640 TRANSACTIONS OF SECTION M. 
worked out by English farmers in the East and South-East of England, prin- 
ciples first expounded by Jethro Tull, though a complete explanation was not 
then possible, even if it is now. In the first place, the tilth must be made both 
deep and fine, thus whatever rain falls will be absorbed and the conditions 
favouring a deep and full root range will have been established. Next, the 
soil below the surface, though finely worked, must be compact, because 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 cultiva- 
tion, 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 pro- 
duce, 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 obtained a ratio of about 300 lbs. of water transpired for each pound 
of dry matter harvested, but the other investigators under more arid conditions 
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 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 
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 utilised 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 experi- 
ence that cereals can be profitably grown with as small a rainfall as 12 inches. 
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 precipitation 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 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 imper- 
fectly informed as to the relation between rainfall and evaporation. At 
Rothamsted there are three drain-gauges side by side, the soil layers being 
20, 40, and GO inches deep respectively. The surface is kept rough and free 
