FORMATION OF DRY-FARM SOILS 55 
solution carbon dioxid and other substances, tends 
to make the soil more fertile. 
The second great chemical agency of soil formation 
is the oxygen of the air. Oxidation is a process of 
more or less rapid burning, which tends to accelerate 
the disintegration of rocks. 
Finally, the plants growing in soils are powerful 
agents of soil formation. First, the roots forcing 
their way into the soil exert a strong pressure which 
helps to pulverize the soil grains; secondly, the acids 
of the plant roots actually dissolve the soil, and third, 
in the mass of decaying plants, substances are formed, 
among them carbon dioxid, that -have the power 
of making soils more soluble. 
It may be noted that moisture, carbon dioxid, 
and vegetation, the three chief agents inducing 
chemical changes in soils, are most active in humid 
districts. While, therefore, the physical agencies 
of soil formation are most active in arid climates, 
the same cannot be said of the chemical agencies. 
However, whether in arid or humid climates, the 
processes of soil formation, above outlined, are essen- 
tially those of the ‘‘fallow” or resting-period given 
to dry-farm lands. The fallow lasts for a few 
months or a year, while the process of soil forma- 
tion is always going on and has gone on for ages; 
the result, in quality though not in quantity, is the 
same — the rock particles are pulverized and the 
plant-foods are liberated. It must be remembered 
