102 
ECONOMIC GEOLOGY 
4 per cent. of coarse sand. Clay is “ heavy,” as it is hard to 
work, and is often left as grassland ; it shrinks during droughts 
and the cracks tear across the plant roots; and clay breaks 
into rough clods which are difficult to cultivate. Lime 
makes a clay soil looser and more tractable. 
Soir Composition—Chemical analysis of soil determines 
its supply of plant foods. The four chief elements which 
plants obtain from the soil are nitrogen, calcium, phos- 
phorus, and potassium.! Calcium is usually present in the 
soil as carbonate, but may be added as gypsum, the sulphate. 
Its chief functions are to coagulate colloidal clay, and to 
neutralize the acids and thus cure ** sourness.” 
Phosphorus is used by plants as phosphoric acid, P,0;, 
and average English soils have about 1 per cent. or up to 
'2 per cent. of it, with about a third more in the soil than in 
the subsoil. The prairie soils of the United States have a 
similar excess. Thus in Illinois the average percentage of 
phosphorus is ‘161 per cent. in the uppermost inch; -149 
per cent. for the depth of 2-3 inches; :143 per cent. for 
4-6 inches and +127 per cent. for 7-12 inches (Alway and Rost, 
Soil Science, ii, 1916, p. 495). The Australian soils are poor 
in phosphorus—the average of many clay soils of Victoria 
is only -047 per cent.—and it is often lower in the soil than 
the subsoil; this abnormal feature is probably due to the 
absence of ordinary mammals, whose litter of bones and dung 
in other continents have enriched the soil with phosphate. 
Potassium, in the form of potash (K,0), is an essential plant 
food and prevents some diseases. It is usually derived in 
soils from potash felspar. 
The other -essential inorganic elements of soil are mag- 
nesia—which is injurious if in excess of the lime as in 
some basic igneous rocks that yield surprisingly poor 
soil—iron, aluminium, chloride, and sulphur. Chlorine is 
chiefly present as sodium chloride, which is present in all 
soils ; most crops can tolerate +25 per cent. in the soil, while 
the vegetation of salt marshes is adapted to a high percentage. 
Sodium carbonate is very injurious except in small amounts. 
Few plants withstand more than one part in 1000, and owing 
to it much land has been ruined bv ill-managed irrigation. 
1 For the effect of the different constituents on plant growth, see 
Sir E. J. Russell, Soil Conditions and Plant Growth, 5th ed., 1026.