346 CHAPTER XXI 



Thus, for the average weed, from five to eight inches of 

 rainfall is required to produce one ton of dry matter. The 

 loss of moisture through v^eeds is one of the chief causes of 

 crop failure in South Africa. Moreover, weeds may reduce 

 to a marked extent the amount of plant food available for the 

 use of the crop, and thus increase the water requirement of 

 the crop. 



Evaporation. — The loss of water referred to under this 

 head is that due to direct evaporation of moisture from the 

 surface o.f the soil. The " evaporation " of a particular 

 locality is usually stated as the number of inches of water 

 evaporated from a free water surface, and in semi-arid parts 

 is generally considerably higher than the rainfall. From a 

 wet soil surface evaporation occurs at an even faster rate than 

 from a free water surface. Fortier found that a wet soil sur- 

 face evaporated 4" 75 inches per week, while a free water 

 surface evaporated only 1"88 inches per week. The difference 

 is probably accounted for by the soil's highest temperature, 

 higher absorptive capacity for heat, and larger surface 

 exposure. 



The rate of evaporation is influenced by a great many 

 factors, chief among which are : — 



(1) The initial percentage of water in the soil, 



(2) Atmospheric temperature. 



(3) Kelative humidity of the atmosphere. 

 ^4) Winds. 



(5) Sunshine. 



(6) Nature of the soil (physical and chemical). 



Ordinarily, about 25 to 35 per cent, of the rainfall is lost 

 through direct evaporation from the soil surface. Loss by 

 evaporation is high during the earlier part of the season, but 

 as the crop develops, the increased protection of the surface 

 by the foliage greatly reduces direct evaporation from the soil 

 surface. 



The efficacy of preventing evaporation by soil and parti- 

 cularly dust mulches is a much disputed point. It is held 

 that in dry regions the moisture in the upner 6 to 10 inches 

 will be lost through evaporation, whether a soil mulch is 

 present or not. The maintenance of a soil mulch does, how- 

 ever, conserve moisture, because in establishing a mulch, 

 weeds are controlled, improved aeration influences the amount 



