86 INFLUENCE OF PHOSPHATES 



It may be that an inadequate supply of available phosphoric acid 

 on the untreated plot prevented the crop from utilising the accumu- 

 lated nitrate when suitable conditions occur, and that the subsequent 

 depressions in the nitrate content are due to rain washing the nitrate 

 down to below the 9 inch level. 



THE INFLUENCE OF PHOSPHATES ON 

 SOIL BACTERIA 



At the suggestion of Dr Russell an attempt was made to ascertain 

 what effect, if any, the application of phosphates has had on the 

 soil bacteria. Preliminary counts were made at Rothamsted during 

 the autumn of 1919, but the results were contradictory. 



Phosphates are essential for the development of all types of 

 bacteria. Fred and Hart (9) in an investigation on the comparative 

 effect of phosphates and sulphates on soil bacteria show that phos- 

 phates increase the number of soil bacteria, and they suggest that 

 increased crop production of a soil resulting from the application of 

 soluble phosphates is in part due to the promotion of bacterial 

 activity. The work of Hoffman and Hammer UO) demonstrates that 

 phosphates greatly increase the amount of nitrogen fixed by Azoto- 

 bacter and they came to the conclusion that for this purpose di- and 

 tri-calcium phosphates are more effective than mono-calcium phos- 

 phates. 



The soils at Martin's Hearne and Horndon are very deficient in 

 phosphoric acid and if any positive results were to be obtained it 

 seemed probable that it would be at these two centres. Samples for 

 bacteriological examination were taken every month from March to 

 August. The samples were secured by means of a small soil sampler 

 which removed a 9 inch core of about \ inch diameter. The sample 

 from each plot consisted of four cores. Before use, the soil sampler 

 was sterilised by means of a methylated spirit lamp, and the samples 

 when drawn were placed in previously sterilised bottles. 



The total counts for the treated and untreated plots at Martin's 

 Hearne and Horndon are given in Table XLV. 



There is very little difference between the bacterial counts repre- 

 senting the two plots at Martin's Hearne, and apparently phosphate 

 has had little effect in this direction. 



The two plots at Horndon show decided differences. During the 

 months of May, June and July there are twice as many bacteria in 



