34 CHAPTER III 



tively uniform ; therefore the deeper the root zone the more 

 uniform is the water supply hkely to be. 



(4) Drained soils have a higher average temperature than 

 similar soils in an undrained condition, consequently an earlier 

 growth is obtained on the former than on the latter. Sandy 

 soils, as compared with clay soils, are looked upon as warm and 

 early, due to the fact that the water-holding capacity of clay 

 soils is higher than that of sandy types. 



(5) The supply of available plant food is increased by the 

 better aeration, higher temperature, deeper root penetration, 

 better tilth, better moisture supply, and more active and fav- 

 ourable bacterial growth in the soil, all of which conditions 

 result from improved drainage of a wet soil. The organisms 

 that cause the decay of roots and manure and those that use the 

 free nitrogen of the air are particularly affected in a beneficial 

 way. 



(6) Drained land is more receptive of moistujre either in 

 the form of irrigation or rain, and because of this incidentally 

 ciiecks erosion. 



(7) The provision of adequate drainage prevents the 

 accumulation of brak salts in the surface soil. This is in fact 

 the only method of keeping land permanently free from harm- 

 ful accumulations of brak salts in areas where the trouble is 

 common. 



The necessity for drainage is indicated when the tilth of 

 the soil is poor, where the crumb structure of the particles is 

 absent, where the water-table is within 2 to 3 feet of the sur- 

 face, and where the growth of the plants is of an unthrifty 

 character and the vegetation of a yellowish colour. Where 

 land is relatively cheap, as in South Africa, artificial drainage 

 will be restricted to land which for some outstanding reason 

 requires it, e.g., horticultural land near a homestead. How- 

 ever, in parts of Natal and Griqualand East, where arable land 

 is often scarce, due to the mountainous character of the country 

 and the high rainfall, drainage efi'ected by open furrows has 

 been profitable and necessary. In many parts of the country 

 land under irrigation might be profitably drained with some 

 cheap form of under-drainage. 



Types of Drains. — The oldest, apart from open surface 

 ditches, but the least efficient, are those which are made by 

 placing loose stone or closely packed logs in furrows which have 

 been dug with the necessary fall, and then filling in with soil. 

 These are usually short lived. 



