120 RELATIONS OF SOIL TO WATER 



the water is thus removed from below the surface. A bare 

 soil dries at its surface, and generally only to a slight depth, 

 while a cropped soil dries from below, and often to a con- 

 siderable depth. 



The distribution of the roots has a great influence in deter- 

 mining the amount of water available to a crop, and the 

 extent to which the soil is dried. Very deeply rooted crops, 

 as lucerne and red clover, draw their supply from so great 

 a depth of subsoil that they are practically independent of 

 summer rains. Such is also the case with many forest trees. 

 With such a crop as wheat, the extent of the development 

 of the roots determines often the whole difference between 

 a good and bad crop. The history of the Rothamsted wheat 

 field shows that the best crops are obtained after a mild 

 winter, followed by an early spring, especially when these 

 seasons are rather dry. Under such circumstances the wheat 

 finds itself in May provided with a maximum of root develop- 

 ment, and it will then require little rain afterwards for its 

 maturation. On the other hand, cold and wet weather during 

 the early part of the plant's life prevents the development 

 of the roots, and the crop consequently suffers in the first 

 drought that occurs. Roots will not develop in a saturated 

 soil : on heavy land, a dry spring goes far to ensure a good 

 crop of corn. 



Examples of the quantity of water consumed by crops 

 have been already given (p. 51), and attention has been 

 called to the fact that the capacity of a plant to evaporate 

 water increases with the amount supplied. A good illustration 

 of the drying effect of a cereal crop is afforded by the deter- 

 minations of water in land growing barley, and in adjoining 

 fallow ground, made at Rothamsted during the drought of 



