n8 
Journal, of Agricultural Research 
Vol. 26, No. 3 
THE CAUSE OF THE LOW INTENSITY OF SOIL ACIDITY 
If the HC 1 formed when potassium is adsorbed from KC 1 remained 
uncombined, then the increase in hydrogen-ion concentration would be 
much larger than it is normally. Calcium silicates are present, and the 
further reaction may be represented as follows: 
HCl+Ca silicate^CaCIj + H silicate. 
The relatively small hydrogen-ion concentration is due to the hypo¬ 
thetical feebly ionized silicic acid or the acid salt. The larger the amount 
of calcium in the soil, even in the form of silicates, the less the hydrogen- 
ion concentration. This is also the reason why in the presence of a small 
intensity of acidity, a relatively large amount of base such as Ca(OH) 2 is 
necessary to shift the equilibrium from a high to a lower hydrogen-ion 
concentration. This fact was shown in several of the preceding experi¬ 
ments. 
HARMFUL EFFECTS OF ALUMINO-SILICATES 
Several investigators (i , io, 20) have shown the harmful effect upon 
plant growth of aluminum in some soils. The net result of the weather¬ 
ing process is the enrichment of aluminum in the mineral matter of the 
soil at the same time that there is an impoverishment of the bases. In 
tropical weathering this is carried to the extreme, as has already been 
shown. The luxuriant vegetation of the Tropics would indicate that 
the accumulation of aluminum is not in itself harmful. Just why 
aluminum compounds should be toxic under some conditions and not 
toxic under such extreme conditions as laterite weathering is a problem 
which apparently has not been investigated. The explanation probably 
lies in the nature of the subsoil. If the subsoil is open, transference of 
matter takes place more readily and a better balance is maintained. 
The addition of calcium compounds removes the toxic effect of alumi¬ 
num ( 20 ). The cycle of aluminum in nature and its effect on the physio¬ 
logical activities of plants is given by Stocklasa (, 42 ). 
ADSORPTION OF CALCIUM AS A PROCESS OF REPLACEMENT 
The presence of such alumino-silicates as are produced by weathering 
probably accounts for the adsorption of calcium from such compounds 
as Ca(OH) 2 and CaC 0 3 . Leaching and treatment with acids, both of 
which may be considered as intensified processes of weathering, increased 
the adsorptive power of the soil (see Experiments 11 and 14). Calcium 
adsorption was the same whether the original acid-treated soil was 
high or low in calcium (see Experiments 14 and 15). The adsorptive 
power is conditioned upon the alumino-silicates that are produced by 
those processes which make soil. Adsorption in soils is a process of 
replacing the lost base. 
RELATIVE ADSORPTION OF CALCIUM AND POTASSIUM 
When KC 1 is present in large amounts the need of these alumino¬ 
silicates for a base is satisfied. When the ratio of calcium to potassium 
was very small no calcium adsorption took place. When the ratio of 
