52 ABSOEPTIVE POWER OF SOILS 



Exactly how the phosphoric acid exists in the rocks and minerals 

 forming a soil, it is difficult to determine. A portion probably occurs 

 as calcium phosphate, Ca 3 P 2 8 , and undergoes a change thus : 



Ca 3 P 2 8 + 2C0 2 + 2H 2 * 



The monohydrogen calcium phosphate, CaHP0 4 , being slightly 

 soluble in water, is available to plants. 1 It is possible, too, that the 

 solution of calcium bicarbonate thus formed, together with that fur- 

 nished by the calcium carbonate in the soil, may react upon such sili- 

 cates as orthoclase and liberate their potash as carbonate, the lime 

 uniting with the other constituents of the mineral : 



Al 2 O 3 .K 2 O.6SiO 2 +Ca(HC0 3 ) 2 = Al 2 O 3 .CaO.6SiO 2 +2KHC0 3 . 



The soluble potassium salt is then either absorbed by the roots of 

 the crop growing on the soil or held in a weak state of combination by 

 the hydrated silicates present. 



The substances which become soluble owing to the action of water, 

 carbon dioxide and other reagents in a soil are not necessarily washed 

 out of the soil by the drainage. The chlorides, sulphates, carbonates, 

 and, perhaps, to a less extent the silicates, especially of lime and soda, 

 are in great measure thus removed, the only action interferipg with 

 their complete removal being apparently the surface attraction exerted 

 by the particles of the soil, an action similar to that exerted by 

 charcoal towards salts in solution. 



Absorption and Retention by Soils. With the potassium and 

 ammonium salts and with phosphoric acid, however, the case is very 

 different. In addition to the physical absorption or adhesion alluded 

 to (adsorption), there is a chemical retention exerted by certain consti- 

 tuents of soils for these substances. It can be shown by direct experi- 

 ment that dilute solutions of potassium or ammonium salts or of 

 phosphates, if filtered through a sufficiently thick layer of soil, are 

 robbed of some of their constituents. In most cases the acids of the 

 ammonium or potassium salts are found in the filtrate in combination 

 with calcium. 



This retentive power is apparently to be attributed mainly to the 

 presence of hydrated silicates analogous in composition to the crystal- 

 line minerals known as zeolites (so called from their frothing, due to 

 evolution of steam, when heated on platinum wire in the blow-pipe 

 flame). It is, however, extremely unlikely that fragments of such 

 minerals should exist in the soil, as they are, as a rule, easily decom- 

 posable and not very abundant in rocks. It seems more probable that 

 the retentive substances are transition bodies produced in the weather- 

 ing of such silicates as felspar and mica, and consequently that they 

 are not permanent ingredients in the soil and are present, not in a 

 crystalline, but in an amorphous state. 



The phenomenon of the retention of certain substances by soils 



1 In the presence of ferric hy Irate, often present in soils, the phosphoric acid 

 of tricalcium phosphate is, by prolonged action of carbon dioxide and water, con- 

 verted into ferric phosphate, the lime being removed as carbonate. G. v. Georgie- 

 vies, Jour. Soc. Chem. Ind., 1892, 254. 



