• Topics: Soil and Water Science  | Soil and Water Basics  | Brown, Randall B 

Retention of Water: Basics of Soil-Water Relationships - Part II ¹

INTRODUCTION

In Part I we addressed the fundamentals of soil structure: particle and pore size, pore shape, total porosity, and soil bulk density. In this fact sheet, we will consider the ways in which these and other attributes of the soil structure influence the retention of water.

WATER RETENTION

The pore space in soil is usually at least partly filled with water. When all pores are water-filled, the soil is said to be water-saturated. Unsaturated conditions occur when water is present only in the finer pores while the larger pores are air-filled. This phenomenon can be explained by considering the capillary processes. When capillaries of different sizes are placed in water, the water will rise to the highest level in the smallest capillary (Figure 1). The smaller the capillary, the greater the suction by which water is held. Stated in another way, the pressure head (h) is more negative in the smaller capillary. It is more difficult (requires more energy) to remove water from finer pores than from larger pores. It is important to know how strongly water is held by the soil at any given time, because this governs not only the rate of water movement but also the availability of water to plants.

Figure 1. 

Water is pulled up into a capillary when the capillary is placed in water; the thinner the capillary, the higher the rise. This illustrates that small pores in unsaturated soil retain more water than large pores. In turn, it is more difficult to remove water from small pores than from large pores.

The pressure head (or suction) can be best measured with a tensiometer, which in its most simple form consists of a water-filled porous cup that is in contact with the soil (Figure 2). At first, the water level in the open end of the tensiometer corresponds with the level of the cup. Gradually, however, water will be drawn by the unsaturated soil. This becomes ever more difficult as the level in the open end of the tensiometer decreases. At

Figure 2. 

Schematic diagram of a tensiometer in its simplest form. At equilibrium, the soil water around the porous cup has a pressure of -h centimeters (or inches). In practice, tensiometers used in the field are more elaborate than the one depicted here. Ordinarily, pressures can be read on the vertical manometer arm of a dial-type pressure guage above the ground surface.

equilibrium this level is a measure of the average

energy status of the water in the soil, which can be expressed as a negative pressure head of h centimeters (or inches). Why is the pressure head negative? If water with atmospheric pressure (h=Ocm) is brought into contact with unsaturated soil, it is drawn into the soil which acts like a sponge. Since water flow occurs from areas of high to low energy, we conclude that the pressure head in unsaturated soil is negative.

These principles are not only used to measure actual energy conditions, with tensiometers, but also to produce them artificially in soil samples to determine how much water can be held in different soils at specific energy levels. Differences among soils are likely because of different pore size distributions. The apparatus used (in its simplest form) consists of a porous-plate which can be subjected to a series of suctions (negative pressure heads). Saturated soil samples can be placed on the plate and the soil-water content can be measured at several different suctions. Different points of a soil-water characteristic curve are thus obtained (Figure 3). Water content at each of the several suctions is obtained by removing a soil sample from the porous-plate apparatus and then weighing the sample before and after oven-drying to determine how much water has been retained by the soil at the suction in question.

Figure 3. 

Soil-water characteristic curves for surface soil and subsoil of Lakeland and Orangeburg soils. Notice that at any given pressure head (suction), the four different samples of soil retain markedly different amounts of water.

Soils with large pores can retain less water at a given suction than soils with smaller pores. This is because the smaller pores act as narrower capillaries, exerting greater suction on soil water than can be exerted by larger pores.

To illustrate the relationships among pore size, pressure head, and water retention, consider the soil-water characteristic curves in Figure 3. At a pressure head of 0 cm, all or nearly all of the pores in each soil are filled with water, and the water contents of the four soils are about the same (35-45%). As the pressure head is reduced (becomes more negative), however, the different soils exhibit different water retention behavior. The Lakeland subsoil, for example, whose pores are predominantly large, has a water content of less than 10% at a pressure head of -100 cm. The Organgeburg subsoil, by contrast, is much more clayey and has much finer pores than the Lakeland subsoil. At a pressure head of -100 cm, the Orangeburg subsoil has a water content greater than 30%.

Thus we see that soils having different structures and pore size distributions can have very different water contents at the same pressure head. In Part III of this series of fact sheets, we consider the application of the principles discussed above to the study of water movement in soil.

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service.

U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Millie Ferrer-Chancy, Interim Dean.

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