salinity

UPFRONT salinity

Pomewest recently completed a root cause analysis to investigate isolated tree decline in a Donnybrook orchard as part of an APAL Future Orchards® Trial Incubator project.

Words Susie Murphy White, Project Manager, Pomewest and Jen Riseley, Project Officer, Pomewest

THE analysis identified salinity in irrigation water as the primary cause, with soil sodicity and aluminium toxicity emerging as secondary complications. These issues developed gradually over several seasons, with rainfall temporarily masking the effects of saline irrigation. As a result, remediation will require multiple years to restore soil conditions and orchard productivity.

Tree distress was seen in the Pink Lady® block during summer and autumn of 2024, following a dry winter and hot preceding summer.

Despite adequate irrigation, the grower reported several concerning symptoms that pointed to underlying soil and water conditions:

• Accelerated starch maturity in fruit, before colour development

• Burning of leaves, appearance of water stress despite ample irrigation (image right)

• Waterlogging even in uphill areas of the block during rainfall and irrigation

• Lack of water infiltration through soil profile, despite recent irrigation

However, salinity is rarely a standalone issue. To avoid premature conclusions, a structured approach was taken to define the problem clearly and guide effective remediation.

The process included:

• Soil sampling at four depths across upper and lower slopes.

• Water testing for salinity indicators like Sodium Adsorption Ratio (SAR) and chloride levels.

• Leaf and fruit analysis to assess nutrient uptake and maturity.

• Calculations to reveal hidden stressors, including Exchangeable Sodium Percentage (ESP) and root zone salinity (ECe).

• Visual soil profile assessments to observe root development and structural issues.

Findings confirmed that saline irrigation water, subsoil sodicity, and poor drainage were driving tree stress. The perched water table and salt build-up in lower slopes explained the uneven maturity and advanced aging.

Next steps include modifying irrigation practices, applying gypsum and lime, improving drainage, and implementing a long-term soil health plan.

This trial highlights the value of a multi-layered diagnostic approach, empowering growers to uncover hidden issues and take informed action.

Investigation

Soil testing

Soil tests were taken in January and submitted to CSBP laboratory, chloride testing was requested additionally. Samples were taken at 10cm, 30cm, 60cm and 90cm.

Key information to come from soil tests:

• pH (CaCl₂)

• Chloride concentration

• Exchangeable sodium (meq/100g)

• Cation Exchange Capacity (CEC)

• Exchangeable Sodium Percentage (ESP) — needs to be calculated

• Electrical Conductivity in soil (ECe) — needs to be calculated

Exchangeable sodium in soil is recommended below 0.5meq/100g in apples. No recommendation could be found for soil chloride.

ECe calculates soil salinity corrected for soil type/ texture. To calculate ECe a conversion factor is applied by soil type; sandy soil is a factor of 10, loam is 8 and clay is 6.

ECe = EC × Conversion Factor

Target ECe is below 1dS/m, many 10cm samples were close to or exceeded 1, variation was seen at different depths. This indicates plants are experiencing salinity in the rootzone when considering the clay/loam soil onsite. As soil tests were taken in January, this was a result of irrigation and evaporation rather than rainfall.

ESP indicates sodicity of soil, showing the proportion of sodium ions to total cation exchange capacity. The calculation to find ESP below:

ESP (%) = (Exchangeable Sodium / CEC) × 100

ESP was within range for many samples at 10cm depth but exceeded 6 at depths of 30cm and some exceeded 15 (highly sodic) at depths of 60cm. Without deeper soil testing, this constraint could easily have been missed (see Table 1).

Levels for pH (CaCl2 ) were acceptable at the surface but became acidic at depth, pH below 4.8 risks aluminium toxicity though this is not a universal threshold.

What is a sodic soil?

Soils become sodic when charged calcium ions are replaced with sodium ions. Calcium ions are important for soil structure in heavy and clay soils to keep soil particles apart and allow water and air infiltration.

Sodium ions carry less charge and are weaker at keeping soil particles apart, too much sodium can eventually lead to soil aggregate breakdown and a collapse in soil structure. Collapsed soil structure can be seen in poor drainage, increased waterlogging, hard setting compacted soils in summer, difficulty for plant roots to grow to depth.

Water testing

Sodium and chloride results were above guidelines, apples are sensitive to sodium levels above 114 mg/L and chloride levels above 178 mg/L when using sprinklers.

Electrical conductivity is recommended below 0.7 dS/m for apples and many other deciduous crops before yield is impacted.

Sodium adsorption ratio (SAR) is a calculation of sodium ions in relation to calcium and magnesium. Search an online calculator or have your agronomist run the calculation. The SAR measures the risk of sodic soils developing as a result of irrigation water. The trial site had an SAR of 5.05 — medium risk.

Treatment

Various treatments are recommended, the most immediate for this article is application of gypsum (calcium sulphate) to restore calcium in the soil profile. Gypsum also bonds with sodium to form soluble sodium sulphate that can be flushed from the soil during periods of high rainfall, leaving calcium behind. There are limitations to how much lime or gypsum can be added at once with trees in the ground, so a regular plan is required for farm management.

Acknowledgment

With thanks to Rachel Lancaster of Environmental and Agricultural Testing Services, Bunbury.

More information

Susie Murphy White, susan.murphy-white@dpird.wa.gov.au Jen Riseley, jen.riseley@dpird.wa.gov.au