82 



SOILS AND FERTILIZERS 



happens that the surface soil and the subsoil have en- 

 tirely different chemical, as well as physical, properties. 

 This is particularly true of the western 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 strong crop-producing powers, but if 

 considered separately each would have weak points. 



Since crops appropriate their food mainly from the 

 silt and clay particles, the amount of plant food pres- 

 ent in these grades of particles determines largely the 

 reserve fertility of the soil. A soil in which 70 per 

 cent, of the total potash is present in the silt and clay, 

 is in better condition for crop production than a sim- 

 ilar soil with a like amount of potash which is present 

 mainly in the sand. Because a soil has a given com- 

 position, it does not follow that all of the different 

 grades of particles have the same composition. In fact 

 the different grades of soil particles may have as varied 

 a composition as is met with among different soils. 25 



