Soil Permeability - Page 6
How does soil water permeability affect soil health?
Waterlogging occurs when soil becomes saturated (soil pores are full of water) because excess water cannot drain away or the groundwater level rises to near the surface. This may be caused by excessive rainfall, poor irrigation management, seepage, and groundwater recharge resulting in a rising watertable.
The effect of waterlogging in the field is extremely complex, with soil physical and chemical properties interacting with the duration and depth of waterlogging, waterlogging frequency, and the growth stage and nutritional status of plants.
Poorly drained or waterlogged soils lack the interconnected air spaces and large pores that are essential for the movement of water and gases, and the growth of plant roots. Soils with poor internal drainage (low hydraulic conductivity) can feature some or all of the following characteristics.
Increasing salinity
Rising groundwater may contain salts or it can dissolve salt that is stored in the soil. When the groundwater comes within 1 m (for sands) or 2 m (for clay loams) of the ground surface, suction from the evaporating water at the surface draws water to the surface by capillary rise. As the water evaporates, the salts are left behind to create a saline layer. This problem is further exacerbated when poor surface drainage leads to ponding, followed by evaporation.
Decreased soil water storage
Impermeable soil layers affect the amount of water percolating through the soil profile and stop water re-filling the subsoil. There is greater potential for perched water tables to result in waterlogging (typically above an impermeable subsoil), as well as increased surface run-off and subsurface lateral flow.
Excessive deep drainage also contributes to the transport of both urban and agricultural pollutants from soil to waterways, with potentially negative impacts (environmental, social and economic).
Increased incidence of waterlogging
Compacted soils have poor aeration and drainage. Low permeability reduces deep drainage resulting in a greater risk of waterlogging. As a soil becomes waterlogged, soil becomes anaerobic and bacteria convert excess nitrate to nitrous oxide (N2O) – a greenhouse gas (read more on ‘Greenhouse Gases’). Under permanently waterlogged conditions ethylene, hydrogen sulphide and methane emissions may also increase. Soil strength is weakened in waterlogged soils, making the soil more susceptible to compaction from traffic and animals.
Provides a less favourable habitat for soil biota, particularly earthworms
Although soil biota can populate a wide range of environments, the majority function best in an aerobic (aerated), well drained habitat. Waterlogged soils lack oxygen (anaerobic) and limit biological activity and root respiration.
Poor plant growth
The hydraulic conductivity of a soil influences the rate at which stored water moves upwards in response to evaporation and has a major impact on the germination and survival of young crops. In soils with a poor ability to transmit moisture when unsaturated, prepared seed-bed tends to dry out too rapidly.
Plants can turn yellow due to nitrogen deficiency caused by denitrification under waterlogged conditions, and may show stunted or uneven growth - death can also occur. Under anaerobic conditions roots generally cannot respire and grow normally, so root extension and exploration of the soil for water and nutrients is reduced. Manganese toxicity associated with waterlogging is also possible.
The net effects of waterlogging are often reduced plant growth and crop yields (Figure 4) due to root damage/death. Estimates of losses in wheat and barley grain yield are up to 100 kg/ha for every day soil is waterlogged to the surface.
Photo: Greg Hamilton (DAFWA)
Figure 4 Poor crop growth associated with waterlogging on a dispersive soil (grey clay) in Western Australia