LOSSES AND GAINS BY THE SOIL 



279 



fluence. A high temperature is favorable to decay only when 

 accompanied by sufficient moisture, but, as a general rule, a low 

 temperature is favorable to accumulation of humus. The less 

 the permeability of the soil, the slower is the oxidation and the 

 greater is the accumulation of humus. The greater the quantity of 

 plant substance produced, the larger the accumulation of organic 

 matter in the soil. Dry plant residues decay more slowly than 

 green, the straws of cereals more rapidly than the leaves of trees. 

 Wood decays more slowly than any of the other materials named. 

 In cultivated soil, in spite of the addition of manure, the 

 accumulation of humus is less than in similar uncultivated soils 

 covered with perennial crops, such as pastures, forests, etc. Both 

 lose organic matter, but the protective covering of the latter de- 

 creases the loss or even causes an increase. The following table 

 shows the carbon content of the soil under three forms of treat- 

 ment : 



GRAMS. OF CARBON PER 1,000 GRAMS. SOIL. 



A loss of over half the organic carbon in the soil occurred with 

 the cultivated crops, while with the uncultivated crop (sanfoin) 

 there was a slight gain. Manure was added to the cultivated plots. 



It appears that cultivated soils become poorer in carbon, no 

 matter how much manure is applied, and this impoverishment 

 ceases and the soil begins to get richer when the land is filled 

 with perennial plants. 



