IV.] STORAGE OF WATER BY BARE FALLOWS 113 



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. 116). 



But in addition to the gain in available nitrogen due 

 to fallowing, the land which does not carry a crop during 

 a season will accumulate a store of water which may be 

 of the utmost service to the succeeding crop. In the 

 preceding section some figures have been given showing 

 how much more water is present at the end of the 

 summer in the fallow land than in the land which had 

 carried a crop, so that in districts where the winter rain- 

 fall is small the fallowed land will start the next season 

 with a great advantage. Indeed, in a semi-arid climate 

 where the annual rainfall is insufficient, satisfactory crops 

 may yet be grown in alternate years by using an inter- 

 mediate fallow period in which to accumulate a reserve 

 of subsoil water. 



The following series of measurements will illustrate 

 this point ; it shows the percentages of water in spring 

 and autumn on fallow and cropped land respectively, 

 also the water present in the same land in the following 

 spring and autumn, when both plots were in oats. 



The effect of the fallowing in retaining more moisture 

 in the soil is seen throughout the whole of the second 

 season. 



