CHAPTER IV 



SOIL MANAGEMENT (continued), FERTILISER 



PRACTICE, GREEN-MANURING AND 



WEED CONTROL. 



FERTILISER PRACTICE. 



Without attempting to take up the subject of fertiliser 

 practice in detail the authors have considered that a very brief 

 discussion of the matter, giving some of the underlying prin- 

 ciples, as applied to South African conditions in particular, 

 would be helpful to some readers. 



A fertiliser may conveniently be defined as a substance 

 which is applied to soil for the purpose of directly increasing 

 the amount of available plant food present. 



The fertiliser itself carries the plant food in question, and 

 is to be distinguished from other substances occasionally added 

 to soils, which may stimulate the production of available 

 plant food from the soil's reserves, but do not themselves 

 contain the plant food in question; e.g., gypsum contains no 

 potash, but when applied to soil it is known to increase the 

 availability of the soil's supply of potash. Such substances 

 are more correctly termed " stimulants." 



Of the various plant foods known to be indispensable for 

 the proper growth and development of crops, nitrogen, phos- 

 phoric oxide and potash are the ones whose supply is most 

 likely to be deficient in soils. In some cases the supply of 

 lime may be insufficient to meet the food requirements of 

 crops; as a general rale, however, the lime supply is of 

 interest chiefly by virtue of its indirect effects on the soil. It 

 is possible that rare cases may occur where the supply of 

 certain other plant foods, such as sulphates and magnesia, is 

 deficient, but none such has been definitely recorded. 



Available Plant Food. — The productivity of a soil, so 

 far as the food supply is concerned, is determined not so much 

 by the total amount of plant food present as by the rate at 

 which the reserves of plant food become available for nutrition 

 of crops. There is, however, generally a close relationship 

 between these two factors, and under normal conditions large 

 reserves of plant food, as determined by chemical analysis, 



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