no THE MOVEMENTS OF SOIL WATER [chap. 



ended is of the greatest importance in getting a heavy 

 clay soil to break down to a mellow crumbly seed bed. 

 The remaining operations of early spring are mostly 

 directed towards obtaining a good seed bed by the use 

 of cultivators of various kinds, and on heavy soils the 

 important thing to be observed is never to work the 

 land when it is in the least wet, or clods of deflocculated 

 clay must inevitably be the result, clods which dry with 

 difficulty and then form hard tough masses which can 

 only be worked down by the action of frost. By 

 implements alone — cultivators, rollers, or clod-crushers 

 — it is impossible to reduce the lumps formed by 

 smeared and deflocculated clay into a fine tilth. 



But while the farmer is reducing his soil to a crumb, so 

 as to secure a fine seed bed in which he can deposit the 

 seeds at an even depth, he is also careful to keep the soil 

 consolidated, because if it becomes loose below the 

 surface there can no longer be any continuity of liquid 

 films between the water in the subsoil and that round 

 the particles in contact with the roots of the young 

 plant. After sowing, the land is generally further 

 consolidated by rolling, and in a dry spring the rolling 

 may be repeated just after the seeds have begun to push. 

 The effect of rolling is to consolidate the whole soil 

 from the surface downwards ; it brings the soil particles 

 into more intimate contact and assists the formation of 

 continuous water films from the wet subsoil below. As 

 these water films are continuous to the surface there 

 will be loss of water by evaporation, with a consequent 

 reduction of temperature; but these disadvantages have 

 to be faced, in view of the vital importance of keeping 

 up the water-supply from below to the germinating 

 seeds or infant plants in time of drought. By experi- 

 ment, we can find that land which has been rolled is (i) 

 moister at the surface but drier at 9 inches or a foot 



