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Minimizing salt problems

Southwestern growers face formidable challenges in managing sales; limited permeability soils, low-quality irrigation water or both.

Excess soil salinity and sodium can reduce the amount of water available to the trees, preventing from taking up the proper balance of nutrients needed for healthy growth and nut production.

For example, a soil salinity level greater than about 2 dS/m (equivalent to 1,280 ppm) can restrict growth of pecan trees, notes James Walworth, University of Arizona soils specialist. A level of 3.5 dS/m (2,200 ppm) can reduce growth by about 25 percents. Branch die-back can occur at soil salinity levels of 5 dS/m (3,200 ppm) and trees may die when soil salinity measures 6 dS/m or more (3,800 ppm). Typically, symptoms of trees suffering from excessive soil salinity are quite similar to sodium or boron toxicity— leaf burning or marginal necrosis on older or younger leaves.

Salinity levels rise when water and fertilizer containing salts are continually added to the soil. Compacted soil layers, heavy clay soils or sodium problems can also cause salt to build up in the soil.

“Salt and sodium problems are easier to prevent than to correct,” Walworth says. “The key to controlling soil salinity is adequate soil drainage which allows salts to be leached below the root zone. There are no amendments that can directly control soil salinity.”

“The most effective means of controlling soil salinity at tolerable levels is to leach excess water and salts below the root zone,” he says.

That means knowing the salinity of your irrigation water. The higher it is, the more leaching required.

If high sodium levels are restricting soil drainage, Walworth recommends injecting sulfuric acid or gypsum into your irrigation water or adding them directly to calcareous soils. In the case of compacted soils, tillage may be needed to improve drainage. “Often a combination of tillage and amendment application will be most effective for eliminating infiltration problems and allowing salts to leach from the rooting zone,” he says.

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