Soil is about half solids and half holes. The holes can contain either air or water and for plant growth, there needs to be a suitable supply of each.

Large holes forming large channels are called “macropores”. Small holes forming fine channels are called “micropores”. Obvious really. Each plays a different role. The macropores are pipes that allow water to move freely into the soil, through it and ultimately to drain out the bottom if there is an excess. The micropores are like blotting paper, and the fine channels can hold water against gravity, keeping it available for plant roots to access.

The diagram below shows how soil moisture level terms relate to one another, and to the amount of water available for plant growth. We measure the amount of water in a soil in millimetres, the same way we measure rainfall. As we’ll see later in this course, we do everything in millimetres! It makes the sums so much easier.

After a large rain or irrigation event, the soil will be saturated. All the pores are full of water, and there is no air for plant respiration. After a day or two, gravity will have drained the soil macropores, but the micropores will be full, and the soil will be quite wet. This is called Field Capacity (FC). It varies with soil texture as described above.

As plants transpire and the soil water evaporates, the water stored in micropores gets used up. When all the plant available water is gone and plants can no longer access the water they require, the soil is said to be at Wilting Point (WP). There is still some water in the soil, but it is too tightly held for the plants to get. At this point there is no growth and plants are starting to die. Crop yield and quality will be seriously affected.

The amount of water between Field Capacity and Wilting Point is known as Plant Available Water (PAW). About half this water is Readily Available Water, and plants will grow at maximum rate. Once the Readily Available Water is used up, plants have to work for more and their growth begins to decrease as stress increases. By the time they have used the Survival Water and get to Wilting Point all growth has stopped.

Different textured soils have different ratios of macropores and micropores, so handle water differently.

• Light (sandy/gravelly) soils have a high proportion of macropores, so water easily moves through them. But they have few micropores so very little water is held and they dry out quickly.
• A loam soil has a good mix of macropores and micropores, letting water in, holding a significant amount for plants, but allowing excess to drain so air can enter the root zone again.
• Heavy soils have many micropores so can hold a lot of water, but not many macropores so water movement is slow.
• Very heavy (clay) soils can hold water so tightly that plants have difficulty accessing it, even when the soil is actually quite damp. With few macropores, water enters very slowly and moves very slowly. It takes a long time for air to get back into the root zone.

The diagram below shows thow the amount of water held and available changes with soil texture.

The soil’s water holding characteristics also change with depth.

Typically, the darker, higher organic matter and better structured topsoil can hold more water than subsoil. We usually talk about the RAW as percent by volume, which is the mm of water held in a 100 mm thick slab of soil. If the roots are deeper, they have more slabs of water, so they can last longer between top-ups. More on that in Soil Water Budgeting later in this course.

The aim of irrigation management is to maintain plants in the Readily Available Water zone, providing enough water to maximise growth, but to avoid saturation and water logging. An excepting is grapes, when management can aim to maintain a level of water stress to reduce leaf growth and encourage development of desired wine characteristics.

Water only moves down though the soil as it reaches and passes Field Capacity. Remember, below FC the micropores hold water against the force of gravity. We can see this in action when we dig into a recently watered dry soil. The upper levels will be darker and moist, and those below paler and drier. There is often a noticeable Wetting Front, which is the advancing edge of the applied water soaking into the profile.

If excess rain falls or too much irrigation is applied the wetting front moves all the way down past the root zone, causing drainage and taking valuable nutrients with it. If only a little rain or irrigation is applied, only the surface soil gets watered. Below this, the soil can be drying out and roots unable to access water or nutrients. We don’t want to apply too much water, but we don’t want to apply too little either.

Movement of water into and through the soil is not uniform. The soil is seldom uniform and this is amplified by uneven irrigation, and by intense rain or irrigation that causes surface flows and ponding. Some area receive more water than others. We can have a crop getting too much water in some spots, and not enough in others. It is common to apply extra to ensure the drier spots get enough, but that means higher pumping costs, wetter area plants saturated for longer, and increased leaching.

Good irrigation management is ensuring the right amount of adequately even irrigation is applied at the right time.

The following video clip discusses soil water concepts as they apply to irrigation management.