Availability of Water to the Soil-Plant System

September 7, 2023 Agric4Profits

Forces that Influence Soil Water

A number of forces influence the way water behaves in the soil. The most obvious is the gravitational force, which pulls water down through the soil.

Other forces, called adhesion and cohesion, work against gravity to hold water in the soil Adhesion is the attraction of soil water to soil particles, while cohesion is the attraction of water molecules to other water molecules.

The water molecule is like a bar magnet-positive on one end, and negative on the other. Like bar magnets, the opposite ends of water molecules attract. The bond between the hydrogen of one water molecule and the oxygen of another, called a hydrogen bond, accounts for cohesion.

Hydrogen bonding also accounts for adhesion. The main chemical in soil minerals is silica (quartz is pure silica). Silica, with the chemical formula S_i O₂, has oxygen atoms on the surface that can form hydrogen bonds with soil water.

Together, adhesion and cohesion create a film of water around soil particles. The film has two parts. A thin inner film is held tightly to the particle by adhesion. The adhesion water is held so tightly it cannot move.

A thicker outer film of water is held in place by cohesion to the inner film. Cohesion water, sometimes called capillary water, is held loosely and can be absorbed by plants. Thus, plants use cohesion water that is clinging loosely to soil particles.

Capillarity

Soil water exists in the small spaces in the soil as a water film around soil particles. The small pores act as capillaries. A capillary is a very thin tube in which a liquid can move against the forces of gravity.

Capillary action, the additive effect of adhesion and cohesion, holds soil water in small pores against the force of gravity. The fact that soil water can move in directions other than straight down is also due to capillary action. The smaller the pores, the greater that movement can be.

Types of Soil Water

Consider what happens after heavy rain. At first, all soil pores are completely filled with water. This is called saturation. All of this water does not normally remain in the soil pores.

In larger pores, some water is too far from the nearest surface to be attracted by the particles to overcome gravity; the gravitational potential exceeds the matric potential.

The extra water, called gravitational water, drains through the soil profile, usually within 24 to 48 hours in well-drained soil. As the soil drains, it pulls air to fill the large soil pores. This action provides new oxygen-rich air to plant root systems.

Eventually, drainage ceases. The soil moisture level at that point in time is called field capacity. At field capacity, the remaining water is close enough to the surface of a particle to be held against the force of gravity.

The soil-water potential is about –1/3 bar. Air fills the large pores, and thick water films (cohesion water) surround each soil particle. Plant growth is most rapid at this ideal moisture level because there is enough soil air yet sufficient water is held loosely at high potential.

Once drainage stops, plant and evaporation continue to remove cohesion water, shrinking the soil water films. As the water films become thinner, the remaining water clings more tightly, being held at a lower potential (larger negative value). It becomes increasingly difficult for plant roots to absorb water.

Eventually, at the permanent wilting point, most of the cohesion water is gone and the plant can no longer overcome the soil-water potential. The plant wilts and dies. The potential at this point varies according to plants and conditions but is generally about –15 bars.

Beyond the wilting point, some capillary water remains but is unavailable to plants. The capillary water may also evaporate, leaving only the thin film of adhesion water. This point is called the hygroscopic coefficient, the point at which the soil is air dry.

This hygroscopic water, as it is called, is held to particles too tightly, between –31 and –10,000 bars, that it can only be removed by drying the soil in an oven.

In fact, the strength of the soil water potential is so great at this stage that if oven-dry soil is exposed to air, it will bind water vapor from the air until the soil moistens to the hygroscopic coefficient.

Available Water

Available water is that part of soil water that can be absorbed by plant roots. Soil scientists consider gravitational water to be largely unavailable because it moves out of the reach of plant roots. If the excess water is unable to drain away, roots become short of oxygen and fail to function.

Hygroscopic water cannot be removed by roots, so it is also unavailable to the plant. Only some cohesion water can be used by plants.

Available water is defined as lying between the field capacity and the wilting point or between –1/3 and –15 bars. In medium soil, available water amounts to about 25% of the water held at saturation.

Water Retention and Movement

Both the retention of water and the movement of water in the soil are governed by the energy relations just described. We can begin by looking at water retention.

1. Water Retention

How much water can a particular soil retain and make available to plants? Actually, these are two separate questions. Not all the water film surrounding a soil particle can be drawn on by plants, so only a portion of the total water-holding capacity of a soil can be said to be plant-available.

Both the total water-holding and the available water-holding capacity are based mainly on soil texture. Let’s look at the effect of each soil separately.

Sand grains are large, so the internal surface area of soil high in the sand is quite low. Thus, there is little surface to hold water films. In addition, the pores are large enough that much of the volume of each pore is too far from a surface to retain water against gravity.

The opposite is true of clay soils – they have small pores and a large internal surface area. Thus, soils high in the sand have a low total water-holding capacity, while soils high in clay have a large water-holding capacity.

Not all of this water is available to plants, however, in soil high in clay, clay particles are crowded together tightly, leaving tiny pores. Any one water molecule occupying one of the pore spaces will be close to a clay surface, therefore tightly bound.

Most of the water in high-clay soil is held at a low water potential. Sand is the opposite. With large pores, much of the water can be fairly distant from grain, and therefore be held at high potential.

This leads to two rules. First, water in fine soils is held at low potential, and water in coarse soils is held at high potential. Thus, it is easier for plants to remove water in coarse soils than in fine soils.

Second, since most of the water in high-clay soils is held at low potential, much of the water is not available to plants. In contrast, most of the water in sandy soil is available.

Silt particles, and to some degree very fine sand, are a special case. They are small enough that there is a high surface area to hold water.

The pores are small enough to hold large amounts of water by capillary force but large enough that much of it is held loosely at high water potentials. Thus soils high in silt hold large amounts of plant-available water.

To hold the largest amounts of plant-available water, then, the soil should have a mixture of large and small pores with many medium-sized pores caused by silt and very fine sand. There are several important points to note in the:

The fine sandy loam holds much more water than regular sandy loam, reflecting the influence of very fine sand.

The finest soil, clay, has the highest total water-holding capacity. But note that it holds no more available water than a sandy loam.

Medium-textured soil has the highest available water-holding capacity. Note that the best soil is silt loam.

2. Water Movement

Horticulturists suggest that trees be watered by letting a hose trickle on the ground under the tree for a few hours. How will the water move into the soil?

First, water infiltrates the soil, then it percolates downward through the soil profile. The distance, direction, and speed of travel are set by gravity, matric forces, and hydraulic conductivity.

Directly below the nozzle of the trickling hose is a column of percolating water. This water is gravitational water, moving under the influence of gravity (moving in response to the gravitational potential). It is called gravitational flow.

Gravitational flow only occurs under saturated conditions, when the matric potential is so low that it cannot hold water against gravity. Because it occurs under such conditions, it is also called saturated flow. Saturated flow resembles the flow of water through water pipes.