FERTILITY AND HUMUS IN DRY AREAS 411 



tilizers. Fertility already in the -soil may be made 

 available by various processes. 



Soils in dry areas are usually rich in mineral mat- 

 ters, hence they do not meanwhile call for additional 

 supplies of phosphate and potash, but they are frequently 

 deficient in nitrogen and humus. These may be best 

 supplied by legumes (see chapter XIII). They take nitro- 

 gen from the air in the process of growth, and much of 

 what has been stored in the soil feeds the crops that 

 follow, at least for a time, usually for 2 or 3 years in 

 humid climates, and frequently it may be for a longer 

 term in dry areas. If the crops are not buried but are 

 harvested and fed or grazed off, such enrichment is 

 without cost to the grower of these crops. Whether 

 the entire crop should be buried will depend more on 

 the extent to which the soil needs vegetable matter than 

 on the extent to which it needs fertilization. Humus 

 may frequently be supplied more cheaply by non-legu- 

 minous plants. In areas where the header is used in 

 harvesting wheat grown on fallowed land in alternate 

 years, the humus thus supplied does not cost anything. 

 Grazing off the crop grown, especially when leguminous 

 in nature, brings enrichment to the land in a very effect- 

 ive form. The pea crop properly grown is one of the 

 best for such a use. 



Farmyard manure is very beneficial to arid soils 

 when properly applied, but "it may be so applied as to 

 do harm, as when, for instance, large quantities are 

 buried in the soil at one time. When dry weather fol- 

 lows, such a soil is kept too open and will not suffi- 

 ciently hold moisture. It should, therefore, be applied 

 in quantities quite moderate, not more usually than ten 

 tons to the acre, and frequently considerably less will 

 be better. It should be applied when practicable with 

 the manure spreader. If the manure is disced in before 

 the land is plowed, its action will usually be quicker 



