104 SOIL CONDITIONS AND PLANT GROWTH 



precisely like a solid solution and is therefore regarded by 

 van Bemmelen as an "absorption compound," SiO 2 , mA! 2 O 3 , 

 nFe 2 O 3 . . . pH 2 O, in which the constituents are not chemic- 

 ally united but are held by the feebler forces manifested by 

 colloids in their attractions one for the other. 1 



The Physical Properties of the Various Fractions. 

 Serious studies of the soil by competent physicists have 

 scarcely been attempted 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 inti- 

 mately 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 numbers. Only a very general summary 

 will therefore be attempted. 



The Clay Fraction. The word "clay" is unfortunately 

 used in many senses ; the soil worker and the ceramic chemist 

 in particular attach widely different meanings to it. In soil 

 investigations clay is the material of less than -002 (or in the 

 U.S.A. '005) mm. diameter : in ceramic work it is the 

 material of cri mm. diameter downwards. Clay in the soil 

 sense may be regarded as a plastic colloid, but its special 

 properties are seen only when a certain amount of water is 

 present. 2 If it is well rubbed with water it becomes very 

 sticky and absolutely impervious to air or water ; it is also 



Gedroiz' method of estimating the zeolitic silicic acids see Bull. 

 Internal. Instit. (Rome), 1917, 8, 1190. 



2 Older work on the constitution of clay is summarised by Rohland in 

 Abegg's Handbuch der Anorganischen Chemie, 1906, 3, 97-119. 



