Chapter 9
SOIL WATER

Water content is one of the most variable characteristics of soil. The soil acts as a reservoir for water, making it available for plants as it is needed. Soil water is very important to the entire soil system, not only because it is necessary for plant growth, but because the nutrients required for plant growth are also present in the soil solution. Most of the important soil reactions (weathering, cation exchange, organic matter decomposition, fertilization) take place in the context of the soil solution. Thus, it is evident that the moisture status of a soil is a key property.

Definitions of Soil Water Terms

Soil water potential - Soil water status is related to energy and the forces that hold and move water within the soil. The best description of soil water includes an analysis of the energy involved. Three major forces are involved in the movement of soil water (Figure 9.1). Gravitational, matric, and osmotic (solute) potential comprise the total soil water potential and give a measure of the differences in energy status between soil water and pure, standing water. Standing water has a potential of zero. Soil water movement occurs when there is a difference in total potential between two points in the soil. The direction of water movement, like energy transfers, will be in the direction of the point having the lowest potential. Matter seeks the lowest energy potential (ie. water runs down hill). A dry soil absorbs water from a wet soil and soil water moves toward an absorbing plant root (Figure 9.1).

Gravitational potential - Gravitational potential describes the force gravity has on water. The greater the height of water above a given reference point, the greater the gravitational potential. Gravitational potential is measured as the height above the reference point. If the reference is the soil surface and there are 10 cm of water sitting on the soil surface, then the gravitational potential is equal to 10 cm. Gravitational potential is responsible for water movement under saturated conditions.

Matric potential - Matric potential describes the surface attraction of soil particles for water. If a dry soil is adjacent to a pool of water, the soil will absorb the water. Because free, standing water (as in a pool of water) has an energy level of zero, the matric potential of the soil must be less than zero. Remember that matter always seeks the lowest energy level. Matric potential is responsible for soil water movement in unsaturated conditions.

Osmotic potential - Osmotic potential is the difference in energy between pure water and water containing dissolved salts. Water flows from areas of pure water to areas of salty water. Again, because the energy level of pure water is zero, osmotic potential is expressed as a negative potential. Within the bulk of the soil, osmotic potential has little influence on water movement. However, movement of water into plant roots is greatly influenced by osmotic potential. In soils, water will move from a wet zone (high potential) to a dry zone (low potential). Plant root potential is normally lower than the soil water potential; therefore, water moves from the soil to the root (Figure 9.1).











Figure 9.1. The three main components of total soil water potential and how they influence soil water movement ( Y _g - gravitational potential, Y _m - matric potential, and Y _o - osmotic potential).

Saturation point - Not all the water present in the soil is available for plant use. Some water drains past the rooting zone and is unavailable. Soil can be viewed as a sponge composed of air and solid particles when dry. When water is added to soil, the pore spaces begin to fill with water, pushing the air out. Usually the smallest pores fill first, followed by the medium-sized, and finally the largest pores. At the point when all the pores are filled with water the soil is said to be at the saturation point. This is an undesirable condition for the growth of most plants because the available dissolved oxygen is quickly depleted. Water at the saturation point in soils is held at a tension of 0 MPa (0 bars) (Figure 9.2).

Field capacity - Field capacity is the amount of water remaining in the soil after the large pores have drained. Medium and small pores are still filled with water held against the force of gravity. Soil water at field capacity is readily available to plants and sufficient air is present for root and microbial respiration. The optimum water content for plant growth and soil microbial respiration is considered to be close to field capacity. Soils at field capacity are generally considered to be holding water at a tension about 0.01-0.03 MPa (0.1-0.3 bars) (Figure 9.2).

Permanent wilting point (PWP) - If water is continually taken-up by plants and no additional water is added to the soil in the form of precipitation or irrigation water, the medium and small soil pores will be emptied of water. With time, the plant will eventually wilt when it cannot extract more water. The soil is said to be at the permanent wilting point when plants can no longer exert enough force to extract the remaining soil water. At the permanent wilting point, water is held in the soil at about 1.5 MPa (15 bars) (Figure 9.2).

Oven-dry - If soil is placed in an oven and dried at 105^oC, additional water will be removed. The oven-dry condition is the reference state used as the basis for expressing most soil characteristics.











Figure 9.2. Typical soil moisture content/potential curve for a sand, loam, and clay soil. From: The Nature and Properties of Soil, 10th edition, N.C. Brady, Macmillan Pub. Inc.

Available and unavailable water - The water held between field capacity and the permanent wilting point (PWP) is termed available water. Water remaining in the soil after the PWP has been reached is termed unavailable water (Figure 9.2).

A complete description of the soil moisture status includes the amount present (either percent moisture on a weight or volume basis) and the energy status of that water. For example, a sandy soil with 10% moisture may be at field capacity, while a loam soil at 10% moisture may be near the PWP (Figure 9.2). Expression of soil moisture status in terms of potential tells us much more than just the amount of moisture. A matric potential of 0.1 MPa (1 bar) tells us that a plant growing in this soil will have adequate moisture, regardless of the soil texture or the amount of water expressed as a percentage.

Measurement of Soil Moisture

Soil moisture measurements are expressed in two different ways, either as amount of moisture (percentage) or as a potential (MPa). Percent soil moisture can be determined by weighing a soil sample before and after oven-drying. This is called the gravimetric method, which is a direct measurement of soil water content. The water loss is divided by the oven-dry weight to obtain percent moisture. Notice that the water loss is divided by the oven-dry weight not the wet weight. If the wet weight of the sample is used as the divisior, the percent moisture will change with the initial weight of the sample. Thus, if soluble phosphorus (in percent of the total soil) was expressed on a wet weight basis, the actual amount of phosphorus in the soil would depend on the amount of moisture in the soil. And, of course, that is not true. Such a mistake could introduce considerable error into soil fertility recommendations and other calculations based o comparisons of soils. The correct calculations of soil moisture, by weight and by volume, are shown below.

By weight:

% moisture = (soil wet weight) - ( soil dry weight) x 100

(soil dry weight)

= water loss x 100

soil dry weight

By volume:

% moisture = (% moisture by weight) x (soil Db)

(density of water)

The oven-dry weight of a soil sample can be calculated, rather than measured, if both the percent moisture by weight and the wet weight of soil are known. This calculation is handy when a moist or air-dry sample is used for analysis, but the results must be expressed on an oven-dry basis.

oven-dry weight = (wet weight of soil) x 100

100 + (% moisture by weight)

The gravimetric method is accurate and few complications arise with technique. However, this method is destructive, in that a measurement can not be repeated on the same sample of soil. This becomes a problem in field studies where determination of soil moisture content on the same soil, over time, is an integral part of the study. If repeated measurements are required then an indirect measure of soil moisture content is used. Neutron scattering and electrical resistance (gypsum blocks) are two indirect methods of determining soil moisture content. Both the indirect methods must be calibrated using gravimetric moisture determinations before they can be used in the field with good results. The use of both neutron scattering and electrical resistance have limitations which will be discussed by the instructor.

Soil moisture potential can be determined using a tensiometers or gypsum blocks. Tensiometers are capable of measuring soil potential from saturation to about 0.10 MPa (1.0 bar). Gypsum blocks are more effective at lower soil water potentials. The gypsum block can be used to determine either amount of soil moisture or soil moisture potential, depending upon how it has been calibrated.

The relation between amount of soil moisture and soil moisture potential is unique for each soil (Figure 9.2). Soil texture is one of the main soil characteristics which controls this relationship. The apparatus used to establish the relationship is called a pressure plate. Saturated soil is set on a plate composed of a porous membrane. The membrane is then placed in a pressure cooker type chamber and sealed. The soil is then subjected to a selected series of pressures. The pressure in the chamber forces the water out of the soil and through the membrane. After equilibrium is established at each pressure step, a soil sample is taken from the chamber and the amount of water in the soil is determined gravimetrically. In this manner, a soil moisture release curve (like Figure 9.2) can be established for any soil.

Laboratory Exercise

A. Gravimetric moisture

1. In the laboratory: Weigh a clean, dry, labeled soil moisture can and record on the data sheet.

2. In the field: Clear the soil surface of any litter or debris.

3. Carefully take a soil core by pushing the sampler vertically into the soil till the mark on the sampler reaches the soil surface.

4. Crumble the soil core into the soil moisture can.

5. In the laboratory: Weight the tin and soil to the nearest 0.01 g and record on the data sheet.

6. Place the can and soil into the drying oven at 105^oC.

7. Return the following day and weigh the can and oven-dry soil.

8. Calculate the percent gravimetric moisture in the soil sample.

B. Soil columns: capillary

1. Set up two soil columns, one a coarse textured

soil and one a fine-textured soil, in a pan

containing water.

2. Observe the height and speed of rise of the water

in each of the columns. Continue to make

observations over several weeks.

C. Gravitational potential

1. Saturate a sponge in a basin of water.

2. Hold the sponge above the water with the shortest dimension in the vertical position (see below). Keep the sponge in this position until the water no longer drains freely.

3. Change the orientation of the sponge so that the next largest dimension is now in the vertical position (see below). Record your observations.

4. Again, shift the sponge orientation so now the longest dimension is in the vertical position (see below). Record your observations.

D. Measurement of amount of soil moisture and soil moisture potential

1. The instructor will demonstrate and discuss the use, calibration, advantages, and limitations of the various methods of measuring amount of soil moisture tension introduced earlier.

DATA SHEET

Chapter 9

A. Gravimetric moisture

1. Weight clean, dry soil moisture can ____g

2. Weight can and field moist soil ____g

3. Weight can and oven-dry soil ____g

4. Gravimetric moisture ____%

B. Soil columns: capillary rise

Observations:

Explanation:

C. Gravitational potential

Observations:

Explanation:

Water Potential Components