106 SOILS AND MANUKES 



more potash from the sulphate than from the chloride, 

 and still more from the carbonate. When sulphates or 

 chlorides of potassium, sodium, ammonium, etc., are 

 applied to the soil, calcium sulphate or chloride appears 

 in the drainage water. The salts first react with lime, 

 which combines with the acid radicles and liberates the 

 bases as carbonates, thus 



K 2 S0 4 + CaC0 3 = CaS0 4 + K 2 C0 3 



The addition of lime to the soil does not, however, 

 increase the absorptive power except when the proportion 

 of that substance originally present is very small. 



The power of absorption is greatest in soils which con- 

 tain clay, humus, and colloidal hydrates of iron and alumina. 

 Pure sands have practically no absorptive power at all. 



It is well known that when aluminium hydrate A1(OH) 3 

 is precipitated by potash it cannot be entirely freed from 

 the reagent by washing with water, and that it is soluble in 

 excess owing to the formation of compounds called alumin- 

 ates in which the aluminium hydrate acts the part of 

 an acid. When carbon dioxide is passed into a solution of 

 potassium carbonate, and a solution of aluminate is added 

 at the same time, a substance called aluminium-alkali- 

 carbonate is precipitated 



A1 2 3 , K 2 + 2 KHCOs = A1 2 3 , K 2 0, 2 C0 2 + 2 KOH 



potassium. alummimn- 



aluminate. alkali-carbonate. 



The retention of potash by aluminium hydrate in the 

 soil may be explained in this way, or it may be due to the 

 formation of simple aluminates such as KH 2 A10 3 or, pos- 

 sibly, compounds of a similar type containing a smaller 

 proportion of potash. The aluminates are readily soluble 

 in acids, and are decomposed even by carbonic acid. Ferric 

 hydrate is also precipitated from salts by potash ; it is not 



