412 Mineral Nutrition of Plants 



out, more work must be done to extract water, and the equivalent 

 negative pressure in the soil water increases. This pressure, as far as 

 soil moisture energy relations are concerned, is equivalent to tensile 

 stress and is referred to as soil moisture tension. 



Other factors being favorable, plants can grow and absorb moisture 

 at soil moisture contents ranging from saturation down to some mini- 

 mum moisture content that is associated with the wilting of plants and 

 which depends on the texture of the soil. As this minimum moisture 

 content is approached and the plant loses turgor, the rate of growth 

 of the plant approaches zero. The wilting percentage may be as low 

 as 2 or 3 per cent for sands or as high as 30 or 40 per cent for clay and 

 peat soils. Normally, in the root zone in the field, soils do not become 

 completely saturated. The soil moisture tension actually drops to zero 

 only in the shallow surface layer. After heavy rain or irrigation, the 

 water contained in the large pores of the soil drains away freely under 

 the influence of gravity. This drainage rate is rapid at first, but after 

 two or three days, in soils that do not have restricted drainage or a 

 water table, the rate of drainage becomes negligible. The moisture 

 content of the soil, after drainage becomes negligibly slow, is designated 

 as the "field capacity." Consequently, it is common to refer to the 

 moisture content of soil from field capacity to the wilting percentage 

 as the "available range," or the range of moisture that is available for 

 producing plant growth. 



The equivalent negative pressure in the soil ranges from zero in 

 saturated soil to something of the order of 0.05 to 0.15 atmosphere at 

 field capacity and on up to about 15 or 20 atmospheres at the wilting 

 percentage for crop plants. This pressure scale is convenient since it is 

 a direct measure of the work that plants must do to absorb water 

 against surface force action in the soil. The relation between the mois- 

 ture tension and the moisture content of soil is hyperbolic in nature 

 (/oj) as illustrated by the curves in Figure 1. It is seen that a con- 

 siderable amount of water may be withdrawn from wet soil before 

 the tension rises more than an atmosphere or two. However, at tensions 

 above 8 or 10 atmospheres a small decrease in moisture content cor- 

 responds to a large increase in soil moisture tension. The curves in 

 Figure 1 illustrate typical moisture-tension relations over the whole 



