THE SOIL 
[QI 
fruits which are out of season. The essential value depends 
on composition, texture, porosity, and colour. 
Soi CownstiTuTiON—SAND, CLAY, AND Sitt—A bulk 
chemical analysis of a soil may be of little agricultural 
significance, as much of the material is not available as 
plant food. The constitution is often more instructive; 
it is determined by mechanical analysis, which after re- 
jecting the pebbles, divides the material that will pass through 
a sieve with holes ;-inch in diameter into sand, silt, and clay. 
Sand has often been regarded as composed of silica, and clay 
as composed of silicate of alumina; but some sand is com- 
posed of carbonate of lime, or of grains of felspar, and some 
clay consists of quartz ground as fine as flour. That the 
difference between clay and sand is physical and not due to 
composition is shown in mining. An ore is crushed to allow 
the extraction of its metal, and sorted into a coarse grade 
(above 04 mm.) ‘the sand,” and a finer, * the slime,” 
which is a clay. Their chemical composition is identical, 
and the difference is due wholly to the size of the particles. 
Sand is loose material in which the particles vary from J 
to gig of an inch (or I to '05 mm.) in diameter! Clay is 
material in which the particles are less than 554 of an inch 
(-005 mm.) in diameter. Material intermediate between sand 
and clay is known as silt. Clay of less than 002 mm. in 
diameter is a colloid and its effects on soils are independent 
of its chemical composition; colloids absorb from solutions 
materials that would otherwise be carried away by drainage ; 
they strengthen the imbibition (cf. p. 228) and control the 
reaction of the soils to lime. Lime may act differently on 
two soils of the same chemical composition, since it coagulates 
colloids into larger particles, and thus improves the texture, 
and confers no such benefit upon the coarser particles of silt. 
A sandy soil is said to be * light,” as it is friable and easily 
worked. A soil with more than 40 per cent. of fine sand 
cakes after rain, and has to be broken by rolling. A soil 
with 40 per cent. of sand and less than 5 per cent. of clay is 
rarely useful unless stable manure is abundant and the water 
conditions favourable. Many good loams hold less than 
L For a classification of the grades, and literature, see Tyrrell, Principles 
of Petrology, pp. 190-1. For methods see Boswell, British Refractory 
Sands, 1918, pp. 18-28.