SOIL REACTION 



By E. a. fisher, M.A.. B.Sc. 

 University, Leeds 



The Nature of Soil Fertility 



The problem of maintaining the fertility of our richer agri- 

 cultural soils and of improving that of the poorer ones is one 

 of ever-increasing importance. Soil fertility, however, is often 

 spoken of as though it were an absolute property of a soil : it 

 is not always sufficiently emphasised that fertility is a particular 

 relationship subsisting between soil conditions on the one hand 

 and crop growth on the other. The particular conditions that 

 conduce to soil fertility are the resultant of two main groups 

 of factors : the intrinsic properties of the soil, which are depen- 

 dent on the actual chemical and physical and biological nature 

 of the soil complex and those extrinsic properties which are 

 impressed on the soil by topographical and climatic factors. 

 These two groups cannot be sharply distinguished from each 

 other, nor can any hard-and-fast line be drawn between the 

 various chemical, physical, and biological factors comprised in 

 them because few are at present susceptible of any exact 

 measurement. In other words, that particular group of con- 

 ditions that make up the fertihty of any particular soil is an 

 equilibrium caused by the interactions of numerous factors, 

 some of which can be varied by the agriculturist by manurial 

 and cultivation operations. It is by these operations (knowledge 

 of which has been evolved empirically throughout the ages) 

 that the farmer is enabled to maintain or to change, to regulate 

 and adjust the relationships between soil conditions and crop 

 growth.* 



Among those soil factors that are most readily susceptible of 

 regulation and adjustment are many that cannot be allowed 

 to vary beyond comparatively narrow limits without becoming 

 limiting or controlling factors in crop production. The capacity 

 of the soil to supply the necessary nutrients for plant growth is 

 of fundamental and obvious importance : the original reserves 

 of foodstuffs, supplemented by biological activity of the soil 



* For an interesting discussion of fertility from this point of view see 

 E. J. Russell, Soil Conditions and Plant Growth (Longmans)^ 4th ed., 192 1, 

 especially chap. viii. 



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