THE WATER OF THE HABITAT 9 



11. Influence of other factors upon water content. The most 

 important differences between habitats are due to differences of 

 water content. The latter arise largely from the effect of the other 

 factors of the habitat. All of these have an influence either direct 

 or indirect upon the amount of water present, but soil, rainfall, 

 physiography, and humidity are the most important. A sandy 

 soil contains less water than a clay soil, even when both receive the 

 same amount of rain. It goes without saying that a habitat in a 

 desert region has a lower water content than one in a humid region, 

 because of differences in precipitation and humidity. Two habi- 

 tats with the same soil and atmospheric conditions may owe their 

 difference to slope, which tends to decrease water content. The 

 latter is likewise affected by cover, which prevents rainfall from 

 running away before it can be absorbed, and also decreases the 

 loss by evaporation from the surface of the soil. Heat and wind 

 decrease water content indirectly by decreasing the humidity, par- 

 ticularly upon exposed, slopes. The plant covering itself has the 

 effect of a dead cover in reducing evaporation from the soil, but on 

 the other hand it tends to decrease water content, owing to the 

 use of water by the plants. 



12. Available and non-available water content. If a rooted 

 plant is allowed to wilt and cUe, a careful examination of the soil 

 shows that some water still remains. The amount depends upon 

 the kind of soil, but all soils agree in the ability to retain some 

 portion of the water content. This is due to the fact that the 

 attraction of soil particles for the water films increases as the film 

 grows thinner, until finally it is greater than the attraction exerted 

 by the root-hairs. At this point the plant is unable to obtain 

 water, and it rapidly dies by wilting. The water thus held by the 

 soil can not be used by the plant, and it is hence called the non- 

 available water, or echard. It is usually but a small part of the 

 water commonly present, particularly in moist or saturated soils. 

 In a fine-grained compact soil, like clay, the amount of non-avail- 

 able water is large; in coarse-grained soils, e.g., sand and gravel, 

 it is very small. Of the total water content, or holard, the larger 

 portion can be absorbed by the plant, and is consecjuently termed 

 available water. The response of the plant to water content is 

 determined ])y the amount available for absorption and not by the 

 total amount present. This availal)lo walei-. or chrcsard, differs 

 for the different soils, and for dissimilar species of plants. It is 



