196 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



heat causes a similar change. Greater growth is commonly obtained 

 wherever a bonfire has been made, and in India it has been the 

 practice from time immemorial to heat the surface of the land before 

 growing the rice crop. Volatile antiseptics are now known to have a 

 like result. 



The converse of the rule is also true : whenever a soil is well 

 supplied with organic matter, with moisture, and kept well warmed, 

 the bacterial numbers do not remain as high as might be expected, but 

 on the contrary they tend to come down. After a time these soils 

 fail to produce their full effect and they are said to become " sick." 

 Instances occur in commercial glasshouses run at a high temperature 

 where the soil after a season's use becomes unsuitable and is therefore 

 thrown out, all its valuable manurial residues being sacrificed (fig. 65). 



Sick soils have been examined in some detail, and the trouble 

 was traced to at least two causes : an accumulation of disease organ- 

 isms, and also an exaggerated activity of the factor limiting bacterial 

 activity in ordinary soils. 



These phenomena afford further evidence of competition among 

 the soil organisms, and indicate that some of the groups, and especially 

 those which are fairly readily killed, are detrimental to the useful 

 soil bacteria. 



Some of these changes affect the soil itself. The jelly-like sub- 

 stances — the colloids — shrink on drying, and may conceivably expose 

 fresh particles of organic matter to the action of the organisms or 

 liberate some of the phosphates they had absorbed. So also intense 

 frost may split up some of the undecomposed organic matter and 

 facilitate the work of the micro-organisms. 



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 generalize about seasonal effects, but as a general 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 temperature 

 limits the activities 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 wetness of the 

 winter, and when the sunny days first cause the temperature to rise. 

 But it is the rain coming after warmth that causes the rush of life. 

 Three factors seem to be involved. During winter the cold and the 

 general unfavourable conditions have had their partial sterilizing 

 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 become warm it has to dry, and by the time it is warm enough for 



