PRESIDENTIAL ADDRESS. 71 



The Influence of Season. 

 The fertility of a natural soil is largely influenced by the 

 moisture content and by the temperature. The bacterial 

 activities, as indicated by the fluctuations in the bacterial 

 numbers, in the carbon dioxide content and in the amount 

 of nitrates, rise and fall generally with the seasons. 1 There 

 may be slight influences brought about by the effect of 

 manuring and cropping, but the general rise and fall of the 

 biochemical activity,as indicated by these co-related factors, 

 varies with the season. It is at a maximum in late spring 

 or early summer, at a minimum in summer, a maximum in 

 late autumn and a minimum in winter. Although we can- 

 not alter these general changes very much, we can, by soil 

 treatment, make certain differences which render the soil 

 more fertile. We endeavour to do so by manuring. Prac- 

 tice has proved that certain fertilisers have a decided effect 

 upon the growth of plants, and the wonder is, that although 

 there may be an abundance of fertilising material already 

 in the soil, the addition of a few hundredweights per acre 

 of a fertiliser has a profound effect upon the crop. The 

 reason has been ascribed to the greater availability of the 

 added fertiliser, but Whitney and others, have suggested 

 that the fertiliser acts more by reason of its power to 

 neutralise soil toxins than by its nutritive capacity. 



This has been rebutted by certain Rothamsted investi- 

 gators, 2 who upon growing plants in the extracts of various 

 soils, found that the growth was proportional to the 

 manurial history, the previous cropping, and the composition 

 of the soil solution. 



Raw Organic Matter in Soil. 

 Experience has shown that it is not good agricultural 

 practice to add raw, that is, unfermented organic matter 



1 Russell and Appleyard, Journ. Agri. Sci., 7, 1. 



2 Hall, Brenchley, and Underwood, Ibid., 6, 278. 



