28 



SOIL TILTH AND CULTIVATION. 



In the Physical Department the studies of tilth under Dr. 

 Keen are being- continued. The work includes exact laboratory 

 studies of the physical factors involved in tilth and also measure- 

 ments of the drawbar pull when land is ploughed under varying 

 conditions. An investigation of this kind is prolonged but already 

 interesting results are emerging. The purpose of the laboratory 

 work is to develop the science of soil physics on which ultimately a 

 scientific soil cultivation can be based, just as scientific manuring 

 is based on chemistry and plant physiology. Mr. Haines has 

 completed some important pioneering work on the physical 

 properties directly concerned in ploughing : cohesion and plas- 

 ticity of soil, and surface friction between soil and metal. But 

 in order to get very far with the investigation it is necessary to 

 study the underlying causes, and so researches are carried out 

 which, while less obvious in their bearing, are no less, but 

 possibly even more, essential than those just mentioned. Good 

 tilth in soil is traditionally associated with the formation of com- 

 pound particles or soil aggregates. These in turn are determined 

 by the colloidal properties of the soil : and so it comes about 

 — as often in agriculture — that progress in a practical problem 

 cannot be made until some abstruse and apparently wholly irre- 

 levant scientific problem is solved. The friction between the 

 plough and the soil is a practical problem of the first importance : 

 but it cannot be adequately studied without a proper under- 

 standing of the colloidal properties and the ultimate constitution 

 of the soil. The three methods of investigating these in the 

 Physics laboratory are : — 



(a) A study of the relative intensity of the forces holding 



soil particles together when the soil has been sub- 

 jected to a variety of treatments that simulate field 

 conditions. The method adopted is the measurement 

 of the amount of soil in suspension after shaking with 

 water under definite conditions; 



(b) Direct measurements of the vapour pressure at different 



moisture contents of soils treated in various ways ; 



(c) Indirect measurements of the vapour pressure using 



a method that depends on the lowering of the freezing 

 point depression of benzene in contact with the moist 

 material (hat has an affinity for water. 



The results show that many of the observed properties of soils 

 can be interpreted on the assumption that the colloidal material 

 is permeated with minute capillaries, analogous to l hose investi- 

 gated by Zsigmondv and Anderson in silica g-el. They also 

 indicate thai compound particles are formed in soil at com- 

 par.itivclv high moisture contents, and that once formed they 

 are not easily disintegrated. This last conclusion has led to a 

 somewhat disconcerting discovery. It is found that complete 

 dispersion of soil is frequently not attained in the standard 

 method of mechanical analysis : hence many of the recorded 



