THE SOIL AND ITS EFFECT ON PLANT GROWTH. 25 



capped until he is supplied with more precise information 

 regarding the physical characters of various types of soil. Un- 

 fortunately the soil is of so complicated a character that some time 

 must elapse before it can be properly understood. The directions 

 in which the problem is being attacked at Rothamsted may, 

 however, be briefly indicated. 



In the first place an extended series of trials is in progress, 

 the object of which is to show exactly how much work — as 

 measured by the dynamometer — is done by a tractor in pulling 

 various implements through the soil. The tractor will work at 

 var\ing speeds and on different types of soil which have been 

 subjected to different systems of manuring. The important 

 physical properties of the soil, such as moisture content, humus 

 content, etc., will be ascertained. Thus the effect of different 

 systems of manuring, including green manuring and the ploughing 

 in of clover residues, will be noted, and the measurements will 

 be extended to include cultivation, cross-ploughing and such 

 other operations as seem suitable. The results of these trials 

 will show how the various modifications of soil type affect the 

 draw- bar pull of the tractor, and as this draw-bar pull is reflected 

 in consumption of fuel and in wear and tear, a small alteration 

 in resistance will affect the cost of every operation, so that the 

 experiments will have a direct practical bearing. Certain of the 

 recent trials, for example, were carried out in a field part of which 

 had been chalked nine years previously, and although the effect 

 of chalking on crop yield had practically ceased, the chalked 

 portion was clearly shown up by the dynamometer records, and 

 the draw- bar pull — and consequenth' the fuel consumption of the 

 tractor — was reduced by over one-tenth. 



Although this practical issue is important, however, it is not 

 the ultimate reason for the experiments. In the cultivation of 

 soil, the main cost to the farmer is for power required to overcome 

 the resistance to the passage of the implement which is being drawn 

 through it. Apart from purely engineering considerations, this 

 resistance may be broadly divided into two parts, first, the 

 resistance offered by the soil itself — which of course varies 

 according to composition, moisture content, humus content, etc. 

 - — and second, the resistance which is inherent in the design of the 

 implement. Clearly, one of the first essentials for further work 

 is to divide the total resistance into these two component parts. 

 A good deal of information will be secured from the experiments 

 described in the preceding paragraph, but it must be supplemented 

 by an intensive laboratory study of certain physical properties 

 of the soil, such as its cohesion and plasticity. The reason for this 

 will appear from a simple illustration. A plough can be drawn 



