2^ 



VII. '^ The Non-Persistence of Bacterio-Toxins in the soil.''' Henry 

 Brougham Hutchinson and Aage Christian Thaysen. 

 Journal Agricultural Science, 1918, 9^ 43-62. 



It has been claimed by other workers that the phenomena of partial 

 steriUsation of soil are due, in part, to the destruction of bacteric- 

 toxins when a soil is subjected to heat. The experiments on which this 

 conclusion is based were, therefore, repeated and extended : in particular, 

 a comparison was made of the growth of a test organism — B. prodigiosus 

 — when inoculated into an extract of untreated soil and into a similar 

 portion of extract which has been heated with the object of destroying 

 any toxins present. The results obtained with six normal English 

 soils show that the initial depression occurring when a culture is carried 

 into an extract of untreated soil is not due to the action of toxic sub- 

 stances in the extract, as has been assumed by other experimenters. 

 but is more probably a starvation effect. When these extracts were 

 subjected to heat, their suitability for growth was still further reduced, 

 but the addition of minute quantities of peptone was sufficient to 

 convert them into media suitable for active growth. 



The only extract which showed improvement on boiling was that 

 of a very acid soil, but in this case the observed toxicity appears to be 

 connected with the presence of acid iron and aluminium compounds, 

 which are liberated by the action of neutral salt solutions, but thrown 

 out of action when the extracts are heated. 



VIII. " The Reaction between Dilute Acids and the Phosphorus 



Compounds of the Soil^ E. J. Russell and James 

 Arthur Prescott. Journal of Agricultural Science, 

 1916. 8, 65-110. 



This reaction is of great importance in soil analysis, as it forms the 

 basis of the methods for determining the amount of phosphate " avail- 

 able " in the soil for the plant. In studying this reaction in the 

 laboratory it was found to throw important light on the constitution 

 of the soil. 



When soil is shaken with a dilute acid a certain amount of phos- 

 phoric oxide (PjO,) is dissoh^ed, the quantity depending on the 

 particular acid and the conditions of the reaction. Under similar 

 conditions the amount varies widely with different acids, being greatest 

 with citric and oxalic acids, which are usually regarded as weak acids, 

 and least with hydrochloric and nitric acids, the strong acids. The 

 investigation cleared up this anomaly. 



It was shown that the action is really complex ; two changes are 

 proceeding simultaneously, a direct and a reverse action, and the 

 observed result is the difference between the two. 



When soil is shaken with a dilute acid some of the phosphorus 

 compounds go into solution, and the amount dissolved by different 

 acids at equivalent concentrations is much the same ; nitric, hydro- 

 chloric and citric acids give the same results ; sulphuric acid gives a 

 somewhat higher result. 



A reverse reaction at once sets in, however. Some of the phosphoric 

 oxide is withdrawn from the solution in spite of the presence of excess 

 of acid. The process is an ordinary adsorption and obeys the usual 

 law expressed by the equation y=Kc*. Its extent varies with the 

 dift'erent acids ; it is much more marked in the presence of nitric than 

 of citric acid. 



