384 THE POPULAR SCIENCE MONTHLY 



to absorb the thawing snow and to prevent it being swept into the gullies 

 and hollow places by the wind. On some of the Eussian steppes it has 

 become customary to leave a long stubble in order to entangle as much 

 snow as possible, but probably a rough ploughing before the snowfall 

 would be even more effective. When the rainfall drops to the region of 

 12 to 16 inches and occurs during the summer months, then dry- farming 

 methods and the summer fallow become of the first importance. The 

 deep cultivation ensures that the water gets quickly down to the subsoil 

 away from danger of evaporation, and the immediate renewal of a loose 

 surface tilth is essential in order to conserve what has thus been gained. 



In connection with this dry-farming there are several matters that 

 still require investigation before we can decide what is the minimum 

 rainfall on which cultivation can be profitable. In the first place, we are 

 only imperfectly informed as to the relation between rainfall and evap- 

 oration. At Eothamsted there are three drain-gauges side by side, the 

 soil layers being 20, 40 and 60 inches deep, respectively. The surface is 

 kept rough and free from growth, though hardly in the condition of 

 looseness that could be described as a soil mulch. Yet the evaporation, 

 even under a moist English atmosphere, amounts to one half of the 

 annual rainfall, and the significant thing is that the evaporation is 

 approximately the same from all of the gauges and is independent of 

 the depth of subsoil within which water is stored. Evaporation then 

 would seem to be determined by surface alone, but we are without syste- 

 matic experiments to show how variations in the surface induced by 

 cultivation will alter the rate of evaporation. A knowledge of the 

 evaporation factor would then inform us of what proportion of the 

 rainfall reaches the subsoil ; we then want to know to what extent it can 

 be recovered and how far it may sink beyond the reach of the crop. 

 It is commonly supposed that the subsoil below the actual range of the 

 roots of the crop may still return water by capillarity to the higher levels 

 that are being depleted, the deeper subsoil thus acting as a kind of regu- 

 lating reservoir absorbing rain in times of excess and returning it when 

 the need arises. But some work of Leather's in India and Alway's on 

 the great plains of North America throw doubt on this view, and would 

 suggest that only the layer traversed by roots, say, down to a depth of 

 6 feet, can supply water to the crop; the water movements from the 

 deeper layers due to capillarity being too slow to be of much effect in the 

 maintenance of the plant. The evidence on either side is far from being 

 conclusive and more experiment is very desirable. 



It would also be valuable to know how far evaporation from the bare 

 soil can be checked by suitable screens or hedges that will break the 

 sweep of the wind across the land. In England hedges have always 

 been looked at from the point of view of shelter from stock; we find 

 them most developed in the grazing districts of the west, while bare 



