THE CONSTITUTION OF THE SOIL 



53 



proportion ; their relative abundance affords the basis on which soils 

 are classified. From the agricultural point of view we thus have : (a) 

 mineral soils consisting mainly of rock material, subdivided into sands, 

 loams and clays ; (d) calcareous soils containing notable amounts of 

 chalk or limestone ; (c) alkali soils rich in soluble, saline matter ; (d) 

 acid humus or peat soils where much organic matter has accumulated 

 in absence of calcium carbonate ; (e) neutral humus soils where much 

 organic matter has also accumulated, but in presence of sufficient cal- 

 cium carbonate to prevent acidity. 



By far the greater proportion of agricultural soils belong to the 

 first group. 



The Mineral Portion of tlie Soil. 



By the method of mechanical analysis described in the appen- 

 dix the particles of soil can be sorted out into fractions, each falling 

 within certain specified limits of diameter; those adopted in Great 

 Britain are given in Table XXIV. : — 



Table XXIV. — Size and Average Composition of Fractions Obtained by 

 Mechanical Analysis. Hall and Russell (124). 



The fractions fall into two broad groups: the sand, silt, and the 

 coarser part of the fine silt are mainly silica, while the clay^ and 

 some of the fine silt are complex silicates containing much iron and 

 alumina. The silica fractions do not represent distinct substances, the 

 lines are all artificial and merely divide up into a convenient number 

 of groups a mixture that shows a perfect graduation from an upper 

 down to a lower limit. So far as is known all these groups are 

 chemically inert. The clay fraction, on the other hand, stands out in 



1 It is unfortunate that no distinct word has been generally adopted for this fraction : 

 " clay " already stands for a particular mineral and also for a heavy soil. 



