112 THE MOVEMENTS OF SOIL WATER [chap. 



An old gardener was accustomed to say that he watered 

 his garden with his hoe ; and in this connection it should 

 be noted that a light watering or a gentle shower of 

 rain during a drought often causes more loss than gain 

 of moisture to the soil. The wetting of the surface soil 

 which ensues may be only sufficient to re-establish the 

 film of water round the soil particles at the top and make 

 that film continuous with the permanent film reaching up 

 from the wetted subsoil. In consequence, as soon as 

 evaporation begins afresh at the surface, water is lifted 

 from the lower layers to takes its place, and the process 

 goes on with considerable loss to the stock of water in 

 the soil unless the continuity of the film near the surface 

 is ruptured by cultivation. Waterings should be done 

 thoroughly or not at all, and they should be followed 

 up by hoeing as soon as the surface is fit to work again. 

 The fact that the crop evaporates or transpires con- 

 siderable quantities of water has already been noted, and 

 also the consequence that the growth of a crop leaves 

 the soil very much drier than it was before. The 

 converse of the proportion is seen in the fact that a year 

 of bare fallow, in which no crop is grown but the surface 

 is kept stirred to check evaporation, will accumulate a 

 considerable proportion of that year's rainfall in the 

 subsoil for the benefit of subsequent crops. In semi-arid 

 areas of deficient rainfall such as prevail in many parts 

 of western America and Canada, in southern Russia, 

 and in regions bordering the Australian deserts, farming 

 is rendered possible with a rainfall of 8 to 12 inches per 

 annum by taking advantage of bare fallows. It is 

 customary to take only one crop in two years, or two 

 crops in three years ; in the intermediate years the soil 

 is kept ploughed so as to establish a surface which will 

 both collect any rain that falls and will also evaporate 

 but little because of its looseness. The chief features of 



