2/O 



SOILS. 



over, a soil once so saturated will yield to rain-water filtering 

 through it, notable amounts of all the ingredients absorbed in 

 it; and, at least so far as the physically condensed soluble in- 

 gredients are concerned, long-continued leaching with pure 

 water will inevitably result in the withdrawal of additional 

 amounts of absorbed ingredients, apparently dividing them- 

 selves up pro rata between the water and the soil. 



It is obviously of the utmost importance to the farmer to 

 know to what extent the soil will retain manurial ingredients 

 against the influence of leaching rains; for unless this is 

 taken into consideration, it may readily happen that the fertil- 

 izer supplied before a rainy season will be washed through be- 

 yond the reach of plant-roots, and so practically become a dead 

 loss. 



Absorptive Power Varies. So far as the mere physical ab- 

 sorption is concerned, it will readily be understood that a 

 coarse sandy soil exercises less retentive influence upon dis- 

 solved substances than clay or humous soils. In the humid 

 region, where sand is substantially nothing but granular silica 

 (see above, chap. 6, page 86), the same may be measurably 

 true as regards the chemical absorption also. In the arid re- 

 gion, on the contrary, a great many sandy or silt soils, very 

 poor in clay, exert fully as much chemical absorption as clay 

 soils, and are no more liable to the washing-out of soluble 

 fertilizers introduced than are the latter. For the chemical 

 absorption lies chiefly in the zeolitic portion of the soil (see 

 above chap. 3, p. 37, which in the humid region accumulates 

 in the clay, while in the arid it remains encrusting the sand and 

 silt grains. 



Generalities regarding Chemical Absorption and Exchange. 

 In regard to the leaching-out and absorption or retention of 

 substances important to agriculture, the following general 

 statement may be made : 



The substances most likely to be leached out of soils are, of 

 bases : soda, magnesia and lime ; of acidic constituents : chlor- 

 ine, sulfuric acid and nitric acid. Lime sometimes passes 

 off with either of the above acidic ingredients, and also in the 

 form of carbonate. 



Substances rather tenaciously retained in soils are: potash 



