PLANT-FOOD MATERIALS IN SOILS 95 



substances to be in solution in order that they shall be 

 absorbed by plants. Soil is not readily soluble. The natu- 

 ral insolubility of soil is modified by various conditions 

 of the soil itself and by the plants that grow on it. The rate 

 of availability of plant nutrients is, therefore, not a constant 

 quantity for any soil. A soil in good tilth will produce much 

 better crops than a soil in poor tilth, which means that the 

 rate of availability of its plant nutrients changes with 

 the physical condition of the soil. 



The available plant-food materials are not necessarily 

 proportional to the quantities of plant-food materials in a 

 soil. One piece of land may contain more plant nutrients 

 than another and yet be less productive. It has been shown 

 that the addition of four or five volumes of quartz sand to 

 one volume of a heavy, but highly productive, black clay soil 

 greatly increased the productiveness, although the conse- 

 quent dilution of plant-food content reduced the potash to 

 0.12 percent and the phosphoric acid to 0.03 percent. The 

 mechanical condition of the soil was better after applying 

 the sand. 



111. Conditions that influence availability. — It is appar- 

 ent that the immediate availability of the plant-food materials 

 in a soil is not so much a matter of their total quantity, as 

 of favorable conditions for the decomposition of both the 

 organic and the inorganic matter in the soil, and for the growth 

 of plants. For this reason good tilth, good drainage, warmth, 

 absence of acidity and the kind and vigor of the plants are 

 factors that influence availability. When any one or more 

 of these conditions is unfavorable, the availability of the 

 plant nutrients may be decreased. 



While all of these conditions influence the availability 

 of the plant-food materials, it still remains true that, other 

 things being equal, the greater the total supply of each of 

 these constituents of a soil, the greater will be the total 



