THE SOIL 87 



is when the soU is saturated and all the au--spaces are 

 filled with water ; the lower Mmit, 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 

 aU 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 — i.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 J pounds ; the same amount of clay increased 35 pounds, 

 and peat as much as 40 pounds. 



II. Soil and Temperature. — The sun shines on aU soils 

 alike, but some become warm naore quickly than others. 

 For example, a dark soil absorbs more heat than a hght 

 soil, and a dry soil becomes warmer than a wet soil. In 



