1 1 



only produced in presence of soil bacteria, thus affording further 

 evidence of an interaction between the growing plant and the de- 

 composition processes. Experiments on the effect of weeds on 

 crops and of cross cropping were started last year in conjunction 

 with Dr. Brenchley to enable the facts to be determined more com- 

 pletely, and these are still going on. 



A further consequence of such an interference between the 

 plant and the soil bacteria is interesting in the study of plant nutri- 

 tion. It has been commonly supposed that the plant must in natural 

 conditions get most or all of its nitrogen as nitrate because the acti- 

 vities of the nitrifying organisms leave it no option, and the argu- 

 ment was justified so long as it could be supposed that nitrification 

 went on independently of the growing plant. But if it turns out 

 that the plant interferes with the production of nitrate and ammonia 

 in soil then the necessity for the supposition disappears and it may 

 equally be possible for the nitrogen to be taken in other forms. 



A beginning has also been made this year with a systematic 

 investigation of the soil as a medium for biological activity. This 

 has involved a study of the constitution of the soil, and already three 

 distinct lines of work are bringing out the biological importance of 

 the soil colloids. Mr. Prescott has been engaged in working out the 

 effect of dilute acids on the soil, and Mr. B. A. Keen has been 

 determining the rate of evaporation of water from the soil, while Mr. 

 Appleyard has been studying the gases absorbed by the soil and 

 given up only in a vacuum. The experiments are not sufficiently 

 advanced to justify discussion in this Report, but they promise to 

 throw light on the constitution of the soil. 



The composition of the soil atmosphere at a depth of 6 inches 

 below the surface has been determined periodically during the year 

 by Mr. Appleyard, and it has been shown to approximate very 

 closely to that of ordinary air, so that organisms growing in the sur- 

 face soil will find an atmosphere with practically normal oxygen 

 content. 



The second great division of the Rothamsted work — the investi- 

 gation of the plant — is still in its opening stages, although marked 

 advances have been made during the year. Dr. Brenchley has 

 closed her work on the effect of inorganic poisons on plant growth 

 and has prepared a monograph in which her own and other experi- 

 ments are set out and the results discussed. The results are 

 definitely against the hypothesis that all such poisons act as stimuli 

 when applied in small quantities. Increased yields that require 

 further examination were, however, obtained in some instances with 

 boric acid and with manganese salts. Dr. Brenchley is now turning 

 to the effect of certain organic substances on plant growth and will 

 also test systematically the substances isolated from the soil by the 

 soil chemists. 



An interesting investigation has been begun by Miss Adam into 

 the anatomical structure of certain of the grasses on the Park grass 

 plots. It has been observed that, where potash manures are with- 

 held, the grasses do not stand up well but tend to become "laid." 

 Microscopic examination is being made to see whether this is 

 accompanied by any modification in the anatomical structure. 



The chemical side of the work has progressed steadily. The 

 usual methods of analysing crops are based on old investigations 



