[04 " ECONOMIC GEOLOGY . 
is attended by a corresponding difficulty as it deals with 
powdered soil which does not indicate the actual texture. 
Heber Green, and Ampt (¥. Agric. Sci., iv, 1911, pp. 1-24) 
have shown that the essential factors can be determined 
and expressed by formule which indicate the amount of pore 
space, the permeability to air and water, and the capillarity. 
This method represents the facts for a soil as it is, and not 
when it is artificially broken up. 
The texture of soils controls their fertility in various ways. 
Soils may be barren owing to the absence of water, which 
drains away quickly from coarse sands; or to its excess in 
water-logged clay; or to deficiency of air in stiff impene- 
trable clay; or to acidity due to absence of carbonate of 
lime. 
The water capacity of soil depends on its interspaces or 
pores. The pores in stiff clay amount to 50 per cent. of the 
bulk, and the total surface of the particles is about 3 acres 
per cubic foot. A coarse sand, on the other hand, has a 
pore space of only from 25 to 30 per cent., and the surface 
area per cubic foot is about half of an acre. As water 
spreads through soils in a film covering the particles, 
the larger their surface the more water the soil will hold. 
Hence clay absorbs more water than sand, and holds it 
more firmly. A clay soil may hold an excess of water, and 
being water-logged, air is excluded, and the soil is barren. 
A sand on the contrary may be drained quickly and may not 
hold sufficient water to feed a crop during dry weather. 
Clay may be relieved of the excess of water by drainage, 
which allows air to enter and aerate the roots, lightens the 
soil by washing away clay particles, and renders it warmer 
by avoiding the chilling process of evaporation. The soil 
is nourished from the water-table {cf. p. 224) during drought 
as the film of water spreads from particle to particle. The 
water rises higher through fine grained rock with numerous 
pores and uniform closely packed grains than through loose 
coarse material. Hence crops separated from the water- 
table by a few feet of coarse gravel may perish from drought ; 
while the soil above an even-grained rock may be well- 
nourished. 
The principle of dry farming is to till the land so that the 
svaporation of water from the surface is kept under full