Soil Permeability - Page 7
How do I improve soil permeability?
Often it is not evident that a site is waterlogged until water appears on the soil surface. However, by this stage, plant roots may already be damaged and plant growth severely affected. The best way to identify problem areas is to dig holes about 40-60 cm deep in winter. If water flows into them the soil is waterlogged (Figure 5). Digging holes for fence posts can often reveal a waterlogged area.
Photo: Tim Overheu (Department of Agriculture and Food Western Australia)
Figure 5 Free water in a subsoil held above an impermeable soil layer
An excess of water in a soil constrains plant growth - aim to provide soil conditions in the root zone that allow rapid return of oxygen to the soil pore system following wetting by rain and/or irrigation.
Management of waterlogging (since it is usually shallow and localised) can be achieved using surface water management. Although waterlogging is a consideration in designing deep drains, it is not a reason for implementing deep drainage.
Modify soil structure to improve surface condition
Zero tillage and controlled traffic help eliminate plough pans and reduce surface pooling of water, decreasing the potential for denitrification and consequent nitrogen loss. Compacted soil can be loosened using mechanical tillage such as deep ripping to increase porosity (see ‘Soil Compaction’) if required.
Root channels or ‘biopores’ formed by soil fauna (e.g. earthworms, ants) are useful for loosening soil and the movement of water and gases in soil. Zero tillage will preserve natural cracks in soil and encourage earthworm activity
Improve water entry and storage
Intact stubble and root systems increase the rate of water entry into soil. Maximise inputs of OM and reduce tillage to improve soil structure and permeability (see ‘Soil Organic Matter’).
Trials have shown improved moisture storage can increase yield up to 15-25% under zero-tillage compared to cultivation – hence reduce the number of tillage operations, or adopt zero tillage. The implementation of controlled traffic is an essential partner to soil loosening in minimising or repairing compaction otherwise the benefits of repair can be lost quickly.
Induced surface roughness through tillage is a short term approach to improving water intake where there is little ground cover
Raised beds and controlled traffic
Raised beds are being increasingly employed as a management response to waterlogged soils, especially those with compacted subsoils, across southern Australia. The better quality topsoil, loosened and formed into a bed, has improved aeration and is protected from re-compaction through the use of controlled traffic.
NB. Raised beds can shed water and may increase off-site nutrient and pesticide loss.
Manage livestock
Reduce soil compaction by removing stock when soil is most prone to damage (i.e. when wet) to parts of the farm where there is less risk of soil compaction or where it is most easily rectified at a later date.
Increase soil water use
In dryland agriculture, changes in the water balance from activities such as cropping (e.g. lower water use) may increase the drainage of water below the root zone into groundwater, and result in the mobilization of salts, nutrient and pesticides in the soil.
Plant crops and increase cropping frequency to more effectively use excess water, and reduce runoff and evaporation. Alternatively, plant deep rooted or perennial vegetation for greater utilisation of soil water and utilise vegetation strips to improve the ability of soil to take in water and improve water use.
Manage irrigation scheduling
Poor irrigation practices, together with leakage from water distribution channels has resulted in rising water tables created by excessive deep drainage and redistribution of salts from groundwater to the soil surface.
Gypsum and other soil ameliorants
Excessive sodium (>6% of CEC) destroys soil structure and decreases internal soil drainage properties. Applying gypsum replaces sodium (Na) ions with calcium (Ca) ions to improve soil structure in a sodic soil (see ‘Soil Instability’). This works by creating larger pore size to increase the hydraulic conductivity of soil, helping water move both up and down. The primary effect is to improve downwards water movement (drainage).
Manage drainage lines for efficient flow of water
A slope of at least 1:1250 is best for draining irrigation water or rainfall from a paddock depending on soil type and land use (improvements to paddock gradients can be achieved through laser levelling). Engineering solutions such as siphons and drains should be designed to remove run-off as quickly as possible to reduce the period of waterlogging stress, but environmental off-site impacts must also be considered.
Improvement in surface drainage is the simplest and cheapest engineering solution. This involves maintaining existing drains and installing additional drains (e.g. ‘W’ drains, spoon drains) that are adequately sized and positioned correctly (usually placed along fence lines or through depressions). Machinery can be worked across the drains, which helps entry of water flowing down cultivation furrows.
The two main forms of subsurface drainage are tile drains (buried PVC pipe that is both slotted and corrugated, surrounded with a permeable backfill such as gravel or blue-metal; Figure 6) and mole drains (unlined temporary drain formed by pulling a torpedo shaped plug through soil to form a compacted channel).
Figure 6 Tile drain: buried corrugated pipe with a permeable backfill (blue-metal)
In irrigation areas, groundwater pumping is a common form of subsurface drainage. In rainfed high-rainfall areas, the type of drainage depends on soil characteristics, rate of drainage required and topography.
