THE ROTATION OF CROPS. 63 



(3) Green Manure Crops. — These are crops grown for 

 the purpose of being ploughed under, the object being to in- 

 crease the organic matter content of the soil, e.g., cowpeas, 

 lupines, soy-beans, vetches, rye and buckwheat. They are 

 used chiefly on heavy clay, sandy and worn-out soils. Legumes 

 are usually preferred, since besides adding to the humus supply, 

 they increase, very cheaply, the nitrogen content of the soil. 



Since this group is used to increase the humus content, it 

 may be well to review briefly the role of humus in the soil. 



Chemically : (a) It is the source of nitrogen for ordinary 

 farm crops. 



(By nitrification the nitrogenous compounds of humus are 

 changed to nitrates, in which form the nitrogen can be assimi- 

 lated by crops.) 



(b) The carbon dioxide liberated by the decomposition of 

 vegetable matter renders insoluble mineral plant food available, 

 e.g., orthoclose is probably decomposed as follows, with libera- 

 tion of carbonate of potash : 



2 KAl SisOs + 2H.0 + C02 = H4A1. Si209 + ASiO. + K2CO3 

 To some extent the organic acids formed may have a similar 

 effect. 



(c) The carbohydrates of humus supply certain useful soil 

 micro-organisms with energy. 



(d) it increases the retentive power of soils for soluble 

 mineral salts. 



Physically : (a) It renders clay soil more friable and thus 

 improves aeration and enables more effective tillage. 



(b) It gives body to sandy soils and makes them more re- 

 tentive of moisture. Incidentally it checks the " blowing 

 out " of light sandy soils. 



(4) Grass or Sod-Forming Crops. — The chief character- 

 istics of the crops comprising this group are that they require 

 little or no cultivation, and that during the time that they 

 occupy the land a turf or mulch of roots and stems is developed, 

 filling the surface soil with vegetable matter ; and that they 

 occupy the land for several successive years. They assist in 

 maintaining the humus content, e.g., paspalum, lucerne, 

 kikuyu and Ehodes grass. Because of their long stay on the 

 land they usually become weedy, consequently they should be 

 followed by a cultivated crop. 



(5) Catch Crops. — These are crops (a) used to take the 

 place of crops w^iich have failed through adverse conditions, 

 such as insect attack or drought ; or (b) grown with regular 



