Moisture in the Soil 111 



important part, is the loss of water. Of the rain that falls upon the 

 surface of the soil, part runs off immediately, part sinks in, and another 

 portion is lost by evaporation. The amount of moisture at any one 

 time and place depends upon the relative rates of the supply of water 

 to the soil and evaporation from it— another example of an ecological 

 factor whose value depends upon an equilibrium. In evaluating the 

 water factor in the terrestrial environment both supply and loss 

 processes must be taken into account. 



Moisture in the Soil. Since the terrestrial environment is so varied, 

 it is not surprising that the water factor is very complex, and the 

 moisture in the soil and in the air will first be considered separately. 

 When rain water enters the soil, it fills the spaces between the par- 

 ticles. The volume that can be filled is known as the pore space and 

 commonly varies between 60 per cent for heavy soils and 40 per cent 

 for light soils. When the rain ceases, a certain portion of the water 

 in the soil soon drains out, and this is known as the gravitational water. 

 The portion of the soil water that is held by capillary forces around 

 and between the particles is the capillary water. That part of the 

 pore space which is not filled by gravitational or capillary water may 

 be occupied by water vapor. Another form of soil water, termed the 

 hygroscopic water, occurs as an extremely thin film on the soil grains 

 but this cannot move as a liquid. A small portion of the soil water is 

 chemically bound with soil materials and is known as combined water. 

 The total amount of capillary, hygroscopic, and combined water plus 

 the water vapor constitutes the field capacity and is the maximum 

 amount of water that the soil can hold after the gravitational water 

 has drained away ( Fig. 4.9 ) . 



The maximum potential supply of water available for the vegetation 

 is represented by the full field capacity of the soil. As plants draw 

 on the soil water, they gradually reduce the amount held in capillary 

 spaces. Below a certain moisture content plants tend to wilt in the 

 hottest part of the day. When the capillary water has been still 

 further depleted, a point is reached at which the plants will not re- 

 cover from wilting until more water is added to the soil, regardless 

 of other environmental conditions. At this point absorption of water 

 by the plants has become too slow to replace the water lost by 

 transpiration. The moisture then remaining in the soil is designated 

 as the permanent wilting percentage, or the wilting coefficient. Since 

 little difference has been found in the abundance of soil water when 

 wilting occurs for plants of various species growing in the same soil, 

 the permanent wilting percentage is primarily a characteristic of the 

 soil, and as such has great ecological significance. The amount of 



