130 



THE PRINCIPLES OF SOIL MANAGEMENT 



soil, it very greatly increases its moisture-holding 

 capacity, which is further increased by the great ca- 

 pacity of humus itself to retain water, which amounts* 

 to 200 or 300 per cent of its dry weight, as compared 

 with 10 or 15 per cent for sandy loam and 25 to 35 per 

 cent for clay soils. It therefore improves the drought 



Fig. 39. The solid disc harrow. Most efficient on medium heavy soil free 



from stone and rubbish. 



resistance of soils by increasing their reservoir for 

 available water. 



(c) The dark color which humus imparts to soils 

 permits them to absorb the heat of the sun's rays very 

 much more than when the humus is absent, and thereby 

 their average temperature is decidedly raised. It is for 

 this reason that the corn first appears in the spring in 

 the low areas of dark-colored soil, which difference 

 in time may amount to several days. 



