132 THE PRINCIPLES OF SOIL MANAGEMENT 



much used for this purpose are the clovers, vetch, 

 field-peas, cowpeas, soy beans, rye, and buckwheat. 

 When any of these crops are planted in the late summer 

 to conserve plant food, they are termed "catch crops," 

 and when used to cover the ground and protect it from 

 erosion, they are termed "cover crops." Many forms 

 of organic manures and waste materials are applied 

 as a source of humus. 



Good tillage and the proper rotation of crops greatly 

 assist the accumulation of organic matter in the soil, 

 and to these may sometimes be added amendments 

 such as lime. Some of the conditions which favor the 

 accumulation of organic matter in the soil are: (1) The 

 presence of an excess of water. (2) Low temperature. 

 (3) Limited aeration. (4) Deficiency of basic elements. 

 (5) Absence of decay organisms. (6) Application of 

 organic manures. (7) Accumulation of plant residues 

 in the soil. (8) Proper rotation of crops. (9) Absence 

 of tillage. 



Some of the conditions which favor the rapid dis- 

 appearance of humus from the soil are: (1) The presence 

 of a moderate amount of water. (2) Thorough aeration. 

 (3) High temperature, from 75 to 110 Fahr. (4) 

 Abundance of available basic elements. (5) Abundance 

 of decay organisms. (6) Failure to maintain the supply 

 of organic matter. (7) Complete removal of all crops. 

 (8) Improper crop rotation. (9) Excessive tillage. 



Good management seeks to adjust these two sets of 

 conditions, so that large crops are produced without 

 imparing the humus supply in the soil. 



