280 THE PRINCIPLES OF SOIL MANAGEMENT 



the vital processes of the plant and are essential to its 

 growth. Others seem to have no specific function, but 

 are generally present. 



143. Substances found in ash of plants. The sub- 

 stances commonly met with in the ash of plants are 

 potassium, sodium, calcium, magnesium, iron, aluminum, 

 phosphorus, sulfur, silicon, and chlorine. In addition 

 to these, nitrogen is absorbed from the soil in the form 

 of soluble salts. 



The substances known to be absolutely essential to 

 the mature growth of plants are potassium, calcium, 

 magnesium, iron, phosphorus, sulfur and nitrogen, 

 while the others are probably beneficial to the plant in 

 some way not yet discovered. 



Of the substances acting as plant nutrients, each must 

 be present in an amount sufficient to make possible the 

 maximum growth consistent with other conditions, or 

 the yield of the crop will be curtailed by its deficiency. 

 To some extent certain essential substances may be 

 substituted by others, as, for instance, potassium bj r 

 sodium; but such substitution is probably possible only 

 in some physiological role other than that of an ele- 

 mental constituent of an organic compound. The sub- 

 stances that are likely to be so deficient in an available 

 form in any soil as to curtail the yield of crops are potas- 

 sium, phosphorus and nitrogen, while the addition of 

 certain forms of calcium is likely to be beneficial on 

 account of its relation to other constituents and proper- 

 ties of the soil. It is for the purpose of supplying these 

 substances, and to some extent to improve the mechani- 

 cal condition of the soil, that mineral manures are used. 



