60 SOIL CONDITIONS AND PLANT GROWTH 



tempted as yet, and the work hitherto done can only be regarded as 

 preliminary. The fundamental difficulty in applying the ordinary 

 physical methods is to synthesise the soil ; numerous studies have been 

 made of the physical properties of sand, silt, clay, etc., considered as 

 separate entities, but no one has worked out the resultant when all the 

 varying grades of sand, silt and clay are intimately mingled, or drawn up 

 a scheme or formula to express the properties of the soil in terms of the 

 mechanical analysis. More useful results are obtained by the method 

 of correlation ; soils of known properties are analysed and the results 

 are correlated so far as is possible with the properties ; even this method, 

 however, can only be used very crudely, because the physical properties 

 of the soil as a whole cannot at present be expressed by definite num- 

 bers. Only a very general summary will therefore be attempted. 



The Clay Fraction. Clay may be regarded as a plastic colloid, but 

 its special properties are only seen when a certain amount of water is 

 present. 1 If it is well rubbed with water it becomes very sticky and 

 absolutely impervious to air or water ; it is also highly plastic, and can 

 be moulded into shapes which remain permanent on drying and baking. 

 It shrinks very much on drying and absorbs heat ; on moistening again, 

 however, there is a considerable swelling and evolution of heat. The 

 reversibility of the process has been studied by van Bemmelen (20, 25), 

 who has also shown that the rate at which water is lost on drying over 

 sulphuric acid is not essentially different from the rate at which evap- 

 oration takes place from a pure water surface under the same condi- 

 tions. The separate particles of clay are so small that, when placed 

 in water, they assume a state of Brownian movement and sink only 

 very slowly in spite of their high specific gravity. Traces of electrolytes 

 have a profound effect on these properties ; small quantities of acids or 

 salts cause the temporary loss of plasticity, impermeability, and the pro- 

 perty of remaining long suspended in water without settling ; the clay is 

 now said to be flocculated. The change can be watched if a small quan- 

 tity of any flocculating substance is added to the turbid liquid obtained 

 by shaking clay with water ; the minute particles are then seen to unite 

 to larger aggregates which settle, leaving the liquid clear. There is, 

 however, no permanent change ; deflocculation takes place and the 

 original properties return as soon as the flocculating agent is washed 

 away. Alkalis (caustic soda, caustic potash, ammonia and their car- 

 bonates) deflocculate clay, causing it to remain suspended in water for 



1 Older work on the constitution of clay is summarised by Rohland in Abegg's 

 Handbuch der Anorganischen Chemie, 1906, 3, 97-119, 



