9 8 



SOILS AND FERTILIZERS 



smaller, but in a few years returns will be greater than 

 if all of the vegetable matter were burned. 



104. Physical Properties of Soils Influenced by 

 Humus. — The physical properties of a soil may be 

 entirely changed by the addition or the loss of humus. 

 The influence of humus upon the weight, color, 

 water, and heat of soils, is discussed in the chapter on 

 the physical properties of soils. Soils reduced in 

 humus content have less power of storing up water 

 and resisting: droug-ht. This fact is illustrated in the 



following table : 3 ° 



Per Cent. Water. 



In soil. 



Soil rich in humus (3.75 per cent.) 16.48 



Adjoining soil poorer in humus (2.50 per cent.) 12.14 



After 10 hours 

 exposure in 

 tray, to sun. 



6.12 



3-94 



105. Humic Acid. — In the absence of calcium 

 carbonate or other alkaline compounds, the vegetable 

 matter may produce acid products destructive to the 

 growth of some crops. The acidity in such cases may 

 be readily corrected by the use of lime or wood ashes. 

 Studies conducted by the Rhode Island Experiment 

 Station indicate that the areas of acid soils are 

 quite extensive. Acid soils can be distinguished by 

 their action upon red litmus paper. A soil may, how- 

 ever, give an acid reaction and contain a fair amount 

 of lime. The subject of acid soils and liming is consid- 

 ered in Chapter IX. 



