SOIL MANAGEMENT. 29 



Per cent. 



Water 5-99 



Organic matter 14'81 



Nitrogen '099 



Lime 1"444 



Potash -473 



Phosphoric oxide '221 



Juritz says : " The specific gravity of the silt is 2 "03 on 

 the basis of the dry specimen, so that a cubic foot would 

 weigh about 127 pounds. One acre of land covered with this 

 silt to a depth of half an inch, thus receiving a deposit of 

 1,815 cubic feet, or 115 Cape tons, would, therefore, be en- 

 riched to the extent of 3,314 pounds of lime, 1,086 pounds of 

 potash and 507 pounds of phosphoric oxide." 



Some irrigation water in South Africa, having its source 

 in dolomitic formations, contains a high percentage of lime. 

 On this account lucerne, a crop preferring calcareous soils, 

 will often thrive on relatively acid soils when irrigated with 

 water of this nature. 



Apart from the value indicated above, the fact that irriga- 

 tion water often carries enormous numbers of beneficial soil 

 micro-organisms should not be overlooked. On the other 

 hand some waters in South Africa carry & ;.'igh charge of dis- 

 solved salts, and the irrigated lands gradually become impreg- 

 nated with those salts to such an extent thc>kt typical " brak " 

 soil results. In this way the productivity of valuable fields 

 may be ruined. In some cases, too, especially on lucerne 

 lands in the Karroo, the silt carried in flood water forms a 

 hard impervious surface which is very detrimental to the crops. 



(c) The Nature of the Soil. — The following table indi- 

 cates the importance of the soil factor in irrigation practice : — 



* Maximum capillary capacity. 



