76 



R. GREIG-SMITH. 



seasonal conditions, which by alternately wetting and dry- 

 ing the soil, disturbs the relative numbers in the various 

 groups. Not only this, but the temperature has consider- 

 able effect upon the activity of certain forms. The bac- 

 terial products are thus continually altering, as the one 

 bacterial group or the other preponderates. This alteration 

 of bacterial products will naturally influence the plant. 

 Thus the fertility may be as much a factor of the plant, as 

 of the soil or the bacterial or the protozoal life. 



When considering fertility, however, we are chiefly con- 

 cerned with an increased growth of crop, and for this to 

 occur we must naturally enquire into how best to annul 

 the effects of anything which will hinder growth, whether 

 it be a lack of nutriment, or an adverse condition, or an 

 oxcess of some poison or toxin. The working of the soil 

 and its consequent exposure to the oxidising action of the 

 air and antitoxic action of the sun, is without doubt the 

 natural method of destroying toxins, and favouring the 

 activity of the beneficial micro-organisms, and possibly also 

 of altering the condition of the soil colloids. In a dry 

 climate such as we have, the high temperature and com- 

 parative dryness of the soil is probably adverse to the 

 accumulation of toxins, while in a wet climate, the rain 

 washes them out of a well-drained soil. Still they are 

 continually being formed, and when they are allowed to 

 oxist, only as traces, they act not as toxins, but as stimu- 

 lants, and while they do so they increase the fertility of 

 the soil. 



A curious feature of these soil toxins is that they can 

 only be detected directly when a certain quantity of water 

 relative to the soil is used in extracting them. This 

 quantity is generally one of water to one of soil. 1 By using 

 a smaller quantity of water the extract is found to be 



1 Proc. Linn. Soe. New South Wales, 1913, 725; 1915, 633. 



