176 



PRINCIPLES OF AGRICULTURAL CHEMISTRY 



Soil E with a very high percentage of phosphoric acid and only 

 a moderate supply of lime, is very productive. Soils F and G 

 the one with fair lime and low phosphoric acid, the other with 

 the proportions reversed, are both about equally productive. 



Potash. According to Hilgard, sandy soils of great depth may 

 contain less than o.io per cent, potash without being deficient 

 therein; sandy loams contain 0.3 to o.i per cent.; loams 0.45 to 

 0.3 per cent., heavy clays and clay loams o.8c to 0.45 per cent. 

 As a rule, soils containing less than 0.25 per cent, potash are 

 likely to require fertilization with potash salts early, while as 

 much as 0.45 per cent, seems to be sufficient for the same soils. 

 Sometimes, however, a soil rich only in lime and phosphoric acid 

 shows good productiveness despite a low potash percentage, and 

 conversely a high percentage of potash may offset a low per- 

 centage of lime. 



The availability of the potash depends upon the general char- 

 acter of the soil. With a good supply of available lime and 

 magnesia, the potash of the soil is usually in an available form. 



The above standards are for potash estimated according to 

 Hilgard's method. The Association method dissolves less potash 

 and calls for lower standards. Exactly what these standards 

 should be, remains to be determined. 



Nitrogen. Nitrogen is present in the soil in organic compounds 

 which cannot be taken up by plants, and which change slowly into 

 compounds which can be assimilated. This change depends 

 largely upon physical conditions, though the composition and 

 nature of the soil also have an influence. The rapidity of the 

 production of active plant food is more important than the 

 quantity of nitrogen. It is thus evident that it is difficult to fix 

 a standard for nitrogen. It has generally been assumed that o.io 

 per cent, is adequate. With less than 0.07 per cent, the soil is 



