SOILS 23 



and by enriching the soil in humus either by manuring 

 or a suitable rotation. Humus affects the physical 

 properties of the soil considerably among other things, 

 enabling it to hold more moisture*) without injury to 

 the plants in a wet time, and to endure drouth in a 



FIG. 8 AN EFFICIENT PULVERIZER! THE DISK HARROW 



dry time. 1 Even where irrigation is practiced the 

 above factors cannot be economically neglected. 



Surface-fitting Tools. The Acme harrow is one 

 of the best tools for making a soil mulch before the 

 crop is planted, and in trials made by Sanborn 2 was 

 shown to be the most efficient type of harrow for pul- 

 verizing soil. On stony land, or where roots of trees 

 interfere, the spring- tooth harrow (Fig. 7) is preferred 

 for deep tillage of the soil, while under other conditions 



Minn. Bui. 68, pp. 576-579. 2 Utah Bui. 4. 



