II.] WATER REQUIRED BY CROPS 39 



by a crop has entailed the supply of from 250 to 300 lb. 

 of water, and this mounts up to a considerable quantity 

 when the whole production per acre is considered. Of 

 mangolds, for example, we may expect to grow with 

 proper treatment 40 tons per acre, about 12 per cent, of 

 which will be dry matter, i.e. 4-8 tons of dry matter per 

 acre will be produced. This entails the evaporation of 

 4-8 X 250, or 1200 tons of water per acre ; and as an inch of 

 rain is approximately equal to 100 tons of water per 

 acre, a mangold crop of 40 tons per acre must evaporate 

 through its leaves as much as 1 2 inches of rain, a very 

 considerable proportion of the annual rainfall in the 

 districts in which mangolds are much grown. Other 

 crops do not produce quite as much dry matter per 

 acre as the mangold does, but still it will be found that 

 most of our farm crops necessitate the evaporation 

 of from 5 to 10 inches of rain. Considering that crops 

 make their chief growth during the hotter periods of the 

 year, when evaporation from the soil is also most active, 

 it becomes evident why the amount of rainfall is one of 

 the biggest factors in crop production, and why the 

 operations of the farmer in cultivating the soil are very 

 largely directed towards conserving for the plant what- 

 ever water reaches the land. 



Because of the large amount of water evaporated by 

 the plant, a growing crop always keeps the ground 

 beneath it in a very dry condition ; after harvest it will 

 generally be found that the subsoil is dry to some 

 considerable depth. For example, in June 1870 at 

 Rothamsted after a long drought, determinations were 

 made of the amount of water in the soil on which a 

 barley crop was being grown, and on an adjacent piece 

 of land which was being fallowed and kept bare. Down 

 to a depth of 54 inches it was found that the bare fallow 

 soil contained 900 tons per acre more water, an amount 



