CHAPTER IX. 



CHEMICAL COMPOSITION OF THE SOU. 



Chemical analysis shows that the greater bulk of the soil 

 is composed of compounds of silica, oxides of iron, and oxides 

 of alumina, in various compounds. These substances have no 

 value as plant food, except iron, and only very small amounts of 

 it is essential. They serve a useful purpose, however, in holding 

 moisture, modifying the supply of plant food, supporting the 

 plant, and, giving it a medium in which to develop its roots. 



The important plant foods nitrogen, phosphoric acid, and 

 potash make up only a small percentage of the soil. The 

 quantity of plant food may be large in pounds per acre, sufficient 

 for several hundred crops, if it were all available for use of the 

 plant. But the proportion of plant food to the total quantity of 

 soil is small. Further, the amount which can be taken up by 

 plants may be only a small proportion of the total amount pres- 

 ent ; so that although several hundred pounds of phosphoric acid, 

 for instance, may be present in the soil, the addition of a few 

 pounds of highly available phosphoric acid may produce a large 

 increase in the crop. 



Methods of Examination. Four chief methods of examining 

 soils have been used. In addition, special methods are used for 

 special analyses. 



(1) Complete decomposition of the silicates. This method 

 gives the total quantity of the constituents of the soil. 



(2) Partial decomposition with strong acid. This method 

 attempts to determine the quantity of plant food which may be- 

 come available to the plant in a series of years. It distinguishes 

 between the most resistant silicates, and those decomposed by 

 acids. 



(3) Weak Solvents. This method attempts to determine the 

 immediate needs of the soil for plant food. 



(4) Water-Soluble Constituents. This method considers the 

 material extracted from the soil by water. 



