54 CHAPTER IV 



The humus content of the soil is best maintained where 

 the cropping system involves comparatively frequent green- 

 manuring, coupled with the use of manure and the return of 

 crop residues to the land. 



Considerations in the Choice of Crop. — The choice of 

 a crop for green-manuring will be governed by the following 

 considerations : — 



(a) The suitability of the climate and soil. In the 

 summer rainfall areas naturally crops like cowpeas will be 

 used, while vetches will find a place in those parts having a 

 winter rainfall. 



(h) The cost of seed, e.g., Sweet Clover, is an excellent 

 crop for this purpose, but the seed is costly as compared with 

 that of cow-peas, consequently the latter is more commonly 

 used. 



(c) On foul lands certain weeds can often be controlled 

 by a green-manuring crop which is also a smothering crop, 

 e.g., Kweek {Cijnodon dactylon) can be sometimes effectively 

 smothered by cowpeas. 



(d) The rooting system. As previously shown, this varies 

 with different crops, and therefore the residual effect of the 

 roots on the succeeding crops also varies. 



(e) The value, quality and quantity of feed or pasture it 

 may produce. The farmer in adverse seasons may be com- 

 pelled to utilise as stock feed the crop intended for green- 

 manuring. Where this possibility is present he should grow 

 legumes, because of their high nutritive value, their extensive 

 and deep root-systems, and their capacity for gathering atmo- 

 spheric nitrogen. 



(/) The possibility of the crop growing with a cash or 

 other crop is also a factor, e.g., cowpeas with maize. The 

 maize may be harvested for silage, or if an early variety, for 

 seed, and the cowpeas then ploughed down during the end of 

 March or early in April. 



Green-Manuring Crops. 



(1) Legumes or Pod-Bearing Plants. — These obtain part 

 of their nitrogen from the atmosphere, through the agency of 

 the nodule bacteria attached to their roots. Other conditions 

 being equal, the amount of atmospheric nitrogen " fixed " 

 depends upon the nitrogen content of the soil — the richer the 

 soil in available nitrogen the less the amount of fixation, and 



