Computing Nitrogen Credits: Make The Most From Soil Data

A lot of growers were undoubtedly nodding as they listened to Dani Gelardi introduce the conclusion of the Washington State Department of Agriculture’s “Making Soil Data Actionable” webinar series. Gelardi, the Senior Soil Scientist and Climate Coordinator for WSDA, hit the nail on the head.

“We love soil data, and we also know that it can be confusing,” she said. “More soil tests and data are available to farmers than ever, and while this can be empowering, it can be difficult to translate into action.”

Galardi introduced sometime translator and fulltime Soil Scientist Rich Koenig, an Extension Specialist in the Department of Crop and Soil Sciences, Washington State University. Koenig outlined the importance of Nitrogen (N) credits in the fertilizer budgeting process.

The chief economic advantage is pretty obvious. If growers can determine, through careful soil sampling, that there is a fair amount of N in their ground already, they don’t need to buy as much fertilizer. Foregoing such tests can be pretty risky, however, especially for growers in certain parts of the world where the government has stepped in to restrict N fertilizer use because of concerns about nitrate levels in groundwater.

In the U.S., growers in California or along the southern Mississippi River can actually face fines for contributing to high N levels. But if they test their soils, they can avoid such problems and become better stewards of the land resource. Growers can use soil tests as a diagnostic tool or to identify trends through time. To obtain meaningful test results, you must sample soil correctly, at the same time each year, and you must maintain records.

Because soils vary so much, and crop needs are so different, Koenig says growers would be wise to avail themselves of the many nutrient management guides available online, which generally either address a certain region or crop. There’s some form of nitrogen crediting process embedded within those nutrient management guides, which cover crops from cranberries to Christmas trees.

Koenig says one thing to be aware of, when measuring soil residual nitrogen, is that it’s a point sample, both in terms of time and space, and therefore has some limitations. The effects will, of course, depend on where they are taken. For instance, the spatial variability would not ordinarily be as great in flat fields as on steep hillsides.

It’s also critical for growers to keep in mind that soil tests are used to measure soil nutrients that are expected to become plant-available, not the total amounts of nutrients in soil. Measurements of total nutrient content are not useful indicators of sufficiency for plant growth, because only a small portion of the nutrients in soil are plant-available. The plant-available forms of nitrogen are ammonium-N (NH4 -N) and nitrate-N (NO3 -N).

Though soil tests might show total N, it includes N in all organic and inorganic forms. Total N does not indicate plant available N and is not the sum of NH4 -N + NO3 -N. Total N is not used for fertilizer recommendations.

For more, click here to continue reading the full article as part of our special report on Soil Health.

In addition, check out the previous reports in Meister’s Global Insight Series covering a range of topics from Irrigation Innovations to Agricultural Technology.