278 SOILS: PROPERTIES AND MANAGEMENT 



195. Depth of a mulch. The depth of a mulch is an 

 important question in humid regions. Not only must 

 all the water in the layer be sacrificed in order to make 

 the mulch effective, but the plant-food of that layer 

 is temporarily, withdrawn from use. In humid areas, 

 where the surface soil is usually not over eight or ten 

 inches in depth, the latter consideration is vital, since 

 the fertility of the soil would be greatly depressed by a 

 deep soil mulch. Another factor to be considered here 

 is the possible root pruning that may occur while the 

 mulch is being formed. While not of importance early 

 in the season, it is worthy of considerable attention when 

 the intertilled crop attains some age. It has been shown, 

 with such crops as corn, that considerable depression in 

 yield may result from the maintenance of a mulch at too 

 great a depth, some of the feeding roots being cut off 

 thereby. For such reasons the average depth of mulch 

 for humid regions and in dry-farming operations has 

 become regulated to about three inches, although in the 

 late cultivation of corn a less depth than this is advocated. 

 In irrigated regions where little rainfall occurs and where 

 the soil is very deep and uniformly fertile, mulches as 

 deep as ten or twelve inches are sometimes found, es- 

 pecially in orchards. As rainfall occurs but few times 

 during the season, such a mulch often needs no attention 

 except for its original formation. With crops having 

 shallow roots a thinner mulch layer must of course be used. 



196. Resume of mulch control. To summarize briefly, 

 the cardinal points in mulch control are : (1) mulches are 

 more effective and more easily maintained in an arid than 

 in a humid climate; (2) their efficiency depends directly 

 on their dryness, looseness, and granulation; (3) sandy 

 soil is more easily maintained as a mulch than clay soil ; 



