Overcoming transient salinity and sodicity

Figure 1. Different forms of salinity in dryland regions

By Pichu Rengasamy

A study aimed at understanding the processes involved in transient salinity - which affects almost 60 percent of the nation"s cropping soils - and sodicity in dryland cropping regions is under way in South Australia.

The project aims to find management options for growers to tackle these subsoil constraints.

The main outcome will be an increase in grain productivity in southern Australian soils with problems of transient salinity (the accumulation of salts in subsoils not associated with groundwater), sodicity and associated subsoil constraints.This will be achieved by identifying better suited genotypes and agronomic practices. Through education, consulting and extension, the project aims to accelerate grower adoption.

Intended project outputs are:

Researchers are working with growers through the Eyre Peninsula Farming Systems project, the Yorke Peninsula Alkaline Soils Group, and the Mallee Farming Systems project.

The term "transient salinity" refers to the accumulation of salts in subsoils without the influence of groundwater processes. It occurs extensively in sodic soils where the watertable is deep, and affects more than 60 percent of the 20 million hectares of cropping soils in Australia.

This form of salinity is different from "dryland salinity" which is a result of shallow watertable. It is important to diagnose the different forms of salinity in the landscape (Figure 1) because each form requires specific and different management strategies.

Figure 1. Different forms of salinity in dryland regions

Water uptake by plants is affected by the osmotic effect due to transient salinity; it is increasingly difficult for crops to take up soil water as the salt concentration of that water rises. This project evaluated the relationship between laboratory-measured soil salinity and the changes in osmotic effect with water content in different soil types of varying texture.

For example, Figure 2 shows that in a loamy soil, when there is no salt, plants are able to take up water until the soil dries to five percent water content (dry soil). Whereas, when the soil salinity measured in the laboratory (EC1: 5) is 0.64 dS/m (deciSiemens per metre), they can get water only up to 15 percent water content. (The total soluble salt content is estimated from the electrical conductivity (EC) of the soil solution.)

Figure 2. Osmotic effect changes with water content in a loamy soil

Figure 2. Osmotic effect changes with water content in a loamy soil

When the salinity increases to 1 dS/m, plants cease to take up water at 18 percent water content (this equates to wet soil).

In a site at Redhill, South Australia, where subsoils had high salt levels, bread wheat (particularly Kirchauff) and lentils were grown successfully but durum varieties failed (this work was done in association with Dr Rathjen and David Cooper).

While reduction in the yield of durums due to salinity levels were moderate, high pH in combination with salinity reduced the yield dramatically. Even the boron-tolerant varieties were affected by high pH. Locally developed wheat varieties (Spear, Frame, Yitpi, Stylet, Pugsley, Aroona, BTSchomburgk, Barunga, Kirchauff, Worrakatta and WI99072) were only slightly affected by high pH, whereas the interstate bread varieties Condor, Meering and Janz suffered significant yield penalty due to high pH.

Solution culture experiments confirmed the toxicity of aluminium to wheat plants when grown in high pH (above 9) solutions.

While aluminium toxicity in acid soils is well known, the toxic effect of aluminium in some alkaline soils of South Australia was a new finding.

Pot experiments were conducted in high pH soils using different soil types and wheat varieties.

Differences in aluminium toxicity effects were found between wheat varieties and between soil types (Figure 3). Analysis of soil solutions indicated an interaction between aluminate ions and soluble carbonate ions.

Figure 3. Effect of aluminium(Al) in high pH soil from Jamestown (Al-No indicates absence of aluminate ion and pH 8.6. Al-yes indicates the presence of aluminate ion and pH 9.3)

Figure 3. Effect of aluminium(Al) in high pH soil from Jamestown (Al-No indicates absence of aluminate ion and pH 8.6. Al-yes indicates the presence of aluminate ion and pH 9.3)

When bicarbonate ions were predominant, aluminium effects were larger than when carbonate ions predominated.

We are continuing to experiment to understand the complex interactions of aluminium with other ions present in soil solution.

The use of gypsum to assist the leaching of salts to below the root zone was examined in pot experiments. Researchers investigated farmers" fields in Eyre Peninsula where gypsum has been applied in the past to compare with the fields where gypsum has never been used.

Generally, salt accumulation within the crop root zone in sodic soils was negligible where gypsum has been applied. Without such treatment, the sodic subsoil has low permeability and thus reduces the leaching fraction (the proportion of rainfall that moves down through the soil profile) to a low level; this is why salts accumulate within the root zone.

Researchers collected soil samples in October 2003 near Cleve, where in replicated trials the grower had applied 5, 10 and 15 tonnes per hectare of gypsum in different plots five years ago and the current crop was canola. The results of soil analysis are shown in Table 1.

Table 1. Salinity levels in soil layers expressed as EC1:5 (dS/m) of soil due to sodium chlorine

These results show that even with 2.5t/ha of gypsum, salts are being leached in an area where the average rainfall is around 350 millimetres/year; with higher rainfall, the effect could be quicker and/or of greater effect. Further analysis showed that application of gypsum at the rate of 10 and 15t/ha reduced salinity levels up to 60 centimetres depth.

Average exchangeable sodium percentage in soil layers from 0cm to 60cm was reduced from 14 (highly sodic) to 5 (non-sodic).

This shows that even in a low-rainfall region, gypsum can be effective in improving the movement of salts to below the root zone; growers also need to assess the economics of this amelioration.

The research team has participated in several growers" meetings in Eyre Peninsula, Yorke Peninsula and the Mallee region to discuss subsoil problems, with special emphasis on transient salinity and sodicity.

We hope to soon deliver a simple diagnostic tool that can be used by growers for on-farm testing of soils, and a training program to identify salinity, alkalinity and sodicity in subsoils.

For more information:
Pichu Rengasamy, School of Earth and Environmental Science, University of Adelaide
08 8303 7418, pichu.rengasamy@adelaide.edu.au

GRDC Research Code: UA 00023