SOIL WATER 75 



moisture lost than was received as rain. While it is not 

 possible to make a definite rule for frequency of stirring a 

 mulch, it may be said that a mulch should never be allowed 

 to remain in a compact condition. However, in arid regions 

 the surface of the soil sometimes becomes completely dry 

 so quickly, even when compact, that capillary connection 

 is destroyed and loss of moisture is prevented. 



90. Depth of mulch. — In considering the depth that a 

 mulch should have, several facts should be kept in mind. 

 The deeper the mulch the more effective it will be, but as 

 it must be perfectly dry, roots cannot obtain nourishment in 

 the zone occupied by the mulch. The surface soil, from 

 which plants derive a large part of their material, is frequently 

 only eight to ten inches deep in humid regions and the deeper 

 the mulch the less top soil remains for roots. In arid regions 

 plants obtain food materials from greater depths and mulches 

 may be made deeper, which is fortunate since they need to 

 be deeper in regions where evaporation is greater. Another 

 consideration is the disturbance of roots in the process of 

 cultivation. Here, again, there is less occasion to cultivate 

 shallow in an arid region, as roots are generally found at 

 greater depths in such soils. 



A good depth for a mulch in humid regions is about three 

 inches, becoming somewhat less during the last cultivations 

 of corn. In irrigated regions a mulch of ten to twelve inches 

 is frequently used, especially in orchards, in which it is often 

 not necessary to renew the mulch, as the rainfall is usually 

 light. 



91. Effectiveness of mulches. — That mulches are effec- 

 tive in conserving moisture and increasing crop yield has 

 lately been called in question by certain writers who claim 

 that corn is not more benefited by tillage than by the 

 removal of weeds without tillage, and by some experi- 

 menters who find that fallow land contains as much moisture 



