334 ECOLOGY 



In the case of trees with a wide-spreading root-system 

 and very large leaf-surface, the amount of water drawn 

 from the soil and given up to the atmosphere as vapour is 

 enormous, and has an important effect in drying the 

 soil and in increasing the humidity of the air. It is esti- 

 mated that, during one season, a Beech wood gives off 

 354 tons of water per acre, and that suitable trees planted 

 in marshy ground play an important part in draining it 

 and bringing it ultimately into a state suitable for higher 

 cultivation. 



If the mineral salts needed by plants are in weak solutions, 

 how is it that after heavy rains the soluble compounds and 

 even added manures are not washed out of the soil ? To 

 some extent this does occur, as may be determined by 

 noting the difference of residue after evaporating (a) rain- 

 water, and (b) spring-water. We have seen (p. 326) how very 

 soluble are the nitrates in the soil and how easily they are 

 washed out. But chemical changes are constantly going 

 on in the soil, which counteract this tendency to depletion 

 of food-materials. By means of these changes soluble com- 

 pounds like salts of potassium, magnesium, calcium, or 

 ammonia displace the alkali in the silicates of the soil and 

 insoluble compounds are formed, and for a time fixed in the 

 soil. The next changes result in these insoluble compounds 

 being slowly acted upon and re-converted into soluble sub- 

 stances which provide a steady supply of mineral food for 

 plants. 



In this manner the soil is constantly storing up plant-food 

 in an insoluble form, preventing its escape, and then giving 

 it up slowly to the plants in a soluble form. It is impor- 

 tant, however, that the solutions should be very weak, for 

 if they exceed a concentration of 3 % the plants are unable 

 to absorb them. 



Effect of hoeing. Hoeing produces a loose, dry, well- 

 aerated layer of soil which conducts heat badly and pro- 



