FORMS OF PLANT FOOD 77 



imately indicated, by the use of other solvents, as ex- 

 plained in section 90. 



The character of the soil, as acid, alkaline, or neu- 

 tral should first be determined, because plant food ex- 

 ists in a different form in each class of soils. If a soil 

 contains from 0.3 to 0.5 per cent, or more of lime (as 

 CaO) and from 0.1 to 0.4 per cent, of carbon dioxide 

 (C0 2 ), and is not strongly alkaline, there is a reason- 

 able content of lime carbonate. If, however, the soil 

 contains only 0.01 or 0.02 per cent, of carbon dioxide, 

 then the lime is not present as carbonate, but is prob- 

 ably present as a silicate, in which case the soil may 

 stand in need of a lime fertilizer. A soil which gives 

 an alkaline or neutral reaction and contains 0.15 per 

 cent, of phosphorus pentoxide, and is well supplied with 

 organic matter and lime, is amply provided with phos- 

 phoric acid, and under such conditions the exten- 

 sive use of phosphate fertilizers is not required, 

 except possibly for special crops. Hilgard states that 

 should the per cent, of phosphoric acid be as low as 

 0.05, there is, in all probability, a poverty of this ele- 

 ment. 



Soils containing less than 0.07 per cent, of total ni- 

 trogen are usually deficient. A soil containing as 

 high as o. 1 5 or o. 2 per cent, of nitrogen may fail to re- 

 spond to crop production. Such cases are generally 

 due to some abnormal condition of the soil, as a lack 

 of alkaline compounds which are necessary for nitri- 



