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Part IV, continued: 

 

Toxicosis of Soils and Biological Factors Causing It

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

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

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

  Soils toxicosis expresses itself in relation to bothhigher plants and lower plants--bacteria, fungi, actinomycetes, algae, etc. Muchdate has been accumulated on fatigue and toxicosis of soils and the significanceof this factor for the fertility of the latter. However, the essence of this phenomenonremained 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 causedby the accumulation of special toxic substances as a result of growing plants againstthe 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 toxinsin the soil is denied, and the fatigue of soils is explained by lack of nutrientsubstances, as a result of their unbalanced withdrawal from the soils in case ofmonocultures (Ressel, 1933; Pryanishnikov, 1928; Kossovich, 1905; Hutchinson andThaysen, 1918).

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

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

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

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

  The soil possesses the ability to rid itself of pathogenicbacteria entering it. The rate of autoliberation from these microbes differs in varioussoils (Table 104).

Table 104
Survival of pathogenic bacteria in various soils
(in days)

Bacteria

minimum

maximum

Bact. typhi

15-20

360

Bact. dysenteriae

6-10

270

Vibrio cholerae

6-12

120

Mocob. tuberculosis

60

210

Bact. necrosis

10

75

Mact. melitensis

3-10

90

Bact. pestis

3

30

Bact. tularense

--

75

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

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

  Root-nodule bacteria introduced into clover-tired soildo not grow, and die comparatively quickly. Thus, from 65,000 bacteria introducedper 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 completelyand only a few cells were found.

  Kazarev (1907) observed, that the fungus Pyronemaconfluens grew well in sterilized soil and did not grow in nonsterile soil. Extractof 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 substancecausing the death of the fungus is thermolabile; it can be destroyed by heating to120° C.

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

  Most strongly expressed and most widespread toxicosisis observed in soils of the podsol zone. According to our observations, there iseither 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 placesof the Soviet Union.

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

Table 105
Toxicosis of turf-podsol soils

Soils and region

Total No of samples studied

Samples toxic to Azotobacter

Kola Peninsula

 

 

Forest

43

0

Humus-ferruginous

105

5

Swamps

22

0

Cultivated garden

215

87

 

 

 

Leningrad Oblast'

 

 

Fields, acid

28

0

Fields, neutral

25

5

Garden

17

15

 

 

 

Arkhangel'sk Oblast'

 

 

Virgin

35

0

Cultivated

40

8

Forest

27

0

Garden

23

16

 

 

 

Kaluga Oblast'

 

 

Forest

17

0

Virgin

25

0

Cultivated

32

10

Garden

35

28

 

 

 

Ryazan' Oblast'

 

 

Forest

13

0

Virgin

18

0

Cultivated

21

8

Garden

18

15

 

 

 

Yaroslavl' Oblast

 

 

Virgin

30

0

Cultivated

30

6

Garden

30

27

 

 

 

Karelo-Fin AASR

 

 

Forest

21

0

Field, virgin

30

0

Field, cultivated

50

10

Garden

20

18

 

 

 

Kalinin Oblast'

 

 

Fields, virgin

47

0

Fields, cultivated

53

12

Garden

23

16

 

 

 

Gor'kii Oblast'

 

 

Virgin

20

0

Cultivated

20

6

Garden

20

12

 

 

 

Moscow Oblast', Volokolamak Region

 

 

Forest

13

0

Virgin

33

0

Cultivated

40

5

Garden

50

42

 

 

 

Dmitrovsk Region

 

 

Forest

33

0

Virgin

53

0

Cultivated

67

12

Garden

67

53

 

 

 

State Farm "Krasnyi Mayak"

 

 

Virgin

150

0

Cultivated

200

12

Garden

70

52

 

 

 

Chashnikovo

 

 

Virgin

250

0

Cultivated

350

18

Garden

50

48

Forest

70

0

  As can be seen from the data given, podsol forest andvirgin field soils are not suitable for Azotobacter. All the 717 fields soilsand 237 forest soils were toxic. Very seldom does one encounter samples of weakly-cultivatedfield soils (116 out 2,100 of studied samples), where Azotobacter grows. Well-cultivatedand fertilized garden soils are less toxic or not toxic at all. Of 1,863 samples1,244 contained Azotobacter at a greater or lesser density and 23 samplesproved to be toxic for it.

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

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

  We determined the toxicity of soils by the viabilityof Azotobacter and by germination of seeds of plants (wheat, beet, etc). Atthe 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-freesoils, there are less in plowed soils and still less in well-cultivated ones.

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

  In Table 106 data are given on the toxic action ofsoils 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; Azotobacterdoes not perish for several days or even weeks. With renewal of the forest, the soil'stoxicity is also restored (Krasil'nikov, Mirchnik and others, 1955).

  The formation of toxic substances in chernozem soilsunder an artificially planted forest in the region of the southern steppes was observedby Runov (1953).

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

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

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

  Toxicity changes considerably with the seasons of theyear, as do many other properties of soil. It is most apparent during the summer-autumnmonths (July-September); in the late autumn and in winter it decreases and approachingspring it reaches its minimum. Azotobacter perishes more quickly in summersoils 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 greaternumbers in spring soils (April-May) than in summer-autumn ones (August-October).

  According to Rybalkina, toxicity of soils in relationto 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 detectedin the autumn-winter months, with a decrease beginning in March.

  One may assume that the increase and decrease of soiltoxicity is caused by quantitative fluctuations in the toxin content. Toxic substancesare 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 lowtemperature in the winter. For the verification of these assumptions we conductedspecial experiments.

  In one series of experiments the soil (forest) wasthoroughly washed with water and studied for toxicity. In washed soil Azotobacterperished at the same rate as in nonwashed soil. As can be seen, the toxic substancespresent in the soil which we studied are not washed out with water or only a partof them is washed out, as may be assumed, the part that is not adsorbed by the soilparticles. 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 washingout by rains and thawing snow but, one may assume, by the action of winter frosts.

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

  The soils investigated by us were not inactivated byheating at 100° C for 30 minutes, Inactivation was not attained even after autoclavingat 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° Cmaintained for 30 minutes, Obviously, the nature of the inhibitory substances inthese soils is different.

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

  Many investigators (Christenson, 1915; Gainey, 1918,1940 and others) ascribe the absence of Azotobacter in podsol soils to theiracidity. They think that Azotobacter cannot grow in soils which have a pHof 5.5 or lower, If one even occasionally finds this microbe, it is considered tobe 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 proliferationof Azotobacter is conditioned, not by acidity, but by the ratio of the anionsCO3 : PO4 (Niclas, Poshenrider and Mock, 1926), by the conditionof the oxide and suboxide salts of metals (aluminum, iron, etc).

  There are indications that, Azotobacter doesnot grow in many acidic soils after chalking, when the pH comes close to neutral(6.5--7.0).

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

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

  Katznelson (1940) studied the viability of Azotobacterin acidic soils of America and tested various organic and mineral fertilizers withand without neutralization of the soil. The author reached the conclusion that therewas 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 improvedconsiderably, but its toxicity was not completely eliminated. Under field conditionschalking also failed to reduce toxicosis of soil. As in the experiments of Brenner,we did not find Azotobacter in podsol, sparsely cultivated and well-chalkedsoils. We did not find it either in the year when chalking was applied, or 1-3 yearslater. Azotobacter was not detected in these soils even after a single introductionof manure. Azotobacter introduced in such soil died out at a slower rate thanin the control, surviving 6-10 days and more, however its multiplication was notobserved. Upon introduction of potassium nitrate or potassium phosphate the deathof the introduced Azotobacter was speeded.

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

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

 

Formation of toxic substances by plants

  Schreiner and Reed (1907) found that roots of certainplants excrete toxic substances. When they grew wheat repeatedly in the same vesselcontaining 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 increasein crops but only after the first sowing. The authors obtained the strongest effectafter 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 fertilizersthe 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 inhibitionof root growth of flax seedlings was especially strong in this solution. When nutrientsubstances were added to this solution, only the growth of the aerial parts improved,but the roots remained undeveloped.

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

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

  Molliard (1915) tested the effect of root excretionsof 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 rootexcretions of oats on the growth of young apricot trees, The effect of root excretionsof potatoes and tomatoes was less strongly expressed. A still smaller effect wasthat of roots of mustard and rape. Root excretions of beans and clover did not suppressgrowth of the above-mentioned trees. Similar phenomena were observed in the horticulturedepartment of the Experimental Station of Woburn (USA), where it was shown that rootexcretions of grass suppressed the growth of young peripheral root tips of applesand pears which constitute the most active part of the root system.

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

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

  Fletcher (1912) observed the high sensitivity of Sesamumindicum L. to root excretions of Andropogon sorghum Brot. According tohis 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 theresults of Fletcher and Sewell. According to his data, sorghum has no toxic effecton plants.

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

  Ahlgren and Aamodt (1930) studied the interaction betweendifferent plants by growing them separately and together in containers. When Timothygrass was grown together with spear grass, or meadow grass with spear grass or withTimothy 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 excretionsof Robinia pseudoacacia L.

  Many investigators connect the formation of toxic substancesin 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 specialsubstances--cholines and blastocholines (blastanein--germination and cholycin--toprevent), inhibiting germination of their own seeds and also of seeds of other speciesof plants. The nature of those substances differs in different plants. They may beexcreted by the roots of seedlings and suppress growth of neighboring plants. Rootexcretions of seedlings of birch suppress the growth of rye grass and lychins: rootexcretions of seedlings of wheat and rye grass suppress germination of moods of certainweeds of Anthemis arvensis L. and Metricaria inodora L; seedlings ofbeans suppress germination of seeds of flax and wheat and seedlings of violets inhibitgermination of wheat seedlings (after Audus, 1953).

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

  Bode (1940) described the poisonous effect of rootexcretions of Artemisia absinthium L. on the growth of fennel, caraway, sageand other plants sown in its vicinity. The height of the anise stem at a 70 cm distancefrom absinth was 5.7 cm at a distance of 100 cm--17 cm and at a distance of 130 cm--39cm. 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 withwater; it is washed out by the rains and enters the soil under the plant crown. Itis preserved for prolonged periods in the soil.

   There are indications that accumulation of toxinsin the soil also takes place under fruit trees. Proebsting and Gilmore (1940) haveshown 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 treesfor a long time. Plants planted on plots that have never before been under citrusgrove a grew 9% faster than plants planted on old citrus-grove soils. Tomatoes andother vegetables grew quite satisfactorily on soils of old lemon groves and showedno 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 with2% sulfuric acid or 2% KOH and subsequent saturation with calcium did he succeedin removing the toxicity and restoring the fertility of the soil.

  The inhibitory effect of root excretions of grassyplants, mustard, tobacco, tomato, and others in noted by many authors. Their effectin expressed in grassy plants and woody varieties as well. The degree of their inhibitoryeffect 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 toxicosisof soil to root excretions. He investigated a great number of plant species, anddetected in many of them the ability to form toxins and to excrete them into thesoil. These plants can be divided into groups according to the intensity of theirtoxic action. If one were to use Brachypodium pinnatum P. B. as a controlplant which does not form toxins, with an index of 100, the remaining plants willhave 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 seedsprogressively germinate less well, and the mature plants yield smaller and smallercrops. According to Guyot and Massenot (1950), Hypericum perforatum L. underthe 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. Similarresults 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 causeda strong inhibition of seed germination, root excretions of other plants showed nosuch effect. According to Schilling (1951), cauliflower grows better if there incelery in its vicinity. Piettre (1950) noticed a strong poisoning of soils undermany-year plantations of coffee plants. He isolated a substance belonging to thefatty 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 toxicityof soils under certain grassy plants. According to his data. seeds of rape--Brassicanapus, and B. rapa L. germinated very weakly or not at all in soil fromunder Agropyrum. repens P. B. or Festuca rubra L. The author succeededin isolating a substance inhibiting germination of rape seeds from roots of the couchgrass. The toxicity of soils on which couch grass or Festuca rubra L. have been grownwas removed by heating at 80-90° C (Oswald, 1949, 1950).

  According to Schuphan (1948), lettuce suppresses growthof 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 substancewas extracted from the leaves with ether and obtained in crystaline form (0. 5 mgfrom 1 g of leaves). It proved to be 3-acetyl- 6-methoxybenzaldehyde (Bonner 1950)

  Lyubich (1955) tested the interaction between variouswoody plants. Planting various species in pairs on the same hole it was found thatcertain 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)

-

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


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

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

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

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

  One cannot combine narcissus and lily-of-valley flowerstogether in one bunch since they would soon wither; similarly, mignonette increasesthe 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 -cinnamicacid. Ten milligrams of this acid in 1.5 kg of soil completely inhibits germinationof guayule seeds. The higher the concentration of this substance, the longer it remainsin the soil. One milligram of cinnamic acid endures 14 days in nonsterile soil andin 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 certaindistance around it. Investigations have revealed that the soil under the crown ofthis shrub is poisoned by the toxic substances, formed by its leaves. An extractof the leaves, or even better, the leaves themselves when introduced into the soilarrest growth of other plants--tomatoes, pepper and rye. These toxins do not acton the growth of Encelia, barley, oats and sunflowers.

  It is well known that a great number of plants producevarious 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 germinationof 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 theyre 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). deadroots of brome grass release toxic substances upon their decomposition which suppressgermination of brome-grass seeds. The above-mentioned absinth and Enceliarelease their toxins upon the decomposition of their aerial parts.

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

  Feldmeier and Guttenberg (1953) obtained alcoholicand ether extracts from seeds and seedlings of beans, which inhibited growth of oatcoleoptiles.

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

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

  Callison and Conn (1927) and McCalla (1948, 1949) foundtoxic 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. Smalldoses of these substances, noticeably inhibited growth of plants.

  Toxic substances are present in plants of many speciesof 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 odorousplants. In some plants they are formed in the seeds and fruits, while in others inthe leaves and stems or in the roots. Essential oils of a series of plants: citrusplants clove, mint, Satureia, thymes, germander, eucalyptus, etc and the resinof coniferous trees, poplar and others inhibit the germination of seeds of variousplants to various degrees (Weintraub and Pricae, 1948, Grümmer, 1955, Molisch1937; Madaus, 1936, Clausen, 1932).

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

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

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

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

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

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

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

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

  The alkaloids are very widespread among plants. Someof 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 substancesexcreted by plants is unknown in the majority of cases. Grümmer (1955) relatesthese substances to a special group of specific substances--the cholines,

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

  Pteroylglutamic acid (4-amino-9-methylpteroylglutamicacid) 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 ofplant growth. At a concentration of 0.01% this compound suppresses the growth ofthe raspberry for 24-38 days and ripening of berries for 16-33 days. This substanceis recommended for use on lawns, i. e., it strongly checks the growth rate of grass.

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

  It should be noted, that many substances of the auxingroup 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 stimulatesand sometimes suppresses seed germination, depending on the concentration used. Para-aminobenzoicacid has a stimulating effect at a concentration of 0.001% and strongly inhibitsseed germination and plant growth at a concentration of 0.05%. Nicotinic acid ata concentration of 0.01% is also a strong poison for plants.

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

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

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

  In sterile soils the active substances are preservedfor considerably longer periods of time than in nonsterile soils. When sterile soilis inoculated with a particle of nonsterile soil, the toxins in it soon become inactivatedas 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 inthe liberation of soil from toxins.

  As can be seen from the above-mentioned data, the accumulationof toxic substances under natural conditions may vary, depending on the type of soil,the nature of the substance and on external conditions. Some substances accumulatein considerable quantities, while others remain in small concentrations or are notaccumulated at all. The concentration of these substances determines the degree oftoxicosis and fatigue of soils.

  Under natural conditions. in soils and other substratesinto which toxic substances constantly enter there is some degree of accumulationof the latter. At certain concentrations of toxins in the soil, poisoning of plantsmay take place.

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

  It was experimentally shown, that a number of toxicsubstances penetrate the plants via the roots and spread to the tissues. The merefact of the poisonous effect of inhibitors in the above experiments show that thesesubstances penetrate the plants. There are also direct experiments with chemicallypure substances obtained from bacteria, fungi and actinomycetes. Gramicidin--a preparationobtained from the sporeforming bacterium--Bac. brevis, possesses stronglytoxic properties; it poisons the tissues of animals as well an those of plants. Smalldoses of it in the nutrient medium cause rapid browning of roots, withering of aerialparts, 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--perishedwithin a few hours under its influence. Substances obtained from many actinomycetesare also toxic for plants: mycetin, lavendulin, actinomycetin, longisporin, etc.Among the fungal metabolites, notatin, glyotoxin, and some others possess herbicidalproperties.

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

 




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