iv.] STORAGE OF WATER BY BARE FALLOWS 125 



but it is not possible as yet to discriminate with any 

 certainty among the actions at work. For similar 

 reasons, when trees are planted in arable land weeds 

 should be kept down, nor should crops like cabbages or 

 mangolds be grown between the rows of trees ; such 

 crops are usually considered to " draw the land " and 

 deplete it of plant food, but the harm they do lies in 

 the water they withdraw just at the most critical season, 

 when the tree is making its first start in its new quarters. 



Bare Fallows. 



The custom of fallowing land, of leaving it entirely 

 bare for a season, during which the land is worked as 

 often as possible, is one of the oldest in agriculture ; a 

 rotation of wheat, wheat, fallow, or of beans, wheat, 

 fallow, being almost universal, until the introduction of 

 turnips gave the farmer a chance of cleaning his land 

 and yet growing a crop at the same time. The objects 

 of a fallow were various : in the first place, the summer 

 cultivations resulted in a thorough cleaning of the land 

 and in a free development of nitrates for the succeeding 

 crop; also on the heavy soils, which are the most suited 

 to fallowing, a good tilth was obtained that was often 

 impossible otherwise. Indeed, at the present day it is 

 found desirable and even necessary to introduce an 

 occasional bare fallow when farming on the heavy clays 

 of the south and east of England, in order to obtain a 

 satisfactory tilth in that dry climate. 



One of the most notable effects of fallowing lies in 

 the production of a stock of nitrates from the stores of 

 combined nitrogen in the soil ; these nitrates are at once 

 available for the ensuing wheat crop if the autumnal 

 rains are not too great to wash them out of the soil 

 (see p. 129). 



But in addition to the gain in available nitrogen due 



