22 AGRICULTURAL RESEARCH AND THE FARMER : 



keeps in view, namely, how to cultivate the soil in such a manner 

 that it provides the most efficient medium for the growth of crops. 

 The physical character of the soil is influenced by many conditions, 

 such as the supply of air and moisture, temperature, the quantity 

 and state of organic matter present, and so on. Until we know 

 exactly what these factors are, how they are inter-related and how 

 they are affected by influences from outside, no definite advance 

 on existing methods of cultivation is probable, and any improve- 

 ments which may be secured from time to time must be based 

 on the purely empirical and slow method of trial and error. 

 Moreover, the present methods of cultivation have been adopted 

 after centuries of experience to suit conditions under which 

 animals supply the motive power of the farm. The change to 

 mechanical power is a fundamental one, and may not only result 

 in a modification of our tillage methods, but may need an entire 

 re-casting of the designs of tillage implements. Doubtless some 

 change of this character would be introduced after prolonged 

 experience, even in the absence of scientific knowledge of the soil. 

 But agriculture cannot afford to be dependent on the slow and 

 not always sure progress of empiricism. The medium in which 

 tillage implements spend the whole of their working lives is the 

 soil, and an intimate acquaintance with this medium should surely 

 be the first essential for the proper study of methods of cultivation 

 and of the design of tillage implements. 



Until comparatively recently the soil was regarded as a 

 mass of mineral particles, on the surface of which were spread 

 thin films of moisture which together formed the water-content 

 of the soil. Later research showed that this view was insufficient. 

 This had been demonstrated, for example, by a somewhat com- 

 plicated and ingenious experiment conducted at Rothamsted, 

 in which an apparatus was designed in the Physical Department 

 to measure the rate of evaporation of moisture from the soil. 

 If the old view regarding soil texture held, evaporation should 

 proceed at a gradual pace until all the water was driven off. 

 Actually the rate was by no means gradual ; the moisture evapo- 

 rated first very quickly and finally very slowly. Evidently, 

 therefore, there is something in the soil which exerts a tenacious 

 hold on moisture ; this is in accordance with what is termed 

 the " colloidal theory " of the soil, strongly supported by Rotham- 

 sted, and the results of recent research go to show that this idea 

 regarding the structure of the soil is in its essentials correct. 

 Briefly the view is that substances of jelly-like properties are 

 important texture -forming constituents of soil. These substances, 

 termed " colloids," consist mainly of particles of fine clay 

 and of organic matter, which clothe the mineral particles and 



