140 HOW CROPS FEED. 
ly with other bases, as lime, oxide of iron, ete. These 
alkali-salts, then, should attack the minerals of the soil in 
a manner similar to carbonic acid. The same is probably 
true of crenic and apocrenic acids. 
d. It scarcely requires mention that the ammonia salts 
and nitrates yielded by the decay of plants, as well as the 
organic acids, oxalic, tartaric, etc., or acid-salts, and the 
chlorides, sulphates, and phosphates they contain, act upon 
the surface soil where they accumulate in the manner al- 
ready described, and that vegetable (and animal) remains 
thus indirectly hasten the solution of mineral matters. 
Action of Living Plants on the Minerals of the Soil._— 
1. Moisture and Carbonic Acid.—The living vegetation 
of a forest or prairie is the means of perpetually bringing 
the most vigorous disintegrating agencies to bear upon 
the soil that sustains it. The shelter of the growing 
plants, not less than the hygroscopic humus left by their 
decay, maintains the surface ina state of saturation by 
.Mmoisture. The carbonic acid produced in living roots, 
and to some extent, at least, it is certain, excreted from 
them, adds its effect to that derived from other sources. 
2. Organic Acids within the Plant.—According to 
Zoller, (Vs. St. V. 45) the young roots of living plants 
(what plants, is not mentioned) contain an acid or acid- 
salt which so impregnates the tissues as to manifest a 
strong acid reaction with (give a red color to) blue litmus- 
paper, which is permanent, and thercfore not due to car- 
bonic acid. This acidity, Zéller informs us, is most in- 
tense in the finest fibrils, and is. exhibited when the roots 
are simply wrapped in the litmus-paper, without being at 
all (?) crushed or broken, The acid, whatever it may be, 
thus existing within the roots is absorbed by porous paper 
placed externally to them. 
Previous to these observations of Zdller, Salm Horst- 
mar (Jour. fiir. Prakt. Chem. XL, 304,) having found in the 
ashes of ground pine (Lycopodium complanatum), 38° |, of 
