THE SOIL 87 
is when the soil is saturated and all the air-spaces are 
filled with water ; the lower limit, when the water present 
is reduced to the hygroscopic film. The former is the 
maximum, and the latter the minimum water-capacity. 
The maximum water-capacity obviously varies as the 
pore-space. Coarse-grained soils have less pore-space 
than fine-grained soils. In the former, the individual 
pore-spaces are larger, but there are fewer of them, and 
the total pore-space is less. For this reason the water- 
capacity of clay is greater than that of sand in the ratio 
of about 7 to 5. Peat has the highest water-capacity of 
all soils. 
3. Water-Retaining Capacity of Soils.—This is measured 
by the amount of water a soil can retain after the excess 
of water is drained off. The water-retaining capacity of 
coarse sand is about 13 per cent. of its volume—7.e., 
100 cubic feet of sand can absorb 13 cubic feet of water, 
and retain it without dripping. Compared with this, 
the water-retaining capacity of clay is 41 per cent. of its 
volume ; that is to say, clay can hold and retain three 
times as much water as sand. 
4. Permeability of Soils——This is measured by the rate 
at which water is able to percolate, or soak through, a 
certain depth of moist soil. Water runs through loose 
sand more quickly than through soils of closer texture. 
Clay is almost impermeable, but the water that is absorbed 
is very tenaciously held, and it takes a long time to drain 
or dry it out. 
5. The Absorptive Power of Soils.—If equal weights of 
different soils be exposed during the night, when the air 
near the ground is nearly saturated with water-vapour, 
they will be found, in the morning, to have increased in 
weight, but each to a different extent. The increase in 
weight is due to the amount of water absorbed: 
in one experiment it was found that 1,000 pounds of 
sand, exposed for twelve hours, increased in weight 
1} pounds ; the same amount of clay increased 35 pounds, 
and peat as much as 40 pounds. 
II. Soil and Temperature.—The sun shines on all soils 
alike, but some become warm more quickly than others. 
For example, a dark soil absorbs more heat than a light 
soil, and a dry soil becomes warmer than a wet soil. In 
