Inorganic Plant Poisons and Stimulants TOC




C. F. CLAY, Manager
Edinburgh: 100 PRINCES STREET


London: H. K. LEWIS, 136 GOWER STREET, W.C.
Bombay and Calcutta: MACMILLAN AND CO., Ltd.
Toronto: J. M. DENT AND SONS, Ltd.

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Fellow of University College, London
(Rothamsted Experimental Station)

Cambridge: at the University Press 1914



During the last century great and widespread changes have been made in agricultural practice—changes largely associated with the increase in the use of artificial fertilisers as supplements to the bulky organic manures which had hitherto been used. The value of certain chemical compounds as artificial manures is fully recognised, yet many attempts are being made to prove the value of other substances for the same purpose, with a view to increase in efficiency and decrease in cost. The interest in the matter is naturally great, and agriculturists, botanists and chemists have all approached the question from their different standpoints. In the following pages an attempt is made to correlate the work that has been done on a few inorganic substances which gave promise of proving useful in agricultural practice. Much of the evidence put forward by different workers is conflicting, and it is clear that no definite conclusions can yet be reached. Nevertheless, examination of the evidence justifies the hope that results of practical value will yet be obtained, and it is hoped that the analysis and coordination of the available data put forward in this book will aid in clearing the ground for those investigators who are following up the problem from both the academic and the practical standpoints.

W. E. B.


October 1914.


I. Introduction
II. Methods of Working
I. Discussion of Methods
1. Water cultures
2. Sand cultures
3. Soil cultures in pots
4. Field experiments
II. Details of Methods
III. Effect of Copper Compounds
I. Presence of Copper in Plants
II. Effect of Copper on the Growth of Higher Plants
1. Toxic effect
(a) Toxic action of copper compounds alone in water cultures
(b) Masking effect caused by addition of soluble substances to solutions of copper salts
(c) Effect of adding insoluble substances to solutions of copper salts
(d) Effect of copper on plant growth when present in soils
(e) Mode of action of copper on plants
2. Effect of copper on germination
(a) Seeds
(b) Spores and pollen grains
3. Does copper stimulate higher plants?
4. Action of copper on organs other than roots
(a) Effect of copper sprays on leaves
(b) Effect of solutions of copper salts on leaves
III. Effect of Copper on Certain of the Lower Plants
IV. Effect of Zinc Compounds
I. Presence of Zinc in Plants
II. Effect of Zinc on the Growth of Higher Plants
1. Toxic effect
(a) Toxic action of zinc salts alone in water cultures
(b) Effect of soluble zinc salts in the presence of nutrients
(c) Effect of zinc compounds on plant growth when they are present in soils
(d) Mode of action of zinc on plants
2. Effect of zinc compounds on germination
3. Stimulation induced by zinc compounds
(a) Stimulation in water cultures
(b) Stimulation in sand cultures
(c) Increased growth in soil
4. Direct action of zinc salts on leaves
III. Effect of Zinc on Certain of the Lower Plants
V. Effect of Arsenic Compounds
I. Presence of Arsenic in Plants
II. Effect of Arsenic on the Growth of Higher Plants
1. Toxic effect
(a) Toxic action of arsenic compounds in water cultures in the presence of nutrients
(b) Toxic effect of arsenic compounds in sand cultures
(c) Toxic effect of arsenic when applied to soil cultures
(d) Physiological considerations
2. Effect of arsenic compounds on germination
3. Do arsenic compounds stimulate higher plants?
III. Effect of Arsenic Compounds on Certain of the Lower Plants
1. Algae
2. Fungi
VI. Effect of Boron Compounds
I. Presence of Boron in Plants
II. Effect of Boron on the Growth of Higher Plants
1. Toxic effect
(a) Toxic action of boron compounds in water cultures
(b) Toxic action of boron compounds in sand cultures
(c) Toxic action of boron compounds in soil experiments
2. Effect of boron compounds on germination
3. Does boron stimulate higher plants?
(a) Water cultures
(b) Sand cultures
(c) Soil cultures
III. Effect of Boron Compounds on Certain of the Lower Plants
VII. Effect of Manganese Compounds
I. Presence of Manganese in Plants
II. Effect of Manganese on the Growth of Higher Plants
1. Toxic effect
(a) Toxic action of manganese compounds in the presence of soluble nutrients
(b) Toxic action of manganese compounds in sand cultures
(c) Toxic action of manganese compounds in soil cultures
2. Effect of manganese compounds on germination
3. Does manganese stimulate higher plants?
(a) Stimulation in water cultures
(b) Stimulation in soil cultures
III. Effect of Manganese Compounds on Certain of the Lower Plants
IV. Physiological Considerations of Manganese Stimulation
VIII. Conclusions

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Inorganic Plant Poisons and Stimulants – Conclusions


In the foregoing chapters a very limited number of plant poisons have been considered, yet there is sufficient evidence to show that even these few differ considerably in their action upon plant-life. This action is most variable, and it is impossible to foretell the effect of any substance upon vegetative growth without experiments. The degree of toxicity of the different poisons is not the same, and also one and the same poison varies in the intensity and nature of its action on different species of plants. While certain compounds of copper, zinc and arsenic are exceedingly poisonous, compounds of manganese and boron are far less deleterious, so that a plant can withstand the presence of far more of the latter substances than of the former. Again, the tested compounds of copper, zinc and arsenic do not seem to stimulate growth, even when they are applied in the smallest quantities, whereas very dilute solutions of manganese and boron compounds decidedly increase growth. But, differentiation occurs even in this stimulative action, for while manganese is the more effective in stimulating barley, boric acid is far more potent for peas, the shoots being particularly improved.

A consideration of the experimental work that has been done on this subject of poisoning and stimulation leads one to the inevitable conclusion that it is not true to maintain the hypothesis that all inorganic plant poisons act as stimulants when they are present in very small quantities, for while some poisons do increase plant growth under such conditions, others fail to do so in any circumstances. It is probable that what has been found true with the few substances tested would prove to be similarly true over a much wider range of poisons, and at any rate the hypothesis must be dismissed in its universal application. A more accurate statement would be that some inorganic poisons act as stimulants when present in small amounts, the stimulating concentrations varying both with the poisons used and the plants on which they act.

It is quite possible for a stimulation in one respect to be correlated with a retardation in another. In the Rothamsted experiments on the action of manganese sulphate on barley the weaker concentrations of the salt improved the vegetative growth, as was shown by the increase in the dry weights, but with the same strengths of the poison the ripening of the grain was retarded, so that, while certain of the physiological functions were expedited, others were hindered by the action of the poison.

Thus it is evident that it is exceedingly difficult sharply to characterise either toxic or stimulant action. In neither case is the reaction simple—many factors may come into play and many processes are concerned, while the effect of a so-called poison may vary in respect of each of the functions and processes concerned. If the poison is presented in great strength the toxic action is dominant, and probably affects many functions in the same sense, so that the action is, so to speak, cumulative. Lower down in the scale of concentration differentiation of action may set in, and while some processes may still be hindered, others may be stimulated. If the two actions balance one another an apparent indifference may be manifested, so that it seems that such strengths of the poison have no effect on growth, either harmful or beneficial. At still lower concentrations, with certain plants and certain poisons, the stimulative action overpowers the toxic effect, so that in some respect or other improvement occurs in growth.

It is quite conceivable, however, that some poisons are truly indifferent in weak concentrations, as no stimulation makes itself evident under any circumstances. In these cases one is inclined to suspect that the action is somewhat more simple, in that the toxic effects gradually diminish until no poisonous action is manifest at very weak concentrations, and as no stimulation is present to bring the growth above the normal with these very weak concentrations the plant is similar to those grown without any addition of the poison.

The modus operandi of these stimulative agents is not yet fully understood. Perhaps at the present time two main theories hold the field: (1) that they act as catalytic agents, being valueless on their own account, but valuable in that they aid in the procuring of essential food substances; (2) that the stimulants themselves are of integral value for nutrition. The French school, with Bertrand at the head, hold strongly to the catalytic theory, maintaining that manganese and boron compounds are able to increase growth if they are present in small quantities, as they act as “carriers” whereby the various functions of the plant are expedited by the increased facility with which the essential nutritive elements are supplied. The manganese in laccase, for instance, is held to be an oxygen carrier, whereby the oxygen is first absorbed and then released for the benefit of the plant, the manganese being regarded as essential for the functioning of the enzyme. But, if these elements are essential, this theory seems to stop short of the truth. If certain functions are dependent for their very occurrence upon the presence of even minute traces of any element, then surely that element is as essentially a nutrient element, as vital to the well-being of the plant as is such an element as carbon or nitrogen or phosphorus, even though the latter occurs in far greater quantity. It is necessary that one should free one’s mind from the idea that the quantity of an element present in a plant is an index of its value to the plant. Naturally enough, in the early days of plant physiology, the most abundant elements first engaged the attention of investigators, and they were divided into essential and non-essential, ten elements being classed in the former category. More recent work is beginning to show that other elements are constantly present in plants, but in such small quantities that the older and cruder methods of analysis failed to reveal them, so that until lately they have been completely overlooked in work on plant nutrition. Even yet we do not know which of these other elements are essential and which are merely accidental. While we do know that the ten essential elements (C, H, O, N, S, P, K, Mg, Fe, Ca) are necessary for the well-being of all plants, it is conceivable that these other substances which only occur in very small quantities may be more individual in their action, and that while a trace of a certain element may be absolutely essential to one plant, that same element may be quite indifferent for another species. If one takes a broad outlook, the two theories seem to be in reality only parts of one, the “nutrition” theory carrying matters a little farther than the “catalytic” idea, broadening its scope and extending its application.

It seems probable that all the experimental work that has been discussed will prove to be simply preliminary to a far greater practical application of the principle of stimulation or increased growth. While the physiologists have been feeling their way towards the conclusions put forth on this subject, the agriculturists have been discovering and extending the application of artificial manures, until at the present time such manuring is coming into its own and is receiving more of the widespread attention that it deserves. The possibility now exists that in some respects the two lines of work are converging and that the more purely scientific line will have a big contribution to make to the strictly practical line. Artificial manuring aims at improvement of the soil and crop by the addition of food substances that are needed in a particular soil, a result that used to be obtainable only by the use of the bulky farmyard manure, seaweed, &c. Apart from any other aspect of the matter the artificials, when intelligently used, are far more easy to handle and to regulate in supply, and they yield excellent results, especially in conjunction with a certain proportion of organic manures. The further prospect now opened up is the possibility of utilising some of these stimulating compounds as artificial manures. As only small traces are beneficial, larger amounts being poisonous, it is obvious that only small quantities would be needed, and, as the compounds are not usually very expensive, a considerable increase of crop for a relatively small outlay might be anticipated if no complicating factors intervened. Very much work will be required in the field to test the value of these substances, as their action may be influenced by the nature of the soil, climatic conditions, general conditions of manuring, and the crops grown. Some tests have already been made, especially in Japan, with boron and manganese, and these indicate a promising field for investigation.

Above all, it is most important to realise that one is approaching an entirely unexplored field, and that it is inevitable that the results of the initial experiments will be contradictory, at least in appearance, so that it is necessary to keep an open mind on the subject, being ready to modify one’s ideas as circumstances require, as improved experimental methods lead on to more accurate results.


Inorganic Plant Poisons and Stimulants – Methods of Working


I. Discussion of Methods.

In the course of the scattered investigations on plant poisons and stimulants, various experimental methods have been brought into use, but these all fall into the two main categories of water and soil cultures, with the exception of a few sand cultures which hold a kind of intermediate position, combining certain characteristics of each of the main groups.

The conditions of plant life appertaining to soil and water cultures are totally different, so different that it is impossible to assume that a result obtained by one of the experimental methods must of necessity hold good in respect of the other method. A certain similarity does exist, and where parallel investigations have been carried out this becomes evident, but it seems to be more or less individual, the plant, the poison and the cultural conditions each playing a part in determining the matter.

1. Water cultures.

This method of cultivation represents the simplest type of experiment. Its great advantage is that the investigator has absolute control over all the experimental conditions. Nutritive salts and toxic substances can be supplied in exact quantities and do not suffer loss or change by interaction with other substances which are beyond control. Any precipitates which may form in the food solution are contained within the culture vessel and are available for use if needed. The results are thus most useful as aids in interpreting the meaning of those from the field experiments, the results of the one method frequently dovetailing in with those of the other in a remarkable way. The disadvantage of the water culture method is that it is more or less unnatural, as the roots of the plants are grown in a medium quite unlike that which they meet in nature, a liquid medium replacing the solid one, so that the roots have free access to every part of the substratum without meeting any opposition to their spread until the walls of the culture vessel are reached. The conditions of aeration are also different, for while the plant roots meet with gaseous air in the interstices of the soil, in water cultures they are dependent upon the air dissolved in the solution, so that respiration takes place under unusual conditions. It is possible that the poverty of the air supply can be overcome by regular aeration of the solution, resulting in decided improvement in growth, as L. M. Underwood (1913) has shown in recent work on barley in which continued aeration was carried out.

2. Sand cultures.

This method has the advantage over water cultures in that the environment of the plant roots is somewhat more natural, but on the other hand the work is cumbersome and costly, while the conditions of nutrition, watering, &c., are less under control than in the water cultures. Sand cultures represent an attempt to combine the advantages of both soil and water cultures, without their respective disadvantages. Generally speaking perfectly clean sand is used varying in coarseness in different tests, and this is impregnated with nutritive solutions suitable for plant growth. The sand is practically insoluble and sets up no chemical interaction with the nutritive compounds, while it provides a medium for the growth of the plant roots which approximates somewhat to a natural soil. It is probable, however, that a certain amount of adsorption or withdrawal from solution occurs, whereby a certain proportion of the food salts are affiliated, so to speak, to the sand particles and are so held that they are removed from the nutritive solution in the interspaces and are not available for plant food, the nutritive solution being thus weakened. The same remark applies to the poisons that are added, so that the concentration of the toxic substance used in the experiment does not necessarily indicate the concentration in which it is presented to the plant roots. On the other hand, undue concentration of the solution is apt to occur on account of the excessive evaporation from the surface of the sand. The sand particles are relatively so coarse in comparison with soil particles that the water is held loosely and so is easily lost by evaporation, thus concentrating the solution at the surface, a condition that does not apply in soil work. With care this disadvantage is easily overcome as it is possible to weigh the pots regularly and to make up the evaporation loss by the addition of water.

3. Soil cultures in pots.

In this case the conditions of life are still more natural, as the plant roots find themselves in their normal medium of soil. But the investigator has now far less control, and bacterial and other actions come into play, while the nutrients and poisons supplied may set up interactions with the soil which it is impossible to fathom. This method is useful in the laboratory as it is more convenient for handling and gives more exact quantitative results than plot experiments. Also the pots can be protected from many of the untoward experiences that are likely to befall the crops in the open field. The conditions are somewhat more artificial, as the root systems are confined and the drainage is not natural, but on the whole the results of pot experiments are very closely allied to those obtained in the field by similar tests.

4. Field experiments.

These make a direct appeal to the practical man, but of the scientific methods employed the field experiments are the least under control. The plants are grown under the most natural conditions of cultivation it is possible to obtain, and for that reason much value has been attached to such tests. Certainly, so far as the final practical application is concerned, open field experiments are the only ones which give information of the kind required. But from the scientific point of view one very great drawback exists in the lack of control that the investigator has over the conditions of experiment. The seeds, application of poison, &c., can all be regulated to a nicety, but the constitution of the soil itself and the soil conditions of moisture, temperature and aeration introduce factors which are highly variable. No one can have any idea of the composition of the soil even in a single field, as it may vary, sometimes very considerably, at every step. Further, no one knows the complicated action that may or may not occur in the soil on the addition of extraneous substances such as manures or poisons. Altogether, one is working quite in the dark as to knowledge of what is going on round the plant roots. It is impossible to attribute the results obtained to the direct action of the poison applied. While the influence may be direct, it may also happen that certain chemical and physical interactions of soil and poison occur, and that the action on the plant is secondary and not primary, so that a deleterious or beneficial result is not necessarily due to the action of the toxic or stimulating substance directly on the plant, but it may be an indirect effect induced possibly by an increase or decrease in the available plant food, or to some other physiological factor. Consequently great care is needed in interpreting the results of field experiments without the due consideration of those obtained by other methods.

II. Details of Methods.

Many details of the sand and soil culture methods have been published by various investigators, e.g. Hiltner gives accounts of sand cultures, while the various publications issued from Rothamsted deal largely with the soil experiments. As this is the case, and as all crucial experiments have always been and must always be done in water cultures, it is only necessary to give here full details of these.

The great essential for success in water culture work is strict attention to detail. Cleanliness of apparatus and purity of reagents are absolutely indispensable, as the failure of a set of cultures can often be traced to a slight irregularity in one of these two directions. Purity of distilled water is perhaps the greatest essential of all. Plant roots are extraordinarily sensitive to the presence of small traces of deleterious matter in the distilled water, especially when they are grown in the absence of food salts. Ordinary commercial distilled water is generally useless as the steam frequently passes through tubes and chambers which get incrusted with various impurities, metallic and otherwise, of which slight traces get into the distilled water. Loew (1891) showed that water which contained slight traces of copper, lead or zinc derived from distilling apparatus exercised a toxic influence which was not evident in glass distilled water. This poisonous effect was removed by filtering through carbon dust or flowers of sulphur. Apparently only about the first 25 litres of distilled water were toxic, in the later distillate the deleterious substance was not evident.

The best water to use is that distilled in a jena glass still, the steam being passed through a jena glass condenser. For work on a large scale, however, it is impossible to get a sufficient supply of such water, while the danger of breakage is very great. Experiments at Rothamsted were made to find a metallic still that would supply pure water. While silver salts are very injurious to plant growth it was found that water that had been in contact with pure metallic silver had no harmful action. Consequently a still was constructed in which the cooling dome and the gutters were made of pure silver without any alloy, so placed that the steam impinged upon the silver dome, condensed into the silver gutter and was carried off by a glass tube into the receptacle. Such water proved perfectly satisfactory so long as any necessary repairs to the still were made with pure silver, but a toxic action set in directly ordinary solder was employed. More recently a new tinned copper still has been employed with good results, but this is somewhat dangerous for general purposes, as in the event of the tin wearing off in any place, copper poisoning sets in at once. The water is always filtered through a good layer of charcoal as a final precaution against impurity.

In the Rothamsted experiments no attempt is made to carry on the cultures under sterile conditions. Bottles of 600 c.c. capacity are used, after being thoroughly cleaned by prolonged boiling (about four hours) followed by washing and rinsing. The bottles are filled with nutritive solution and the appropriate dose of poison, carefully labelled and covered with thick brown paper coats to exclude the light from the roots and to prevent the growth of unicellular green algae. The corks to fit the bottles are either used brand new or, if old, are sterilised in the autoclave to avoid any germ contamination from previous experiments. Lack of care in this respect leads to diseased conditions due to the growth of fungi and harmful bacteria. Two holes are bored in each cork, one to admit air, the other to hold the plant, and the cork is cut into two pieces through the latter hole.

The seeds of the experimental plants are “graded,” weighed so that they only vary within certain limits, e.g. barley may be ·05–·06 gm., peas ·3–·35 gm., buckwheat ·02–·03 gm. In this way a more uniform crop is obtained. Great care is needed in selecting the seeds, the purest strain possible being obtained in each case. With barley it has always proved possible to get a pure pedigree strain, originally raised from a single ear. In this way much of the difficulty due to the great individuality of the plants is overcome, though that is a factor that must always be recognised and reckoned with. The seeds are sown in damp sawdust—clean deal sawdust, sifted and mixed up with water into a nice crumbly mass—and as soon as they have germinated and the plantlets are big enough to handle they are put into the culture solutions. Barley plants are inserted in the corks with the aid of a little cotton wool (non-absorbent) to support them, care being taken to keep the seed above the level of the water, though it is below the cork. With peas it is impossible to get a satisfactory crop if the seed is below the cork, as the plant is very prone to bacterial and fungal infection in its early stages, and damp cotyledons are fatal for this reason. Consequently the mouths of the bottles are covered with stout cartridge paper, the pea root being inserted through a hole in the paper, so that the root is in the liquid while the cotyledons rest on the surface. As soon as sufficient growth has been made the papers are replaced by corks, the remnants of the seeds still being kept on top in the air. Other plants are treated according to their individual needs and mode of germination.


Fig. 1. Diagrammatic sketches showing methods of setting up water cultures.

A.   a.   Seedling of cereal.
b. Cork bored with two holes, and cut into two pieces through one hole.
c. Food solution.
B. a. Pea seedling.
b. Paper shield which supports the seedling.
c. Brown paper cover over bottle of food solution.

The constitution of the nutritive solution is important, and it is becoming more and more evident that different plants have different optima in this respect. For several years a solution of medium strength was used, containing the following:

Potassium nitrate 1·0 gram
Magnesium sulphate   ·5    „
Sodium chloride   ·5    „
Calcium sulphate   ·5    „
Potassium di-hydrogen phosphate   ·5    „
Ferric chloride   ·04  „
Distilled water to make up 1 litre.

This is an excellent solution for barley plants, giving good and healthy growth. While peas grew very well in it, they showed some slight signs of over-nutrition. A weaker solution is being tested which gives very good results. Peas grow very strongly in it and it also seems to be sufficiently concentrated to allow barley to carry on its growth long enough for the purposes of experiment. The solution is as follows:

Sodium nitrate   ·5 gram
Potassium nitrate   ·2    „
Potassium di-hydrogen phosphate   ·1    „
Calcium sulphate   ·1    „
Magnesium sulphate   ·1    „
Sodium chloride   ·1    „
Ferric chloride   ·04  „
Distilled water to make up 1 litre.

The latter solution was made up so that the quantity of phosphoric acid and potash approximated more or less to the amount of those substances found by analysis in an extract made from a good soil.

The experiments are usually carried on for periods varying from 4–10 weeks, six weeks being the average time. Careful notes are made during growth and eventually the plants are removed from the solutions, the roots are washed in clean water to remove adherent food salts, and then the plants are dried and weighed either separately or in sets. In order to reduce the error due to the individuality of the plants, five, ten or even twenty similar sets are grown in each experimental series, the mean dry weight being taken finally. Also the same experiment is repeated several times before any definite conclusions are drawn.

Another method of water cultures is used by some investigators, in which the experiments only last for a few hours or days, usually 24–48 hours. While such experiments may not be without value for determining the broader outlines of toxic poisoning, they fail to show the finer details. The effect of certain strengths of poison is not always immediate. Too great concentrations kill the plant at once, too weak solutions fail to have any appreciable immediate action and so appear indifferent. Between the two extremes there exists a range of concentrations of which the effect varies with the plant’s growth. A solution may be of such a nature and strength that at first growth is seriously checked, though later on some recovery may be made, while it is also possible that a concentration which is apparently indifferent at first may prove more or less toxic or stimulant at a later date, according to circumstances. Consequently too much stress must not be laid upon the results of the short time experiments with regard to the ultimate effect of a poison upon a particular plant.

An examination of the various experimental methods shows that while no one of them is ideal, yet each of them has a definite contribution to make to the investigation of toxic and stimulant substances. Each method aids in the elucidation of the problem from a different standpoint, and the combination of the results obtained gives one a clearer picture of the truth than could be obtained by one method alone. Water cultures, with their exactitude of quantitative control lead on by way of sand cultures to pot cultures, and these to field experiments in which the control is largely lost, but in which the practical application is brought to the front.