CHAPTER VI. 



THE BIOLOGICAL CONDITIONS IN THE SOIL. 



IT is now necessary to study the soil conditions that determine 

 the growth not only of plants but also of the micro-organisms 

 that, as we have seen, make new plant food out of old plant 

 residues, and render possible the continuance of vegetable life 

 on the earth. These conditions are water supply, air supply, 

 temperature, food supply, and absence of injurious factors. 



The Water Supply of the Soil. 



The water content of the soil is the difference between the 

 amount supplied and the amount lost. The supply may 

 come from rain, from irrigation, and from the subsoil : the loss 

 is by evaporation or drainage from the soil or transpiration 

 through the plant. The broad general facts are that a soil 

 tends to maintain its water supply within fairly definite 

 seasonal limits, any excess draining rapidly away (unless 

 there is some mechanical hindrance), and any deficiency below 

 the lower limit setting in only slowly. This is well shown in 

 the curves of Fig. 17 (p. 131), where the winter level is never 

 greatly exceeded and the summer level is fairly well maintained. 

 The fluctuations, however, are of great importance. 



The moisture range depends on the structure of the soil. 



The balance retained in the soil is obviously determined by 



the resistance the soil can offer to the forces involved in 



drainage or evaporation, which depends on its composition, 



, its texture and its colloidal matter. To a considerable degree 



I evaporation can be controlled by cultivation : this forms the 



* basis of the so-called Dry Farming. 1 



1 See British Ass. Repts., 1914, p. 645. 



