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Part IV, continued: 

  The phenomenon of toxicosis of soils has been known for a long time in agricultural practice and has always attracted the attention of many investigators.

  Toxicosis expresses itself in the suppression of the growth and development of higher plants, and in the lowering of crop yields. The phenomenon of toxicosis is frequent under monocultures. In such cases, one speaks of the soil exhaustion as the reason for the suppressed growth of plants.

  Tiring of soils was observed by agriculturists and scientific specialists. Plank (1795), De Candol (1813), Daubeni (1845), Uzral (1852), and others, indicated the lowering of soil fertility under monocultures and explained it by an accumulation of toxic substances. Later more attention was devoted to this phenomenon by many investigators: Kossovich (1905), Pryanishnikov (1928), Timiryazev (1941), Vorob'eva and Shchepetil'nikova (1936), Krasil'nikov and Garkina (1946) and others (of Krasil'nikov and Mirchink, 1955, Grummer, 1955).

  Soils toxicosis expresses itself in relation to both higher plants and lower plants--bacteria, fungi, actinomycetes, algae, etc. Much date has been accumulated on fatigue and toxicosis of soils and the significance of this factor for the fertility of the latter. However, the essence of this phenomenon remained obscure until now. There are different points of view concerning its causes, but they can all be reduced to two basic ones.

  According to one opinion, soil toxicosis is caused by the accumulation of special toxic substances as a result of growing plants against the rules of agrotechnique (Ishcherekov, 1910; Whitney and Cameron, 1914; Greig-Smith, 1913, 1918 and others). According to the other point of view, the existence of toxins in the soil is denied, and the fatigue of soils is explained by lack of nutrient substances, as a result of their unbalanced withdrawal from the soils in case of monocultures (Ressel, 1933; Pryanishnikov, 1928; Kossovich, 1905; Hutchinson and Thaysen, 1918).

  Whitney and Cameron (1914) during their studies of soil fatigue found that plants in such soils do not suffer from lack of food but from accumulation of large amounts of toxins. Ishcherekov noted the possibility of removing toxic substances from the soil by washing with water. After washing, the plants grow better and produce normal crops. Thorough studies by Greig-Smith show that in Australian soils toxic substances accumulate in considerable amounts. Their concentration depends upon the type of soil, season of the year and other external factors. According to his observations, the toxins are thermolabile and are destroyed upon boiling and by drying.

  Hutchinson and Thaysen (1918) studied European soils and found that the toxic substances which accumulate in them are thermostable, unlike those formed in Australian soils.

  Many investigations have shown that soil toxicosis in relation to microorganisms is observed much more often and expressed more strongly than toxicosis in relation to higher plants.

  It has long been known that microorganisms, pathogens and saprophytes which enter the soil do not grow and sooner or later perish. The bactericidity of soils was noted by Garre (1887), Freudenreich (1889) and others, They showed that pathogenic bacteria of the colon group, pyogenic cocci and diphtheria bacilli perish in the soil. Microbes such as the tubercle bacillus, the bacillus of anthrax and many others also perish in the soil (cf. Mishustin and Perteovskaya, 1954).

  The soil possesses the ability to rid itself of pathogenic bacteria entering it. The rate of autoliberation from these microbes differs in various soils (Table 104).

  Many pythopathogenic fungi and bacteria cannot remain in soils for prolonged periods of time. The survival of certain bacteria of this group upon being introduced into the soil is as follows: Bact. armeniaca --6-8 days, Bact. citri --6-40 days, Bact. aroideae --3-15 days and Bact. tabacum --7-14 days.

  Bact. malvacearum, Bact. citriputeale, Bact. amylovorum and others die relatively quickly in the soil (cf. Gorlenko, 1950). The death of the fungus Pythium ultimum in forest humus was observed by Peitsa (1952). According to this author, humus from under various woody plants possessed different toxicity; the strongest toxicity was found in extract of humus from under pine, next from under beech and the the weakest of all, from under birch. The antimicrobial properties of soil are not less sharply expressed in relation to saprophytic bacteria, fungi, actinomycetes and other microorganisms. Members of the soil microflora, which come from other soils often also perish in the soil.

  Root-nodule bacteria introduced into clover-tired soil do not grow, and die comparatively quickly. Thus, from 65,000 bacteria introduced per one gram of a soil:

after two days 15,000 bacteria remained
after three days 1,000 bacteria remained
after five days, 10 bacteria remained.

  After ten days the bacteria disappeared almost completely and only a few cells were found.

  Kazarev (1907) observed, that the fungus Pyronema confluens grew well in sterilized soil and did not grow in nonsterile soil. Extract of nonsterile soil added to the sterile soil made the latter unsuitable for the fungus. A similar phenomenon was also observed by Novogrudskii (1936 a). The toxic substance causing the death of the fungus is thermolabile; it can be destroyed by heating to 120° C.

  Numerous data are available concerning the adaptability of Azotobacter, rootnodule bacteria and certain sporeforming bacteria to soil.

  Most strongly expressed and most widespread toxicosis is observed in soils of the podsol zone. According to our observations, there is either no Azotobacter growth in these soils, or it dies fairly quickly.

  During our many years of study of soil microflora, we have investigated thousands of samples of podsol soils taken from various places of the Soviet Union.

  Selected indexes of soil-toxicosis distribution in the various districts are given in Table 105.

  As can be seen from the data given, podsol forest and virgin field soils are not suitable for Azotobacter. All the 717 fields soils and 237 forest soils were toxic. Very seldom does one encounter samples of weakly-cultivated field soils (116 out 2,100 of studied samples), where Azotobacter grows. Well-cultivated and fertilized garden soils are less toxic or not toxic at all. Of 1,863 samples 1,244 contained Azotobacter at a greater or lesser density and 23 samples proved to be toxic for it.

  Of all the podsol soils studied by us, those which were studied in greatest detail were the soils of the Moscow district on the fields of the experimental station in Chashnikovo and the Academy of Agricultural Sciences im. Timiryazev. These soils are loams with considerable leaching, Soils from under forests, with different woody species (spruce grove, birch wood, aspen grove, oak grove, etc), and soils of glades covered by grassy vegetation, soils weakly -cultivated which were plowed one or two years ago, soils cultivated for a long time (15-20 years and more) and soils of a renewed forest were studied.

  In all cases the investigations were conducted all the year round; the samples for analysis were taken at 6-10 day intervals during the summer months and once a month during the winter. The soils were analyzed while fresh.

  We determined the toxicity of soils by the viability of Azotobacter and by germination of seeds of plants (wheat, beet, etc). At the same time a total count of the microflora was made, including microbial inhibitors, which form toxic substances.

  Studies have shown that many soils contain toxic substances. In forest soils, as a rule, there are more of these substances than in forest-free soils, there are less in plowed soils and still less in well-cultivated ones.

  The toxicity of forest soils is determined by the varieties of trees growing in it. The greatest amount of toxic substances is found under spruce grove, and in a smaller amount, under pine and aspen grove. Soils under birch wood and oak grove are weakly toxic or nontoxic at all.

  In Table 106 data are given on the toxic action of soils on germination of seeds of beet and wheat and on Azotobacter.

Table 106
Toxic action of forest podsol soils of the Moscow Oblast'

Soils	Germination of beet seeds, %	Germination of wheat seeds, %	Time of death of Azotobacter cells (in hours)
From under a spruce-grove	1	5	2-4
From under a pine-grove	5	25	20
From under a birch-grove	50	80	72
From under an oak-grove	80	90	82
Fallow cultivated soil	72	90	30 days

  After clearing a forest and plowing, the soil becomes less toxic; Azotobacter does not perish for several days or even weeks. With renewal of the forest, the soil's toxicity is also restored (Krasil'nikov, Mirchnik and others, 1955).

  The formation of toxic substances in chernozem soils under an artificially planted forest in the region of the southern steppes was observed by Runov (1953).

  Plots covered by forests in the chestnut-soil zone of the Trans-Volga region, lose their Azotobacter. We observed a similar picture upon afforestation of soils in Central Asia, Kirghizia, Vakhsh valley of the Tadzhik SSR, Moldavia and other places. Soils rich in Azotobacter, lose them as soon as certain varieties of trees start growing in them.

  Formation of toxic substances in soils is observed under certain grassy plants, particularly often in monocultures.

  While studying the clover-exhausted soils of the experimental fields of the Agricultural Academy im. Timiryazev, we observed that they were obviously toxic for Azotobacter and for root-nodule bacteria, as well as for plants (Krasil'nikov and Garkina, 1946).

  Toxicity changes considerably with the seasons of the year, as do many other properties of soil. It is most apparent during the summer-autumn months (July-September); in the late autumn and in winter it decreases and approaching spring it reaches its minimum. Azotobacter perishes more quickly in summer soils than in winter ones, while in spring soils, it even grows (Table 107).

Table 107
Toxicity of soils in relation to the season of the year
(Survival of Azotobacter in different months, hours)

Soils	July	Aug.	Sept.	Oct.	Dec.	Jan.	Feb.	April	June
Forest	2	6	2	2	24	24	72	216	96
Weakly cultivated	24	24	24	72	72	120	--	216	96
Well cultivated: under Timothy grass	24	24	2	6	24	40	24	24	--
Well cultivated: under clover	72	96	96	96	120	216	--	216	--

  Similarly, seeds of beet germinate with greater energy and in greater numbers in spring soils (April-May) than in summer-autumn ones (August-October).

  According to Rybalkina, toxicity of soils in relation to Bac. mycoides is lost after cool rainy weather.

  According to the observations of Reiner and Nelson-Jones (1949), the greatest toxicity in the forest fields of Wareham (England) is detected in the autumn-winter months, with a decrease beginning in March.

  One may assume that the increase and decrease of soil toxicity is caused by quantitative fluctuations in the toxin content. Toxic substances are either washed out by rain waters in the autumn and thaw waters in the spring, as it was assumed by Reiner and Nelson-Jones, or they are inactivated by the low temperature in the winter. For the verification of these assumptions we conducted special experiments.

  In one series of experiments the soil (forest) was thoroughly washed with water and studied for toxicity. In washed soil Azotobacter perished at the same rate as in nonwashed soil. As can be seen, the toxic substances present in the soil which we studied are not washed out with water or only a part of them is washed out, as may be assumed, the part that is not adsorbed by the soil particles. The majority of the toxic substances are probably in the adsorbed state. Therefore, the decrease of the soil toxicity in the spring is not caused by washing out by rains and thawing snow but, one may assume, by the action of winter frosts.

  In another series of experiments we subjected forest soil to freezing at minus 15-20° C for two months, and after thawing, an Azotobacter culture was introduced into it and the soil was incubated at 25° C. The results showed that in control soil, maintained at room temperature, Azotobacter died after one and a half hours while in soil which had been subjected to freezing, it did not die for 96 hours.

  The soils investigated by us were not inactivated by heating at 100° C for 30 minutes, Inactivation was not attained even after autoclaving at 120° C for 30 minutes. In "exhausted" soils as we have shown earlier, the toxic substances are destroyed and disappear, at a temperature of 100° C maintained for 30 minutes, Obviously, the nature of the inhibitory substances in these soils is different.

  Cultivation of podsol soils exerts a large influence on their toxic properties. Azotobacter grows better in chalked soils than in nonchalked soils (cf. Sushkina, 1940).

  Many investigators (Christenson, 1915; Gainey, 1918, 1940 and others) ascribe the absence of Azotobacter in podsol soils to their acidity. They think that Azotobacter cannot grow in soils which have a pH of 5.5 or lower, If one even occasionally finds this microbe, it is considered to be of a special acidophilic species (Az. indicum). According to these authors, as soon as one neutralizes acid soils, the ordinary Azotobacter (Az. chroccoocum) will start to grow and accumulate in them.

  There exists another opinion, according to which proliferation of Azotobacter is conditioned, not by acidity, but by the ratio of the anions CO₃ : PO₄ (Niclas, Poshenrider and Mock, 1926), by the condition of the oxide and suboxide salts of metals (aluminum, iron, etc).

  There are indications that, Azotobacter does not grow in many acidic soils after chalking, when the pH comes close to neutral (6.5--7.0).

  Levinskaya and Malysheva (1936) observed that introduction of CaCO₃ into acidic soil (podsol of Murmansk Oblast') does not improve the growth of Azotobacter.

  Brenner (1924) chalked acid soils of Finland and introduced Azotobacter. This measure did not decrease the soil toxicity. The introduced microbe perished as fast as in the nonchalked soils. According to the author. toxicosis of podsol soils is caused by special toxic substances. formed upon decomposition of plant residues (moss, etc) and also by toxic iron compounds.

  Katznelson (1940) studied the viability of Azotobacter in acidic soils of America and tested various organic and mineral fertilizers with and without neutralization of the soil. The author reached the conclusion that there was no strict correlation between soil acidity and viability of Azotobacter. At pH 5.9 its number may be higher than at pH 6.6. At the same pH value of soil, Azotobacter proliferates in some cases and does not grow in others.

  In our experiments on neutralization of soils by CaCO₃, MgO, NaOH the conditions of growth of Azotobacter in podsol forest soil improved considerably, but its toxicity was not completely eliminated. Under field conditions chalking also failed to reduce toxicosis of soil. As in the experiments of Brenner, we did not find Azotobacter in podsol, sparsely cultivated and well-chalked soils. We did not find it either in the year when chalking was applied, or 1-3 years later. Azotobacter was not detected in these soils even after a single introduction of manure. Azotobacter introduced in such soil died out at a slower rate than in the control, surviving 6-10 days and more, however its multiplication was not observed. Upon introduction of potassium nitrate or potassium phosphate the death of the introduced Azotobacter was speeded.

  Therefore, the suppressing factor consists, not only of soil acidity, but also of many agents: physical, chemical and mechanical ones. For instance suboxide salts of aluminum, iron and other compounds, which, by the way, are in direct relationship to the pH of the environment, may inhibit growth of Azotobacter.

  However, the main factors which cause soil toxicosis are, in our opinion, in many (if not in all) cases, excretion products of plants and microbial metabolites.

  Schreiner and Reed (1907) found that roots of certain plants excrete toxic substances. When they grew wheat repeatedly in the same vessel containing sand or soil, they observed a decrease in crops after each new sowing; this differed in various soils (Table 108).

Table 108
Wheat crops upon repeated sowing in the same soil (after Schreiner and Reed)
(in % of the first sowing)

Soil and region	First sowing	Second sowing	Third sowing	Fourth sowing
Clay--Cecille	100	68	57	44
Loamy--Leonardstown	100	30	37	23
Clay--Tacoma	100	53	53	46
Sandy--Portsmouth	100	64	30	--

  Application of fertilizers causes a certain increase in crops but only after the first sowing. The authors obtained the strongest effect after the application of lime and manure. The crop of the first sowing was as follows (per cent of control):

Control (without fertilizer), 100
Mineral fertilizer, 130
The same plus lime, 205
Manure, 200
Manure plus lime, 238

  Upon repeated sowing with the same amount of fertilizers the crop decreased. The solution from under wheat was not suitable for this plant: seeds did not germinate well in it, and seedlings were clearly retarded. The inhibition of root growth of flax seedlings was especially strong in this solution. When nutrient substances were added to this solution, only the growth of the aerial parts improved, but the roots remained undeveloped.

  Toxic substances obtained from the substrate (from the solution or from the soil or sand) in which wheat grew, are thermolabile, inactivated by boiling, and absorbed by charcoal and chalk. If a toxic solution In filtered through charcoal or chalk, or subjected to heating, plants grow normally in it. The addition of pyrogallol to the soil extract removes its toxic properties. The same effect is obtained with the use of naphthylamine. These substances differ in their chemical composition and belong to the picolinic acid, salicylaldehyde, vanillin, and dihydroxystearic acid.

  The experiments of Schreiner and Reed were repeated by Periturin (1911, 1912) with the same results. According to his data, the root excretions of wheat suppress not only the growth of the wheat seedling but also that of the oat. Schmuck (1911) grew cereals in sand after wheat; in some cases he cut the wheat at the root, while in others he let it grow to the end. After the harvest of crops, wheat or oats were sown again. In some containers roots of wheat were introduced into the sand. These experiments showed that the presence of wheat roots lowered the crop of oats to 76.8% and that of wheat to 45.2%.

  Molliard (1915) tested the effect of root excretions of peas and corn, grown under sterile conditions, on seedlings of the same plants. The data obtained by him agree with those of Schreiner and Reed.

  Hedrick (1905) observed an inhibitory effect of root excretions of oats on the growth of young apricot trees, The effect of root excretions of potatoes and tomatoes was less strongly expressed. A still smaller effect was that of roots of mustard and rape. Root excretions of beans and clover did not suppress growth of the above-mentioned trees. Similar phenomena were observed in the horticulture department of the Experimental Station of Woburn (USA), where it was shown that root excretions of grass suppressed the growth of young peripheral root tips of apples and pears which constitute the most active part of the root system.

  Jones and Morse described the inhibitory action of the nut tree (gray nut--Jualans cinerea L.) on the growth of the creeping cinquefoil shrub. The latter does not grow in the vicinity of this tree to a distance of approximately twice the diameter of the foliage. Jensen has experimentally established that the root excretions of maple, cornel, cherry, tulip, and pine suppress the growth of wheat; the strongest suppression was observed in the summer, during the period of plant growth, when root excretions were more abundant than in autumn (after Schreiner and Reed, 1907).

  Pickering (1903-1913) observed the deleterious effect of grasses on growth of fruit trees. He grew grasses in baskets with soil, hung under the foliage of an apple tree. The water coming through during downpours irrigated the soil around the apple tree. The roots of the latter were poisoned and the plants perished.

  Fletcher (1912) observed the high sensitivity of Sesamum indicum L. to root excretions of Andropogon sorghum Brot. According to his observations, this plant cannot ripen in the vicinity of sorghum. Sewell (1923) found that roots of sorghum are toxic to wheat. Shull (1932) did not confirm the results of Fletcher and Sewell. According to his data, sorghum has no toxic effect on plants.

  Mashkovtsev (1934) observed the thinning out of rice plantations after 2- 3 years of monoculture. The author thinks that the reason for this was the presence of toxic substances in the soil.

  Ahlgren and Aamodt (1930) studied the interaction between different plants by growing them separately and together in containers. When Timothy grass was grown together with spear grass, or meadow grass with spear grass or with Timothy grass, much lower yields were obtained than when they were grown separately. The dry weight in grams of the plants in isolated cultures was as follows:

Spear grass, 0.396
Meadow grass, 0.343
Flat meadow grass, 0.450
Timothy grass, 0.577
Mixture:
Spear grass, 0.244
Timothy grass, o.360
Mixture:
Meadow grass, 0.195
Spear grass, 0.311
Mixture:
Flat meadow grass, 0.260
Meadow grass, 0.264

  Waks (1039) found toxic substances in the root excretions of Robinia pseudoacacia L.

  Many investigators connect the formation of toxic substances in soil with the growth of plants (Jakes, 1937; Rippel, 1936, Winter and Bublitz, 1953, Dimond and Waggoner, 1953, Nutman, 1952; Grümmer, 1955 and others).

  It has been found that in many plants there are special substances--cholines and blastocholines (blastanein--germination and cholycin--to prevent), inhibiting germination of their own seeds and also of seeds of other species of plants. The nature of those substances differs in different plants. They may be excreted by the roots of seedlings and suppress growth of neighboring plants. Root excretions of seedlings of birch suppress the growth of rye grass and lychins: root excretions of seedlings of wheat and rye grass suppress germination of moods of certain weeds of Anthemis arvensis L. and Metricaria inodora L; seedlings of beans suppress germination of seeds of flax and wheat and seedlings of violets inhibit germination of wheat seedlings (after Audus, 1953).

  Benedict (1941) observed the death of Bromus inermis Leyss, after its repeated sowing for many years in the field. The soil of such fields was toxic for the plant itself and for certain other plants. an well. Introduction of fertilizers did not abolish the toxicosis but only somewhat diminished it.

  Bode (1940) described the poisonous effect of root excretions of Artemisia absinthium L. on the growth of fennel, caraway, sage and other plants sown in its vicinity. The height of the anise stem at a 70 cm distance from absinth was 5.7 cm at a distance of 100 cm--17 cm and at a distance of 130 cm--39 cm. A toxic substance was isolated, which was found to be a glucoside. This glucoside, called absinthin, is formed in the leaves of absinth and in easily extracted with water; it is washed out by the rains and enters the soil under the plant crown. It is preserved for prolonged periods in the soil.

   There are indications that accumulation of toxins in the soil also takes place under fruit trees. Proebsting and Gilmore (1940) have shown that soil from under old peach trees is toxic for young peach saplings. Martin (1950-1951) found the same to be true for soils that have been under lemon trees for a long time. Plants planted on plots that have never before been under citrus grove a grew 9% faster than plants planted on old citrus-grove soils. Tomatoes and other vegetables grew quite satisfactorily on soils of old lemon groves and showed no signs of repressed growth.

  According to Martin, the toxicity of much "exhausted" soils was not eliminated by washing with water for six weeks. Only by treatment with 2% sulfuric acid or 2% KOH and subsequent saturation with calcium did he succeed in removing the toxicity and restoring the fertility of the soil.

  The inhibitory effect of root excretions of grassy plants, mustard, tobacco, tomato, and others in noted by many authors. Their effect in expressed in grassy plants and woody varieties as well. The degree of their inhibitory effect varies from 6 %to 97 % depending on plant species and on external conditions (Livingston, 1023; Breazeal, 1924; Conrad, 1927 and others, cf. Grammer, 1955).

  Guyot (1951) ascribes a decisive role in toxicosis of soil to root excretions. He investigated a great number of plant species, and detected in many of them the ability to form toxins and to excrete them into the soil. These plants can be divided into groups according to the intensity of their toxic action. If one were to use Brachypodium pinnatum P. B. as a control plant which does not form toxins, with an index of 100, the remaining plants will have the following indexes:

Helianthemum vulgare Gärten., 85, weakly toxic
Barkhausia foetids Mönch., 81, weakly toxic
Thymus serpyllum L., 75, moderately toxic
Hieracium pilosella L., 70, moderately toxic
Origanum vulgare L., 70, moderately toxic
Asperula cyanchica L., 69, moderately toxic
Teucrium chamaedrys L., 68, moderately toxic
Picris hieraciodies L., 65, moderately toxic
Papaver rhoeas L., 61, moderately toxic
Achillea millefolium L., 59, moderately toxic
Hieracium umbellatum L., 39 strongly toxic
Solidago virga aurea L., 30, strongly toxic
Hieracium vulgatum Fr., 29, strongly toxic

  Upon repeated sowings of the same plants, their seeds progressively germinate less well, and the mature plants yield smaller and smaller crops. According to Guyot and Massenot (1950), Hypericum perforatum L. under the same sowing conditions had in the first year a density of 4,200 plants per plot, the year after--1,100 and on the third year--500 plants on the same plot. Similar results are given by Curtis and Cottham (1950) with various species of sunflower. Hurtis (1953) tested the effect of root excretions of corn, peas, wheat. oat, rye, and lucerne, on the germination of mustard seeds. Excretions of barley roots caused a strong inhibition of seed germination, root excretions of other plants showed no such effect. According to Schilling (1951), cauliflower grows better if there in celery in its vicinity. Piettre (1950) noticed a strong poisoning of soils under many-year plantations of coffee plants. He isolated a substance belonging to the fatty acids from these soils. The most ubiquitous among them is lignoceric acid (C₂₄H₄₈O₂) which strongly suppresses the growth of plant seedlings.

  The Swedish scientist Oswald (1947) studied the toxicity of soils under certain grassy plants. According to his data. seeds of rape--Brassica napus, and B. rapa L. germinated very weakly or not at all in soil from under Agropyrum. repens P. B. or Festuca rubra L. The author succeeded in isolating a substance inhibiting germination of rape seeds from roots of the couch grass. The toxicity of soils on which couch grass or Festuca rubra L. have been grown was removed by heating at 80-90° C (Oswald, 1949, 1950).

  According to Schuphan (1948), lettuce suppresses growth of radish seed and radish is toxic to the development of lettuce seeds. Lettuce excretes, according to this author, saponin, and radish excretes mustard oil. The toxic substance was extracted from the leaves with ether and obtained in crystaline form (0. 5 mg from 1 g of leaves). It proved to be 3-acetyl- 6-methoxybenzaldehyde (Bonner 1950)

  Lyubich (1955) tested the interaction between various woody plants. Planting various species in pairs on the same hole it was found that certain species inhibit the growth of others (Table 109).

Table 109
Interaction between woody plants

Plants	English oak	Green ash (Fraximus viridus)	Box elder	Indian bean	Japanese pagoda tree	Elm (Ulmus pinnato- ramosa
English oak	+	+	+			-
Green ash (Fraximus viridus)	+	+	-	+	-	-
Box elder	+	-	+	-
Indian bean		+	-	+
Japanese pagoda tree		+	-
Elm (Ulmus pinnato-ramosa)	-	-

Note; a plus sign means good growth (no suppression), a minus sign means suppression of growth.

  An is evident from the table, each of the tested plants suppressed the development of shoots of any other type.

  According to Guyot, root excretions of Hieracium pilosella L., even at low concentrations, are toxic to many plants. Vigorov has found that by its root excretion Agropyrum repens P. B. inhibits the growth of seedlings of pine and Caragana (after Grümmer, 1955).

  Rodygin (1955) observed a 40-80% cancer morbidity of the lime tree when it was grown in the vicinity of asp. In the neighborhood of other plants (pine, spruce, fir) the percentage of cancerous lime tree did not exceed 2-3%.

  According to Bordukova (1947) the kind of plants that grow in the vicinity of potatoes is important. In the vicinity of sunflower, tomatoes, apple, cherry, raspberry, pumpkin or cucumber, the resistance of potatoes to Phytophthora was lowered. Potatoes grown in the vicinity of a birch wood rot more easily than potatoes grown in the vicinity of pine.

  One cannot combine narcissus and lily-of-valley flowers together in one bunch since they would soon wither; similarly, mignonette increases the withering of flowers in a vase.

  The prolonged studies of Bonner and his co-workers (1938-1948) have shown that guayule roots excrete a toxic substance--trans -cinnamic acid. Ten milligrams of this acid in 1.5 kg of soil completely inhibits germination of guayule seeds. The higher the concentration of this substance, the longer it remains in the soil. One milligram of cinnamic acid endures 14 days in nonsterile soil and in sterile soil for an even longer period of time.

  The shrub Encelia parinosa Adana, (of the compositae) grows in deserts. It is characterized by the fact that no grass grows for a certain distance around it. Investigations have revealed that the soil under the crown of this shrub is poisoned by the toxic substances, formed by its leaves. An extract of the leaves, or even better, the leaves themselves when introduced into the soil arrest growth of other plants--tomatoes, pepper and rye. These toxins do not act on the growth of Encelia, barley, oats and sunflowers.

  It is well known that a great number of plants produce various compounds possessing toxic properties in relation to bacteria and to plants. Substances such as glucosides, saponin and coumarin are very widespread among plants. Inhibitors of the saponin group were found in hay stacks which suppress the germination of seeds and growth of algae (Moewus and Bonnerjee, 1951, Lindahl, Cokk et al, 1954), These substances reach the soil with plant residues. Upon decay of the latter they re liberated and may exist in the soil, in active form for a certain length of time, affecting the microflora and the higher plants. According to Benedict (1941). dead roots of brome grass release toxic substances upon their decomposition which suppress germination of brome-grass seeds. The above-mentioned absinth and Encelia release their toxins upon the decomposition of their aerial parts.

  Golomedova (1954) studied the effect of aqueous extracts of many grasses and shrubs on plant growth. Tartar honeysuckle, maple, ash tree, buckthorn and amorpha inhibit growth of fescue and Euagropyrum, Extracts of couch grass, root and green parts of Austrian absinth inhibit oak saplings.

  Feldmeier and Guttenberg (1953) obtained alcoholic and ether extracts from seeds and seedlings of beans, which inhibited growth of oat coleoptiles.

  Bublitz (1954) tested the effect of extracts of pine branches, decomposing in soil, on the germination of Lepidium sativum L, seeds and on the growth of certain bacteria in compost. The extracts noticeably inhibited growth of bacteria and germination of seeds, more so in an acid medium (pH-5.6) than in a neutral one.

  Lindahl, Cook et al, (1954) obtained a toxic substance of the saponin type from lucerne hay, According to Mishustin (1956) these substances are excreted by the roots of lucerne during vegetation,

  Callison and Conn (1927) and McCalla (1948, 1949) found toxic substances of the soil in the form of decomposition products of plant residues. They established the presence of the following compounds among these substances: vanillin, coumarin, dehydrostearic acid, salicylic acid and other compounds. Small doses of these substances, noticeably inhibited growth of plants.

  Toxic substances are present in plants of many species of the umbellate family: poison hemlock, poisonous cicuta, water dropwort, marshwort, Anthriscus and others. In their tissues one finds phthalides, various tars, esters, acids and other compounds. In Cruciferae plants one finds mustard oils.

  Various volatile substances are present in many odorous plants. In some plants they are formed in the seeds and fruits, while in others in the leaves and stems or in the roots. Essential oils of a series of plants: citrus plants clove, mint, Satureia, thymes, germander, eucalyptus, etc and the resin of coniferous trees, poplar and others inhibit the germination of seeds of various plants to various degrees (Weintraub and Pricae, 1948, Grümmer, 1955, Molisch 1937; Madaus, 1936, Clausen, 1932).

  Lebodev (1948) has found that absinth inhibits growth of flax, peas, beans, sage and clove. Roots of ash excrete volatile substances which inhibit growth of the oak.

  Golubinski (1946) observed the stimulation of pollen germination by volatile substances formed by plants.

  Solov'ev (1954) found that volatile substances from certain plants (Agropyrum pectiniforme R. et Sch.) stimulate the germination of lucerne pollen, those of other plants (Bromus intermis Leyss, Phelum Pretense L.) inhibit and still others have no effect at all.

  The volatile substances excreted by onion, garlic and horse radish are well known (cf. Tokin, 1951).

  In agricultural practice plants forming volatile substances have, since ancient times, been used as preservatives against spoilage of foodstuffs, For instance peasants put pieces of garlic into the cornbins for protection against the weevil, and against Agrostis segetum they use branches of the bird cherry tree (Grimm, 1950).

  Volatile substances excreted by plants have a definite effect on phytopathogenic microflora and may play a protective role, Fahlpahl (1949) observed a protective effect of hemp, The volatile substances which it excreted inhibited growth of certain pathogenic microorganisms, due to which plants growing in the vicinity of hemp were less subjected to diseases. Schilling (1951) gives data on the protective effect of celery. When cabbage grows in the vicinity of this plant it is less affected by microorganisms.

  Pirozhkov (1950) noted the lethal effect of the volatile substances of tomatoes on certain insects attacking the gooseberry shrub, as Tenthrodinodea and Pyralididae. According to this author's observations gooseberry shrub in the vicinity of tomatoes do not suffer from these insects.

  Certain organic acids of plant origin are also toxic for seedlings of a number of plants, They are often found in the fruits. Malic and citric acids, in apples; 3.4-dihydroxycinnamic acid and 3-methoxy-4-hydroxycinnamic acid in tomatoes, transcinnamic acid, in guayule, etc (Akkerman and Veldstra, 1947),

  The alkaloids are very widespread among plants. Some of them inhibit growth of plants. The most well known are cocaine, physiostigmine, aconite, caffein and quinine,

  It should be noted, that the nature of the toxic substances excreted by plants is unknown in the majority of cases. Grümmer (1955) relates these substances to a special group of specific substances--the cholines,

  Recently, artificially produced substances have penetrated more and more into agricultural practice; these substances exert a certain inhibitory effect on plants, Substances have been obtained that put put potato tubers "to sleep." To these belong a series of compounds--methyl esters of alpha-naphthylaceatic acid, ß-naphtyldimethyl ester, isopropylphenylcarbonate and certain other esters of phenylcarbonic acid (Krylov, 1954). Small doses of these substances (0.1% of aqueous solution) keeps tubers from sprouting in storehouses (Moewus and Schader, 1951). Preparation M-1 (methyl ester of ß-naphthylacetic acid) is offered for use in the preservation of potatoes, A dosage of 1.5- 3 kg of this substance is enough to protect 1 ton of potatoes from sprouting, increasing their yield by 10-14%, and decreasing weight losses 2.5-5 times, it also protects the starch and vitamin C, decreases the accumulation of the glucoside and solanin in the tubers (Krylov, 1954).

  Pteroylglutamic acid (4-amino-9-methylpteroylglutamic acid) and indolyl-3-acetic acid suppress growth of roots and, to a lesser extent, inhibit the growth of the aerial parts of the plants.

  Maleic hydrazide causes a long-lasting inhibition of plant growth. At a concentration of 0.01% this compound suppresses the growth of the raspberry for 24-38 days and ripening of berries for 16-33 days. This substance is recommended for use on lawns, i. e., it strongly checks the growth rate of grass.

  Suppression of plant growth by chemical preparations in widely used in agriculture, In a number of cases it is necessary to arrest the development of buds and especially the beginning of flowering in fruit trees (apples, pears, aprocits, peaches, etc), These substances can also be used in decorative plant breeding, when it is necessary to arrest the growth of plants, etc.

  It should be noted, that many substances of the auxin group may act as inhibitior or herbicides and as stimulants as well. For example, the well-studied compound- 2.4-dichlorophanoxyacetic acid (2.4-D) sometimes stimulates and sometimes suppresses seed germination, depending on the concentration used. Para-aminobenzoic acid has a stimulating effect at a concentration of 0.001% and strongly inhibits seed germination and plant growth at a concentration of 0.05%. Nicotinic acid at a concentration of 0.01% is also a strong poison for plants.

  The toxic substances differ in their stability. Some of them are quite stable, are not destroyed upon prolonged stay in the soil, and may be concentrated to a greater or smaller extent. Other substances are easily destroyed and vanish from the soil. In such cases, plowing is enough to remove the toxicity of the soil. The vegetative cover, fertilizers and other measures also change actively the toxic substances in the soil,

  Certain substances are easily leached by rains and are removed from the soil, while others are in the adsorbed state and are not eluted by water.

  Studies show that there is a direct relationship between the concentration and length of time of preservation of toxins in the soil, and the qualitative and quantitative composition of the soil microflora,

  In sterile soils the active substances are preserved for considerably longer periods of time than in nonsterile soils. When sterile soil is inoculated with a particle of nonsterile soil, the toxins in it soon become inactivated as in nonsterile soil (Brown and Mitchell, 1948, Audus, 1953, Jorgensen and Hammer, 1946),

  Stapp and Spicher (1955) and Jensen and Peterson (1932) have described bacteria which strongly destroy herbicides. Their activity aids in the liberation of soil from toxins.

  As can be seen from the above-mentioned data, the accumulation of toxic substances under natural conditions may vary, depending on the type of soil, the nature of the substance and on external conditions. Some substances accumulate in considerable quantities, while others remain in small concentrations or are not accumulated at all. The concentration of these substances determines the degree of toxicosis and fatigue of soils.

  Under natural conditions. in soils and other substrates into which toxic substances constantly enter there is some degree of accumulation of the latter. At certain concentrations of toxins in the soil, poisoning of plants may take place.

  The question of whether toxins can themselves enter plants is answered in the affirmative.

  It was experimentally shown, that a number of toxic substances penetrate the plants via the roots and spread to the tissues. The mere fact of the poisonous effect of inhibitors in the above experiments show that these substances penetrate the plants. There are also direct experiments with chemically pure substances obtained from bacteria, fungi and actinomycetes. Gramicidin--a preparation obtained from the sporeforming bacterium--Bac. brevis, possesses strongly toxic properties; it poisons the tissues of animals as well an those of plants. Small doses of it in the nutrient medium cause rapid browning of roots, withering of aerial parts, and death of the whole plant.

  Pyocyanin, a substance formed by the blue pus bacillus--Ps. pyocyanea, is endowed with strongly toxic properties. Plants-- wheat and clover--perished within a few hours under its influence. Substances obtained from many actinomycetes are also toxic for plants: mycetin, lavendulin, actinomycetin, longisporin, etc. Among the fungal metabolites, notatin, glyotoxin, and some others possess herbicidal properties.

  Toxins and antibiotics may enter the plants through their leaves. If a drop of the solution of a toxic substance is placed on the surface of the plant, after some time one observes symptoms of poisoning, not only in the tissues that have been in direct contact with the solution, but in distant parts as well. Sometimes the whole branch withers away. This phenomenon of poisoning of branches and leaves was observed by us in birch, due to the action of the toxin produced by the fungus Botrytis cinerea (Krasil'nikov, 1953b).

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