172 



PRINCIPLES OF AGRICULTURAL CHEMISTRY 



plants is, to a large extent, proportional to the total quantity 

 present, though this depends on the changes going on in the soil. 



A comparatively small amount of plant food in the soil is 

 sufficient to make it productive, if present in an available form. 

 If we assume the weight of one acre of soil to the depth of one 

 foot to be $y 2 million pounds, then o.oi per cent, corresponds to 

 350 pounds per acre foot. A crop of 40 bushels of corn, including 

 ears, stalk and leaves, requires about 25 pounds phosphoric acid, 

 which would be about 0.0007 per cent., or 7 parts per million. Con- 

 sequently as much as o.oi per cent, phosphoric acid could supply 

 14 crops of corn of this size, if the plant could get it. But soils 

 containing only this quantity of phosphoric acid usually respond 

 to applications of phosphoric acid greatly. 



The analysis by strong acids does not differentiate between com- 

 pounds which may have different values to crops. Hence two 

 soils may have the same composition but react differently to 

 fertilizers. The chemical composition as determined in this way, 

 is more closely related to the wearing qualities of the soil than to 

 the immediate needs of the soil for plant food. 



Number of Crops the Plant Food Will Supply. This depends 

 on the size and kind of crop, as well as on the composition of the 

 soil, Assuming the entire removal of a crop of 40 bushels corn 

 per acre, the following is the number of crops which could be 

 supplied by the acid-soluble phosphoric acid and potash, and the 

 total nitrogen in some Texas soil types, 1 if they were in such 

 forms that they could be used by the crops without any loss. 



Bulletin 126, Texas Station ; see also No. 99. 



