192 



PRINCIPLES OF AGRICULTURAL CHEMISTRY 



The actual amount of potash removed, on an average, from the 

 different soils, is given in the following table. In order to make 

 the table more concrete, the amount of potash is also expressed 

 in bushels of corn per acre which could be produced by this quan- 

 tity, both stalk and grain included : 



Importance of the Active Plant Food. The active plant food 

 is thus related, on an average, to the ability of the soil to supply 

 plant food. Variations undoubtedly occur, due to variations in 

 the nature of the active plant food. The relation of the active 

 plant food to field results must be studied and worked out. 

 Deficiency as shown in pot experiments must be considered as 

 relative deficiency, and in applying the results to field conditions, 

 the possibilities of the soil under the prevailing climatic condi- 

 tions must be considered. For example, in our pot experiments, 

 soils containing ioo to 150 parts per million of active potash were 

 deficient in potash in 54.3 per cent, of the tests, and yet on an 

 average, they gave up enough potash for 102 bushels of corn, 

 the maximum being 352 bushels. In other words, the pot experi- 

 ments demanded more potash than would suffice for the crop 

 indicated above. Had the demands for potash been smaller, the 

 soil would not have been deficient. 



The importance of the estimation of the active potash and 

 phosphoric acid is to show the relative deficiency of the soil for 

 these elements. The tables we have given are an aid in this con- 

 sideration. For example, a soil containing 10 parts per million 

 of active phosphoric acid and 50 parts of active potash, would 



