to8 TILLAGE MOVEMENTS OF SOIL WATER [chap. 



wet and then dries rapidly, as it often will do in March, 

 it forms obstinate clods which no operation will reduce 

 to a good tilth. Again, the additional surface which is 

 exposed to the action of frost and rain causes increased 

 weathering, and some of the dormant mineral plant food 

 is brought into a more available condition. 



Spring Cultivation. 



In such climates as prevail in parts of England, 

 where it is necessary to retain as much of the winter's 

 rainfall in the land as possible, and where spells of 

 drying weather are apt to set in with the spring, it is 

 desirable to cross plough or otherwise move any land 

 that is destined for a summer crop at as early a date 

 as it will bear cultivation. This spring working is 

 necessary for two reasons : to obtain a mulch, or layer 

 of loose soil, which will conserve moisture in the sub- 

 soil during the dry periods that follow, and to give the 

 surface soil an opportunity of drying gradually into a 

 condition that will yield a good tilth. The land, even 

 though ploughed in the autumn, will become consoli- 

 dated again to a considerable degree by the beating 

 rains of winter. In this closely-packed material capil- 

 lary water can move freely, and as the surface layer 

 dries under the action of the sun and wind, fresh supplies 

 of water are lifted from the subsoil by surface tension, 

 with the result that there is a steady and continuous 

 drain of subsoil water through its connection with the 

 exposed and rapidly evaporating surface. But if the 

 top layer of soil is broken up and left loose upon the 

 land by the cultivator, there is no longer a continuous 

 film joining the exposed surface and the subsoil water ; 

 surface tension can only lift water as far as the film is 

 unbroken, i.e., as far as the unstirred soil extends, and 

 this layer is protected from evaporation by the loose 



