METHODS OF INTKRSEASON TILLAGE. 



25 



in the season a very effective dust mulch on the surface of the soil, 

 which is not likely to be disturbed by summer rains and which re- 

 quires only enough cultivation to keep weeds from growing. (See 

 PI. II, fig. 1.) The maintenance of this surface mulch is an expen- 

 sive operation even under these conditions, for weed seeds germinate 

 readily in the moist soil below the surface, push up through the dry 

 mulch, and unless quickly killed dissipate large quantities of valuable 

 soil moisture. It is quite as important to keep weeds froui developing 

 to any considerable size as it is to maintain a surface mulch that will 

 check direct evaporation. If these weeds are killed at short intervals 

 by light surface cultivation they use but little water, and it is also 

 much cheaper to destroy them when small than after they have grown 

 to considerable size. 



It must always be taken into account that the surface soil mulch, 

 which is so necessary to moisture conservation, is possible only where 

 the soil is sufficiently heavy not to be blown away by summer winds. 

 These summer winds, which in some regions are almost continuous 

 and sometimes of high velocity, often make a clean summer fallow 

 altogether impracticable. It is also extremely doubtful if the requi- 

 site summer fallowing is practicable on very light soils, for even the 

 milder summer winds would do serious damage where such a soil 

 in a fine state of tilth is exposed. 



THE IMPORTANCE OF SOIL TEXTURE. 



The water-holding capacity of a soil depends largely upon its 

 physical texture and is also influenced to some extent hj the amount 

 of organic matter present. For instance, a heavy clay soil contain- 

 ing a large amount of humus might hold 30 per cent of water and 

 still be in workable condition, while sanely soil having 10 per cent of 

 water might seem very wet. It is therefore obvious that where 

 success in dry farming depends upon the storage in the soil within 

 the reach of plants of enough water to carry a crop to maturity, the 

 water-holding capacity of the soil becomes a matter of the first 

 importance. 



Table II. — Increased moisture occasioned hy 1 inch of rain distributed through 

 various depths of soil, tveighing 85 pounds per cuhic foot. 



A cubic foot of heavy loam soil perfectly dry is estimated to 

 weigh 85 pounds. A cubic foot of water weighs about 62| pounds. 



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