150 SOILS: PROPERTIES AND MANAGEMENT 



soil by the plowing-under of green materials or of barn- 

 yard manures. In general, the percentage content of 

 inorganic matter increases as the organic matter decays. 



101. Effects of organic matter on soil. — The effects 

 of the organic matter on soil and plant conditions are as 

 numerous as they are complex. Some of the influences 

 are direct, others are indirect. As the specific gravity of 

 organic matter is low, the first effect of its addition would 

 be to lower the absolute and the apparent specific gravity 

 of the soil. As the water capacity of humus is very high, 

 a soil rich in organic constituents usually possesses a 

 high water-holding power. This makes possible greater 

 volume changes both on drying and in the presence of 

 excessive moisture. The granulating effects of wetting 

 and drying and freezing and thawing are therefore ac- 

 celerated. The organic matter tends also to spread 

 the individual particles of soil farther apart, especially 

 in a clay. Its loosening effects are immediately ap- 

 parent in such soil. On the other hand, because organic 

 matter has a higher cohesive and adhesive power than 

 sand, it performs the function of a binding material 

 with the latter soil, a condition much to be desired in a 

 material possessing such textural characteristics. 



The better tilth induced by the presence of organic 

 matter in any soil tends to facilitate ease in drainage 

 and to encourage good aeration. These two conditions 

 are of course necessary for the promotion of soil sanita- 

 tion. Root extension and bacterial activity are thus 

 increased. It is of especial importance that the splitting- 

 up of the organic matter shall take place in the presence 

 of plenty of oxygen, in order that toxic compounds may 

 not be generated and that a humus highly favorable to 

 plant growth shall be produced. The increased water 



