58 SOIL CONDITIONS AND PLANT GROWTH 



of the original silicates and also of easily soluble substances including 



r ... ... , . molecules of SiO 2 . r ^ 



a fusible group with- ratio * varying from 3 to 6, 



molecules of Al O 



and a silicate resembling kaolin with ratios varying between 2 and 3. 

 The easily soluble material represents the products of weathering since 

 it does not occur in rocks. If it were a definite chemical compound the 

 ratio of its constituents should be constant whole numbers, but this is 

 not the case. It behaves, however, precisely like a solid solution and 

 is therefore regarded by van Bemmelen as an " absorption compound," 

 SiO 9 , mAl 2 O 3 , nFe 2 O 3 . . . pH 2 O, in which the constituents are not 

 chemically united but are held by the feebler forces manifested by 

 colloids in their attractions one for the other. 



The Physical Properties of the Various Fractions. Serious 

 studies of the soil by competent physicists have scarcely been at- 

 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. 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 



