218 SOILS AND FERTILIZERS 



The factors, within the control of man, that affect the 

 availability of fertilizers are the following : (1) the kind of 

 crops ; (2) soil moisture content ; (3) soil acidity ; (4) tilth 

 of the soil ; (5) organic matter in the soil. 



An undesirable condition of any one or more of these 

 factors is a very common occurrence, and yet fertilizers are 

 expected to produce profitable returns, in spite of these 

 adverse conditions. It must be remembered that the supply 

 of nutrients is only one of the conditions that influence plant 

 growth. Furthermore, an economical use of fertilizers 

 requires that they merely supplement the natural supply 

 of plant nutrients in the soil, and that the latter should fur- 

 nish the larger part of the nutrient material used by the 

 crop. Finally, most fertilizers are rendered less readily 

 soluble by the absorptive properties of the soil, and the re- 

 lease of these substances for plant use depends to a great 

 extent on the conditions enumerated above. 



276. Response of sandy and of clay soils to fertilizers. — 

 It is generally recognized that a sandy soil responds more 

 promptly to the application of fertilizers than does a clay 

 soil. There are probably two reasons for this : (1) Absorp- 

 tion may not be so complete both on account of the particles 

 being larger, and because in many sandy soils the particles 

 are largely composed of quartz, which does not have the 

 property of forming combinations with bases, as does clay; 

 (2) Drainage and aeration are likely to be better, as are most 

 of those conditions that make plant-food materials more 

 available. For these reasons, a sandy soil generally makes 

 a greater response to fertilizers the first year, but shows less 

 effect in subsequent years unless the treatment is repeated. 

 On the other hand, less fertilizing material is lost from a clay 

 soil by leaching. 



277. Cumulative need for fertilizers. — It is often re- 

 marked that on land habitually fertilized there is a gradually 



