15 



the diffusion of the nutrients to the points in the liquid film which 

 had been exhausted by contact with the roots. Growth in such 

 nutrient solutions forming a film over sand particles was much 

 superior to the growth in a water culture of equal concentration, 

 but the growth in the water culture was similarly increased if a 

 continuous current of air was kept passing through it. 

 From these data it is concluded : — 



(1) The composition of the natural soil solution as regards 

 phosphoric acid and potash is not constant, but varies significantly 

 in accord with the composition of the soil and its past history. 



(2) Within wide limits the rate of growth of a plant varies with 

 the concentration of the nutritive solution, irrespective of the total 

 amount of plant food available. 



(3) When other conditions, such as the supply of nitrogen, 

 water, and air are equal, the growth of the crop will be determined 

 by the concentration of the soil solution in phosphoric acid and 

 potash, which, in its turn, is determined by the amount of these 

 substances in the soil, their state of combination, and the fertiliser 

 supplied. 



(4) On normal cultivated soils the growth of crops like wheat 

 and barley, even when repeated for 60 years in succession, does not 

 leave behind in the soil specific toxic substances which have an 

 injurious effect upon the growth of the same or other plants in the 

 soil. 



The net result of these investigations is to restore the earlier 

 theory of the direct nutrition of the plant by fertilisers. The com- 

 position of the soil solution which determines the growth of the plant 

 is dependent upon the amount and the mode of combination of the 

 phosphoric acid and potash in the soil, both of which are affected by 

 the fertiliser supply, though to what extent is not yet determinable. 



V. "The Effect of Partial Sterilisation of Soil on the Pro- 

 duction of Plant Food. Part II. The Limitation of 

 Bacteria Numbers in Normal Soils and Its Consequence" 

 E. J. Russell and H. B. Hutchinson. Journal of 

 Agricultural Science, 1913. 5, 152—221. 



The conclusions reached in the previous paper have been con- 

 firmed and extended. Fresh evidence is adduced that bacteria are 

 not the only inhabitants of the soil, but that another group of 

 organisms occurs, detrimental to bacteria, multiplying more slowly 

 under soil conditions and possessing lower power of resistance to 

 heat and to antiseptics. 



In consequence of the presence of these detrimental organisms, 

 the number of bacteria present in the soil at any time is not a 

 simple function of the temperature, moisture content and other con- 

 ditions of the soil. It may, indeed, show no sort of connection with 

 them ; thus rise of temperature is found to be ineffective in increas- 

 ing the bacteria in the soil ; increase in moisture content has also 

 proved to be without action. The number of bacteria depends on 

 the difference in activity of the bacteria and the detrimental 

 organisms. 



But when soil has been partially sterilised the detrimental 

 organisms are killed and the bacteria alone are left. It is then 

 found that increase in temperature (up to a certain point) favours 



