direct attack. The growers concerned were so satisfied with the 

 results that with the aid of the Board of Agriculture they have set 

 up an Experiment Station in their midst where other of their 

 problems can be investigated. 



The laboratory experiments do not conclude an investigation : 

 pot experiments are made to test any conclusions bearing on the 

 growth of plants and, if these indicate a sufficiently important bearing 

 on farm practice, they are followed by small plots, and then finally 

 by larger plots to show to farmers, or demonstration pots on a 

 practical scale for horticulturists. In this manner a green manure 

 rotation experiment has this year passed from the pot culture house 

 to the demonstration plot stage. Further, in order to broaden the 

 outlook, investigations are periodically carried on outside Roth- 

 amsted ; these, however, necessarily deal with special cases where 

 ordinary methods fail, and as a preliminary they usually require the 

 working out of new methods of attack. 



In dealing with problems of soil fertility agricultural chemists for 

 some years confined themselves largely to one aspect only : the 

 supply of plant nutrients. The experiments made by County 

 Councils and other experts were almost entirely restricted to this 

 phase of the problem, and when a soil was sent in for analysis the 

 advice given commonly had reference only to the suitability of 

 certain artificial manures. 



It soon appeared, however, that other factors had also to be 

 taken into account, and that the ordinary soil analysis or manurial 

 demonstration in itself was insufficient to give the help the farmer 

 was justified in asking. The water and air supply, the tilth, and the 

 soil type were equally important, and they are now claiming con- 

 siderable attention from the investigator. 



The water supply is a factor of the first importance in deter- 

 mining not only the growth of plants but also of micro-organisms, 

 the activity of which has been shown in previous reports to play so 

 large a part in soil fertility. It can be modified by suitable cultiva- 

 tions and manures, but, notwithstanding its vital importance, it has 

 not proved easy to study. A considerable mass of data has been 

 accumulated at Rothamsted showing the percentage of water in 

 soils under various conditions, but it was impossible to discuss the 

 results adequately because we did not know T in what state the water 

 existed in the soil. Physical investigations made elsewhere indi- 

 cated that the water was present as free films merely suspended 

 on the particles just as sea water is suspended on grains of sand 

 or as oil on leaden bullets. This view had the advantage of 

 simplicity, and it allowed deductions to be made direct from the 

 known laws of evaporation, diffusion, etc. ; but several field obser- 

 vations indicated that it was not correct. It was useless to go on 

 multiplying field observations when we could not deal with those 

 we had already got, and Mr. Keen accordingly began on this problem 

 directly he took up his post as Goldsmiths' Company's Soil Physicist. 

 The method adopted was to study the rate of evaporation of water 

 from the soil. The relationship of water to soil was found to differ 

 considerably from its relationship to sand. The soil colloids diminish 

 the tendency to evaporation : the effect is so definite that it can be 

 represented by a mathematical formula from which a curve can be 

 drawn that agrees entirely with the experimental curves. One of 



