Ii 6 
Journal of Agricultural Research 
Vol. 26, No. 3 
The carbon dioxid needed for this weathering process is derived 
partly from that brought to the soil in rain (9), and perhaps to a larger 
extent from the decay of organic matter in the soil. In this way organic 
matter indirectly brings about an acid condition of the soil. 
IMPOVERISHMENT OF CALCIUM, THE END RESUI/T OF WEATHERING 
The net result of the weathering process is the removal of bases, 
mostly in carbonate form, the production of free silica, and the accumu¬ 
lation of the alumino-silicates. The presence of calcium in soil in the 
noncarbonate form indicates that all of the calcium has not been re¬ 
moved from these silicates. A large part of this calcium is soluble in 
dilute hydrochloric acid and in a 0.5N solution of KC 1 . In this process 
the potassium is taken up by the silicate and the calcium is set free in 
equivalent amounts ( 36 ). This process is usually called basic exchange 
(43, 46 , 47 ). The ionic and adsorption theories show that calcium is 
made soluble because of the excess of hydrogen and chlorin ions when the 
adsorption of potassium takes place. The weathering process produces 
a soil poor in calcium. Plants need calcium for growth and development. 
What is often called an acid condition exists when calcium is so firmly 
combined with the alumino-silicates that it is unavailable for plant use. 
Under such a condition calcium supplied even in the form of CaSi 0 3 
may be beneficial, not because it reduces the hydrogen ions, but because 
it satisfies the need of plants for calcium. When calcium in the form 
of Ca(OH) 2 is added to soils which contain these alumino-silicates the 
basic condition of the original mineral tends to be restored. Such re¬ 
combination may be purely chemical. If this is true, there are an in¬ 
definite number of alumino-silicates in which calcium is only partly 
taken up. This was shown in several experiments in which the amount 
of calcium adsorbed was definitely related to the amount added. 
ACID NATURE OF WEATHERED AEUMINO-SIUCATES 
The acid nature of weathered silicates, or the property which makes 
them combine with bases, can be seen* from the following explanation 
based on Clarke’s theory (« 8 ). The simplest method of representing 
many, perhaps all, of the orthosilicates of aluminum is as substitution 
derivatives of the normal salt Al 4 (Si 0 4 ) 3 derived from the normal silicic 
acid H 4 Si 0 4 . The existence of this silicate has not been proved, but 
several minerals such as nephelite, NaAlSi 0 4 , leucite, KAlSi 0 4 and 
Kryptolite, HAlSi 0 4 , in which one or more atoms of aluminum are 
replaced by other metals, are known. The acid nature of the latter 
is suggested both by the formula and by the substitution of hydrogen 
for the bases potassium and sodium. 
The structural formulas (8) of the normal aluminum silicate and 
kaolinite suggests the acid nature of the latter. 
Normal silicate* Kaolinite. 
/Si 0 4 =Al /OH 
Afc^—Si 0 4 =Al Al^—Si 0 4 =H 2 
\Si 0 4 =Al \Si 0 4 =Al 
It is well known that kaolin has an acid reaction toward indicators. 
Fuller’s earth, a substance of the same chemical nature as kaolin, gave 
