PLANT-FOOD MATERIALS IN SOILS 



99 



always possible, as the result of a chemical analysis, to esti- 

 mate the -degree of productiveness of a soil, or to say that it 

 should have a certain kind of fertilizer treatment, or that it 

 is adapted to certain crops. 



115. Absorptive properties of soils. — If a solution of 

 certain substances required by plants be poured on soil they 

 will not leach through the soil unaltered, but part will be 

 held by the soil. On the other hand, the drainage water 

 may contain an increased quantity of some other substance 

 in place of the one added in solution. As an example of 

 this we may take the following case. An application of 

 200 pounds to the acre of a potash fertilizer was made 

 annually for five years to soil contained in one of the large 

 tanks previously referred to. The composition of the drain- 

 age water from the tank so treated, and of the drainage 

 water from an untreated soil is shown in the following table : 



Table 21. — Annual Average Pounds to the Acre of Lime, 

 Magnesia and Potash in Drainage from Soil Treated 

 with Potash Fertilizer and from Untreated Soil 



In this case the effect of the application of the potash 

 fertilizer was to increase the quantities of lime and magnesia 

 in the drainage water, but not the quantity of potash. . 



116. Selective absorption. — Some substances are retained 

 by soils only in small part. Among these are nitrates, 

 which, as we shall see later, are very important forms of 

 nitrogen, and sulfates, which are also required by plants 



