240 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



is removed. Thus, even with a soil of low absorptive power, on 

 account of the great mass of soil which enters into consideration, 

 almost all the material is absorbed. 



With a soil of good absorptive power, phosphoric acid is mainly 

 retained by the uppermost layers, the first 9 inches ; with potash, 

 although the uppermost 9 inches contains chiefly the greater por- 

 tion of the unused fertilizer, a considerable amount penetrates to 

 and is retained by the second and third 9 inches. 



The absorptive power of soils also enters into consideration 

 when the land is to be irrigated, in which case it is desirable to 

 know whether any of the fertilizer may be washed out or carried 

 to too great depth by application of the water, and how long a 

 time should elapse before irrigation will cause a loss of plant food. 



Some sandy soils have little absorptive power, and no doubt 

 losses of plant food occur where heavy rains fall shortly after 

 applications of potash and soluble phosphates. 



Cause of Absorption. The causes of absorption are different 

 for the basic radicles, potash, soda, lime, magnesia, etc., and for 

 the acid radicle, phosphoric acid. It is accordingly necessary to 

 consider these two separately. 



In offering an explanation for the absorption of bases, we must 

 consider the following facts : 



(1) The amount of absorption is often related to the quantity 

 of silicates in the soil decomposed by hydrochloric acid. 



(2) When a base is absorbed, it is replaced by equivalent 

 quantities of other bases. Treat a soil with sodium chloride, for 

 example, and the absorbed sodium will be replaced largely by 

 lime, magnesia, soda, and potash. 



(3) When a soil is treated with an acid, it loses its absorptive 

 power almost entirely, but the addition of calcium carbonate 

 restores it in great part. 



(4) Hydrated oxides of iron, and aluminum, hydrated alum- 

 inum silicate, and sand have slight absorptive powers, but not 

 sufficient to account for the absorptive power of soils containing 

 them. Humus has a comparatively high absorptive power, but 

 the small quantity in most soils will not account for the results. 



