Oct. 20, I923 
Soil Reaction in Relation to Calcium Adsorption 
115 
The reaction is similar when calcium or some other strong base is present 
in the original mineral. The base sodium has been removed from soils 
in humid regions to such an extent that very little is left. Under present 
climatic conditions calcium is the base removed in largest amount ( 22 , 
p. 22-23) . Since the earth's crust contains nearly the same percentage 
of potassium and sodium (9), and also since soils of mineral origin con¬ 
tain more potassium than sodium, it is indicated that potassium is less 
readily removed. 
In the Tropics, where the conditions of weathering are more intense, 
the process goes further than that represented by the above equation. 
The equation for this more intense transformation may be written 
(34)- 
K 2 0 • A 1 2 0 3 • 6 Si 0 2 + 3 H 2 0 + C 0 j = 2 A 1 ( 0 H) 3 + K 2 C 0 3 + 6 Si 0 2 . 
The similarities and differences of the two equations are apparent. 
Both show that the bases are transformed into soluble forms, and in 
this condition may be readily removed by leaching. However, in the 
formation of the alumino-silicate, kaolin, only two-thirds of the silica is 
set free from the original mineral. In tropical weathering, according to 
the above equation, all the silica is split off and the aluminum is found 
as the hydroxid, gibbsite, the characteristic mineral of laterite. 
REMOVAL OF BASES THE FUNDAMENTAL CAUSE OF ACIDITY 
The elements potassium, sodium, and calcium in those minerals which 
form soil are in a condition of unstable chemical equilibrium under 
humid climatic conditions. They form the weak link in the mineral 
molecule. Under the conditions of a humid climate they are removed 
by various agencies, and the more stable alumino-silicates found in clay 
are produced. This removal of bases is the fundamental cause of the 
acid condition of soil of mineral origin. 
Cameron and Bell (6) give another picture of the chemical transforma¬ 
tions which take place in the weathering of silicates. They write the 
equation as follows: 
KAlSi 3 0 8 + HOH=KOH + HAlSi 3 0 8 
The successive removal of silica is represented as follows: 
HAlSi 3 0 8 — Si 0 2 = HAlSi 2 0 6 , pyrophyllite. 
HAlSi 2 O e — Si 0 2 = HAISi 0 4 , kaolinite. 
HAlSi 0 4 — Si 0 2 = HA 10 2 , diaspore. 
This chemical reaction explains the alkaline reaction of finely ground 
suspension of orthoclase in water. Under humid condition the base is 
removed and the acid alumino-silicate is left. This is very slightly solu¬ 
ble and feebly ionized. 
The equations also show the splitting off of silica, and, at present, 
considerable amounts are being leached from the land surface ( 22 , p. 
22-23). This may be removed, at least in part, in the colloidal form 
{24). But the serious loss is that of calcium, which under climatic con¬ 
ditions is leached in large amounts from cultivated land. (30) 
60374—23—3 
