The Composition and Use of Fertilizers. 215 



season by plants may enter into such, insoluble combination with 

 the soil as to become unavailable. On such soils it is, there- 

 fore, better to apply phosphates each year in quantities suitable 

 for a single crop, rather than attempt to make one heavy applica- 

 tion to cover a period of years. 



(c) Potash in Potassium Compounds is not apt to be lost to any 

 extent in drainage waters, since most soils have the power of 

 changing soluble forms of potash into forms less soluble, which 

 are gradually redissolved and given up for the use of plants. 



In addition to the preceding statements, it may be said, in 

 general, that loss of plant food is greatest in sandy soils; the 

 coarser the sand, the greater the loss, other conditions being the 

 same. Clay and humus have very marked power in retaining 

 plant food. 



Relations of Plants and Soils. — We have seen that a very small 

 part of the soil furnishes the most important constituents of 

 plant food. The soil also performs other functions than furnish- 

 ing plant food. We can summarize as follows the general offices 

 which the soil fulfills in its relation to plants : 



1. The soil acts as a mechanical support for plants; the 

 roots of the plants penetrate the soil downwards and sideways, 

 and brace the plant firmly to its upright position. 



2. The soil furnishes directly all the soil-derived ele- 

 ments used by the plant, and is thus immediately connected with 

 the nutrition of plants. In addition, the soil seiwes as a medium 

 for conveying to the plant a considerable portion of the air-de- 

 rived elements. 



3. The soil contributes to the development of plants by 

 modifying and storing the heat of the sun, by regulating sup- 

 plies of food, and, in various ways, by securing those conditions 

 which must be present and unite to produce the fully developed 

 plant. 



4. The soil acts like a sponge to hold water for the use of plants. 



Analysis of Soils. — The value of chemical analysis for ascer- 

 taining the plant-food needs of a soil is liable to be overestimated. 

 Soil analysis has not yet reached such a stage of perfection that 

 its results can always be interpreted correctly. It gives definite 

 results when it shows the complete absence of one or more necessary 

 plant-food constituents. Its chief weakness lies in the present 

 inability to estimate accurately the amount of available plant food 



