NOTES ON THE SOIL, 489 



Soil- Water. Water occurs in soil under three con- 

 ditions : (1) free water, filling the spaces between the soil- 

 grains and free to move under gravity and to trickle through 

 the soil; (2) capillary water, adhering to the surfaces of 

 the soil- grains and to plant-roots in films thick enough to 

 allow surface tension to move it slowly from place to place, in 

 any direction where the soil is becoming drier ; (3) hygroscopic 

 water, which is held around the soil- grains so firmly that it 

 does not move about like the capillary water, and can only be 

 removed by heating to 100 C., when it passes off as steam. 

 Even an " air-dry " soil retains a good deal of hygroscopic 

 water. The free water of the soil is injurious to plants 

 (except those specially adapted e.g. bog- or water-plants), 

 because it interferes with the proper respiration of the root. 

 It is the object of drainage to remove it. 



Water-Content of Soils. The amount of water 

 present in the soil occupied by the roots is called the water- 

 content of the habitat, or home, of the plant or plant- 

 association. The water-content of a habitat depends on the 

 thickness of the water-films around the soil particles. In 

 saturated soils the films run together to form drops and 

 masses of water, while in air-dry soil there is still a thin film 

 around even the smallest soil particles. Hardly any water 

 is present in impervious rocks until cracks are formed by 

 weathering ; sandy fields or hills and other dry habitats have 

 from 3 to 15 per cent. ; moist meadows, cultivated fields, 

 and woods have from 15 to 30 per cent. ; wet habitats, like 

 stream-banks, are more or less saturated, their water- 

 percentage varying from 30 to 80 per cent, according to the 

 texture of the soil. 



As we shall see later, the most important differences 

 between the various habitats of plants are chiefly due to 

 differences of water-content. 



Available and Non-available Soil- Water. If a 



rooted plant is kept without water until it wilts and dies, we 

 find that the soil still contains some water. This non-available 

 water, which the plant cannot use, is retained by the soil 

 because the attraction of the soil-particles increases as the 

 water-films get thinner, until it is greater than the attraction 



