CULTIVATION IN DRY AREAS 167 



cipitation, the greater is the necessity for the dust mulch. 

 It has also an important place in areas with a sufficiency 

 of rainfall for the season, were it properly distributed, 

 but which are more or less subject to dry periods during 

 the season of growth. Moreover it will prove very help- 

 ful in irrigated areas after each application of water to 

 crops that are of that kind, and in that stage of growth, 

 that will admit of some kind of surface cultivation while 

 they are growing. 



It is always in place: (1) On summer-fallowed land 

 in dry areas and during the entire season. (2) In these 

 areas it is also more commonly in place on small-grain 

 crops during the early stages of growth, but in seasons of 

 unusual moisture during the growing period it may not 

 be necessary. (3) On small-grain crops in humid areas 

 in seasons of more than usual drought during the early 

 stages of growth in the grain. (4) On alfalfa lands in 

 dry areas, especially in the early part of the season. 

 (5) On all crops properly known as cultivated crops, and 

 during almost the entire period of growth. Of course, the 

 dust mulch on small cereals and alfalfa land is very much 

 less complete than on land that is fallow or that is grow- 

 ing a cultivated crop, but on the former it i nevertheless 

 a dust mulch in a modified form. 



On the summer-fallow the dust mulch is maintained 

 on many soils almost entirely by the aid of the harrow. 

 But on some soils the aid of the disc is necessary in some 

 instances to aid in killing weeds, to break up impaction 

 resulting from- a heavy rain or to break up a crust formed 

 beneath the soil mulch made by the harrow. Such a 

 crust will form in some soils in the absence of rain. 

 When it forms, it should invariably be broken up by some 

 deeper form of cultivation, as when present it excludes 

 aeration and therefore stays proportionately the active 

 working of bacteria in the soil. 



