348 CHAPTER XXI 



{h) Land summer tilled — h inch absorbed out of 4 inches 

 total. 



Thus vegetation impedes the flow, and consequently 

 enhances penetration. 



Soil rich in humus and sandy soils absorb water much 

 more rapidly than do clay soils. 



In practice, run-off can be decreased by keeping the land 

 in a rough state, rather than in a state of fine tilth, and by 

 cultivating along contours. 



In connection with this loss, it must be borne in mind 

 that valuable plant food constituents, as well as valuable 

 material in the form of sediment, is also carried away by the 

 run-off. 



Percolation. — Loss from this source is seldom met with 

 in South Africa in dryland areas, since our rainfall is insuffi- 

 cient there. 



In semi-arid parts the water table is often so low that 

 the soil moisture is not maintained from below by capillary 

 action. The amount of water available to plants is, there- 

 fore, limited to the rainfall, minus : 



(1) that evaporated from the soil, 



(2) the wilting coefficient, and 



(3) that lost by run-off. 



The amount, then, at the disposal of crops is very much 

 below the annual precipitation, and in arid regions it is easy 

 to see why normal crops cannot be expected to materialise. 



In an area where the rainfall is, say, 20 inches, the fol- 

 lowing losses are possible, and often probable : — 



'20 per cent, lost by evaporation, i.e., 4 inches. 



20 per cent, lost by run-off', i.e., 4 inches. 



1 ton of weeds grown in crop, say 6 inches. 



The total loss is then 14 inches, leaving 6 inches on which 

 the crop is to mature ; and since maize requires 8 to 12 inches, 

 or more, in South Africa, the reason for many crop failures 

 is easily understood. 



The following conclusions, arrived at by investigators in 

 America,'* pertain to dry land agriculture : — 



" The principal objects which are sought by ploughing 

 and the subsequent preparation of the soil are, to prepare a 

 suitable seed-bed for the germination of the seed and the 

 early growth of the seedling, to make the surface soil more 

 receptive to water falling as rain, and to prevent the growth 

 of weeds. 



