SOILS AND THEIR PROPERTIES. 69 



less resistance to the passage of water, and, therefore, facili- 

 tate rapidity of movement. The most suitable condition is 

 one intermediate, where neither the resistance to passage nor 

 the lack of capillary attraction are too pronounced. Where 

 soils have been recently broken up by tillage there will 

 always be a space in the soil which is too large to permit of 

 capillary attraction. The water will, therefore, be obliged 

 to take circuitous routes when it rises, but the open spaces 

 permit the penetration of the roots, which are thereby enabled 

 to go down after the water. Deep tillage, whilst facilitating 

 deep rooting, checks the upward movement of the water 

 supply to the surface. Where rainfall is scanty, deep tillage 

 is not satisfactory, because the seeds that are sown do not 

 easily get enough water for their early stages of growth. 

 Very shallow tillage dries up an inch or so of the surface, 

 but protects the subsoil from loss by evaporation. In some 

 special cases it is possible to obtain a combination of these 

 different effects. When turnips are sown on land which has 

 been put up into riggs and subsequently rolled, the roller 

 only compresses the tops of the riggs, the furrows being 

 untouched. With a " Cambridge " roller the pressure is 

 mostly on the top of the riggs. Capillarity is, therefore, 

 increased about the region where the seed is sown, but a 

 mulch of loose earth remains in the furrows, and hinders 

 the development of the weeds. 



A point to be noted is that evaporation of water from a 

 thoroughly wet soil is greater than that from an equal area 

 of water itself, because the surface of a pond is practically 

 smooth, whilst the surface of a soil is very irregular. As, 

 however, a soil is by no means always thoroughly wetted, 

 but is often dry, the total evaporation in a year from a soil 

 is less than that of an equal area of water surface. At 

 Rothamsted, 14 inches per annum represents the evaporation 

 from the soil, and 18 inches per annum from a water surface. 

 In many parts of the British Isles evaporation is greater than 

 at Rothamsted, and in hot, dry countries the amount is 

 still greater. At Alice Springs, in South Australia, evapora- 

 tion amounts to 103 inches per annum, and at Bombay it 



