SOILS 



23 



and by enrichiug 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.' Even where irrigation is pradliced the 

 above fadlors cannot be economically negledted. 



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'' was 

 shown to be the most eflBcient 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. 



