Water-Wise Gardening Science
|Particle type||Diameter of particles in mm||Number of particles per gm||Surface area in sq. cm.|
|Very coarse sand||2.00-1.00||90||11|
|Very fine sand||0.10-0.05||772,000||227|
|Soil Texture||Average Amount|
|Very coarse sand||0.5|
Source: Fundamentals of Soil Science.
Adhering water films can vary greatly in thickness. But if the water molecules adhering to a soil particle become too thick, the force of adhesion becomes too weak to resist the force of gravity, and some water flows deeper into the soil. When water films are relatively thick the soil feels wet and plant roots can easily absorb moisture. "Field capacity" is the term describing soil particles holding all the water they can against the force of gravity.
At the other extreme, the thinner the water films become, the more tightly they adhere and the drier the earth feels. At some degree of desiccation, roots are no longer forceful enough to draw on soil moisture as fast as the plants are transpiring. This condition is called the "wilting point." The term "available moisture" refers to the difference between field capacity and the amount of moisture left after the plants have died.
Clayey soil can provide plants with three times as much available water as sand, six times as much as a very coarse sandy soil. It might seem logical to conclude that a clayey garden would be the most drought resistant. But there's more to it. For some crops, deep sandy loams can provide just about as much usable moisture as clays. Sandy soils usually allow more extensive root development, so a plant with a naturally aggressive and deep root system may be able to occupy a much larger volume of sandy loam, ultimately coming up with more moisture than it could obtain from a heavy, airless clay. And sandy loams often have a clayey, moisture-rich subsoil.
Because of this interplay of factors, how much available water your own unique garden soil is actually capable of providing and how much you will have to supplement it with irrigation can only be discovered by trial.
How Soil Loses Water
Suppose we tilled a plot about April 1 and then measured soil moisture loss until October. Because plants growing around the edge might extend roots into our test plot and extract moisture, we'll make our tilled area 50 feet by 50 feet and make all our measurements in the center. And let's locate this imaginary plot in full sun on flat, uniform soil. And let's plant absolutely nothing in this bare earth. And all season let's rigorously hoe out every weed while it is still very tiny.
Let's also suppose it's been a typical maritime Northwest rainy winter, so on April 1 the soil is at field capacity, holding all the moisture it can. From early April until well into September the hot sun will beat down on this bare plot. Our summer rains generally come in insignificant installments and do not penetrate deeply; all of the rain quickly evaporates from the surface few inches without recharging deeper layers. Most readers would reason that a soil moisture measurement taken 6 inches down on September 1, should show very little water left. One foot down seems like it should be just as dry, and in fact, most gardeners would expect that there would be very little water found in the soil until we got down quite a few feet if there were several feet of soil.
But that is not what happens! The hot sun does dry out the surface inches, but if we dig down 6 inches or so there will be almost as much water present in September as there was in April. Bare earth does not lose much water at all. Once a thin surface layer is completely desiccated, be it loose or compacted, virtually no further loss of moisture can occur.
The only soils that continue to dry out when bare are certain kinds of very heavy clays that form deep cracks. These ever-deepening openings allow atmospheric air to freely evaporate additional moisture. But if the cracks are filled with dust by surface cultivation, even this soil type ceases to lose water.
Soil functions as our bank account, holding available water in storage. In our climate soil is inevitably charged to capacity by winter rains, and then all summer growing plants make heavy withdrawals. But hot sun and wind working directly on soil don't remove much water; that is caused by hot sun and wind working on plant leaves, making them transpire moisture drawn from the earth through their root systems. Plants desiccate soil to the ultimate depth and lateral extent of their rooting ability, and then some. The size of vegetable root systems is greater than most gardeners would think. The amount of moisture potentially available to sustain vegetable growth is also greater than most gardeners think.
Rain and irrigation are not the only ways to replace soil moisture. If the soil body is deep, water will gradually come up from below the root zone by capillarity. Capillarity works by the very same force of adhesion that makes moisture stick to a soil particle. A column of water in a vertical tube (like a thin straw) adheres to the tube's inner surfaces. This adhesion tends to lift the edges of the column of water. As the tube's diameter becomes smaller the amount of lift becomes greater. Soil particles form interconnected pores that allow an inefficient capillary flow, recharging dry soil above. However, the drier soil becomes, the less effective capillary flow becomes. That is why a thoroughly desiccated surface layer only a few inches thick acts as a powerful mulch.
Industrial farming and modern gardening tend to discount the replacement of surface moisture by capillarity, considering this flow an insignificant factor compared with the moisture needs of crops. But conventional agriculture focuses on maximized yields through high plant densities. Capillarity is too slow to support dense crop stands where numerous root systems are competing, but when a single plant can, without any competition, occupy a large enough area, moisture replacement by capillarity becomes significant.
How Plants Obtain Water
Most gardeners know that plants acquire water and minerals through their root systems, and leave it at that. But the process is not quite that simple. The actively growing, tender root tips and almost microscopic root hairs close to the tip absorb most of the plant's moisture as they occupy new territory. As the root continues to extend, parts behind the tip cease to be effective because, as soil particles in direct contact with these tips and hairs dry out, the older roots thicken and develop a bark, while most of the absorbent hairs slough off. This rotation from being actively foraging tissue to becoming more passive conductive and supportive tissue is probably a survival adaptation, because the slow capillary movement of soil moisture fails to replace what the plant used as fast as the plant might like. The plant is far better off to aggressively seek new water in unoccupied soil than to wait for the soil its roots already occupy to be recharged.
A simple bit of old research magnificently illustrated the significance of this. A scientist named Dittmer observed in 1937 that a single potted ryegrass plant allocated only 1 cubic foot of soil to grow in made about 3 miles of new roots and root hairs every day. (Ryegrasses are known to make more roots than most plants.) I calculate that a cubic foot of silty soil offers about 30,000 square feet of surface area to plant roots. If 3 miles of microscopic root tips and hairs (roughly 16,000 lineal feet) draws water only from a few millimeters of surrounding soil, then that single rye plant should be able to continue ramifying into a cubic foot of silty soil and find enough water for quite a few days before wilting. These arithmetical estimates agree with my observations in the garden, and with my experiences raising transplants in pots.
Lowered Plant Density: The Key to Water-Wise Gardening
I always think my latest try at writing a near-perfect garden book is quite a bit better than the last. Growing Vegetables West of the Cascades, recommended somewhat wider spacings on raised beds than I did in 1980 because I'd repeatedly noticed that once a leaf canopy forms, plant growth slows markedly. Adding a little more fertilizer helps after plants "bump," but still the rate of growth never equals that of younger plants. For years I assumed crowded plants stopped producing as much because competition developed for light. But now I see that unseen competition for root room also slows them down. Even if moisture is regularly recharged by irrigation, and although nutrients are replaced, once a bit of earth has been occupied by the roots of one plant it is not so readily available to the roots of another. So allocating more elbow room allows vegetables to get larger and yield longer and allows the gardener to reduce the frequency of irrigations.
Though hot, baking sun and wind can desiccate the few inches of surface soil, withdrawals of moisture from greater depths are made by growing plants transpiring moisture through their leaf surfaces. The amount of water a growing crop will transpire is determined first by the nature of the species itself, then by the amount of leaf exposed to sun, air temperature, humidity, and wind. In these respects, the crop is like an automobile radiator. With cars, the more metal surfaces, the colder the ambient air, and the higher the wind speed, the better the radiator can cool; in the garden, the more leaf surfaces, the faster, warmer, and drier the wind, and the brighter the sunlight, the more water is lost through transpiration.
Suppose you are growing a conventional, irrigated garden
and something unanticipated interrupts your ability to water. Perhaps you are homesteading
and your well begins to dry up. Perhaps you're a backyard gardener and the municipality
temporarily restricts usage. What to do?
Internet Readers: In the print copy of this book are color pictures of my own "irrigationless" garden. Looking at them about here in the book would add reality to these ideas. I suggest you look at "Carrots," "Endive," and "Kale" now.