58 Keport of the Acting Director and Chemist of the 



plants absorb nitrogen very readily. Some nitrogen may be lost alsO' 

 by organic matter in the process of decay, escaping into the air as 

 free nitrogen. 



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

 extent in drainage waters, since most soils liave tlie power of chang- 

 ing 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 gi'eater 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 furnishing 

 plant-food. We can summarize as follows the general offices which 

 the soil fulfills in its relation to plants : 



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

 of the plants penetrate the soil downwards and sideways, and brace 

 the plant fii-mly to its upright position. 



Second. The soil furnishes directly all the soil-derived elements 

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

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

 conveying to the plant a considerable portion of the air-derived 

 elements. 



Third. The soil contributes to the development of plants by modi- 

 fying and storing the heat of the sun, by regulating supplies of 

 food, and, in various ways, by securing those conditions which must 

 be present and unite to produce the fully developed plant. 



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

 plants. 



Analysis of Soils. — It is ordinarily supposed that a chemist has 

 only to make an analysis of a soil in order to tell just what the soil 

 needs and what elements should be added to it to make it most pro- 

 ductive. What chemical analysis does actually tell is what elements 

 are present in the soil and in what quantities they are present ; it 

 does 7iot tell whether the elements are available as plant-food, and 

 it is just this point which one should know in order to supply to a soil 

 what is needed. Few agricultural chemists to-day place unlimited 

 confidence in the chemical analysis of a soil to find out its needs in 

 the line of plant-food. 



