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Soil
 
                   
 
 
What causes plants to wilt?
Storage of water in soil
How much water is too much,
and why should we worry about drainage?
How much water do I need to apply?
 
 
 
 
 
 
 
 
 
 
 
 

Soil water

Water is essential to all forms of life and plants are no exception. They need to access water from the soil in order to live. Soil water also contains nutrients (food) that plants need to grow, so access to soil water is of vital importance for plant growth and survival.

In the 1940s researchers began to develop the concept of ‘potential evaporation’ as they finally worked out how to describe the relationship between changes in sunshine and temperature and the potential for water to evaporate. Howard Penman, a British scientist, defined an equation that described how water evaporates from free water surfaces. This was considered a breakthrough and it sparked off a lot of scientific research – particularly during the 1960s and 1970s, which ultimately led to us having a much better understanding of how plants are able to extract water from the soil and use it.

We now know that plants are able to extract water from the soil via a process known as ‘transpiration’. During this process, plants suck up water from the soil into their roots, then they transport it up through their stems and expel it into the atmosphere from tiny pores (stomata) on the surface of their leaves

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The transport of water from the soil and through the plant and its release out into the atmosphere takes place in a series of processes that occur independently, like links in a chain. The water flows along a pressure gradient, moving from zones of high potential to zones of low potential. Ultimately it evaporates from the surface of the leaves because there is a difference in the vapour pressure between the surrounding air and the leaf surface. Energy is required to change water from a liquid to gaseous state, so for this process to occur energy is supplied by the sun in the form of radiant heat. At least 2454 Joules (540 calories) of energy are needed to do this for every gram of water that evaporates! Consequently how fast this whole process (known as evapotranspiration) occurs depends on the climatic conditions in the environment, i.e. it will be affected by changes in temperature, radiation from the sun, humidity and wind speed.

Just as clothes hanging on a washing line will dry best on a warm, dry windy day, the evapotranspiration process occurs most rapidly under similar conditions. Although it varies in rate by day and by season, evapotranspiration is a continuous process, which means that plants need to take up a tremendous amount of water to satisfy their continuing demand. For example, for every kilogram of plant dry matter produced, a plant will, over a course of time, take up and transpire anywhere between 200 and 500 litres of water!

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What causes plants to wilt?

Plants do have some ability to cope with a reduction in the amount of water being taken up by the roots, e.g. they have the ability to open and close the tiny pores (stomata) on their leaves to reduce the rate of water evaporation from their leaves. However, as a soil dries out, the plant needs to exert a greater amount of suction to be able to extract the water that is still remaining in the soil. Once the amount of suction gets too much for the plant to cope with, water uptake will cease and the plant will start to wilt. At this point, if additional water is not supplied to the soil soon enough, the plant tissues will lose their ability to recover and the plant will die.

So the transpiration process can be thought of as the motor of plant life so the ability of a soil to store water becomes a prime concern for a gardener.

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Storage of water in soil

Soils mainly store water in two different ways – either within the soil organic matter component (the dead and decaying plant and animal remains as well as the humus in the soil) or in a thin film upon the surface of each of its mineral particles (the sand, silt and clay components of soil). As different soils have varying amounts of organic matter and as each soil contains variable amounts of sand, silt and clay, some soils are better able to store water than others.

Sand particles (comparatively few and relatively big) do not have as big a total surface area as do millions of tiny particles of clay. Since water can be stored on the surface of these particles sandy soils have been shown to be less able to store water than are clay soils. However, although soils with greater amounts of clay are generally better able to store more water, plants can find it harder to extract the water from them as more suction is needed to get the water out. As far as plant roots are concerned, sand has less water available, but it is easily accessible, whereas clay has more but is miserly about letting it go.

For similar reasons, water drains faster through some soils than others. Therefore, a gardener’s ‘ideal soil’ is half-way between sandy and clay, i.e. a loamy soil that is sandy enough to drain well but has enough clay to store essential nutrients and water (see Figure 2).

Whatever type of soil you have in your garden (whether it has a greater proportion of sand or clay) there is a ready way to improve its water storage ability – by adding more organic matter. Adding organic matter to a sandy soil will generally allow it to store more water, whereas adding it to a clay soil will help the water to spread through the root zone and become more accessible to plants. Organic matter may, for example, be added in the form of a mulch (a thick layer of organic material applied to the soil surface). Mulches can act as an insulating layer, reducing surface evaporation of water from the soil surface whilst recycling nutrients back into the soil as the mulch decays (the amount of nutrients returned will vary quite markedly depending upon the type of organic material that is used as a mulch).


Figure 2: Diagram showing how far 25 mm of water is likely to penetrate into clay, loam, and sandy soils in about 24 hours. The water moves quickly through sandy soil, is held near the surface by clay soil, and spreads more evenly through the loamy soil.

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How much water is too much,
and why should we worry about drainage?

Surely if water is so essential to plants, the more they have the better? It sounds a great waste of effort to water plants and then at the same time to take elaborate precautions to drain the water away. The reason is that roots need access to air as much as they need water. Unless plants are specially adapted to water-logged conditions (e.g. water lillies), roots kept for too long under water will die because they will not be able to breathe. As water drains down through soil, it takes fresh air down with it and renews the air in the cavities and spaces between the soil particles, leaving as it drains a healthy moist soil atmosphere with plenty of air for the plant roots and the soil organisms to use.

When excess amounts of water are applied, soils with low amounts of organic matter and/or those that drain rapidly are more likely to lose nutrients (both nutrients that are naturally present in the soil and/or those previously applied in the form of fertilisers/ mulches, etc). Nutrients that readily dissolve in water (such as nitrogen) will move with the water to wherever it moves to. Depending upon the situation/location, sometimes water will be more likely to run off the surface of the soil and end up in local surface waterways (this is known as ‘surface run-off of nutrients’). In other situations, the excess water will drain down through the soil profile to the groundwater, taking nutrients with it as it goes (a process known as ‘leaching’ of nutrients).

As gardeners, we should therefore be concerned about adding water in excess of plant requirements because plants may die due to waterlogging and valuable nutrients may be lost into surface or ground water. Consideration of the loss of such nutrients is not only of economic concern (you will ultimately need to buy more fertilisers to replace those that are lost), but it also has obvious implications in terms of environmental pollution where dissolved nutrients and spay residues, etc., can get unnecessarily washed into local waterways and/or reach and accumulate in the groundwater beneath our gardens.

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How much water do I need to apply?

You only need to apply as much water as is needed to maintain plant growth. Generally you will only need to apply water during the spring and summer months when the amount of water that evaporates from the plant and soil usually exceeds the amount replaced by rainfall. Information on the current amounts of this ‘potential evapotranspiration’ are usually published in your local daily newspapers. The amounts are usually quoted in millimeters of water, telling you how much water has been lost from your soil via evapotranspiration each day. You can add up the daily published figures to see how many millimetres of water you have probably lost to the atmosphere over a given time (e.g. add up the daily figures over the last week). Next you will need to know how much rainfall has fallen in your garden. This is where a rain gauge can come in very handy to get an accurate measurement of how much rain has fallen in your garden. But if you do not have one, you could again use the figures that are published in the newspaper to work out how much rain there has been over the same time period (e.g. over the last week).

Now let’s say that for example we calculate from the newspaper that the total potential evapotranspiration in the last week has been about 25 mm, but there has only been 5 mm of rainfall over the same time interval, then we know that we need to apply about 20 mm of water to replace the water that has evaporated from the garden. If you place a rain gauge under your water sprinkler you will know when you have applied enough water – the plants will be happy as they will have been supplied with sufficient but not too much water and you will not be losing those valuable nutrients. As a rough guide, the maximum amount of water that your garden will need (including rainfall) per week will be around 15 mm in October and this will increase (dependent of course on the weather conditions) up to about 35 mm (includes rainfall plus irrigation) per week in December and January.

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