Soil Sodium: the salt of the earth

Most farmers disregard the sodium level in their soil, unless there is too much. Sodium problems can, however, impact on production. International soil expert Neal Kinsey unpacks the importance of sodium and ways to address imbalances. Glenneis Erasmus reports.
Issue date : 22 August 2008

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Excessive sodium in the soil is usually the consequence of irrigating with water that has a high sodium chloride content. But sodium is fortunately one of the cations that can be removed most easily. Only anions such as nitrates and chlorates move through the soil more rapidly than sodium ions do, provided the soil texture is sufficiently loose for unimpeded water movement.

One of the first steps in addressing excessive sodium would be to ensure that calcium represents between 60% and 70% of the total capacity of the soil and magnesium between 10% and 20%. Together, the two should not exceed 80% of the exchange capacity. nce calcium and magnesium are balanced, the soil will open up so sodium can move freely through it.

“Any soil that can maintain calcium at 60% and higher and provided that water can move freely through it, can tolerate far more sodium without causing a problem to the crop,” soil expert Neal Kinsey explains. S ulphates and nitrates can be used to strip the soil of excess sodium. “But this approach should only be used after other cations have been restored to their desired percentages, as it often automatically corrects other cations to their desired percentages,” Neal says. ”Once these have been restored, sulphates and nitrates will strip an excess of any cation from the soil.”

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Analyse your water
Neal advises that if irrigation water is the main reason for a sodium build-up, then farmers should have the water analysed to establish exactly how much sodium and other minerals it contains. Those applied by irrigation can be calculated and then deducted from what would have been applied in the fertiliser programme. “But at excessive levels of certain nutrients, the fertiliser programme must be adapted to offset or neutralise the impact of nutrients supplied via irrigation. The key is to know what you are applying to the soil,” Neal stresses. Next, a farmer must establish the cost of offsetting or correcting the problem.

“On some farms and with some crops, it may be economically feasible to offset the impact of water with a high sodium concentration by continuously correcting the soil balance, or by providing sufficient nutrients via a foliar feed to the crop,” he recommends. “However this may not be economically viable when the water is extremely alkaline, so in this case rather consider special water treatment, or sourcing water from elsewhere.” “Alternatively a farmer can seek an alternative source of income that doesn’t entail crop or animal production. If neither of these is viable, then sell the farm and buy land with better quality water. It highlights the importance of getting your soil and water tested before buying a farm.”

Juggling cations
Neal explains that in soil with excessive magnesium and sulphur and deficient in calcium, it’s difficult to treat excessive sodium that isn’t caused by irrigation. The magnesium could make the soil so dense that water and plant roots have difficulty penetrating it. He says this can be corrected, but it will take longer and require a greater investment in time and money than with other soil types. The calcium level should be corrected first to open up the soil.

Don’t apply more than 4t/ha of lime per year, as calcium tends to decrease the exchange capacity. Therefore farmers should test the soil a year later to determine its percentage of calcium and then calculate the amount that still needs to be released from the previous application. Neal advises farmers to add only half the amount of calcium needed, according to the soil analysis and the amount of calcium previously applied. Test the soil again a year later before adding the balance.

Sodium and potassium
Sodium and potassium levels should be monitored together as they directly impact on one another. The Albrecht system maintains that sodium should represent between 0,5% and 3%, and potassium between 2% and 7,5% of the exchange capacity of the soil. Together the two elements should not exceed 10%, as this will block the plants’ uptake of manganese and boron. The sodium level should not exceed the potassium level, as this will make it difficult for the plant to differentiate between the two and it will absorb large amounts of sodium instead.

In warm and humid conditions, the sodium will pull water into the cells, damaging the cell walls and causing them to burst, ultimately killing the plant. This damage is often confused with nematode damage. Neal says the problem is easily rectified by  correcting the potassium level or with a potassium foliar feed (See this series’ article on potassium in Farmer’s Weekly, 15 August 2008). |fw