42 LAND DRAINAGE 



only an indifferent crop in a year of ordinary rainfall, 

 and will yield a very poor crop, or may even fail utterly, 

 in a dry year. The reasons for this difference may be 

 very briefly summed up as follows : 



The physical structure of the former field is good, that 

 of the latter is poor. When the structure of a soil is good, 

 its capacity to hold capillary water is greater than when 



FIG. 16. The effect of lumps lying on the surface of a field. In all 

 cases the lumps reflect a part of the sun's radiation. Of the radia- 

 tion which enters the lump, portions are distributed and radiated 

 from the surface of the lump above ground. When the lump rests 

 upon other smaller lumps, leaving air cushions between the lower 

 surface and the soil, other losses occur. That part of the soil surface 

 heavily shaded receives no direct radiation. 



it is bad the storage capacity is greater. The soil of 

 good structure is able to retain its supply of capillary 

 water from evaporation much more successfully than can 

 the soil of poor structure. It offers a much better oppor- 

 tunity for the development of the root system of the crop 

 than does the soil of poor structure. Hence it is that a 

 soil of good physical structure has been known to produce 

 a large crop of wheat without rainfall between planting 

 and harvest, when adjacent fields failed. 



