agent, chalk is not. Lime begins by depressing bacterial numbers 

 and protozoa : later on, however, it is converted into chalk and the 

 bacterial numbers begin to rise with production of ammonia in the 

 usual way. This distinction in biological effect, in addition to the 

 distinction in chemical effects, afforded a satisfactory explanation of 

 some of the discrepancies observed in practice. A method was de- 

 vised for rapidly determining the quantity of lime required per acre ; 

 pot experiments showed that the results are reliable. A further 

 method was worked out for determining the amount of calcium car- 

 bonate (or of lime) needed to correct "sourness" and ensure an ample 

 supply of base for the needs of bacteria and plants. 



The zoological study of the soil protozoa has continued, and 

 Mr. Lewin has further developed the methods which Mr. Martin 

 and he have been using for demonstrating the existence of an active 

 protozoan fauna in the soil. In conjunction with Mr. Martin he has 

 published an account of two of the more interesting types of amoebae 

 that are found in the trophic state. Mr. Goodey has also published 

 an account of three new amoeboid organisms obtained from the soil 

 and apparently widely distributed, that feed on bacteria and other 

 small forms. They are remarkable for their large size, one of them 

 varying up to no less than 1 m.m. in breadth. Our interest in these 

 organisms is to study their activities in the soil : the work is very 

 difficult, but satisfactory progress is being made. 



It has already been pointed out that ordinary soil analysis often 

 yields incomplete information through ignoring the other factors 

 essential to soil fertility. There is, however, another source of 

 trouble well recognised by experts': the selection of a method of 

 extraction. 



The first methods, founded wholly on mineral analysis, proved 

 of little value in this country. A marked improvement was effected 

 when dilute acids were substituted for strong acids, but many ano- 

 malous cases still arose. The underlying assumption always was 

 that the soil was a mass of mineral fragments with the phosphates 

 in the ordinary mineral form. We have seen that this view led to 

 a wholly inaccurate conception of the water relationships of soil. 

 In like manner all attempts to interpret the action of dilute acids on 

 soil phosphates as an ordinary chemical reaction have failed. This 

 year Mr. Prescott has studied it in detail and finds that it includes 

 two separate changes. 



The first action is that of the acid on the phosphate, which, so 

 far as can be gathered, is normal, resembling the ordinary action of 

 acid on any other phosphate. But almost instantaneously there 

 appears another action which gives its distinctive character to the 

 behaviour of the soil : an absorption of the dissolved phosphoric 

 acid. This absorption closely resembles the adsorption shown by 

 colloids, and is entirely expressed by the ordinary adsorption formula 

 generally attributed to Freundlich : the agreement is so satisfactory 

 as to afford strong presumptive evidence that the main features of 

 the reaction are accounted for. 



The reaction between dilute acids and soils thus appears to be 

 essentially a displacement of absorbed material by something which 

 is itself absorbed ; and it falls into line with other displacements 

 from colloids. 



