PLANT NUTRIENTS 13 



INORGANIC PLANT TOXINS AND STIMULANTS 



Much study has been given during recent years to the ques- 

 tion of the supposed poisonous, or toxic, effects upon plants of 

 various soil constituents. There seems to be no doubt that certain 

 organic compounds which are injurious to plant life are often pres- 

 ent in the soil, either as the normal excretions of plant roots or as 

 products of the decomposition of preceding plant growths. A con- 

 sideration of these supposedly toxic organic substances would be 

 out of place in this discussion of mineral soil nutrients. But there 

 seems to be no doubt that there may also be mineral substances 

 in the soil which may sometimes exert deleterious influences 

 upon plant growth. In fact, most metallic salts, except those 

 of the few metals which are required for plant nutrition, appear 

 to be toxic to plants. The exact nature of the physiological effects 

 which are produced by these mineral toxins is not clearly under- 

 stood; indeed, it is probably different in the case of different metals. 

 Further, it is certain that both the stimulating and the toxic 

 effect of metallic compounds upon low forms of plants is quite 

 different from the effects of the same substances upon the more 

 complex tissues of higher plants, a fact which is utilized to advan- 

 tage in the application of fungicides for the control of parasitic 

 growths on common farm crops. 



Among the elements whose physiological effects upon higher 

 plants, such as the cereal crops, etc., when their soluble compounds 

 are present in the soil, have been carefully studied, there are three 

 fairly distinct types of injurious mineral elements. The first of 

 these, represented by copper, zinc, and arsenic, apparently exert 

 their toxic effect regardless of the proportion in which they are 

 present in the nutrient solution which is presented to the plant; 

 although the degree of injury varies with the amount of injurious 

 substance present, of course. The second type, of which boron 

 and manganese are representatives, apparently exerts a definite 

 stimulating effect upon plants when supplied to them in concen- 

 trations below certain clearly defined limits; but are toxic in con- 

 centrations above these. The third includes many soluble salts 

 of magnesium, sodium, potassium, etc., which while either innocu- 

 ous or else definite sources of essential plant foods when in lower 

 concentrations, become highly toxic, or corrosive, when present 

 in the soil solution in concentrations above the limits of " tolera- 



