PLANT NUTRIENTS AND THEIR ABSORPTION 123 



The manner in which these toxic substances check growth is shown 

 by a study of the absorption of mineral constituents. Though absorp- 

 tion is always decreased, the various toxins have more or less specific 

 effects. Cumarin and salicylic aldehyde depress potash and nitrate 

 absorption more than phosphate absorption; quinone depresses phosphate 

 and nitrate more than potash , dihydroxystearic acid and perhaps vanil- 

 lin, retard phosphate and potash more than nitrate absorption. 



Protecting Against Toxins. — The harmful effects of these toxins may 

 be counteracted in numerous ways. Fertilizer treatment is efficacious; as 

 might be expected, various salts act differently in overcoming the respec- 

 tive effects of the toxic substances. Phosphatic fertilizers, for example, 

 are most efficient in overcoming the effects of cumarin, potassic fertilizers 

 in overcoming the effects of quinone and nitrogenous fertilizers in over- 

 coming the effects of vanillin. 



Another way of ameliorating the effects of toxic substances in the soil 

 is treatment with absorbing agents. Roots appear able to oxidize 

 organic materials in such a way that their toxic properties are lost. The 

 large amount of root surface which most plants have makes this oxidizing 

 power important in relation to the destruction of toxic substances through 

 crop rotation. 



Schreiner, Reed and Skinner^^^ found that toxic solutions lost much of their 

 toxicity after plants had been grown in them. They state: "The vanillin' 

 solution, for example, was so reduced in toxicity that a solution originally con- 

 taining 500 parts per million was no more toxic to the second set of plants than 

 a solution of 50 parts per million was to the first. It has been found that an 

 equal number of wheat plants can remove in a similar length of time not more 

 than 30 to 50 parts per million of nitrates from solution and there is no reason 

 to believe that toxic substances should be removed at a much more rapid rate." 



Breazeale^^ reports that peat extract in dilute concentrations (20 parts 

 per million) and calcium carbonate protect citrus seedlings against the 

 toxicity of distilled water, usually associated with the presence of small 

 amounts of copper. Sodium carbonate on the other hand augments the 

 toxicity of soluble organic matter. 



Thus, "When soluble organic matter which is acid in reaction and stimulating 

 to citrus seedUngs in concentrations up to 1,000 parts per million or more is added 

 to a sodium carbonate solution of 400 parts per milUon which in itself is not 

 toxic, a highly toxic solution is formed which will kill the root tips of citrus 

 seedlings. This reaction appears to be of importance in connection with the 

 toxicity of soils containing small amounts of sodium carbonate."'* 



Importance in the Fruit Plantation. — To just what extent organic soil 

 toxins are important in the fruit plantation is not known. That they are 

 of greater significance than is generally realized there can be no question. 



