12 An Account of the Soil [pt. i, ch. i 



air and do not figure at all in the above list. But the 

 term survives because of its convenience. 



In later chapters we shall discuss the effect of the 

 various substances on the plant. For the present it is 

 sufficient to point out that each individual constituent 

 element is subject to the same laws as any of the other 

 soil factors : each must be present to an adequate extent, 

 and lack of any one cannot be made good by putting in 

 more of any other. When a soil is deficient in plant food 

 it need not necessarily receive a complete food: often 

 only one or two constituents are required. The discovery 

 of this fact completely revolutionised the practice of 

 manuring and has enabled farmers to maintain and even 

 to increase the efficiency of their soils as crop producers 

 at a minimum of cost. Such partial manuring, however, 

 has obviously to be done intelhgently or an insufficiency 

 of something that has been left out may operate as a 

 limiting factor and prevent the crop making proper 

 growth. In order to understand the principles involved 

 it is necessary to make a careful study of the soil and 

 of the different manures in common use. 



