THE CHEMICAL COMPOSITION OF SOILS QI 



form in each class of soils. If a soil contain from 0.3 

 to 0.5 per cent or more of lime and from o.i to 0.4 per 

 cent of combined carbon dioxide, and is not strongly 

 alkaline, there is a reasonable content of lime car- 

 bonate. If, however, the soil contain only a trace of 

 carbon dioxide, the lime is not present as carbonate, but 

 probably as a silicate, in which case the soil may be 

 deficient in active lime compounds. 



In the case of phosphoric acid, a soil which gives an 

 alkaline or neutral reaction, contains 0.15 per cent of 

 phosphorus pentoxide and is well supplied with organic 

 matter and lime, is amply provided with phosphoric acid, 

 and under such conditions the use of phosphate ferti- 

 lizers 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 

 deficiency of this element. It frequently happens that 

 in acid soils the phosphoric acid is unavailable until a 

 lime fertilizer is used to neutralize the acid. 



Soils containing less than 0.07 per cent of total 

 nitrogen are usually deficient, and one containing as 

 high as 0.15 or 0.2 per cent may fail to respond to 

 crop production, but such a case is generally due to 

 some abnormal condition of the soil, as lack of alkaline 

 compounds which are necessary for nitrification. The 

 appearance of the crop is one of the best indications as 

 to deficiency of nitrogen. 



A soil which contains less than o.io per cent of 

 potash soluble in hydrochloric acid is quite apt to be 



