THE CHEMICAL COMPOSITION OF SOILS 93 



available potash. This may be due to some abnormal 

 condition of the soil, as an insufficient amount of other 

 alkaline compounds, as calcium carbonate, to take the 

 place of the potash which has been withdrawn by the 

 crop, or lost by leaching, in which case the deficiency of 

 available potash can be remedied without purchasing 

 soluble potash fertilizer. Where a soil contains only 

 0.04 per cent of acid-soluble potash, the purchasing of 

 potash fertilizers is more necessary, but with 0.40 per 

 cent the way is open to render this available for crops. 

 The various ways of rendering acid-insoluble potash and 

 other compounds available for crop production, as by 

 rotation of crops, use of farm manures, use of lime and 

 green manures, or by different methods of cultivation 

 have not been sufficiently studied as yet to offer a solu- 

 tion to all of the problems of rendering inert plant food 

 available. 



106. Distribution of Plant Food. In studying the 

 chemical composition of a soil, the surface soil and sub- 

 soil both require consideration. It frequently happens 

 that these have entirely different chemical, as well as 

 physical, properties, and that a soil fault, as lack of 

 potash in the surface soil, is corrected by a high per 

 cent of that element in the subsoil. This is particularly 

 true of some of the prairie soils, where the surface soils 

 generally contain less potash and lime, but more nitrogen 

 and phosphoric acid, than the subsoils. When jointly 

 considered the surface and subsoil have a good supply 



