October 8, 1914J 



NATURE 



159 



I 



no more than 500 tons of water per acre or 5 in. of 

 rain will have been consumed in the production of this 

 crop. 



It is, of course, impossible to ensure that all 

 the rain falling- within a year shall be saved for the 

 crop ; much must evaporate before it reaches the sub- 

 soil where it can be stored, and only when the crop 

 is in full possession of the land can we expect that 

 all the water leaving- the soil shall go through the 

 crop. What proportion the waste bears to that which 

 is utilised will depend not only on the degree of cultiva- 

 tion, but upon the season at which the fall occurs; 

 summer showers, for example, that do not penetrate 

 more than a few inches below the surface will be 

 dissipated without any useful effect. When the 

 climatic conditions result in precipitation during the 

 winter, the water will be in the main available for 

 crop-production ; and it has been found by experience 

 that cereals can be profitably grown with as small a 

 rainfall as 12 in. The necessary cultural operations 

 consist in producing such a rough surface as will 

 ensure the water getting into the subsoil ; hence 

 autumn ploughing is desirable. Where the precipita- 

 tion is largely in the form of snow, a broken surface 

 also helps both to absorb the thawing snow and to 

 prevent it being swept into the gullies and hollow 

 places by the wind. On some of the Russian steppes 

 it has become customary to leave a long stubble in 

 order to entangle as much snow as possible, but prob- 

 ably a rough ploughing before the snowfall would be 

 even more effective. When the rainfall drops to the 

 region of 12 to 16 in., 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 essen- 

 tial 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 cultiva- 

 tion can be profitable. In the first place, we are 

 only imperfectly informed as to the relation between 

 rainfall and evaporation. \t Rothamsted there are 

 three drain-gauges side by side, the soil layers being 

 20, 40, and 60 in. deep respectively. The surface is 

 kept rough and free from growth, though scarcely 

 m 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 

 the gauges and is independent of the depth of 

 subsoil within which water is stored. Evaporation 

 then would seem to be determined bv surface alone, 

 but we are without systematic experiments to show 

 how variations in the surface induced by cultivation 

 will alter the rate of evaporation. .A^ knowledge o^ 

 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 regulating 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 ft., 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. 



NO. 2345, VOL. 94] 



The evidence on either side is far from being con- 

 clusive, and more experiment is very desirable. 



It would also be valuable to know how far evapora- 

 tion 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, open fields prevail 

 in the east and south. Yet the enormous value of a 

 wind-screen to vegetation can be readily observed, 

 and the market-gardeners both in England and the 

 still drjer districts of the south of France make great 

 use of them. Lastly, we must have more knowledge 

 about the relation between transpiration-water and 

 growth ; we do not know if the high ratios we have 

 spoken of hold for all plants. Xerophytic plants are 

 supposed to be possessed of protective devices to 

 reduce loss of water. Are they merely effective in 

 preserving the plant from destruction during the fierce 

 insolation and drj'ing it receives? and do they enable 

 a plant to make more growth on a given amount of 

 water? Wheat, for example, puts on its glaucous, 

 waxy bloom under dry conditions : Is this really 

 accompanied by a lower rate of transpiration per unit 

 surface of leaf? and is it more than defensive, con- 

 noting a better utilisation of the water the plant 

 evaporates ? 



The cultivation of these soils with a minimum rain- 

 fall necessitates varieties of plants making a large ratio 

 of dry matter to water transpired, and also with 

 a high ratio between the useful and non-useful parts 

 of the plant. Mr. Beaven has shown that the differ- 

 ence in the yields of various barleys under similar 

 conditions in England are due to differences in their 

 migration factors : the same amount of dry matter 

 is produced by all, but some will convert co per cent, 

 and others only 45 per cent, into grain. This migra- 

 tion ratio, as may be seen by the relation betw^een 

 corn and straw on the plots at Rothamsted, is greatly 

 affected by season ; nevertheless, Mr. Heaven's work 

 indicates that under parallel conditions it is a con- 

 genital characteristic of the variety, and therefore one 

 that can be raised by the efforts of the plant-breeder. 

 The needs of drv'-land farming call for special atten- 

 tion on the part of the breeder to these two ratios 

 of transpiration and migration. 



Closely linked up with the problems of drA-land 

 farming are those which arise in arid climates from 

 the use of irrigation water on land which is either 

 impregnated with alkaline salts to begin with or 

 develops such a condition after irrigation has been 

 practised for some time. The history of irrigation 

 farming is full of disappointments due to the rise of 

 salts from the subsoil and the subsequent sterility of 

 the land, but the conditions are fully understood, and 

 there is no longer any excuse for the disasters which 

 have overtaken the pioneers of irrigation in almost 

 every country. Sterility may arise from two causes — 

 overmuch water, which brings the water-table so close 

 to the surface that the plants' roots may be asphyxi- 

 ated, or the accumulation by evaporation of the soluble 

 salts in the surface layer until plants refuse to grow. 

 The annual cutting off of the cotton crop in Egypt 

 as the water-table rises with the advance of the 

 Nile flood affords a good example of asphyxiation, 

 but in the neighbourhood of irrigation canals we 

 also find many examples of sterility due both to the 

 high water-table and an accompanying rise of salts. 

 The governing principle is that drainage must accom- 

 pany irrigation. Even if free from salts at the outset, 

 the land must accumulate them bv the mere evapora- 

 tion of natural waters, and they will rise to the surface 

 where they exert their worst effect upon vegetation, 

 unless from time to time there is actual washing 



