24O SOILS AND FERTILIZERS 



In many soils nitrogen and phosphoric acid are more 

 abundant in the surface soil while potash and lime 

 predominate in the subsoil. When such a condition 

 exists, the alternating of deep and shallow rooted 

 crops is very beneficial, because the surface soil is re- 

 lieved of continuous heavy drafts upon the elements 

 present in scant amounts. 



310. Humus-consuming and Humus-producing 

 Crops. — When grain or hoed crops are grown con- 

 tinuously, oxidation of the humus occurs, and the 

 chemical and physical properties of the soil may be 

 entirely changed by the loss of the humus. The ro- 

 tating of grass and grain crops and the use of stable 

 manure serve to maintain the humus equilibrium. On 

 some soils lime may be required along with the humus 

 to prevent the formation of humic acid, and in such 

 cases the best conditions exist when both lime and hu- 

 mus materials are supplied. The alternation of hu- 

 mus-producing and humus-consuming crops is one of 

 the essential matters to consider in a rotation. 



311. Crop Residue. — Crop residues should always 

 be placed at the disposal of weak feeding crops. For 

 example, after a light clover and timothy sod, wheat 

 or flax should be grown in preference to barley or 

 mangels. The weak feeding crop should then be fol- 

 lowed by a strong feeding crop, and each is properly 

 supplied with food. It would be poor economy, on an 

 average soil, to follow clover and timothy with mangels, 



