***Soils differ in how much water they hold and how readily that water is available to plants. Knowing the physical properties of your soils is essential for optimal irrigation scheduling. By Anna Mouton***
Soil consists of particles with air- and water-containing spaces – called pores – between them. The total volume of all the spaces in a certain volume of soil is called its porosity. Soil is said to be saturated when all the pores are completely filled with water and there is no space to add more water. Porosity, therefore, determines the total amount of water that a given soil can contain.
But porosity is not the whole story. The size of the pores also matters. Large pores allow rapid water and air movement, whereas small pores limit it. Pores that are so big that water runs straight through them are called macropores. Micropores are on the other end of the spectrum – so small that they hold onto water – and mesopores are somewhere in the middle.
Sandy soils are free-draining but hold little water, because they contain mostly macropores. Clay soils hang onto their water because they have mostly micropores.
Read More## Why does soil hold water?
Why does water flow through macropores, but stay in micropores? Surely the power of gravity should cause all the water to drain away? The answer lies in capillary action and adhesive forces.
Capillary action explains how liquids are drawn along tiny spaces. It is the mechanism by which paper towels mop up spills and water moves from wet to dry soil. Capillary action occurs when liquids stick to the sides of spaces while still remaining a unit – the liquid molecules in contact with the sides climb up and pull the rest of the liquid along.
In small spaces – like micropores in soil – adhesion between the liquid molecules and the sides of the space is initially stronger than the pull of gravity on the whole column of liquid. But as the column rises, it will eventually become too heavy. Gravity will win over capillary action and the liquid will not rise any further.
In larger spaces – like macropores in soil – the ratio of the circumference to the surface area is too small to support the column against gravity. There are too few liquid molecules clinging to the sides and too many in the middle being pulled down.
Soil particles also bind to water molecules due to electrical charges on the soil particles. Clay particles have especially high negative charges that cause them to hold tightly onto water molecules. This is why removing all the water from clay soils is difficult – the clay particles usually manage to hang on to at least some water molecules.
## Matric potential and water content
Soil-water potential reflects how hard plants need to work to extract soil water. Matric potential – the sum of the capillary action and the adhesive forces in soil – is the most important component of soil-water potential.
Plants also have to overcome gravity and osmosis to access soil water. The osmotic potential of soil is related to the concentration of dissolved salts. Plants must spend extra energy to extract water from brackish soils with high dissolved-salt concentrations.
Water naturally flows from areas with a high, to areas with a low soil-water potential. Wet soils generally have a higher soil-water potential than dry soils, but this is due to differences in matric potential as opposed to differences in the volume of water in the soil.
Consider the hypothetical soil-water characteristic curves illustrated in Figure 1. All three soil types started with a water content of 0.5 m3 per m3 of soil and a matric potential of zero. The matric potential – shown on the X-axis – increases rapidly as soil-water content drops, until it becomes so high that plants can no longer extract any water. This is known as the permanent wilting point.
A general guideline is that the permanent wilting point occurs somewhere around 1 500 kPa. Figure 1 shows that the water content of sand will drop below 0.1 m3 per m3 of soil before plants reach the permanent wilting point. But clay soils will get there when they still contain about 0.3 m3 water per m3 of soil. Why the difference?
Consider a sandy and a clay soil that each contains exactly the same volume of water. The sandy soil has a small matric potential because it contains mostly macropores, and water molecules bind only weakly to sand particles. Plants find it easy to extract water from the sandy soil. They just need to hurry up and get it before it runs away.
The clay soil has a large matric potential because it contains mostly micropores and clay particles bind water molecules tightly. The water in the clay soil is not going to run away but plants will have to work harder to extract it.
## Using a soil-water characteristic curve
By now it should be clear that knowing the water content of the soil is not enough, because not all the water is available to plants. This is illustrated in Figure 2.
Field capacity – the amount of water that the soil will hold against gravity – increases from sandy through loam to clay soils. Water in excess of field capacity runs off and water that runs off is of no use to plants. This is all the water in the yellow zone on the chart.
Water in the red zone – below the permanent wilting point – is also no good to plants because they are not able to extract it.
Irrigation aims to keep soil moisture in the blue zone because this is the zone in which plants can use water. The boundaries of the blue zone – field capacity and permanent wilting point – are determined by soil type. Hence the need to know the relationship between soil moisture and matric potential when scheduling irrigation.
The Department of Soil Science at Stellenbosch University currently has the only laboratory in the Western Cape that determines soil-water characteristic curves.
Departmental chair Dr Eduard Hoffman and his team are working on a model to predict the soil-water characteristic curve based on the soil texture. Similar models are used in other parts of the world but so far, they have not performed well when applied to all South African soils.
