INTRODUCTION 21 



retains 41 per cent, of its volume or three times as 

 much as sand. Similarly, the permeability of clay is 

 less than that of sand, the water taking far longer to 

 drain off or to be absorbed. The power of soils to 

 absorb water which collects at night as water vapour 

 above the soil is variable for each soil, and this 

 variation depends on the foregoing physical charac- 

 teristics. 



In relation to temperature soils v^ry in their power 

 of retention of heat. This depends on colour, dark 

 soils retaining heat more than light, and water con- 

 tent, dry soils being warmer than wet soils. The 

 latter, e. g. clay soils, are usually cold, and therefore 

 unproductive. 



The need of aeration in the soil is an important 

 factor, for the roots require, like all parts of the 

 plants, to respire, and oxygen in the ground air 

 must enter by the root-hairs, or the lenticels, or air- 

 spaces. 



The chemical properties of the soil are important, 

 as has been seen. There are two cases especially in 

 which soil is markedly important. Maritime plants 

 live in a soil in which saline salts are present. This 

 would be inimical to all other types of plants. In 

 the case of calcareous soil with calcium carbonate 

 certain plants are fond of calcium, and are calcicole, 

 as chalk plants, whilst others are calcifuge or calci- 

 phobe, and cannot live on such soils. They, how- 

 ever, occur on sandy soils. 



