72 Bulletin No. 34. 



lower irrigated levels in injurious quantity, are chiefly of the 

 "white" or least harmful kind. 



It has been found, however, that nitrogen and humus are 

 almost always deficient. The average for twenty soils examined 

 is .045 per cent of nitrogen and .65 per cent of humus or veg- 

 etable mould. When we consider that o. 1 per cent of nitrogen 

 and 1 to 5 percent of humus (according to the physical texture of 

 the soil) is desirable, it is at once evident that the desert farmer 

 stands in need of some remedy for a real defect. Associated with 

 these peculiarities is a prevailing dense and packed condition of 

 desert soils which is far from the light, loose condition of virgin 

 turf or forest soils in humid regions. 



It so happens, fortunately, that the deficiency of humus and 

 nitrogen may be corrected, and the tilth of desert soils improved, 

 by the one operation of green manuring. The humus or decom- 

 posed vegetable matter resulting from this process contains from 

 5 to 15 per cent of nitrogen, so that the one implies the other. The 

 nitrogen of humus, also, is in a form not easily dissolved in soil 

 water and carried away, which is far from being the case with Chile 

 saltpeter and some other commercial fertilizers. 



Also, humus improves the tilth of dense and lifeless soils 

 because it separates the soil particles one from another and per- 

 mits the circulation of the air and water needed by plant roots. 

 The water holding and retaining power of soils is also increased 

 because of the absorbent quality of humus, thus making them less 

 droughty in character. 



In this and other ways humus improves our desert soils both 

 in chemical fertility and in their behavior with water, the latter 

 being specially important in regions where water is often difficult 

 to obtain. 



In addition to these benefits the green manuring crops used 

 for the production of humus, such as "sour" clover and alfalfa, 

 send their roots deeply and in every direction into the soil. In 

 this way they improve the drainage of heavy soils, bring the 

 mineral elements of fertility from below to the surface, and open 

 up the way for the tenderer roots of other crop plants. 



The best kinds of green-manuring crops for southern Arizona, 

 so far as known, and the ways of handling them, are pointed out 



