8 



all the manures are equally effective. The yield was only 18^ tons 

 per acre, and was clearly being kept down by some factor outside 

 the nitrogen supply. It has already been stated that the season was 

 not a good one for mangolds at Rothamsted. 



In the Laboratory and Pot Culture House a considerable amount 

 of work is being carried out on the production of plant food in the 

 soil. As the supply. of nitrogenous food frequently constitutes the 

 limiting factor in crop production our work is largely restricted to 

 the nitrogen cycle in the soil ; the other factors are, however, always 

 taken into account and come in for a number of subsidiary investi- 

 gations. It has been shown that the production of ammonia and 

 nitrate (two very important nitrogenous foods) is largely the work of 

 bacteria. In normal soils, however, the bacteria are not working at 

 their full efficiency. A factor has been discovered limiting the 

 numbers of bacteria and therefore of the amount of decomposition 

 they effect. All the available evidence goes to show that this factor 

 is biological : it is capable of growth, is put out of action by heat or 

 antiseptics, and can only be set up again by infection from outside : 

 it does not, however, appear to consist of bacteria and is provisionally 

 identified with the protozoa, of which numbers have been found in 

 all the soils examined. Partially sterilised soils from which the 

 factor has been eliminated are found to contain larger numbers of 

 bacteria than untreated soils, and to accumulate ammonia and nitrates 

 at a greater rate : they are, as might be expected, more productive. 

 Methods are being worked out for applying this kind of soil treat- 

 ment on the large scale, but instead of setting up a large number of 

 field plots to discover some cheap and convenient process the simpler 

 alternative is adopted of inducing horticulturists who go in for in- 

 tensive culture to adopt some of the methods known to work. 



The tomato and cucumber growers have responded well to this 

 invitation, and during the past three seasons a number of experiments 

 have been carried out in our Laboratory and Pot Culture House to 

 solve some of the problems arising out of the application of partial 

 sterilisation to their particular work. The arrangement has been 

 advantageous in many ways. Certain very interesting lines of 

 investigation have been opened up that promise to throw much light 

 on our general fertility problems and that we should probably not 

 have found otherwise. The growers also have expressed their 

 interest in the very practical way of organising an Experiment 

 Station to be started in the Lea Valley district and devoted ex- 

 clusively to the investigation of problems connected with the glass 

 house industry. Finally, the method of partial sterilisation has now- 

 passed out of the laboratory into the hands of the practical man, and 

 each season becomes cheaper and applicable to a wider range of 

 growers ; we are also learning what are the difficulties attendant on 

 the use of the method in practice. 



Our new conception is that the soil organisms may be divided 

 roughly into two groups in their relation to the processes of food 

 production : a useful group and a detrimental group. The latter are, 

 speaking generally, more readily killed than the former. Conditions 

 that are harmful to active life in the soil tend therefore to reduce 

 their numbers and lead to an increased activity of the useful bacteria. 

 On the other hand, conditions favourable to active life tend to keep 

 up the detrimental organisms and therefore to reduce the useful 



