CULTIVATION OF WASTE LAND 383 



cause only thus can the water present travel to the roots of the plant. 

 Lastly, a loose layer must be maintained on the surface, which, though 

 dry itself, acts as a screen and a barrier to prevent loss of water from the 

 effective soil below by any other channel than that of the plant. Granted 

 these methods of cultivation, the new feature about " dry-farming," 

 which has been introduced by settlers in the arid districts of Australia 

 and North America, is the use of a year of bare fallow in which to 

 accumulate a supply of water for the next year's or two years' crop. 

 This raises the fundamental question of how much water is necessary 

 for the growth of an ordinary crop. The first investigation that Lawes 

 and Gilbert carried out at Rothamsted dealt with this very point ; they 

 grew the usual field crops in pots, protected the surface of the soil from 

 evaporation so that all the loss of water proceeded through the plant, 

 weighed the water that was supplied from time to time, and finally 

 weighed the produce, expressing their results as a ratio between the 

 dry matter produced and the water transpired by the plant. These 

 experiments have been repeated under different climatic conditions by 

 Hellriegel in Heidelberg, by Wollny in Vienna, by King and others in 

 America, Now the two processes in the plant, carbon assimilation and 

 transpiration, are not causally connected, though as both are carried out 

 in the leaf and have some factors in common they are found to show 

 some constancy in their relative magnitudes. Lawes and Gilbert ob- 

 tained a ratio of about 300 lbs. of water transpired for each pound of 

 dry matter harvested, but the other investigators iinder more arid con- 

 ditions found much higher figures, up to 500 and even 700 to 1. Now, 

 a crop yielding 20 bushels of wheat per acre will contain about a ton of 

 dry matter per acre, so that, taking the high ratio of 500 to 1, no more 

 than 500 tons of water per acre or 5 inches of rain will have been con- 

 sumed in the production of this crop. It is, of course, impossible to en- 

 sure that all the rain falling within a year shall be saved for the crop, 

 much must evaporate before it reaches the subsoil 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 utilized will depend 

 not only on the degree of cultivation 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 inches. The necessary cultural operations con- 

 sist in producing such a rough surface as will ensure the water getting 

 into the subsoil, hence autumn ploughing is desirable. Where the preci- 

 pitation is largely in the form of snow, a broken surface also helps both 



