Water leaves soil in three ways: evaporation from a surface; drainage; and movement through a plant. Evaporation is due to heat, drainage is due to gravity, and movement through a plant begins with root uptake and ends with evaporation through the stems, leaves and flowers (a process called transpiration).
Evaporation can be controlled to some extent by using mulches to cool the soil and reduce exposure to the heating influence of the sun. Mulches from shredded plant materials such as leaves and grass are preferred over materials such as plastic sheeting, as these allow infiltration of water into the soil as well as breaking down to fertilise the soil and build structure. Do make sure the mulch doesn’t form an impenetrable mat that forces water to pool on the surface and wash away.
Water that drains out of a soil is removed via gravity. There is little control over this except to allow a soil rich in organic matter and structure to develop, which will naturally hold more water than a poorly-structured infertile one.
The water left after drainage is what resists that pull of gravity, and such ‘just-drained’ soils are said to be at field capacity. As this water resists the pull of gravity, roots must ‘pull’ a little harder again to overcome this resistance and extract this water.
Water in the largest pores is the easiest to remove, then that in subsequently smaller and smaller pores. As smaller and smaller pores are accessed, the amount of water becomes less and less, to a point that it cannot meet a plant’s needs. A plant begins to take on a wilted appearance that progresses during the heat of the day, but recovers during the cooler night. Eventually there is a point at which a plant cannot extract more water — this could be because the soil is too dry, or the water that is present is too tighly held to extract, or roots cannot reach further sources of water quickly enough.
When a soil is so devoid of water, or is held so tightly in the smallest pores (as small as 0.0002 mm across) that a plant cannot overcome the force needed to access it, the soil is at permanent wilting point. If water isn’t returned to the soil, a plant will shed leaves to reduce transpiration, and die if this condition persists.
The water held in soils between permanent wilting point and field capacity is called the available water. Soils with good structure will have more available water than those with poor structure.
Available water is measured as a concentration rather than a volume, often expressed as mm of water held per cm depth of soil. This concept helps to understand the interaction between water, soils and plants.
For example, sand has a lower ability to retain water than a well-structured loamy soil, and would have a lower concentration of available water because of this. However, if that sand is deep and consistently exposed to water, such as in sandy hills by a beach, infiltrating water will aggregate at those lower depths and the whole profile in general will be consistently at or near field capacity.
That field capacity will still be lower than that of a typical well-structured loam, but if the plants growing on those sandy hills have deep roots that reach deep down into the profile, there is actually more water available to them than if that well-structured loam is no more than a shallow layer over solid rock.
A plant needs to exert a pressure of around 10 kPa to pull, or suck the first water from a soil at field capacity. Higher and higher suctions are required to remove more and more water, as this water becomes more tightly bound within smaller and smaller pores. A soil at permanent wilting point requires a suction of at least 1,500 kPa to remove that water.
Sandier soils are able to release more water at lower suctions than heavier soils, as water does not cling to their larger particles as tightly as in clays. This means that while heavier soils hold larger volumes of water comparatively, more of it is locked away in the capillary pores to be of any use to plants.
Understanding the implications of soil type and depth on water availability will help with planting decisions. Plants in shallow sandy soil will need small, frequent waterings to reach the happy medium between fast drainage and subsoil pooling of water. It would probably be best to not plant in such areas in the first place, especially if the plants aren’t drought-tolerant. Plants in a deep sandy soil will benefit from long waterings to encourage deep rooting to where the water will pool.
Conversely, plants in well structured soils of high field capacity and which drain moderately can handle less frequent waterings, but of higher volumes. This too will encourage deep rooting. Plants in shallow soils that don’t drain quickly are best watered more frequently but with smaller amounts, to avoid waterlogging and deprivation of air to the roots.
In this ‘Water’ section so far, we have covered how water enters and moves through soil, and its availability or otherwise to plants. We’ve also seen how a soil’s texture and structure features in all three of these. It follows that allowing your soil to develop a sound, undisturbed structure rich in organic matter can only improve water infiltration, retention and availability to plants with time.