Grape growers across the Eastern U.S. often run into a puzzling scenario: low potassium levels in their soil tests but high potassium in their plant tissue reports. To make sense of this, Fritz sat down with Dr. Jaclyn Fiola, soil scientist and Assistant Professor at Delaware Valley University, for a conversation about how potassium functions in vineyard soils and what grape growers need to consider when interpreting nutrient data.
Potassium isn’t like nitrogen or phosphorus, which mostly come from organic matter. Instead, potassium is largely released from the minerals in the parent rock beneath the soil. This means the amount of potassium available to grapevines often depends on soil depth, moisture, and the types of minerals present.
Eastern vineyards, especially those in the Piedmont and Blue Ridge regions, are commonly built on potassium-rich rocks like schist, phyllite, and mica. But high potassium in the soil doesn’t always lead to better vine health. In fact, too much potassium can interfere with magnesium and calcium uptake and raise the pH of the grapes — causing problems for winemakers by affecting wine stability and flavor.
Dr. Fiola’s research looked at more accurate ways to test for soil potassium. Traditional soil tests, especially those using dried samples, may either underestimate or overestimate potassium availability. She found that testing soil in a moist state using the Malik 1 extraction method gave a better correlation with potassium levels in grapevine tissues — especially when sampling deeper in the soil profile.
From a plant management perspective, Dr. Fiola recommends focusing on tissue samples over soil tests once vines are established. Whole-leaf samples taken at veraison were found to give the clearest picture of a vine’s potassium status. For younger vines, particularly those on sandy or dry sites, occasional supplementation might be needed, but over-application can cause more harm than good.
Growers are encouraged to evaluate potassium carefully, particularly before applying fertilizers or composts that may contain high levels of it. As Dr. Fiola explains, just because a soil test shows low potassium doesn’t mean it’s not there — it may just be hiding in the minerals below.
For those planning a new vineyard or troubleshooting an existing one, combining deep soil sampling, proper timing of tissue tests, and understanding the geology of the site can help make nutrient management more accurate and avoid unnecessary potassium buildup. If you want to learn more about potassium, check out https://www.vineyardundergroundpodcast.com/vu073.
