114 



THE PRINCIPLES OF SOIL MANAGEMENT 



posed organic matter are generally in better physical 

 condition than soils low in organic content. The marked 

 effect of the absence of this material in many long cul- 

 tivated soils is well known. For example, in much of 

 southern New York the hill soils are now recognized 

 to have a much different relation to crop growth than 

 they had for a few years after they were cleared. Their 

 color has changed, and with the decay of the humus 



Fig. 36. The spring-toothed harrow. A type of cultivator adapted to all 

 classes of soil and more efficient than any other in rough and stony ground. 



has come a decided physical change in the soil, which 

 is largely corrected by the restoration of the humus 

 content. In certain prairie soils the effect of humus 

 depletion on structure is even more marked. The actions 

 of humus are many, as will be noted in the more com- 

 plete discussion of that topic yet to follow; but one 

 of those actions is on the granular nature of the soil. 

 (1) As will appear, humus is somewhat plastic, and 

 tends to hold the soil in a more loose condition than 



