Farmnote 78/93 [Reviewed July 2005]
By Don McFarlane, Senior Research Officer, Resource Science, Albany and Bob Belford, Senior Research Officer, Crop Industries, South Perth
Waterlogging is too much water in the root zone of a plant. The roots cannot absorb enough oxygen to breathe, so the plant stops growing within a few days and may die. Other gases, such as carbon dioxide and ethylene, may also accumulate around and affect the plants.
Inundation is water ponding on the ground surface. Those parts of the plant that are submerged cannot breathe or photosynthesise. Short plants, such as young crops or heavily grazed pastures, may be completely covered and usually die. Inundation is often associated with waterlogging.
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 the yield potential severely affected (Figure 2).
Figure 1. Inundation and waterlogging in a crop grown on a duplex soil in early winter, 1993, in the Upper Great Southern
Figure 2. The same crop two months later. The crop failed where it was inundated and severe growth reductions occurred where it was waterlogged
The cost of waterlogging and inundation
Most data on the cost of waterlogging and inundation are from the Upper Great Southern, although the problems are widespread.
Cereal crop yields decrease by about 150 kg/ha for every 10mm of rainfall during August in the Upper Great Southern. In eight Shires from the region, excess rainfall costs farmers an average of $13 million in lost production each year.
Only the most obvious losses are attributed to waterlogging. Remote sensing methods show that symptoms of waterlogging are easiest to detect in the infrared part of the electromagnetic spectrum, which is invisible to the human eye. In 1988, a year with slightly above average rainfall, waterlogging affected about 32 per cent of cereal crops in part of the Cuballing Shire. Cereal losses in four Shires in 1988 were about $23 million.
Waterlogging and inundation slow pasture growth in winter and delay the spring flush. Pasture growth in winter is at least five times more valuable than extra production in late spring. Waterlogged legumes grow more slowly than waterlogged grasses, so waterlogged pastures become grassy and weedy.
In wet years, waterlogging reduced the area that can be cropped. When paddocks are waterlogged shortly after seeding, germination and emergence f crops are often reduced; crops may have to be resown when the soil is form enough to support machinery.
Waterlogged and inundated areas contribute recharge to saline aquifers, are very susceptible to water erosion and are prone to soil structure decline if cultivated or stocked when too wet.
Identifying problem areas
The best way to identify problem areas is to dig holes about 40cm deep in winter and see if water flows into them (Figure 3). If it does, the soil is waterlogged. Digging holes for fence posts often reveals waterlogging.
Figure 3. Waterlogged duplex soil-sandy loam topsoil overlying a sandy clay subsoil at 40 cm. Seepage is entering the hole above the clay base. Seepage faces can be unstable, especially when exposed in gullies and drains
Some farmers put slotted PVC pipe into augered holes. They can then monitor the water levels in their paddocks.
Symptoms of waterlogging include:
- yellowing of crops and pastures;
- the presence of weeds such as toad rush, cotula, dock and Yorkshire fog grass;
- grey or greenish coloured soil; and
- the presence of orange mottles in soil profiles (although some mottles may have been caused by waterlogging in previous climates).
Areas that are most susceptible to waterlogging and inundation are those where:
- the soil has a shallow clay subsoil (particularly on flat land or in water-gaining areas such as rock outcrops, roads, lower slopes and areas where water converges); and
- the soil allows water to infiltrate only slowly, such as clays, loams, soils that set hard and soils with a surface seal (Figure 4).
Figure 4. Inundation and waterlogging of clay flats that have low infiltration and poor surface drainage
Effects on plant growth
Low levels of oxygen in the root zone trigger the adverse effects of waterlogging on plant growth. Waterlogging of the seedbed and inundation most affect germinating seeds and young seedings. Established plants are most affected when they are growing rapidly. Therefore, if a soil becomes waterlogged in July, final yields may not be greatly reduced; soils are cold, the demand for oxygen is low and plant growth is slow at this time of year.
When plants are growing actively, root tips begin to die within a few days of waterlogging. The shallow root systems that then develop limit the uptake of nutrients (particularly nitrogen) and water, particularly when the soil profile starts to dry in spring. As a result plants may ripen early and grains may not fill properly in areas that do not receive seepage water late in the season. Shallow roots also result in more recharge, and this increases salinity.
Nitrogen is lost from waterlogged soils by leaching and denitrification (degassing). These losses, together with the lowered ability of plants to absorb nutrients from waterlogged soil, cause the older leaves to yellow. Waterlogging also directly reduces nitrogen fixation by the nodules of legume crops and pastures.
Waterlogging is most obvious on salt-affected land. Unless salt levels are very high, most plants growing in well-drained soils can keep salt out of their roots. Since waterlogging prevents the roots making enough energy to keep the salt out, the salt enters the roots and the plants die.
Solving these problems
Drainage can be improved on many sites and is the first thing to consider once a waterlogging problem has been identified. Agronomic options are also available. Consult your local adviser.
Note: Reducing the effects of waterlogging and inundation in both crops and pastures is explained in Farmnote No. 80/93 'Managing waterlogging and inundation in crops' (Agdex 130/550) and No. 79/93 'Managing waterlogging and inundation in pastures' (Agdex 130/550).
Editorial assistance: Jo McFarlane, Albany and Eliza Compton, Information and Media Services, South Perth.