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 sojl 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 chmates, the 

 processes of soil formation, above outhned, 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 



