68 CHAPTER V 



(2) Another experiment gave the following figures for 

 maize, following cowpeas used as a green-manuring crop : — 



Check plot. 1st Year. 2nd Year. 3id Year. 



1,430 1,901 1,546 1,430 



The importance of frequent graen-manuring in South 

 African maize farming is shown by the large gain due to the 

 ploughing down of the cowpeas, and by the transient charac- 

 ter of the residual effect of the green manure. These South 

 African experiments, while inconclusive because of their short 

 duration, give ample proof of the necessity for rotative 

 cropping. 



So far the results of the American, English and South 

 African experiments give abundant evidence that in a good 

 rotation provision must be made for the addition of humus 

 and for the application of manures. Besides the detrimental 

 effect on the soil, continuous cropping is generally character- 

 ised by excessive weed growth, largely because certain weeds 

 thrive better in conjunction with some crops than with others. 

 Lucerne is usually ploughed down when the weed growth 

 becomes predominant, and in small grains no inter-cultivation 

 is possible, consequently if continuously cropped to these 

 crops, the fields tend to become infested with weeds. There- 

 fore, while cultivated crops deplete soil humus, they should be 

 included in a rotation to effect weed control. 



To summarise, then, the essentials in a good crop rota- 

 tion are : (1) To have a cultivated or cleaning crop, i.e., one 

 grown in relatively wide rows ; (2) to provide for the maintefi- 

 ance of the soil humus ; and (3) to restore and to increase the 

 available plant food by the application of manures, commercial 

 or otherwise, to one or more crops in the rotation. 



The chief T'cas(-)ns for 7-otative crnpjnrtg are briefly : — 



(1) IJM)our is more evenly distributed tJiroii^houiL-thii 

 year, and'consequently is more economically utilised. Where 

 o'nty one crop is grown ab norm aTt^it" i y ii i adB'o'n the available 

 labour during certain periods, e.g., planting and harvesting. 

 However, if a diversity of crops is grown, the times of plant- 

 ing, cultivating and harvesting will vary, and the requirements 

 from labour will be continuous rather than spasmodic. 



(2) It must be apparent that, where several crops are 

 grown, the risks of failure are minimised, since adverse 

 seasonal differences affecting one crop may not affect another 

 so severely, because crops vary in the time of year at which 

 they make their maximum growth. 



