284 BIOCHEMICAL PROCESSES OF THE SOIL 



These observations throw important light on the effects of 

 season and climate on the production of nitrate in the soil. 

 It is notoriously difficult to generalise about seasonal effects, 

 but as a rule the activity of micro-organisms is greatest in late 

 spring and in autumn, and lowest in summer and winter. 



The winter minimum is easily intelligible : the low tempera- 

 ture limits the activity of the organisms, and as we have already 

 seen, any rise in temperature immediately evokes a response, 

 so that the curves for the production of carbon dioxide run 

 closely parallel to the temperature curve. 



The spring maximum is remarkably interesting. It begins 

 to show itself when the soil is drying after the cold and wet- 

 ness of the winter, and when the sunny days first cause the 

 temperature to rise. Three factors seem to be involved. 

 During winter the cold and the general unfavourable conditions 

 have had their partial sterilising effect on the soil population, 

 and also have resulted in a certain amount of disintegration of 

 the soil organic matter. Everything is therefore ready for a 

 great outburst of activity. 



But in our climate this does not come suddenly. Before 

 the soil can be warm it has to be dry, and. by the time it is 

 warm enough for much bacterial action the chances are that 

 it has dried too much. It is therefore necessary to wait for 

 rain to supply the needful water and to renew the dissolved 

 oxygen in the water round the soil particles. It appears to be 

 this combination of temperature, moisture and oxygen supply, 

 following on the beneficent changes effected in the winter 

 months in the soil organic matter and the soil population, that 

 causes the great outburst of soil life in spring. 



The summer minimum may be attributed to dryness, and 

 the autumn maximum to the effect of rain coming after the 

 heat and drought of the summer. 



In consequence of these various activities the soil is left 

 pretty rich in nitrates at the end of the autumn, provided the 

 summer has been reasonably dry. If these remain they form 

 a good supply for the young plants of the following season. 



