FACTORS WHICH DETERMINE 

 FERTILITY IN SOILS 



By EDWARD J. RUSSELL, D.Sc. (Lond.) 

 Rothamsted Experimental Station, Harpenden 



The profound effect on the fertility of soils which is exercised 

 by their organic contents is not yet sufficiently appreciated. 

 When plants die, they fall back on the soil as decaying masses 

 of substances of varying stability, and give rise to products many 

 of which are highly complex in character : some of the products, 

 the simpler ones in particular, are of great value as plant food 

 whilst others may be deleterious. 



Undecomposed plant residues, leaves, stems, etc., " open up " 

 the soil if present in any quantity, destroying that continuity of 

 the soil masses on which a satisfactory supply of water and a 

 proper root range for the plant depend. If the soil mass be 

 continuous, water tends to distribute itself uniformly ; if it be 

 discontinuous, one part may be too wet while another is too dry. 

 As the organic matter is decomposed, not only does this detri- 

 mental mechanical effect cease but a distinctly beneficial effect is 

 exercised. One of the products — the black substance or mixture 

 of substances known as humus, to which the colour of a rich 

 garden soil is due — has the remarkable property of increasing 

 the water-holding capacity of the soil, thus reducing the loss by 

 drainage, which is one of the most serious of soil losses. Humus 

 also facilitates the production of the fine crumbly condition or 

 " tilth " essential for successful cultivation. 



The effect of the decomposition of organic matter is therefore 

 that the productiveness of the soil is increased by the conversion 

 of useless or even harmful substances into others which are 

 valuable either as plant food or as agencies for improving the 

 texture and water-holding capacity of the soil. The process of 

 change has always interested agricultural chemists. In early 

 years it was thought to be purely chemical in character — a slow 

 combustion or " eremacausis." Even after Pasteur had de- 

 monstrated the presence of micro-organisms in the soil, and 



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