Protecting groundwater quality from nitrate contamination when fertilizing walnut trees requires being on top of your game in managing not only your nitrogen applications but also your irrigation system.

The goal is to allocate your total N budget through the season so that you feed the trees only the amount of N they need and only when they need it, explains Allan Fulton, University of California Cooperative Extension irrigation and water resources advisor for Tehama County. 

Previous research suggests that about 20 pounds of N per 1,000 pounds of dry in-shell walnuts may be exported from an orchard at harvest, he notes. Because these studies of N use by walnut were conducted more than two decades ago, researchers are now doubling checking this rule of thumb for modern walnut varieties.

 

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Apply too much N or put it on at the wrong time and unused portions of the nutrient can leach out of the root zone in the form of nitrate, threatening groundwater quality.

The depth of the root zone depends on the age of the walnut tree and soil characteristics. In uniform, loose soils, mature walnut trees may sink their roots as deep 8 to 10 feet into the ground. In layered, compacted soils, these roots may be able to push no deeper than about 3 to 5 feet, Fulton reports.

“The younger, finer roots that take up nutrients most efficiently are usually found in top 3 feet or so of the soil profile,” he says. “The deeper roots are not as dense and are more involved with taking up water and anchoring the tree than taking up nutrients.”

 

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Even if your timing and rate of nitrogen application is optimum, an inefficient irrigation system can still put ground water quality and your profits at risk. Consider the case where faulty emitters are applying more water in some areas of your orchard than your trees need, causing the excess N to leach out.

“Then, you’ve set yourself up for a double whammy,” Fulton says. “If you’ve limited nitrogen applications to no more than the crop needs, but you lose some to leaching, you can add nitrates to groundwater. Plus, the trees won’t get all the nitrogen they need for proper growth and nut production.”

Timing of your N applications should coincide with the trees’ nutrient needs, he notes. University of California research shows that prior to bud break, walnut trees draw N from reserves stored in the tree from the previous season and not from fertilizer applied to the soil in the current season. So, rather than risk the loss of any N applied during dormancy or near bud break, Fulton recommends waiting to make the first N application until after bloom is complete.