356 THE PRINCIPLES OF SOIL MANAGEMENT 



227. Factors affecting the efficiency of fertilizers. 



The potentially available nutrients in a soil, whether 

 natural or added in manures or fertilizers, are only in 

 part utilized by plants, and the extent of their utilization 

 depends upon the operation of certain limiting factors. 

 This is a very important consideration in the manuring of 

 land, for under conditions as they frequently exist the 

 use of fertilizers is wasteful and extravagant. 



The factors within the control of man that effect the 

 availability of fertilizing material are the following: 

 (1) Soil moisture content. (2) Soil acidity. (3) Organic 

 matter in the soil. (4) Structure or tilth of the soil. 



An undesirable condition of any one or more of these 

 factors is a very common and apparent occurrence, and 

 yet fertilizers are expected to produce profitable returns, 

 in spite of these adverse conditions. It must be remem- 

 bered that fertilizers are primarily only nutrient materials, 

 and that the supply of nutrients is only one of the con- 

 ditions that influence plant growth. Furthermore, an 

 economical use of fertilizers requires that they merely 

 supplement the natural supply in the soil, and that the 

 latter should furnish the larger part of the soil material 

 used by the crop. Finally, most fertilizers are ren- 

 dered more or less difficultly soluble, or in some cases 

 practically insoluble in pure water, by the absorptive 

 properties of the soil, and the release of these sub- 

 stances for plant use depends to a great extent upon the 

 factors mentioned above. 



For instance, when a potassium fertilizer, as potas- 

 sium sulfate or chloride, is placed in the soil, a consider- 

 able portion of the potassium is (page 297) fixed by ab- 



