Be savvy about salt

Brak management in crop production. By Grethe Bestbier.

In South Africa, the term ‘brackish’ refers to any salt-related issues. While salts are a natural part of any farmer’s soil, it is important to manage it correctly to reduce risks of crop damage. Mico Stander and Karen van der Westhuizen, soil scientists from Agrimotion Consulting, explained the key concepts regarding brak — salinity — management and its importance in crop production.

Calcium, magnesium, and sodium — according to Stander, these are the elements you need to consider when thinking about brackish soils. These primary cations are found in soil and water, and are usually accompanied by chlorine, sulphate, and bicarbonate anions.

Salinity refers to the concentration of all salts. If the salt concentration becomes high enough it will affect plant function and crop production. Sodicity, on the other hand, refers to the composition of the salts — the ratio of sodium to calcium and magnesium — in the soil. “It’s important to note that you can have a medium that is saline without being sodic and vice versa,” said Stander.

Salinity is usually caused by one or more of the following: inherent soil mineralogy, poor water quality, poor drainage, shortage of water for leaching, and over-fertilisation.

According to Stander, we can distinguish between direct and indirect impacts of brackish conditions on crop production. The first direct impact is reduced water potential in the soil, making it harder for a plant in saline soil to take up water. The second involves the toxicity of prevalent elements, like sodium or chlorine, which the plant struggles to eliminate. The final direct impact is deficiency of certain elements due to competing root uptake.

Indirect effects include the degradation of soil structure. This can lead to poor water infiltration causing run-off and erosion, poor gaseous exchange, and poor root development.

Managing brackish soils

“The easiest way to deal with brak conditions is to do a soil survey, to analyse your soil and your water sources, so you can avoid moving to these areas in the first place,” said Van der Westhuizen. For situations where salinisation cannot be escaped, she discussed a number of mitigation strategies.

• Ridging – In brackish conditions, ridging is a very useful tool to improve surface drainage and to move the effective rooting depth higher, out of potentially waterlogged and salt-affected sub-soils.
• Rootstock choice – For stone fruit in brackish conditions, Van der Westhuizen recommends choosing either a GF677 or a Viking rootstock, while for pome fruit a more vigorous rootstock should be considered.
• Irrigation system – An irrigation method with a low delivery rate will promote a better infiltration depth and less run-off. This is where drip irrigation should be considered.
• Effective drainage – When leaching, you’re putting water into the soil to dissolve, dilute and transport excess salts, but you now need to remove the saline water. It is important to consider drain spacing, depths, maintenance, and safe disposal of water.
• Gypsum application – When adding gypsum, you’re adding calcium to displace the sodium and magnesium from the soil. This will increase the soil electrical conductivity. If the soil is saline and sodic and has a high pH, then gypsum should not be added. Rather use sulphur-based products.
• Fertilisation – Avoid fertilisers that contain chlorine. Rather use fertilisers that are sulphur- and sulphate-based.
• Mulching – Organic matter in the form of mulch decreases surface evaporation thereby reducing salt movement to the surface. Mulch also helps decrease run-off and improves soil structure and water infiltration.
• Leaching and flushing – Salt accumulation at the edge of the root zone is a natural part of fertigation and drip irrigation. A longer irrigation flushes this accumulation of salts away from the root zone. Leaching removes salts from the profile so it can be carried away by drains.
• Consistent monitoring – Hand-held meters can be used to monitor soil electrical conductivity every ten days. Laboratory analyses of your soil should be done at least every two years to determine the percentages of different cations. Also analyse your water sources two to four times a year. Lastly, soil moisture probes should be used to monitor soil moisture, as well as leaching depths.
• Running at a higher easily-available water percentage – Salts in the soil solution reduce the potential for water uptake by roots. Maintaining a slightly wetter profile, but not saturated, will improve water uptake.
• Applying irrigation after rainfall – Apply irrigation either during or just after rainfall, to avoid salts being pushed back into the root zone by the rain.

Image: Leaf scorch is one of the signs of brackish soils.

Supplied by Mico Stander: Agrimotion.