70 PLANT PRODUCTS 



is 8^ inches. Any green stuff growing on the surface of 

 soil will increase the evaporation, hence weeds rob the soil 

 of water. lyoose stones on the surface decrease the rate of 

 evaporation. In some parts of India stones that have been 

 collected from the surface are carefully put back again as a 

 mtilch, but such a method is only possible in small types of 

 cultivation. When water evaporates the soil shrinks in 

 volume, owing to the removal of the water films, which 

 separate the particles. In sandy soils this shrinkage is 

 very slight ; with humus soils the shrinkage is very large 

 indeed. Clay soils shrink to an intermediate extent, but do 

 not shrink in a regular manner, and generally develop cracks. 

 These cracks tend to break the roots of plants, and, therefore, 

 do harm at the time. The surface soil collects in the cracks 

 and a slow inversion of the soil takes place. In other types 

 of soil cracks rarely develop. Whenever water evaporates 

 from a soil, loss of heat results, owing to the latent heat of 

 steam, hence wet soils are also cold soils. When the surface 

 is loosened by slight tillage, the water is kept in the soil. 



At Cockle Park the moisture content on one occasion was 

 1 1*01 per cent, of water where tilled, and 8*84 per cent, of water 

 where untilled, and on another occasion I3'0i per cent, where 

 tilled, and 9*53 per cent, where untilled. In very hot, dry 

 climates the capacity of dry soil to take moisture from damp 

 air has some distinct value. Dry soil is distinctly hygro- 

 scopic. During the night, soils will radiate heat, but should 

 they condense moisture on their surface the latent heat of 

 the water vapour will check the drop in temperature. During 

 the day the deposited water will evaporate once more, but 

 this time the latent heat will check a rise in temperature. 



Chemistry of Soils. — When any soil is heated, at 

 first water is driven off, then complex gases are produced, 

 and a certain amount of black charcoal left behind. The 

 charcoal slowly burns off and leaves an ash, which is generally 

 dark red in colour. During the first of these stages the 

 amount of water that will be given off will depend upon the 

 atmospheric conditions prevailing when the sample of soil 

 was taken. When soils have been wetted by rain and allowed 



