26 AGRICULTURAL RESEARCH AND THE FARMER : 



through a light sandy soil much more easily than through a 

 heavy clay soil, mainly because in the former case the soil particles 

 are comparatively large and adhere loosely together, while in 

 clay the particles are much smaller ana adhere more closelw The 

 difference is a difference in cohesion — in so far as the adherence 

 of the particles to one another is concerned — and in plasticity, 

 in so far as one kind of soil is " stickier " than another When 

 land is ploughed, the cohesion of the soil determines the ease with 

 which the furrow slice is cut; its plasticity determines the 

 resistance offered to the furrow slice as it slides over the mould- 

 board When it comes to defining in accurate scientific terms 

 the relative cohesion or plasticit}^ of soils, however, the question 

 is much more complicated, for it is found that these qucJities 

 cannot be accurately forecasted from a knowledge of the type of 

 soil, and thej^ bear a very complex relation to the treatment to 

 which the soil has been subjected. One of the important difficul- 

 ties which the Physical Department is trjing to solve at the present 

 time is, in fact, that of arriving at some standard method of 

 determining such properties of the soil as its cohesion and its 

 plasticity. Bound up with this is the necessity for studying 

 the exact causes which produce these properties, primarily the 

 " colloids " which have already been referred to. The field of 

 work is largely unexplored, and it is necessary, all along the way, 

 to invent apparatus which will give the necessary information. 

 The knowledge must, however, be gained, for until it is forth- 

 coming no progress can be made towards the ultimate end of the 

 physicist, which is to supply the agricultural engineer with such 

 formulae as will enable him to design a tillage implement in the 

 confident anticipation that it will do the work required of it 

 under any specified conditions of soil and climate. 



Soil Acidity. 

 We may conclude this chapter by a brief reference to the 

 progress of investigations into the acidity, or sourness, of soils, 

 which is, of course, closely connected with their need for lime. 

 Few questions are of more immediate importance to the farmer, 

 and at the same time few are so little understood. Sometimes a 

 soil is so obviously sour that it will not grow crops, and in such 

 cases the cause of the trouble is easily found, and heavy liming 

 may be safely ad\ised as the first essential to cultivation. These 

 are fortunately the exceptions, but there remain vast areas which 

 appear to be in need of lime, but by no known tests can an 

 agricultural ad\iser say even approximately what sort of dressing 

 ought to be applied. When lime was plentiful and cartage not a 

 serious consideration, a difference of a hundredweight or so per 



