South Yardley Allotments Society
Allotment sites in Acocks Green and South Yardley

Soil Water and Drainage

Water is essential for all life processes, and is especially important for plant growth and development, as it helps, in combination with carbon dioxide, through photosynthesis, to capture the sun’s energy and convert it to form the start of the world’s food chain. It also dissolves mineral salts, enabling them to be assimilated by plants as nutrients.

For the gardener, water is essential, either through rainfall or irrigation, for the successful growing of both ornamental plants and food crops. However, too little, or too much water, will result in a failure of the plants to thrive and can result ultimately in their death. The ‘trick’ to successful growing of plants is to allow the plants to have adequate water for their needs, but neither a scarcity nor an excess. This can be a relatively straightforward matter when growing crops under protective cover, with controllable irrigation, but is not so easy on open land. On open land, the gardener is dependent upon rainfall for much of a crop’s water needs, able to resort to irrigation to supplement inadequate rainfall, but helpless to prevent excess rainfall. As discussed in the section on soil properties, plants are also dependent on an adequate supply of soil air to provide oxygen for respiration and to allow dispersal of carbon dioxide produced by that respiration. For air to diffuse into the soil then water needs to drain away through the soil, to remove excess, whilst leaving sufficient water held by the soil to keep the plants adequately supplied until the next rainfall. So the gardener needs to observe how much water is being supplied to the land, and how quickly it drains away.

Water Input
To obtain a guide to the quantity of rainfall (or irrigation) is being supplied, a simple rain gauge can be installed. This need not comprise anything more sophisticated than an open-top can or jar, being sunk up to its rim in the soil alongside the growing crop. The amount of water collecting in it indicating the quantity of water being supplied to the land. Emptying the container after each reading will enable the gardener to log the amount of water being supplied over a given period of time. Not all of a given area of land may receive the same amount of water by rainfall or overhead irrigation. The falling water is affected by the wind, causing it to drift, and obstructions such as hedges, fences, buildings or even the vegetation of the growing crop can shield areas of soil and cause a ‘rain shadow’ on their leeward side where the quantity of water reaching the soil is reduced. This needs to be taken into consideration when positioning the rain gauge, or choosing the siting of crop plants.
Similarly the gardener can measure how quickly the water is draining away through the soil by a simple percolation test, which is described later.

Water Drainage
The rate at which water will drain away through the soil is governed by its porosity, which in turn is influenced by the texture and structure of the soil. A sandy soil will drain much more quickly than a clay soil, and a cultivated soil will drain more effectively than a compacted soil. This pre-supposes that the soil water has got somewhere to drain to, for even a fast draining soil will become waterlogged if the water cannot find an outlet.
On sandy free-draining soils it may be necessary to slow down the rate of drainage in order to retain sufficient water for utilisation by the crop plants.This is accomplished by the addition of bulky organic matter, compost or manure, which holds onto the water more effectively and slows down the rate of ‘drying out’. A similar effect can be obtained by the addition of vermiculite, clay, or water absorbent polymer (‘water storing’ granules) to potting composts.

Poor Drainage 

On the other hand poor drainage is not so easy to overcome, as it is easier to add water to free-draining soils than to take excess water out of poorly draining soils. Some of the symptoms indicating poorly draining soils include:-

1 Water lying in surface puddles
2 Soil constantly wet
3 Dark coloured soil with a blue/black hue
4 The growth of surface algae, mosses and sedges
5 Poor plant growth (though this will also occur if soil too dry)

There are enormous benefits for the plants by improving the drainage of badly draining soils and these include:-

a The build up of an improved soil structure, making it more friable and easier to work. It also makes it easier for greater root penetration, enabling roots to travel faster and further to ‘mine’ the soil for nutrients
b Improved aeration which makes conditions more favourable for microorganisms to thrive. This increases the rate at which organic matter is broken down into humus and plant nutrients are mineralised into an available form. It also provide the necessary supply of air for root cell respiration. 
c The increased aeration increases the ease with which the soil can be warmed. Water takes more heat energy to raise its temperature than air requires to achieve the same temperature. This can make possible earlier sowing of seeds, with greater likelihood of improved germination.

When poorly drained soil is encountered, the cause of the poor drainage needs to be identified before remedial action can be taken. The texture and structure of the soil may not be the sole reason for poor drainage, it may also be caused by surface or sub-surface compaction, a high water table, or by topographical features such as run-off from adjacent (higher) land.
Clayey and silty soils are slower draining as the fine soil particles pack more densely together, leaving impaired porosity for water to drain through. Soils that have suffered high traffic levels may have serious compaction of the top few centimetres of surface soil, providing a barrier to the free draining of surface water. Repeated ploughing or rotavating to a fixed depth may form a ‘smear’ layer of compacted soil at that depth, again forming a barrier to the free movement of water. This is often referred to as a ‘plough pan’. On some soils, in some cases quite sandy soils, iron salts may be dissolved out of the surface soil and re-deposited, or precipitated, as insoluble salts lower down in the soil, to form a barrier layer, called an ‘iron pan’, or ‘fox bench’. This usually occurs under highly acid anaerobic conditions. Where soil is low lying, there may be a high water table, and or surface water from adjacent areas may run-off and collect here. The run-off may not necessarily come from land but can arise from non-porous areas such as driveways, patios, or the roof of an adjacent shed or greenhouse. The soil itself might be quite porous and free-draining, but the water has nowhere to drain to.

Once the cause of the poor drainage has been identified, then the appropriate remedial action can be undertaken.

Remedial Action
In the case of clayey or silty soils the addition of organic matter, and the addition of lime to counteract high acidity, will cause the aggregation of individual soil particles into larger crumbs, either through the action of gel-like humus, or the flocculating action of the lime. In effect this causes the crumbs to behave like larger particles increasing the porosity and drainage. Silt soils are particularly prone to forming a surface layer or crust under the action of rainfall and subsequent drying. This process is termed ‘capping’. In this case there is increased run-off of rainwater from the surface or increased lying surface water, but insufficient water in the deeper root zone. Again the addition of organic matter will improve matters and reduce ‘capping’.
Where soil is compacted either by surface traffic or by repeated ploughing or digging at a fixed depth the only remedy is to break up the compacted soil by cultivation. In the case of plough or iron pans the remedy is deep digging, to break through the compaction, or on a larger scale by ‘sub-soiling’. Incorporation of organic matter, as well as remedying high acidity and varying cultivation depths will help prevent a recurrence of the problem.
Where the soil is situated over a high water table or subject to run-off from an adjacent area then the remedy is to install drainage. If there is nowhere that the water can be drained to, in order to lower the water table, then the only way is to pump water away, as occurs in fen lands of East Anglia and the Netherlands. In the case of water run off from an adjacent area, an intercept drain or ditch can be installed to divert the water away before it reaches the land. If the soil is still too poorly drained, then the actual area itself needs drainage installing with the drained water being led to an outfall in a soakaway. The other alternative is to create a series of raised beds to raise the soil level higher, but this may not always be practicable.

Percolation Drainage Test
The assessment of soil water drainage by the percolation method is a quite simple test. A reasonable spell of fine weather needs to be chosen to avoid the soil being excessively wet or too dry when carrying out this test.

1. Dig a hole between 20-24” (50-60 cm.) deep. The diameter of the hole is not critical, but try to keep it to about 9 -10” (25 cm.). While doing this, examine the sides of the hole for signs of any change in the soil makeup, such as change from topsoil to subsoil, or for the presence of a compacted layer or a change in soil texture such as a seam of clay, which may provide clues for any poor drainage. If water starts to fill the hole then there is a high water table, or a ‘hanging’ water table.
2. Place a marker cane centrally and vertically in the hole and tie it in position with a second cane resting across the top of the hole.
3. Fill the hole with water to the brim and allow it to drain for several hours, ideally overnight.
4. Then refill the hole with water for a second time and measure how much the water level has dropped after 30 minutes, one hour, and if possible after several hours, or overnight. It is a good idea to cover the hole with a temporary cover in case of rain to prevent anomalies in the readings.
5. Work out the average drop in water level per hour. Refer to the following to interpret the results:-

Less than ½ “ (1 cm.) per hour indicates poorly draining soil, suitable for bog plants
Greater than ½ “ (1 cm.) but less than 1” (2.5 cm.) per hour indicates moderately well draining soil, suitable for many plants
Greater than 1” (2.5 cm.) but less than 2”” (5 cm.) per hour indicates a well drained soil suitable for all species.
Over 2” (5 cm.) per hour indicates a very fast draining soil that is likely to cause problems in periods of drought and need additional irrigation.

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