“If you put on nitrogen before that and then it rains, there’s a good chance of nitrogen leaching out of the root zone,” he says.

Fulton recommends making the initial N application within the first 30 days or so of bloom. That way it’s available from early May through June, when N uptake by walnut trees is the most intensive of the season.

More frequent applications of N at lower rates throughout the season result in more efficient use of N than one or two large applications, he reports. He suggests following the post-flowering treatment with at least three more applications treatments on about a 30-day interval.

 

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When applying N through the irrigation system, uniform application of water and nutrients is critical for making the most efficient use of N, Fulton adds “It’s better to put on N in the last half or even last quarter of the irrigation set,” he says. “You want to run enough water through the system to distribute all of the N but not so much that you begin leaching it out of the root zone.”

For added assurance in monitoring how well your N application rates are in line with tree uptake, Fulton advises confirming that your N fertilizer strategy is on track with an annual summer leaf tissue test.  In other nut crops, such as almonds, spring leaf tissue sampling methods have been developed to give growers earlier insight into their N management strategies. “Research is underway to develop an earlier leaf tissue sampling method for walnuts,” he says. “It should be available as a management tool to growers in a few years.”

Fulton also suggests soil tests to measure N levels in the soil.  He recommends sampling at incremental depths to evaluate the N distribution in the root zone. Collecting composite soil samples will give you more representative results and repeated sampling every few years at the same time each season will be easier to interpret and more informative.  “If your N budget results in adequate N tissue levels and your soil tests show N is not excessive, then you can be confident your N management is on track” he says.

 

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