Waterlogging can limit agricultural productivity in many areas of Tasmania as the state enjoys relatively high rainfall which normally occurs with an excess of rainfall over evaporation in winter and spring. A range of soil orders experience parts of the year when they are saturated due to high regional water tables, low rates of water conductivity, perched water tables or seepage.
Waterlogging occurs whenever the soil is so wet that there is insufficient oxygen in the pore space (anaerobic) for plant roots to be able to adequately respire. Other gases detrimental to root growth, such as carbon dioxide and ethylene, also accumulate in the root zone and affect the plants. Plants differ in their demand for oxygen and a plant’s demand for oxygen in its root zone will vary with its stage of growth.
Lack of oxygen in the root zone of plants causes their root tissues to decompose. Usually this occurs from the tips of roots, and this causes roots to appear as if they have been pruned. The consequence is that the plant’s growth and development is stalled. If the anaerobic circumstances continue for a considerable time the plant eventually dies.
Most often, waterlogged conditions do not last long enough for the plant to die. Once a waterlogging event has passed, plants recommence respiring. As long as soil conditions are moist, the older roots close to the surface allow the plant to survive. However, further waterlogging-induced root pruning and/or dry conditions may weaken the plant to the extent that it will be very poorly productive and may eventually die.
Many farmers do not realise that a site is waterlogged until water appears on the soil surface (figure 1). However, by this stage, plant roots may already be damaged and yield potential severely affected.
Waterlogging occurs when the soil profile or the root zone of a plant becomes saturated. In rain-fed situations, this happens when more rain falls than the soil can absorb or the atmosphere can evaporate.
Figure 1: Waterlogging in a crop grown on a duplex soil in late winter, near Carrick.
Diagnosing your waterlogging problem is the key to achieving success with any drainage. You need to know the source of the water and where it is moving in the soil. This will ensure correct selection of drain type to install and the appropriate depth at which to place drains. In winter it is easier to identify the limits of wet areas, particularly seepage areas, and to identify soil horizons on which a perched watertable occurs.
For the initial investigation dig a series of holes up to one metre deep in and around wet areas (figure 2). A number of pegs are useful to mark out drainage lines and potential drain locations. Signs of waterlogging to look for on the soil surface include ponding; pugging by stock and ruts from machinery; poor crop establishment and growth; and patches of excessive weed growth.
Figure 2: Waterlogged soil near Hagley – seepage rapidly entered the hole when dug.
Reducing the length of time soils remain waterlogged by the installation of appropriate drainage systems, results in greater ease of soil management, increased plant growth by improving aeration and soil temperature, plus control of plant and animal diseases.
Drainage is carried out either on the surface or underground, depending on the diagnosis of the problem (see Identifying problem areas). Surface drainage can take the form of open arterial ditches, grassed waterways, reverse bank interceptor drains or hump and hollow drainage. Surface drains are a minimal investment, last a long time, provided stock are excluded, and can always be deepened or moved. Underground drainage can take the form of pipe drains, mole drains, or deep ripping.
Subsurface drainage schemes are only warranted for intensive cropping or dairying farms. Strategic subsurface drains that use the topography may be worthwhile for less intensive farms but large amounts of money can be sunk in subsurface drainage that may not work.
Different soil types require different solutions to drainage problems. It is important to remember to investigate and plan your drainage in the winter, and install drains in the summer.
The draining of saline or sodic soils should be considered as special cases. Seek professional advice for the most appropriate drainage solution.
Belford R and McFarlane D (1993) Managing waterlogging and inundation in crops. Department of Agriculture and Food, Western Australia. Farmnote 80/93.
Cotching WE (2009) Soil Health for Farming in Tasmania. Bill Cotching, Devonport, Tasmania
Authors: Bill Cotching (Tasmanian Institute of Agriculture) and Derk Bakker (Department of Agriculture and Food, Western Australia)
The National Soil Quality Monitoring Program is being funded by the Grains Research and Development Corporation, as part of the second Soil Biology Initiative.
The participating organisations accept no liability whatsoever by reason of negligence or otherwise arising from the use or release of this information or any part of it.