Sodic soils knowledge gaps
Professor Neal Menzies identified several knowledge gaps that remain in our understanding of sodic soils:
- the ability to predict which soils will show an economic benefit from the application of gypsum (including the rate and frequency of application);
- strategies for the amelioration of sodic subsoils, and the ability to predict when subsoil amelioration will be economically attractive;
- water and nutrient management approaches for sodic soils; and
- alternative amelioration strategies (for example, organic matter management).
The adverse effects of sodicity on plant growth are reasonably well understood; however, differences in soils and plant characteristics mean that this understanding cannot be converted into a single set of reliable rules
Australia has 340 million hectares of sodic land. The rest of the world – combined – has 200 million hectares.
Addressing this stark statistic, Professor Neal Menzies, presenting at the GRDC Update in Coonabarabran, said growers and agronomists needed to complement their knowledge of the underlying soil system with careful observation to craft a solution appropriate to each situation. He said this needed to be done while also being aware of knowledge gaps in this area.
Professor Neal Menzies from the University of Queensland’s School of Agriculture and Food Sciences advises growers and agronomists dealing with sodic soils to complement their knowledge of the underlying bio-physical soil system with careful observation to craft an appropriate solution to their situation.
PHOTO: Sarah Clarry
Simply stated, sodicity is the presence of too much sodium (Na) in the soil.
Unfortunately, knowing how much is too much is not easily determined, Professor Menzies said.
The difficulty is partially due to the usual differences that occur between soils – clay content, organic matter, mineralogy – and partially because of the range of effects that sodium has on soils and on plant growth.
The most common effect is decreased structural stability and dispersive soils, which can lead to surface crusting, reduced water penetrability and water availability, and increased runoff. The risks of erosion and poor plant establishment are increased.
Sodic soils are also difficult to cultivate and have poor load-bearing characteristics.
However, sodicity is a problem that affects only the clay fraction of the soil.
In a sandy soil with little clay, sodicity will not result in adverse physical conditions – although there may still be adverse chemical effects.
The most common ameliorant applied to sodic soils to correct soil structural problems is gypsum, which promotes flocculation.
This increases the ionic strength of the soil solution by supplying calcium ions to displace sodium ions from cation exchange sites.
The first of these effects can be achieved by relatively low rates of gypsum; however, the effect is short-lived.
Gypsum application can be most effective as a means of improving soils’ surface conditions at sowing, providing better soil tilth and reducing crusting.
However, the rates of gypsum required to displace sodium with calcium to depth in the soil to improve subsoil conditions can be substantial and economically prohibitive.
Even if it was economical, Professor Menzies says, the time it would take for the gypsum-derived calcium to move through to the subsoil is considerable. In dryland agriculture only limited rainfall infiltrates to depth in the soil, so it can take decades for enough gypsum to leach into the subsoil to improve structure.
Generally, gypsum is applied at much lower rates than are required to displace all of the sodium.
The expectation is that these smaller additions will help to ameliorate the surface soil, increasing infiltration and encouraging more uniform crop establishment.
Repeat applications may be needed to sustain the soil surface improvement and these repeat applications will gradually have a positive impact on subsoil sodicity.
Such smaller applications can also be more economic.
In the GRDC-funded Subsoil Constraints project, one-time surface-applied gypsum at 2.5 tonnes per hectare increased cumulative gross margins by $207/ha over four crops, reduced 115t of sodium chloride from the rooting depth and increased plant-available water capacity by 15 millimetres.
Unfortunately, gypsum application is not always profitable and more effective prediction of gypsum response is needed.
Lime application to acid soils will address both the acidity and the sodicity, but will have little or no effect on neutral or alkaline soils.
Some of the adverse soil structural aspects of sodicity may be addressed by increasing organic matter in the soil. Organic matter acts to bind soil aggregates, sustaining soil structure.
And while there are many acknowledged benefits to increasing organic matter, it is
not easy to achieve.
Effects of sodium
As the extent of sodium saturation of the cation exchange capacity increases, important plant nutrients such as calcium, magnesium and potassium are diminished.
The most important sodicity-induced nutrition problem is calcium deficiency, where high sodium can interfere with plant uptake of calcium.
Calcium deficiency has a direct impact on root growth, restricting the root system’s capacity to access nutrients.
A crop growing in a soil where sodicity-induced calcium deficiency has restricted root proliferation will be more susceptible to drought and less able to obtain nutrients at depth, rather than showing actual calcium-deficiency symptoms (see photos).
Cowpea root growth (above) where adequately supplied with calcium (Ca) and (above right) plants grown at the same Ca concentration but with sufficient magnesium (Mg) to induce Ca deficiency.
Ca-deficient plants cannot produce healthy root hairs, which profoundly affects the plant’s capacity to capture nutrients such as phosphorus (P).
All of the physical and chemical effects of sodicity, such as alkalinity, can occur simultaneously in a sodic soil.
Furthermore, the observable effects may be similar, so it is difficult to isolate the cause.
For example, poor soil structure will result in a soil susceptible to waterlogging, with roots killed by low oxygen availability, but these damaged roots would not be readily distinguished from roots damaged by calcium deficiency or alkalinity.
That said, it is not always necessary to know the precise nature of the problem, as the same amelioration strategy – the application of gypsum – will address most of the limitations.
Nevertheless, some knowledge of the specific problems faced will guide the implementation of a remediation strategy.
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