Waterlogging (excess water in the root zone) and inundation (surface ponding) greatly reduce pasture growth in winter and early spring in areas with more than about 450 mm of annual rainfall.
Often the reduced growth is not attributed to waterlogging, which mainly affects the roots. Waterlogging in the coldest months does little damage to dormant pastures.
Waterlogged pastures may retain moisture for a longer period through spring. However, the extra growth that occurs may be at the expense of more valuable growth in winter and early spring.
Figure 1. A W drain on a fine-textured (clayey) flat valley. Water enters the drain from both sides. The soil from the drain channels is stored between the channels Deep roots are killed, so the plant takes up less water and nutrients. Leaching and denitrification cause nitrogen deficiency, which is seen as a yellowing of the older leaves.
Nitrogen deficiency increases plant stress. Weeds and disease, particularly root rots, increase under these conditions. Nitrogen fixation by pasture legumes is also severely reduced.
Waterlogging decreases pasture’s/crop’s tolerance to salt. Even small amounts of salt will kill plants if the soil is waterlogged. Waterlogging reduces pasture growth in duplex soils that have a clayey subsoil within 50 cm of the soil surface.
The water lies on top of the subsoil and kills the deep roots. Most damage occurs when the water is less than about 20 cm below the soil surface and when the pasture is growing rapidly. The early stages of plant development are the most sensitive.
In most areas drains are the best way to overcome waterlogging and inundation. Where drains can only partially overcome the problem, change the pasture varieties and management. In many cases, drains have paid for themselves within a few years of installation.
For example, increased grazing rates of only 4 per cent were needed to recoup the cost of drainage in 1990.
Drainage of sloping duplex soils:
About two-thirds of the soils in the Great Southern have perched water within 30 cm of the soil surface in years with average rainfall. About three-quarters of these are duplex soils on slopes which can be drained by seepage interceptor drains. Grade banks, which control water erosion, are usually too shallow to intercept the water flowing on top of the clayey subsoil.
If waterlogging is suspected in a pasture:
- dig shallow holes 30 to 40 cm deep and see whether free water is in them (if so, the soil is waterlogged);
- observe where soils are boggy and pastures (particularly grass species) are yellow;
- mark out the affected areas, either with posts in the ground or on an accurate map;
- survey seepage interceptor drains, placing them immediately above the affected sites; and
- install the drains when the soils are moist.
It may be necessary to fence off badly affected areas and treat them separately. Note: Seepage interceptor drains are only suitable where the slope is more than 1.5 per cent and there is a safe outlet for the water from the drains, such as a non-eroded creek, and clay within 50 cm of the soil surface. Community landcare technicians are available in most districts for surveying work.
Drainage of flat clayey soils:
Waterlogging and inundation are also common on flat, clayey soils. If the water causing the problem is runoff from higher up in the catchment, divert it using W drains (Figure 1) or levees. If the water comes from seepage in nearby duplex soils, then drain the soils with seepage interceptors.
Make sure that the drains empty into a well-defined waterway. Poor soil structure often prevents the rain from soaking into the soil or the land may be too flat for adequate runoff.
Soil structure can be improved on some soils by:
-
using gypsum (see Farmnote 32/85 ‘Gypsum improves soil stability’)
- reducing both the amount of tillage needed in the cropping phase
- preventing stock pugging the area in winter.
Figure 2. A V or spoon drain on a fine-textured (clayey) flat valley. Water enters the drain from both sides. The soil from the drain channels is spread each side of the channel, infilling low areas. Compared with W drains, spoon drains remove less land from production but their channel capacities are usually smaller.
Surface drainage can be improved by connecting low areas with spoon or W drains (Figure 2). The low areas need to be pegged out in winter. See Farmnote 120/84 ‘Spoon and W drains’.
In areas with poor surface drainage, it is possible to create a slope towards drain channels using ‘beds’ or ‘lands’ formed from soil taken out of the channels.
The channels are aligned in the direction of maximum fall and the beds are formed using a grader, disk plough or mouldboard plough placing soil from the channel into the area between the channels.
The distance between the channels is determined by the width of any machinery to be used. Pastures growing near the drain channels may mature more slowly than those on the bed, causing problems for hay cutting.
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