Profit suckers - understanding salinity sodicity and deep drainage
Author: Mark Crawford, Department of Natural Resources and Mines, QLD Government | Date: 05 Mar 2015
Take home message
Develop an integrated management strategy by having a thorough understanding of landscape systems, soil profile properties, plant salt tolerance/capabilities and how current/past farming practices contribute to the salinity and sodicity of your soils.
The presence of salinity and sodicity within Australian agricultural areas is not a new phenomenon. The Australian landscape has accumulated salts from weathering of rocks and deposition by rainfall. The high evaporation, low rainfall and runoff of most inland landscapes means salts (sodium, chloride, sulphate, carbonates) tend to accumulate in our soils – particularly clay soils – our most common type of cropping soil. Rengasamy (2010) estimated that 5.7 million hectares of Australia’s agricultural and pastoral zones are currently affected by groundwater associated salinity and rising watertables and that transient salinity (not influenced by groundwater processes) exists in 2.5 billion hectares of the agricultural area, being 33% of the total. It is therefore essential that the right approach is undertaken when management options are being considered.
Understanding both the processes involved in salinity occurrence and how sodicity interacts or contributes is the initial step that must be taken. Acknowledging how current and past agricultural practices contribute to salinity is the key to successful management. This is important because high concentrations of chloride below 0.9m (0.9–1.5 m) restricts the water extracted by crops through osmotic effects and can be associated with reduced grain yields (Dang et al 2011). The investigation process that identifies the presence of salinity in a landscape and determining the extent of current salting can be aided by tools such as electromagnetic induction (EM38), yield mapping and good old fashioned soil and landscape knowledge. Electromagnetic induction (EM) such as EM38 or EM31 provides a valuable resource when initially determining the extent of the problem area. It provides measurements of apparent soil electrical conductivity (EC), which can be related to soil properties such as salt content, soil water, and texture. The use of EM can only provide a surrogate measure of salinity and is not a direct measure. EM data must be supported by soil sampling to ground truth and make sense of what is otherwise just another pretty map. In order to develop an integrated management strategy it is essential to have a thorough understanding of landscape systems, soil profile properties and plant salt tolerance and capabilities.
Soils with an exchangeable sodium percentage (ESP) ≥6 are classified as sodic (Isbell 2002). Poor drainage, surface crusting, hardsetting and poor trafficability or workability are common when the soil has a large proportion of sodium ions (Na+), leading to reduced crop yield. Crop growth is affected by salinity in two ways: firstly the osmotic potential effect and secondly specific ion toxicity. Salts lowers the osmotic potential (ie makes it more negative) or increases osmotic pressure leading to yield losses as plants cannot extract water from soils when soil solution has lower osmotic potential than the plant cell. The impact on grain productivity of rising electrical conductivity (EC) and exchangeable sodium percentage (ESP) values at different depths is shown in Figure 1, which demonstrates that identifying the complete picture is essential to applying the management option.
Figure 1. Impacts of salinity and sodicity on productivity (Source: Dalal et al. 2002)
A number of options exist when managing salt affected areas that are caused by catchment scale interactions. They can be utilised on an individual basis or in combination and include: Managing the land use in its current state (fence and forget), reducing recharge by retaining native or perennial vegetation, intercepting water in the transmission area and increasing water use in the discharge area by using salt tolerant plant species. Paddock management includes using salt tolerant plants and utilising the available water. The Importance of these measures also depends on the mechanism(s) causing salinity expressions (e.g. Landform features).
The goal of these management options is to re-establish a hydrological relationship closer to the pre-development one and allow the continuation of agricultural production. Crop species that have a high salt tolerance such as: barley (Hordeum vulgare), wheat (Triticum aestivum) and cotton (Gossypium hirsutum) can be grown if maintaining current management practices. Allowing a portion of water to reach the subsoil and below will accommodate salts being leached. However, salinity is largely self-regulating in terms of it is a mass balance relationship (input = output). If salinity increases too much, the crop will struggle to use the soil water, which leads to increased deep drainage and hence reduced salinity. Rarely do opportunities arise to deliberately flush the soil profile in dryland cropping systems. Most ‘flushing’ is incidental/natural and associated with major rainfall events. The other factor to consider is that soils can have a number of saline-sodic combinations.
Sodic soils are prone to poor soil structure, particularly if the natural equilibrium between salinity and sodicity are out of balance. High salinity helps to counteract the effects of sodicity, but as described above, can cause yield issues. Both acidic–sodic and alkaline–sodic soils occur within the northern grains zone, often within the one soil profile. Sodic soils often disperse more after mechanical disturbance (e.g. compaction) and erosion. Gypsum application to these soils improves the soil structure facilitating leaching of salts, even under dry land conditions (Rengasamy 2010). Correcting cation imbalances requires providing a source of the ‘good’ cations, Ca2+ and/or Mg2+, which might come from gypsum, lime, dolomite applications. The choice will depend on considerations such as cost, the existing cation balance in the soil and the speed at which a change is required. The application of gypsum will generally give quicker results as it has a relatively high solubility, whereas agricultural lime has a very low solubility and therefore takes longer to observe results. It is also dependent on the pH of the soil.
The use of decision process models such as Gypsy© can be used as a guide when deciding on the cost of gypsum applications. Gypsy was originally designed by CSIRO to help Australian sugarcane growers with the decision process of what rates of gypsum to apply to sodic soils but can be used as a guide to attain ball park costs of remediation. Like all models though, it is only an estimate and the best measure can be obtained by small in-field trials of different application rates.
Dalal R.C, Blasi M, So H.B (2002) High sodium levels in subsoil limits yields and water use in marginal cropping areas. (Grains Research & Development Corporation project no. DNR 6, final report).
Dang Y. P., Dalal R. C., Pringle M. J., Biggs A. J. W., Darr S., Sauer B., Moss J., Payne J., and Orange D (2011) Electromagnetic induction sensing of soil identifies constraints to the crop yields of north-eastern Australia. Soil Research, 49, 559–571.
Isbell R, (2002) The Australian soil classification: CSIRO Publishing, Collingwood, Vic, Australia.
Rengasamy P, (2010) Soil processes affecting crop production in salt-affected soils. Functional Plant Biology, 37, 613–620.
Andrew Biggs and Peter Binns for editorial advice
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