhizae increase theuptake of water and nutrients especially in less fertile soils. Roots colonized bymycorrihizae are less likely to be penetrated by root-feeding nematodes since thepest cannot pierce the thick fungal network. Mycorrhizae also produce hormones andantibiotics, which enhance root growth and provide disease suppression. The fungibenefit from plant association by taking nutrients and carbohydrates from the plantroots they live in.
Actinomycetes: Actinomycetes (ac"-ti-no-my'-cetes) are thread-likebacteria that look like fungi. While not as numerous as bacteria, they also performvital roles in the soil. Like the bacteria, they help decompose organic matter intohumus, releasing nutrients. They also produce antibiotics to fight diseases of roots.These same antibiotics are used to treat human diseases. Actinomycetes are responsiblefor the sweet, earthy smell of biologically active soil noticed whenever a fieldis tilled.
Algae: Many different species of algae also live in the upperhalf-inch of the soil. Unlike most other soil organisms, algae actually produce theirown food through photosynthesis. They appear as a greenish film on the soil surfacefollowing a good rain. Algae improve soil structure by producing slimy substancesthat glue soil together into water-stable aggregates. Some species of algae (theblue-greens) can fix their own nitrogen, some of which is later released to plantroots.
Protozoa: Protozoa are free-living microorganisms that crawlor swim in the water between soil particles. Many soil protozoa are predatory, eatingother microbes. One of the most common is an amoeba that eats bacteria. By eatingand digesting bacteria, protozoa speed up the cycling of nitrogen from the bacteria,making it more available to plants.
Nematodes: While nematodes are abundant in most soils, only afew species are harmful to plants. The harmless species eat decaying plant litter,bacteria, fungi, algae, protozoa and other nematodes. Like other soil predators,nematodes speed the rate of nutrient cycling.
All these organisms–from the tiny bacteria up to the large earthworms and insects–interactwith one another in a multitude of ways in a whole soil ecosystem. Organisms notdirectly involved in decomposing plant wastes may feed on each other or each other'swaste products or the other substances they release. Among the other substances releasedby the various microbes are vitamins, amino acids, sugars, antibiotics, gums, andwaxes.
Roots can also release various substances into the soil that stimulate soil microbes.These substances serve as food for select organisms. Some scientists and practitionerstheorize that plants use this means to stimulate the specific population of microorganismscapable of releasing or otherwise producing the kind of nutrition needed by the plants.
|Research on biological life in the soil has determined that there are ideal ratios for certain key soil organisms in productive soils.|
Research on biological life in the soil has determined that there are ideal ratiosfor certain key soil organisms in highly productive soils (soil foodweb). The SoilFoodweb lab, located in Oregon, tests soils and makes fertility recommendations thatare based on this understanding. Their goal is to alter the makeup of the soil microbialcommunity so it resembles that of a highly fertile and productive soil. There areseveral different ways to accomplish this goal, depending on the situation. For moreon the Soil Foodweb Lab see the Additional InformationResources Section of this publication. Because we cannot see most ofthe creatures living in the soil and may not take time to observe the ones we cansee, it is easy to forget about them. See Table 4 for estimates of typical amountsof various organisms found in fertile soil. There are many websites that providein-depth information on soil organisms. Look for a list of these web sites in theAdditional Information Resources sectionof this publication. Many of these web sites have color photographs of soil organismsand describe their benefits.
|Table 4. Weights of soil organisms in the top 7 inches of fertile soil.|
Pounds of liveweight/acre
|From: Bollen, 1959. (10)|
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Critical to any model for sustainable soil management, is understanding the rolethat soil organisms play, and that farmers focus on strategies that build both theirnumbers and their diversity. As with cattle and other farm animals, soil livestockrequire proper feed. That feed comes in the form of organic matter.
Organic matter and humus are terms that describe somewhat different but relatedthings. Organic matter refers to the organic fraction of the soil that is composedof both living organisms and once-living residues in various stages of decomposition.Humus is only a small portion of the organic matter. It is the end product of organicmatter decomposition and is relatively stable. Further decomposition of humus occursvery slowly in both agricultural and natural settings. In natural systems, a balanceis reached between the amount of humus formation and the amount of humus decay
As with cattle and other farm animals, soil livestock require proper feed.
The benefits of a soil rich in organic matter and humus are many. They include:rapid decomposition of crop residues, granulation of soil into water stable aggregates,decreased crusting and clodding, improved internal drainage, better water infiltration,and increased water and nutrient holding capacity. Improvements in the soil's physicalstructure facilitate easier tillage, increased soil water storage capacity, reducederosion, better formation and harvesting of root crops, and deeper, more prolificplant root systems. Improvements in nutrient cycling also reduce the fertilizer bill.
Soil organic matter can be compared to a bank account for plant nutrients. Soilcontaining 4% organic matter in the top 7 inches has 80,000 pounds of organic matterper acre. That 80,000 pounds of organic matter will contain about 5.25% nitrogen,amounting to 4,200 pounds of nitrogen per acre. Assuming a 5% release rate duringthe growing season, the organic matter could supply 210 pounds of nitrogen to a crop.If the organic matter is allowed to degrade, purchased fertilizer will be necessaryto prop up crop yields due to lost organic-matter nitrogen.
Ultimately, building organic matter and humus levels in the soil is a matter ofmanaging the living organisms in the soil–something akin to wildlife management oranimal husbandry. This entails working to maintain favorable conditions of moisture,temperature, nutrient status, pH, and aeration. It also involves providing a steadyfood source.
|Ultimately, building organic matter and humus in the soil is a matter of managing the living organisms in the soil.|
All the soil organisms mentioned previously, except algae, depend on organic matteras their food source. Therefore, to maintain their populations, organic matter mustbe renewed from plants growing on the soil, or from animal manure or other materialsimported from off site. By feeding the soil livestock, fertility is built up in thesoil and the soil will feed the plants.
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A soil that drains well, does not crust, takes in water rapidly, and does notmake clods is said to have good tilth. Tilth is the physical condition of the soilas it relates to tillage ease, seedbed quality, easy seedling emergence, and deeproot penetration. Good tilth is dependent on aggregation–the process whereby individualsoil particles are joined into clusters or "aggregates."
Aggregates form in soils when individual soil particles are oriented and broughttogether through wetting and drying, freezing and thawing, and by plant growth andearthworm activity. The weak electrical forces from calcium and magnesium hold thesoil particles together when the soil dries. When the aggregates become wet again,however, their stability is challenged and they may break apart once again. In thecase of earthworm-created aggregates, they are stable once they come out of the worm.An aggregate formed by physical forces becomes stabilized (will remain intact whenwet) through microbial processes involving organic matter decomposition and its by-products–chieflygums, waxes, and other glue-like substances. These by-products cement the soil particlestogether forming water-stable aggregates (Figure 2). The aggregate is then strongenough to hold together when wet–hence the name "water-stable."
USDA soil microbiologist Sara Wright named the glue that holds aggregates together"glomalin" after the Glomales group of common root dwelling fungi (12).These fungi secrete a gooey protein known as glomalin through their hair-like filamentscalled hyphae. When Sara Wright measured glomalin in soil aggregates she found levelsas high as 2% of their total weight in eastern US soils. Soils from the west andMidwest had lower levels of glomalin. She found that tillage tends to lower glomalinlevels. Higher glomalin levels and higher aggregation were found in no-till cornplots than in soil from tilled plots (12). Sara has a brochuredescribing glomalin and how it benefits soil entitled Glomalin, a ManageableSoil Glue. To order this brochure see the AdditionalInformation Resources section of this publication
A well-aggregated soil has increased water entry into the soil, increased aeration,and increased water-holding capacity (13). Plant roots occupy alarger volume of well-aggregated soil that is high in organic matter as opposed toa finely pulverized and dispersed soil, low in organic matter. Roots, earthworms,and soil arthropods can pass more easily through a well-aggregated soil (14).Aggregated soils also prevent glazing or crusting of the soil surface. Finally, well-aggregatedsoils are more erosion resistant, because aggregates are much heavier than theirparticle components. For a good example of the effect of organic matter additionson aggregation and subsequent increase in water entry into the soil, see Table 5.
|Table 5. Water entry into the soil after 1 hour|
Manure Rate Tons/acre
Inches of water
|Boyle, et al. 1989. (13)|
The opposite of aggregation is dispersion. In a dispersed soil, each individualsoil particle is free to blow away with the wind or wash away with over-land flowof water.
Figure 2 Microbial byproducts glue soil particles into water-stable aggregates.
Soils with poor aggregation also tend to be sticky when wet and cloddy when dry.If the clay particles in these soils can be aggregated together, better aerationand water infiltration will result. Sandy soils can benefit from aggregation by havinga small amount of dispersed clay that tends to stick between the sand particles andslows the excess downward movement of water.
Crusting is a common problem on soils that are poorly aggregated. Crusting resultschiefly from the impact of falling raindrops. Rainfall causes clay particles on thesoil surface to disperse and clog the pores immediately beneath the surface. Followingdrying, a sealed soil surface results in which most of the pore space has been drasticallyreduced due to clogging from dispersed clay particles. Subsequent rainfall is muchmore likely to run off than to flow into the soil (Figure 3).
Figure 3 Effects of agggregation on water and air entry into the soil.
Derived from: Land Stewardship Project Monitoring Toolbox (15)
Since raindrops start crusting, any management practices that protect the soilfrom this impact will decrease crusting and increase water flow into the soil. Mulchesand cover crops serve this purpose well as do no-till practices which allow the accumulationof surface residue. Also a well-aggregated soil will resist crusting because thewater-stable aggregates are less likely to break apart when a raindrop hits them.
The best-aggregated soils are those that have been in long-term grass production(16). A grass sod extends a mass of fine roots throughout the topsoil,contributing to the physical processes that help form aggregates. For example, rootscontinually remove water from soil microsites, providing local wetting and dryingeffects that promote aggregation.
|The best aggregated soils are those that have been in long-term grass production.|
Meanwhile, the roots are also producing food for the soil microorganisms and earthworms,thus generating the compounds that bind the aggregates into water-stable units. Additionally,a perennial grass sod provides protection from raindrops and erosion while theseprocesses are occurring. This combination of factors creates optimal conditions forthe establishment of a well-aggregated soil under a perennial cover.
Conversely, cropping sequences that involve annual plants and extensive cultivationprovide less vegetative cover and organic matter, and usually result in a rapid declinein soil aggregation and organic matter. For more information on aggregation, seethe soil quality information sheet entitled Aggregate Stability at the SoilQuality Institute's homepage. From there, click on Soil Quality Information Sheets,then click on Aggregate Stability: http://www.statlab.iastate.edu/survey/SQI/sqihome.shtml
Farming practices can be geared to conserve and promote soil aggregation. Becausethe binding substances are themselves susceptible to microbial degradation, organicmatter needs to be replenished to maintain an aggregated status. Practices shouldalso conserve aggregates once they are formed, by minimizing factors that degradeand destroy aggregation. Some factors which destroy or degrade soil aggregates are:
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Several factors affect the level of organic matter that can be maintained in asoil. Among these are organic matter additions, moisture, temperature, tillage, nitrogenlevels, cropping, and fertilization. The level of organic matter present in the soilis a direct function of how much organic material is being produced or added to thesoil versus the rate of decomposition. The objectives of this balancing act entailslowing the speed of organic matter decomposition, while increasing the supply oforganic materials produced on site and/or the addition of organic matter from offsite.
Moisture and temperature also profoundly affect soil organic matter level. Oneonly has to compare organic matter present in soils of the northern states with thoseof the southern states to see the decline in organic matter due to higher temperatureand moisture in the soil. Generally speaking, soils in the dryer western states havemore organic matter than those in the eastern states receiving more rainfall. Themost dramatic example of the effects of temperature and moisture may be seen by comparingthe organic-matter-rich soils of the arctic region with the organic matter poor soilsof the tropics. The warmer and wetter it gets, the more difficult to maintain soilorganic matter.
Tillage can be beneficial or harmful to a biologically active soil depending onwhat type of tillage is used and when it is done. Tillage affects both erosion rateand soil organic matter decomposition rate. Croplands having organic matter levelsbelow 1% are biologically dead. These low levels are due primarily to tillage. Cleantillage involving moldboard plowing and disking breaks down soil aggregates and leavesthe soil prone to erosion from wind and water. The moldboard plow can bury crop residueand topsoil to a depth of 14 inches. At this depth, the oxygen level in the soilis so low that decomposition cannot proceed adequately.
Surface dwelling decomposer organisms suddenly find themselves suffocated andsoon die. The crop residue that was originally on the surface but has now been turnedunder will putrefy in the oxygen-deprived zone as it rots in the absence of oxygen.This rotting activity may give a putrid smell to the soil. Furthermore, the top fewinches of the field is now often covered with subsoil having very little organicmatter content.
The topsoil is where the biological activity happens–that's where the oxygen is.That's why a fence post rots off at the surface. In terms of organic matter, tillageis similar to opening the air vents on a wood-burning stove; adding organic matteris like adding wood to the stove. Ideally, organic matter decomposition should proceedas an efficient burn of the "wood" to release nutrients and carbohydratesto the soil organisms and create stable humus. Shallow tillage incorporates residueand speeds the decomposition of organic matter by adding oxygen that microbes needto become more active. In cold climates with a long dormant season, light tillageof a heavy residue may be beneficial; in warmer climates it is hard enough to maintainorganic matter levels even without tillage.
As indicated in Figure 4, moldboard plowing causes the fastest decline of organicmatter, no-till the least. The plow lays the soil up on its side, increasing thesurface area exposed to oxygen. The other three types of tillage are intermediatein their ability to foster organic matter decomposition. Oxygen is the key factorhere. The moldboard plow increases the soil surface area, allowing more air intothe soil and speeding the decomposition rate. The horizontal line on Figure 4 representsthe replenishment of organic matter provided by wheat stubble. With the moldboardplow, more than the entire organic matter contribution from the wheat straw is gonewithin only 19 days following tillage. Finally, the passage of heavy equipment increasescompaction in the wheel tracks, and some tillage implements themselves compact thesoil further, removing oxygen and increasing the chance that deeply buried residueswill putrefy.
Figure 4. Organic matter lossesafter various tillage practices (17)
Tillage also reduces the rate of water entry into the soil by removal of groundcover and destruction of aggregates. Table 6 shows three different tillage methodsand how they affect water entry into the soil. Notice the direct relationship betweentillage type, ground cover, and water infiltration. No-till has more than three timesthe water infiltration of the moldboard-plowed soil. Additionally, the no-till fieldwill have higher aggregation from the organic matter decomposing on-site. The surfacemulch typical of no-till fields acts as a protective skin to the soil. This soilskin reduces the impact of raindrops and buffers the soil from temperature extremesas well as reducing water evaporation.
|Table 6. Tillage effects on water infiltration and ground cover.|
|Boyle, et al. 1989. (13)|
Both no-till and reduced tillage systems provide benefits to the soil. The advantagesof a no-till system include superior soil conservation, moisture conservation, reducedwater runoff, the long-term buildup of organic matter, and increased water infiltration.A soil managed without tillage relies on soil organisms to take over the job of plantresidue incorporation formerly done by tillage. On the down side, no-till can fostera reliance on herbicides to control weeds and can lead to soil compaction from thetraffic of heavy equipment. Pioneering development work on chemical-free no-tillfarming is proceeding at several research stations in the eastern US and on farms.Pennsylvania farmer Steve Groff has been farming no-till with minimal or no herbicidesfor several years. Steve is a 15-year no-till farmer in Lancaster County, Pennsylvania,who uses cover crops extensively in his crop fields. In the spring he rolls the covercrops down using a 10-foot rolling stalk chopper. This rolling chopper kills therye or vetch cover crop and creates a nice no-till mulch into which he plants a varietyof vegetables and grain crops.
After several years of no-till production, his soils are mellow and easy to plantinto. Steve farms 175 acres of vegetables, alfalfa and grain crops on his Cedar MeadowFarm. Learn more about Steve's operation in the Farmer Profilessection of this publication, by visiting his website, or by ordering his video (seeAdditional Information Resourcessection).
Other conservation tillage systems include ridge tillage, minimum tillage, zonetillage, and reduced tillage, each possessing some of the advantages of both conventionaltill and no-till. These systems represent intermediate tillage systems, allowingmore flexibility than a no-till or conventional till system might. They are moreor less beneficial or harmful to soil organisms depending on the type and amountof tillage and when it is done.
Adding manure and composts is a recognized means for improving soil organic matterand humus levels. In their absence, perennial grass is the only crop that can regenerateand increase soil humus (18). Cool season grasses build soil organicmatter faster than warm-season grasses because they are growing much longer duringa given year (18).
When the soil is warm enough for soil organisms to decompose organic matter, cool-seasongrass is growing. While growing, it is producing organic matter and cycling mineralsfrom the decomposing organic matter in the soil. In other words, there is a net gainof organic matter because the cool-season grass is producing organic matter fasterthan it is being used up.
With warm-season grasses, organic matter production during the growing seasoncan be slowed during the long dormant season from fall through early spring. Duringthe beginning and end of this dormant period the soil is still biologically active,yet no grass growth is proceeding (18). Some net accumulation oforganic matter can occur under warm-season grasses, however. In a Texas study, switchgrassgrown for 4 years increased soil carbon content from 1.1% to 1.5% in the top 12 inchesof soil (19). In hot and moist regions, a cropping rotation thatincludes several years of pasture will be most beneficial.
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Excess nitrogen applications stimulate increased microbial activity that speedsorganic matter decomposition. The extra nitrogen narrows the ratio of carbon to nitrogenin the soil. The native soil carbon to nitrogen ratio (C:N ratio) is around 12:1.At this ratio, populations of decay bacteria are kept at a stable level (20).When large amounts of inorganic nitrogen are added, the C:N ratio is reduced, whichincreases the populations of decay organisms and allows them to decompose more organicmatter. While soil bacteria can efficiently use moderate applications of inorganicnitrogen accompanied by organic amendments (carbon), excess nitrogen causes bacteriapopulations to explode, decomposing existing organic matter at a rapid rate.
Excess nitrogen stimulates increased microbial activity that speeds organic matter decomposition.
Eventually, soil carbon content may be reduced to a level where the bacterialpopulations are on a starvation diet. With little carbon available, bacterial populationsshrink and less free soil nitrogen is absorbed. Thereafter, applied nitrogen, ratherthan being cycled through microbial organisms and re-released to plants slowly overtime, becomes subject to leaching. This can greatly reduce the efficiency of fertilizationand lead to environmental problems.
To compensate for the fast decomposition of native soil organic matter, carbonshould be added with nitrogen. Typical sources–such as green manures, animal manureand compost–serve this purpose well. Amendments containing too high a carbon to nitrogenratio (25:1 or more) can tip the balance the other way, resulting in nitrogen tiedup in an unavailable form. The soil organisms consume all the nitrogen in an effortto decompose the abundant carbon. The nitrogen is unavailable because it is tiedup in the soil organisms themselves. As soon as one dies and decomposes, its nitrogenis consumed by another soil organism until the balance between carbon and nitrogenis achieved again.
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What are the soil mineral conditions that foster biologically active soils? Drawingfrom the work of Dr. William Albrecht (1888 to 1974), agronomist at the Universityof Missouri, we learn that balance is the key. Albrecht advocated bringing soil nutrientsin balance so that none were in excess or deficient. Albrecht's theory (also calledbase-saturation theory) is used to guide lime and fertilizer application by measuringand evaluating the ratios of positively charged nutrients (bases) held in the soil.The positively charged bases include calcium, magnesium, potassium, sodium, ammoniumnitrogen, and several trace minerals. When optimum ratios of bases exist, the soilis believed to support high biological activity, becomes resistant to leaching, andhas optimal physical properties (water intake and aggregation). The plants growingon such a soil are also balanced in mineral levels and are nutritious to humans andanimals alike.
Through extensive research, Albrecht determined the desirable percentages of basesaturation in the soil. These percentages, he maintained, were optimal for the growthof most crops. These levels are:
Fertilizer and lime applications should be made at rates that will bring soilmineral percentages into this ideal range. Through this approach, soil pH shiftsautomatically into a desirable range without creating nutrient imbalances. The basesaturation theory also takes into account the effect one nutrient may have on anotherand avoids undesirable interactions. For example, excess phosphorus is known to tieup zinc.
The Albrecht system of soil evaluation contrasts with the approach used by manystate laboratories often called the "sufficiency method." Sufficiency theoryplaces little to no value on nutrient ratios, and lime recommendations are typicallybased on pH measurements alone. While in many circumstances base saturation and sufficiencymethods will produce identical soil recommendations and similar results, significantdifferences can occur on a number of soils. For example, suppose we tested a cornfieldand found a soil pH of 5.5 and base saturation for magnesium at 20% and calcium at40%. Base saturation theory would call for liming with a high-calcium lime to raisethe % base saturation of calcium; the pH would rise accordingly. Sufficiency theorywould not specify high calcium lime and the grower might choose instead, a high magnesiumdolomite lime that would raise the pH but worsen the balance of nutrients in thesoil. Another way to look at these two theories is that the base saturation theorydoes not concern itself with pH to any great extent but rather with the proportionalamounts of bases. The pH will be correct when the levels of bases are correct.
Albrecht's ideas have found their way onto large numbers of American farms andinto the programs of several agricultural consulting companies. Neal Kinsey, a soilfertility consultant of Charleston, MO, is a major proponent of the Albrecht approach.Kinsey was a student under Albrecht and is one of the leading authorities on thebase-saturation method. He teaches a short course on the Albrecht system and providesa soil analysis service (21). His book, Hands On Agronomy,is widely recognized as a highly practical guide to its understanding and implementation.For more information on the Albrecht theory request the ATTRA publication on Albrechtand Reams Fertility Management Systems.
Several firms–many providing backup fertilizer and amendment products–offer abiological-farming program based on the Albrecht theory. Typically these firms offerbroad-based soil analysis and recommend balanced fertilizer materials consideredfriendly to soil organisms. They avoid the use of some common fertilizers and amendmentssuch as dolomite lime, potassium chloride, anhydrous ammonia, and oxide forms oftrace elements because they are considered harmful to soil life. The publicationHow to Get Started in Biological Farming presents such a program. See theAdditional Information Resources sectionfor ordering information. For more names of companies offering consulting and productsorder the ATTRA publications Alternative SoilTesting Laboratories and Sources forOrganic Fertilizers and Amendments. Both of these are also available onthe ATTRA web site located at: http://www.attra.org.
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Commercial fertilizer can be a valuable resource to farmers in transition to amore sustainable system and can help meet nutrient needs during times of high cropnutrient demand, or when weather conditions result in slow nutrient release fromorganic resources. Commercial fertilizers have the advantage of supplying plantswith immediately available forms of nutrients. They are often less expensive andless bulky to apply than processed natural fertilizers.
Not all conventional fertilizers are alike, however. Many appear harmless to soillivestock but a few are problematic. Anhydrous ammonia contains approximately 82%nitrogen and is applied subsurface as a gas. Anhydrous speeds the decomposition oforganic matter in the soil, leaving a soil more compact as a result. The additionof anhydrous contributes acidity to the soil, requiring 148 pounds of lime to neutralize100 pounds of anhydrous ammonia or 1.8 pounds of lime for every pound of nitrogencontained in the anhydrous (22, 23). Anhydrous ammonia initiallykills many soil microorganisms in the application zone. Bacteria and actinomycetesrecover within one to two weeks to levels higher than those prior to treatment
Potassium chloride (KCl) (0-0-60 and 0-0-50), also known as muriate of potash,contains approximately 50 or 60% potassium and 47.5% chloride (24).Muriate of potash is made by refining potassium chloride ore, which is a mixtureof potassium and sodium salts and clay from the brines of dying lakes and seas. Thepotential harmful effects from KCl can be surmised from in the salt concentrationof the material. Table 7 shows that, pound for pound, KCl is surpassed only by tablesalt on the salt index. Additionally, some plants such as tobacco, potatoes, peachesand some legumes are especially sensitive to chloride. High rates of KCl must beavoided on such crops. Potassium sulfate, potassium nitrate, sul-po-mag, or organicsources of potassium may be considered as alternatives to KCl for fertilization.
Sodium nitrate, also known as Chilean nitrate, or nitrate of soda, is anotherhigh salt fertilizer. Because of the relatively low nitrogen content of sodium nitrate,a high amount of sodium is added to the soil when normal applications of nitrogenare made with this material. The concern is that excessive sodium acts as a dispersantof soil particles, degrading aggregation. The salt index for KCl and sodium nitratecan be seen in Table 7.
|Table 7. Salt index for various fertilizers.|
Salt index per unit
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Topsoil is the capital reserve of every farm. Ever since mankind started agriculture,erosion has been the single largest threat to the soil's productivity and consequently,the profitability of the farm. This is still true today. In the US, the average acreof cropland is eroding at a rate of 7 tons per year (2). To sustainagriculture means to sustain the soil resource because that's where the farmer'slivelihood comes from.
Protecting the soil from erosion is the first step toward a sustainable agriculture.
The major costs to the farm associated with soil erosion come from the replacementof lost nutrients and reduced water holding ability, accounting for 50 to 75% ofproductivity loss (2). Eroded soil typically contains about threetimes more nutrients than the soil left behind and is 1.5 to 5 times richer in organicmatter (2). This organic-matter loss not only results in reducedwater holding capacity and degraded soil aggregation but also the loss of plant nutrients,which must then be replaced with fertilizer.
Five tons of topsoil (the so-called tolerance level) can easily contain 100 poundsof nitrogen, 60 pounds of phosphate, 45 pounds of potash, 2 pounds of calcium, 10pounds of magnesium, and 8 pounds of sulfur. Table 8 shows the effect of slight,moderate, and severe erosion on organic matter, soil phosphorus level and plant availablewater on a silt loam soil in Indiana (25).
|Table 8. Effect of erosion on organic matter phosphorus and plant-available water.|
|Erosion level||Organic matter||Phosphorus||Plant-available water|
|From: Schertz et al. 1984. (25)|
When erosion by water and wind occurs at a rate of 7.6 tons/acre/year it costs $40per year to replace the lost nutrients as fertilizer and around $17/acre/yr to pumpwell irrigation water to replace the soil water holding capacity of that lost soil(26). The total cost of soil and water lost annually from U.S.cropland amounts to an on-site productivity loss of approximately $27 billion eachyear (2).
Protecting the soil from erosion is the first step toward a sustainableagriculture. Erosion is initiated by raindrop impact on bare soil. Any managementpractice that protects the soil from raindrop impact will decrease erosion and increasewater entry into the soil. Mulches, cover crops, and crop residues serve this purposewell. Additionally, well-aggregated soils resist crusting because water-stable aggregatesare less likely to break apart when the raindrop hits them. Adequate organic matterwith high soil biological activity leads to aggregation.
Many studies have shown that cropping systems maintaining soil-protecting plantcanopy or residue cover have the least soil erosion. This is universally true. Long-termcropping studies begun in 1888 at the University of Missouri provide dramatic evidenceof this concept. Gantzer and colleagues (27) examined the effectsof a century of cropping on soil erosion. They compared depth of topsoil remainingafter 100 years of cropping (Table 9). As the table shows, the practices that maintainedthe highest amount of permanent ground cover (Timothy grass) had the greatest amountof topsoil left.
The researchers commented that subsoil had been mixed with topsoil in the continuouscorn plots from plowing, making the real topsoil depth less than was apparent. Inreality, all the topsoil was lost from the continuous corn plots in only 100 years.The rotation lost about half the topsoil over 100 years. How can we feed future generationswith this type of farming practice?
|Table 9. Topsoil depth remaining after 100 years of different cropping practices.|
|Crop Sequence||Inches of topsoil remaining|
|Cont. Corn |
Cont. Timothy grass
|*Corn, oats, wheat, clover, Timothy, Timothy (27)|
In a study of many different soil types in each of the major climatic zones ofthe US, researchers showed dramatic differences in soil erosion when comparing rowcrops to perennial sods. Row crops consisted of cotton or corn, and sod crops werebluegrass or bermuda grass. On average the row crops eroded over 50 times more soilthan did the perennial sod crops. The two primary influencing factors are groundcover and tillage. The results are shown in Table 10.
|Table 10. Effect of cropping on soil erosion rates.|
|Soil type|| |
Row crop soil loss
Sod soil loss
|Silt loam|| |
|Silt loam|| |
|Fine sandy loam|| |
|Clay loam|| |
|Fine sandy loam|| |
|Silt loam|| |
|Adapted from Shiftlet and Darby, 1985. (28)|
So, how long do fields have before the topsoil is gone? Roughly 8 tons/acre/yearsoil erosion loss amounts to the thickness of a dime spread over an acre. Twentydimes stack up to 1-inch high. So a landscape with an 8-ton erosion rate would losean inch of topsoil about every 20 years. Since this amount is barely detectable withina person's lifetime, it is not likely to be noticed.
Forward thinking researcher Wes Jackson, of the Land Institute, waxes eloquentlyabout how tillage is engrained in human culture ever since we began farming. Beatingour swords into plowshares surely embodies the triumph of good over evil. Someonewho creates something new is said to have "plowed new ground." "Yetthe plowshare may well have destroyed more options for future generations than thesword (29)."
Tillage, for the production of annual crops, is the major problem in agriculturedue to soil erosion and the loss of soil quality. Any agricultural practice thatcreates and maintains bare ground is inherently less sustainable than those whichkeep the ground covered throughout the year.
The only exception that this writer is aware of is wetland rice, where soil canactually be gained by pumping silt-laden surface irrigation water onto the fieldand draining off clear water at the end of the season. Dr. Wes Jackson has spentmuch of his career developing perennial grain crops and cropping systems that mimicthe natural prairie. Perennial grain crops do not require tillage to establish, andthe ground is left covered.
Ultimately, this is the future of grain production and truly represents a newvision in how we produce food. The greatest research need in agriculture today isbreeding work to develop perennial crops that will replace annual crops requiringtillage.
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Soil management involves stewardship of the soil livestock herd! The primary factorsaffecting organic matter content, build-up, and decomposition rate in soils are:oxygen content, nitrogen content, moisture content, temperature, and the additionand removal of organic materials. All these factors work together at any one time.Any one can limit the others. These are the factors that affect the health and reproductiverate of organic matter decomposer organisms.
Managers need to be aware of these factors when making decisions about their soils.Let's take them one at the time.
Increasing oxygen speeds decomposition of organic matter. Tillage is the primaryway extra oxygen enters the soil. Texture also plays a role, with sandy soils havingmore aeration than heavy clay soils. Nitrogen content is influenced by fertilizeradditions. Excess nitrogen without the addition of carbon speeds the decompositionof organic matter. Moisture content affects decomposition rates. Soil microbial populationsare most active over cycles of wetting and drying. Their populations increase followingwetting as the soil dries out. After the soil becomes dry, their activity diminishes.Just like humans, soil organisms are profoundly affected by temperature. Their activityis highest within a band of optimum temperature. Above and below optimum temperaturetheir activity is diminished. Adding organic matter provides more food for microbes.
To achieve an increase of soil organic matter, additions must be higher than removals.Over a given year, under average conditions, 60 to 70 percent of the carbon containedin organic residues added to soil is lost as carbon dioxide (20).Five to ten percent is assimilated into the organisms that decomposed the organicresidues and the rest becomes 'new' humus. It takes decades for new humus to developinto stable humus which imparts the nutrient holding characteristics humus is knownfor (20). The end result of adding a ton of residue would be 400to 700 pounds of new humus. With a 7-inch depth of topsoil over an acre weighing2 million pounds, you can see that building organic matter is a slow process. Onepercent organic matter weighs 20,000 pounds.
Building stable humus is a slow and long-term process. It is more feasible tostabilize and maintain the humus present before it is lost than to try to increaseit. The value of humus is not fully realized until it is severely depleted (20).If your soils are high in humus now, work hard to preserve what you have. The formationof new humus is essential to maintaining old humus and the decomposition of raw organicmatter has many benefits of its own. Increased aeration caused by tillage coupledwith the absence of organic carbon in fertilizer materials has caused greater than50% decline in native humus levels on many US farms (20).
Appropriate mineral nutrition needs to be present for soil organisms and plantsto prosper. Adequate levels of calcium, magnesium, potassium, phosphorus, sodiumand the trace elements should be present but not in excess. The cation balance theoryof soil management helps guide decision making toward achieving optimum levels ofthese nutrients in the soil. Several books have been written on balancing soil minerallevels and several consulting firms provide soil analysis and fertility recommendationservices based on that theory.
Commercial fertilizers have their place in a sustainable agriculture. Some appearharmless to soil livestock and provide nutrients at times of high nutrient demandfrom crops. Anhydrous ammonia and potassium chloride have limitations, however. Anhydrouskills soil organisms in the injection zone. Bacteria and actinomycetes recover withina few weeks but fungi take longer. The increase in bacteria, fed by high availablenitrogen from the anhydrous speeds the decomposition of organic matter. Potassiumchloride has a high salt index and some plants are particularly sensitive to chloride.
Topsoil is the farmer's capital. Sustaining agriculture means sustaining the soilresource. Maintaining ground covers like cover crops, mulch or crop residue for asmuch of the annual season as possible achieves the goal of sustaining the soil resource.Any time the soil is tilled and left bare it is susceptible to erosion. Even smallamounts of soil erosion are harmful over time. It is not easy to "see"the effects of erosion over a human lifetime and therefore erosion may go unnoticed.Tillage for production of annual crops created most of the erosion associated withagriculture. Perennial grain crops not requiring tillage provide a promising hopefor drastically improving the sustainability of future grain production.
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1. Assess Soil Health and Biological Activity onYour Farm
A basic soil audit is the first and sometimes the only monitoring tool used toassess changes in the soil. Unfortunately, the standard soil test done to determinenutrient levels (P, K, Ca, Mg, etc.) provides no information on soil biology andphysical properties. Yet, most of the farmer-recognized criteria listed on the firstpage of this publication for healthy soils include, or are created by, soil organismsand soil physical properties. A better appreciation of these biological and physicalsoil properties, and how they affect soil management and productivity, has resultedin the adoption of several new soil health assessment techniques which are discussedbelow.
The U S D A Soil Quality Test Kit
The USDA Soil Quality Institute provides a Soil Quality Test Kit Guide developedby John Doran and associates at the Agricultural Research Service's office in Lincoln,Nebraska. The kit was designed for field use. Components necessary to build a kitinclude many items commonly available such as pop bottles, flat bladed knives, agarden trowel, and plastic wrap. Also necessary to do the tests is some equipmentusually not locally available such as hypodermic needles, latex tubing, a soil thermometer,an electrical conductivity meter, filter paper, and an EC calibration standard. Thekit allows the measurement of water infiltration, water holding capacity, bulk density,pH, soil nitrate, salt concentration, aggregate stability, earthworm numbers, andrespiration. The Soil Quality Test Kit Guide can be ordered from the USDA throughthe Soil Quality Institute's webpage: http://www.statlab.iastate.edu/survey/SQI/sqihome.shtml
The 88-page on-line version of the guide is available in Adobe Acrobat Readerformat through the above web page and may be printed out. A summary of the testsis also available from the web page. To order a paper version, see the Soil QualityInstitute reference under the Additional InformationResources section. Section II of the test kit provides an interpretiveguide for individual tests described in section I.
A greatly simplified and quick soil quality assessment is also available at theSoil Quality Institute's web page by clicking on "Getting to Know your Soil,"near the bottom of the homepage. This greatly simplified method involves digginga hole and making some observations. Here are a few of the procedures shown at thiswebsite: Dig a hole 4 to 6 inches below the last tillage depth and observe how hardthe digging was. Inspect plant roots for lots of branching and fine root hairs ora balled up condition. A lack of fine root hairs indicates oxygen deprivation, whilesideways growth indicates a hardpan. The process goes on to tell about earthworms,smelling the soil, and assessing the aggregation. Another useful hands-on procedurefor assessing pasture soils is available from the ATTRA publication entitled Assessingthe Pasture Soil Resource.
Early Warning Monitoring for Croplands
A cropland-monitoring guide has been published by the Center for Holistic Management(30). The monitoring guide contains a set of soil health indicatorsthat are measurable in the field. No fancy equipment is needed to make the assessmentsdescribed in this monitoring guide. In fact, all the equipment is cheap and locallyavailable on almost any farm. Simple measurements can help determine the health ofcroplands in terms of the effectiveness of the nutrient cycle, water cycle, and thediversity of some soil organisms. Some of the assessments you can make using thisguide are living organisms, aggregation, water infiltration, ground cover, and earthworms.The monitoring guide is easy to read and understand, and comes with a field sheetto record observations. It is available for $12 from the Center for Holistic Management(See Additional Resources).
Direct Assessment of Soil Health
Some quick ways to identify a healthy soil include feeling it and smelling it. Graba handful and take a whiff. Does it have an earthy smell? Is it a loose, crumblysoil with some earthworms present? Dr. Ray Weil, soil scientist at the Universityof Maryland describes how he would make a quick evaluation of a soil's health injust 5 minutes (31).
Look at the surface and see if it is crusted, which tells something about tillage practices used, organic matter, and structure. Pushing a soil probe down to 12 inches, lift out some soil and feel its texture. If a plow pan were present it would have been felt with the probe. Turn over a shovelful of soil to look for earthworms and smell for actinomycetes, which are microorganisms that help compost and stabilize decaying organic matter. Their activity leaves a fresh earthy smell in the soil.
Two more easy observations are to count the number of soil organisms in a squarefoot of surface crop residue and to pour a pint of water on the soil and record thetime it takes to sink in. Comparisons can be made using these simple observationsalong with Ray Weil's evaluation above to determine how farm practices affect soilquality. Some of the soil quality assessment systems discussed above utilize theseand other observations and provide record keeping sheets to record your observationson.
A simple erosion test
This test demonstrates the value of ground cover. Tape a white piece of paper nearthe end of a 3-foot-long stick. Hold the stick in one hand so as to have the paperend within 1 inch of a bare soil surface (see Figure 5). Now pour a pint of wateronto the bare soil within 2-3 inches of the white paper and observe the soil accumulationon the white paper. Tape another piece of white paper to the stick and repeat theoperation, this time over soil with 100% ground cover, and observe the accumulationof soil on the paper. Compare the two pieces of paper. This simple test shows howeffective ground cover can be at preventing soil particles from detaching from thesoil surface.
Figure 5. Simple erosion test.
Drawing from Cropland Monitoring guide (Center for Holistice Management, 98)
Detachment of soil particles occurs when falling rainwater collides with bareground. After enough water builds up on the soil surface, following detachment, overlandwater flow transports suspended soil down slope (Figure 6). Suspended soil in therunoff water abrades and detaches additional soil particles as the water travelsoverland. Preventing detachment is the most effective point of erosion control becauseit keeps the soil in place. Other erosion control practices which seek to slow soilparticle transport and cause soil to be deposited before it reaches the stream areless effective at preventing erosion. These latter practices are the ones typicallyimplemented (terraces and diversions). Terraces, diversions and many other erosion"control" practices are basically unnecessary if the ground stays coveredyear round. For erosion prevention, a high percentage of ground cover is a good "earlywarning" indicator of success, while bare ground indicates a high risk of erosion(Croplands monitoring guide, 98). Muddy runoff water and gullies are "too-late"indicators. The soil has already eroded by the time it shows up as muddy water andit's too late to save soil already suspended in the water.
Figure 6. Raindrops fallingon bare ground intitiate erosion.
Drawing from Cropland Monitoring guide, 1998, Center for Holistic Management
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Can a cover crop be worked into your rotation? How about a high-residue crop orperennial sod? Are there economical sources of organic materials or manure in yourarea? Are there ways to reduce tillage and nitrogen fertilizer? Where feasible, bulkyorganic amendments may be added to supply both organic matter and plant nutrients.It is particularly useful to account for nutrients where organic fertilizers andamendments are utilized. Start with a soil test and a nutrient analysis of the materialyou are applying. Knowing the amount of nutrients needed to supply the crop to begrown guides the amount of amendment applied and can lead to significant reductionsin fertilizer purchase. The nutrient composition of organic materials can be variable,which is all the more reason to determine the amount you have with appropriate testing.In addition to containing the major plant nutrients, organic fertilizers can supplymany essential micronutrients. Proper calibration of the spreading equipment is alsoimportant to ensure accurate application rates.
Manure is an excellent soil amendment, providing both organic matter and nutrients.Typical rates for dairy manure would be 10 to 30 tons per acre or 4,000 to 11,000gallons of liquid for corn. At these rates the crop would get between 50 and 150pounds of available nitrogen per acre. Additionally, lots of carbon would be addedto the soil, resulting in no loss of soil organic matter. High crop residues grownfrom this manure application would also contribute organic matter.
However, a common problem with using manure as a crop nutrient source is thatapplication rates are usually based on the nitrogen needs of the crop. Because somemanures often have about as much phosphorus as they do nitrogen, this often leadsto buildup of soil phosphorus. A classic example is chicken litter applied to cropsthat require high nitrogen levels, such as pasture grasses and corn. Broiler litter,for example, contains approximately 50 pounds of nitrogen and phosphorus and about40 pounds of potassium per ton. A common fertilizer application for established fescuepasture would be about 50 pounds of nitrogen and 30 – 40 pounds of phosphorus peracre. If a ton of poultry litter were applied to supply the nitrogen needs of thefescue, an over-application of phosphorus would result. Several years of litter applicationcan build soil phosphorus up to excessive levels. One easy answer is to this dilemmais to adjust the manure rate to meet the phosphorus needs of the crop and to supplythe additional nitrogen with fertilizer or a legume cover crop.
Composting farm manure and other organic materials is an excellent way to stabilizetheir nutrient content. A significant portion of raw-manure nutrients are in unstable,soluble forms. Such unstable forms are more likely to run off if surface applied,or to leach if tilled into the soil. Therefore compost is not a good source of readilyavailable plant nutrients like manures are. Compost releases its nutrients slowly,thereby minimizing losses. Quality compost contains more humus than its raw componentsbecause primary decomposition has occurred during the composting process. It alsodoes not contribute the sticky gums and waxes that aggregate soil particles togetheras much as does raw manure because these substances are also released during theprimary decomposition phase. Unlike manure, compost can be used at almost any ratewithout burning plants. In fact some greenhouse potting mixes contain 20 to 30% compost.Compost (like manure) should be analyzed by a laboratory to determine the nutrientvalue of a particular batch and insure its wise use.
|A common problem with using manure as a crop nutrient source is that application rates are usually based on the nitrogen needs of the crop.|
Composting also reduces the bulk of raw organic materials–especially manures whichoften have a high moisture content. However, while less bulky and easier to handle,composts can be expensive to buy. On-farm composting cuts costs dramatically comparedwith buying compost. For more comprehensive information on composting at the farmor municipal level, request the ATTRA publication entitled On-FarmComposting Resource List.
Many types of plants can be grown as cover crops. Some of the more common onesinclude: rye, buckwheat, hairy vetch, crimson clover, subterranean clover, red clover,sweet clover, cowpeas, millet, and forage sorghums. Each of these plants has advantagesover the others and their area of adaptability. Cover crops can maintain or increasesoil organic matter if they are allowed to grow long enough to produce high herbage.All too often, people get in a hurry and take out a good cover crop just a week ortwo before it has reached its full potential. Hairy vetch or crimson clover can yieldup to 2.5 tons per acre if allowed to go to 25% bloom stage. A mixture of rye andhairy vetch can produce even more.
In addition to the organic matter benefits, legume cover crops provide considerablenitrogen for crops that follow them. Consequently, the nitrogen rate can be reducedfollowing a productive legume cover crop taken out at the correct time. For example,corn grown following 2 tons of hairy vetch should produce high yields of grain withonly 50% of the normal nitrogen rate.
When small grains such as rye are used as cover crops and allowed to reach theflowering stage, additional nitrogen may be required to help offset the nitrogentie-up caused from the high carbon addition of the rye residue. The same would betrue of any high carbon amendment such as sawdust or wheat straw. Cover crops alsosuppress weeds, help break pest cycles, and through their pollen and nectar providefood sources for beneficial insects and honeybees. They can also cycle other soilnutrients making them available to subsequent crops as the green manure decomposes.For more information on cover crops request the ATTRA publication entitled Overviewof Cover Crops and Green Manure. This publication is comprehensive and providesmany references to other available resources on growing cover crops.
Humates and humic acid derivatives are a diverse family of products, generallyobtained from various forms of oxidized coal. Coal-derived humus is essentially thesame as humus extracts from soil but there has been a reluctance in some circlesto accept it as a worthwhile soil additive. In part, this stems from a belief thatonly humus derived from recently decayed organic matter is beneficial. It is alsotrue that the production and recycling of organic matter in the soil cannot be replacedby coal-derived humus. However, while sugars, gums, waxes and similar materials derivedfrom fresh organic-matter decay play a vital role in both soil microbiology and structure,they are not humus. Only a small portion of the organic matter added to the soilwill ever be converted to humus. Most will return to the atmosphere as carbon dioxideas it decays.
Many studies have shown positive effects of humates, while other studies haveshown no such effects. Generally, the consensus is that they work well in low organicmatter soils. In low amounts they do not produce positive results on soils high inorganic matter. At high rates they may tie up soil nutrients.
There are many humus products on the market. They are not all the same. Humateproducts, should be evaluated in a small test plot for cost effectiveness beforeusing. Sales people sometimes make exaggerated claims for their products. ATTRA canprovide more information on humates.
While tillage has become common to many production systems, its effects on thesoil can be counter-productive. Tillage smoothes the soil surface and reduces naturalsoil aggregation and earthworm channels. Porosity and water infiltration are decreasedfollowing most tillage operations. Plow pans may develop in many situations. Tilledsoils have much higher erosion rates than soils left covered with crop residue.
Due to all the problems associated with conventional tillage operations, acreageunder reduced tillage systems is increasing on the American landscape. Any tillagesystem that leaves in excess of 30% surface residue is considered a "conservationtillage" system by USDA (32). Conservation tillage includesno-till, zero till, ridge-till, zone till, and some variations of chisel plowingand disking. These conservation till strategies and techniques allow for establishingcrops into the previous crop's residues, which are purposely left on the soil surface.The principal benefits of conservation tillage are the reduction of soil erosionand improved water retention in the soil, resulting in more drought resistance.
Additional benefits, which many conservation tillage systems provide, includereduced fuel consumption, flexibility in planting and harvesting, reduced labor requirements,and improved soil tilth. Two of the most common conservation tillage systems areridge tillage and no-till.
Ridge tillage is a form of conservation tillage that uses specialized plantersand cultivators to maintain permanent ridges on which row crops are grown. Afterharvest, crop residue is left until planting time. To plant the next crop, the planterplaces the seed in the top of the ridge after pushing residue out of the way andslicing off the surface of the ridge top. Ridges are re-formed during the last cultivationof the crop.
Often, a band of herbicide is applied to the ridge top during planting. With bandedherbicide applications, two cultivations are generally used: one to loosen the soiland another to create the ridge later in the season. No cultivation may be necessaryif the herbicide is applied broadcast rather than banded. Because ridge tillage relieson cultivation to control weeds and reform ridges, this system allows farmers tofurther reduce their dependence on herbicides, compared with either conventionaltill or strict no-till systems.
Maintenance of the ridges is key to successful ridge tillage systems. The equipmentmust accurately reshape the ridge, clean away crop residue, plant in the ridge center,and leave a viable seedbed. Not only does the ridge-tillage cultivator remove weeds,it also builds up the ridge. Harvesting in ridged fields may require tall, narrowdual wheels to be fitted to the combine. This modification permits the combine tostraddle several rows, leaving the ridges undisturbed. Similarly, grain trucks andwagons cannot be driven randomly through the field. Maintenance of the ridge becomesa consideration for each process.
Conventional no-till methods have been criticized for a heavy reliance on chemicalherbicides for weed control. Additionally, no-till farming requires careful managementand expensive machinery for some applications. In many cases, the spring temperatureof untilled soil is lower than that of tilled soil. This lower temperature can slowgermination of early-planted corn or delay planting dates.
Also, increased insect and rodent pest problems have been reported. On the positiveside, no-till methods offer excellent soil erosion prevention and decreased tripsacross the field. On well-drained soils that warm adequately in the spring, no-tillhas provided the same or better yields as conventional till.
A recent equipment introduction into the no-till arena is the so-called "no-tillcultivator." These cultivators permit cultivation in heavy residue and providea non-chemical option to post-emergent herbicide applications. Farmers have the optionto band herbicide in the row and use the no-till cultivator to clean the middlesas a way to reduce herbicide use. ATTRA can provide a number of resource contactson cultural methods, equipment, and management for designing conservation till croppingsystem.
If at all possible, add carbon with nitrogen sources. Animal manure is a goodway to add both carbon and nitrogen. When nitrogen fertilizer is used, try to doit at a time when a heavy crop residue is going onto the soil, too. For example,a rotation of corn, beans, and wheat would do well with nitrogen added after thecorn residue was rolled down or lightly tilled in. Spring planted soybeans wouldrequire no nitrogen. A small amount of nitrogen could be applied in the fall forthe wheat. Following the wheat crop, a legume winter-annual cover crop could be planted.In the spring, when the cover crop is taken out, nitrogen rates for the corn wouldbe reduced to account for the nitrogen in the legume. The addition of legume residuewould also be adding carbon. Avoid continual hay crops accompanied by high nitrogenfertilization. The continual removal of hay accompanied by high nitrogen speeds thedecomposition of soil organic matter. Heavy fertilization of silage crops, whereall the crop residue is removed (especially when accompanied by tillage), speedssoil decline and organic matter depletion.
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As you experiment with new practices and amendments, continue to monitor the soilfor changes using some of the tools discussed in the Assessing Soil Health and BiologicalActivity section. Several of these monitoring guides have data sheets you can usein the field to record data and use for future comparison after changes are madeto the farming practices. Review the principles of sustainable soil management andfind ways to apply them in your operation. If the thought of pulling everything togetherseems overwhelming, start with only one or two new practices and build on them. Seekadditional motivation by reading about the people who have successfully built theirsoils discussed in the next section.
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Steve Groff: Steve and his family produce vegetables, alfalfaand grain crops on 175 acres in Lancaster County, Pennsylvania. When Steve took overoperation of the family farm 15 years ago, his number one concern was eliminatingsoil erosion. Consequently, he began using cover crops extensively in his crop fields.In order to transform his green cover crop into no-till mulch, Steve uses a 10-footBuffalo rolling stalk chopper. Under the hitch-mounted frame, the stalk chopper hastwo sets of rollers running in tandem. These rollers can be adjusted for light oraggressive action and set for continuous coverage. Steve says the machine can berun up to 8 miles an hour and does a good job of killing the cover crop and pushingit right down on the soil. It can also be used to flatten down other crop residuesafter harvest. Groff improved his chopper by adding independent linkages and springsto each roller. This modification makes each unit more flexible to allow continuoususe over uneven terrain. Other farmers report similar results using a disc harrowwith the gangs set to run straight or at a slight angle. Following his cover cropchopping, Groff transplants vegetable seedlings into the killed mulch; sweet cornand snap beans are direct-seeded. Since converting to a cover crop mulch system,his soils are protected from erosion and have become much mellower. For more informationon his system, order Steve's videos listed under the AdditionalInformation Resources section of this publication, or visit his webpage: http://www.cedarmeadowfarm.com/about.htmlAt Steve's web site you can see photos of his cover crop roller and no-till transplanterin action, and test-plot results comparing flail mowing, rolling, and herbicide killingof cover crops.
Bob Willett: Bob started no-tilling 20 years ago on his Pride,KY, corn and soybean farm. Willet not only reduced his machinery costs by switchingto no-till, but also made gains in conserving topsoil. His goal is to develop a healthylevel of humus in the top 2 inches, which keeps the seed zone loose. He has stoppedsidewall compaction in the seed slot that still plagues his neighbors during wetsprings. He attributes this to the increase in humus and organic matter. His soilsurface layer is crumbly and doesn't smear when the disk openers pass through. Willettproclaims that earthworms take the place of tillage by incorporating residue andconverting it to humus. Worms help aerate his soil and improve internal drainagewhich contributes to good rooting for his crops (33). Read moreon Bob's success in "Building Soil with Residue Farming" published in theAugust 1998 issue of Tennesee Farmer (33).
David Iles: On the Iles' North Carolina dairy farm the soil hasactually changed from red to a dark, almost black color since converting to no-tillin 1970. David first learned about no-tillage from his college professor at NorthCarolina State University in 1964. Before Iles switched to no-tillage, his corn silageyielded between 12 and 15 tons per acre in years with adequate rainfall and between4 to 5 tons in dry years indicating moisture was his major limiting factor (34).David realized that his water runoff losses and soil erosion were a direct resultof tillage. Addressing the root cause of the problem, he switched to no-till andbegan to spread manure on 1/3 of his land annually. Since these changes, soil wateris no longer limiting. If he gets adequate rainfall he makes nearly 20 tons of silagenow. David says his land is vastly more productive with increased cation exchangecapacity and increased phosphorus levels due to the humus present in his soil. Thoughhis soil pH ranges in the 5.6 to 5.8 level, he applies no lime. His fields are moreproductive now than when he applied lime in the 70s and more productive than thoseof his neighbors, who currently use lime and fertilizer.
David laments that in his area, "We've lost half of our topsoil in less than100 years. What are future generations going to do if we do not do something to savethe soil?" (34). North Carolina State agronomist Bobby Brockagrees and says that for the first time in history we have the opportunity to producefood and build soil at the same time. Iles reasons that no-till is the way to improvethe soil structure, increase tilth, and increase productivity while still practicingintensive agriculture. He realizes that organic matter is the engine that driveshis system and provides food for earthworms and microorganisms. Iles built his soilby fallowing out 20 to 25 acres of his 380-acre farm each year . On these fallowacres he spreads manure and then sows crops that are not harvested but grown justfor their organic matter.
Even weeds are not clipped but left for their organic matter. David loves hisearthworms and says they are the best employees he has. "They work all the timeand eat dirt for a living" (34).
His best field is one he cleared himself in the 70s. In spite of traditional nativepHs in the high 4s in his area, he did not lime this new ground but instead justplanted rye on it. He had a fine rye crop that year, so he applied liberal manureto it and planted rye a second time. His second rye crop was excellent as well andwas followed by corn the third year. That field yielded the highest corn on the entirefarm. This field has been in continuous corn since 1981 and has never been fertilizedwith conventional products or tilled (34). This field has a pHof 6.1 at a 6-inch depth, an exchange capacity of 8 and an 80% base saturation. Davidbelieves this field's productivity is high because it has never been harmed by tillage.For a complete story of the Iles farm see: Dirnburger, J.M. and Johm M. Larose. 1997.No-till saves dairy farm by healing the harm that tillage has done. National ConservationTillage Digest. Summer. p. 5-8.
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No-till Vegetables by Steve Groff. 1997. This video leads you through selectionof the proper cover crop mix to plant into to how to control cover crops with littleor no herbicide as shown on Steve Groff's Pennsylvania farm. You will see mechanicalcover-crop-kill methods, vegetables being planted right into this mulch using a no-tilltransplanter. You'll also hear comments from leading researchers in the no-till vegetablearea.
Order this video for $21.95 + $3.00 shipping from:
Cedar Meadow Farm
679 Hilldale Road
Holtwood, PA 17532
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Ernst, David T. 1995. The Farmer's Earthworm Handbook: Managing Your-UndergroundMoney Makers. Lessiter Publications, Brookfield, WI. 112 p.
To order this book send $15.95 + $4.00 shipping and handling to:
P.O. Box 624, Brookfield, WI 53008-0624
Gershuny, Grace, and Joe Smillie. 1995. The Soul of Soil: A Guide to EcologicalSoil Management, 3rd edition. agAccess, Davis, CA. 158 p.
To order this book send $16.95 + $4.00 shipping and handling to:
Fertile Ground Books
P.O. Box 2008
Davis, CA 95617
Kinsey, Neil. Neal Kinsey's Hands-On Agronomy. 1993. Acres, USA. Austin,TX. 340 p.
To order this book send $24.00 + $3.00 shipping and handling to:
P.O. Box 91299
Austin, TX 78709
Magdoff, Fred. 1992. Building Soils for Better Crops. University of Nebraska Press,Lincoln, NE. 176 p.
Out of print. Try to locate via interlibrary loan at your local library.
Sachs, Paul D. 1999. Edaphos: Dynamics of a Natural Soil System, 2nd edition.The Edaphic Press. Newbury, VT. 197 p.
To order this book send : $14.95 + $1.50 shipping and handling to:
North Country Organics
P.O. Box 372
Bradford, VT 05033
USDA. 1998. Soil Quality Test Kit. Soil Quality Institute. 82 p.
This publication has detailed step by step instructions with accompanning color photographson how to assess in your own fields: soil quality, soil respiration, soil water infiltration,bulk density, electrical conductivity, soil pH, soil nitrate, soil aggregate stability,slaking, and earthworms. It also covers soil physical observations and estimations,water quality tests, with background information on the tests and apendices.
To order this free test kit publication, paid for by your federal tax dollars,contact:
Cathy A. Seybold
NRCS Soil Quality Institute
Soil Science Department
Agriculture and Life Sciences Building
Oregon State University
Corvallis, OR 97331-7306
NRCS Soil Quality Institute
National Soil Dynamics Lab
411 S. Donahue Drive
Auburn, AL 36832
334-844-4741, ext 176
Sullivan, Preston G. 1998. Early Warning Monitoring Guide for Croplands. Centerfor Holistic Management, Albuquerque, NM. 22 p.
To order this guide send $12.00 ppd. to:
Center for Holistic Management
1010 Tijeras, N.W.
Albuquerque, NM 87102
Lamotte Company. 1994. LaMotte Soil Handbook. Chestertown, MD. 81 p.
Covers soil basics, nutrients, pH, acidity and alkalinity, and principles of theLaMotte soil testing system. Has relative nutrient and pH requirements for commoncrops and plants.
To order this handbook send $4.50 to:
P.O. Box 329
Chestertown, MD 21620
Gempler's Inc. 1998. Gempler's Soil Management Guide. Belleville, WI. 41 p.
An information source for on-site sampling and soil testing. Included in the guideis information on soil quality indicators, visual review, sampling soil, pH, conductivity,compaction, infiltration, nutrients, soil moisture and bulk density, biological activity,and organic matter. In the appendices are a number of USDA soil quality informationsheets.
To order this guide send 4.95 each plus $5.90 shipping to:
100 Countryside Drive
P.O. Box 270
Belleville, WI 53508
National Conservation Tillage Digest. The official publication for state ConservationTillage Associations and Districts.
To order a one-year subscription to this magazine send $25 to:
National Conservation Tillage Digest
P.O. Box 468
Perryville, MO 63775
Zimmer, Gary F. No date. How to get Started in Biological Farming. 11 p.
To order this publication send $3 to:
Highway ID, Box 126
Blue Mounds, WI 53517
Wright, Sara. 1999. Glomalin, a Manageable Soil Glue. 1-page brochure.
To order this free publication contact:
Bldg. 001, Room 140, BARC-W
10300 Baltimore Avenue
Beltsville, MD 20705-2350
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Life in the Soil
This is an excellent website which includes brief overviews of many subjects including:nutrient transformation, biological degradation, soil structure, crop rotation, tillage,soil testing for microbes and organic matter turnover. Color photos of many soilcritters with short descriptions appear on the front webpage. Other drawings andblack and white photos of soil microbes and their effects on soil are on other pagesat this site.
Davy Jone's Homepage: Plant-Soil-Microbial and Rhizosphere Interactions
This webpage contains a complete course in soil microbiology, for free! Davy Jonesis a soil professor who teaches at the University of Wales, Bangor. The microbiologycourse contains detailed notes from 24 lectures in soil microbiology. From the moduleD2503's homepage click lecture notes, then click lecture notes main index, then clickon lecture notes to see the microbiology class. Also available are lecture noteson soil management, soil fertility and world soils, and soil plant interactions andthe role of trees. You can also view microbes in action from quick animated bug moviesshowing dividing bacteria, a bacteria swimming along, and root hairs. There is alot more than I can mention here. Don't miss it at: http://www.safs.bangor.ac.uk/dj/
The Pedosphere and its Dynamics: A Systems Approach to Soil Science
A complete on-line soils textbook covering what soil is, ecological functions ofsoil, soil texture, structure and color, soil formation, Canadian soil classificationsystem, mineralogy, soil reaction, soil water, soil air, soil ecology, soil organicmatter and soil survey. To view this textbook click on the textbook icon at the homepage.Much more information is available from the homepage including educational resources,tutorials, workshops, publications, etc. Visit the University of Alberta's Soil ScienceServer at: http://www.soils.rr.ualberta.ca/
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Soil Foodweb Inc.
1228 NE 2nd Street
Corvallis, OR 97330
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Prepared by Preston Sullivan
ATTRA Technical Specialist