ABSORPTION BY SOILS. 



271 



and ammonia among the bases, and phosphoric acid among the 

 acids. 



Thus (as stated above) when a weak (one or two per cent) 

 solution of potassic chlorid or sulfate is poured upon a 

 column of good soil several inches thick, it will be found that 

 the first portions passing through are free from potash, but 

 contain the chlorids or sulfates of magnesium and calcium. 

 If potassic nitrate be used, lime and magnesia will pass off as 

 nitrates ; while in the case of potassic phosphate, both ingredi- 

 ents will be retained. A solution of gypsum (calcic sulfate) 

 will usually cause the passing-off of some of the magnesia, soda 

 and potash contained in the soil, in the form of sulfates; but 

 the amount of potash thus dissolved soon diminishes to a mere 

 trace. Solutions of potassic or ammonic phosphates will be 

 absorbed and retained by the soil to a very considerable extent, 

 before the soil becomes saturated. 



While it is true that the degree to which the soil retains 

 the several ingredients may serve in a very general way to 

 indicate their richness or poverty in the same, the attempt to 

 make such experiments serve to determine the agricultural 

 needs of soils has met with but little practical acceptance. 



Drain Waters. The table on p. 22, chapter 2, illustrates 

 forcibly the working of the above principles, which are verified 

 by the composition of drain-waters. In all, the chief nutritive 

 ingredients of plants, except nitrogen, are present in traces 

 only; chlorids, nitrates and sulfates of sodium and mag- 

 nesium form the bulk of the permanently soluble matter, with 

 usually a considerable proportion of calcic (and magnesic) 

 carbonate, depending upon the amount of the earth-carbonates 

 present in the soil, as well as upon that of oxiclizable organic 

 matter from which carbonic acid can be formed. That calcic 

 carbonate filters readily through the soil has already been some- 

 what elaborately discussed (see chap. 3, p. 41); one of the 

 results being that the surface soil is sometimes almost com- 

 pletely depleted of this important substance, while it accumu- 

 lates at a greater or less depth in the subsoil, or in under- 

 drains, as the case may be. 



Of the ingredients appearing in the above list, the one of 

 greatest agricultural importance is nitric acid, since chlorine 

 and sulfuric acid, as well as soda, are required only in very 



