102 TILLAGE MOVEMENTS OF SOIL WATER [chap. 



fair yield of the crop in question, the percentage of 

 water contained in the crop, the weight of dry matter 

 produced per acre, then the water transpired as deduced 

 from the dry matter produced, and in the last column 

 this same amount of transpired water recalculated as 

 inches of rain. 



It will be seen that in all cases the amount of water 

 transpired by the crop is a notable fraction of the total 

 annual rainfall, particularly so in the case of a root crop 

 like mangolds, which in the south and east of England 

 will often require a full half of the total rain falling 

 within the year. As much of the rainfall runs straight 

 off the surface into the ditches, and another portion 

 is lost to the land by percolation into the springs, as 

 again a considerable fraction is evaporated at certain 

 seasons from the bare surface of the soil, it is evident 

 that the water supply, even in our humid climate, is 

 far from sufficient for the maximum of production, and 

 may easily fall below that which is required for an 

 average crop. Indeed, we may take it as a truism that 

 the yield is more often determined by the water avail- 

 able than by lack of the other essentials of growth 



