What is the best way for Washington State grape growers to analyze nitrogen levels in their vineyard?
Washington State University soil scientist Dr. Joan Davenport believes that growers will get more out of a spring soil test than spring tissue analysis when trying to measure the nitrogen status of their vineyards.
"I definitely walk a very different path about leaf tissue testing and sampling than the guidelines that come from California," she said, noting that California vineyardists typically take petiole samples at bloom.
"In Washington, we’d be better off taking a spring soil test to find out what we have available and what we may need to do, totally ignoring the concept of bloom tissue samples and waiting until veraison to take a leaf sample—whether it’s a blade or whole leaf—and using that in conjunction with the soil test in the spring to make the fertilizer plans," she said.
Davenport takes that position based on extensive leaf and petiole sampling done at bloom and veraison from Pacific Northwest vineyards under irrigated systems. The trial found a wide disparity in nitrogen levels in the petiole samples taken at bloom, with nitrogen ranging from 5 to 1,500 ppm.
"And I can tell you that the vineyard with the 5 ppm level was not nitrogen deficit," she said. "So I’m just a little skeptical about of the value of bloom-time samples because sometimes our soils are so dry at that time that not much is moving around."
By veraison, however, the nitrogen that’s available will be in the leaves, she said, adding that she prefers taking whole leaf or leaf blade samples than petiole samples.
WSU is in the process of developing nutrient management guidelines and target levels for soil and tissue samples. Until the guidelines are published, she suggests that growers should be very concerned if leaf tissue levels taken at veraison are below 2 ppm or above 3 ppm.