Global: Potatoes Need Boron Only Below 0.5 ppm Soil Test, and It Should Go on the Soil, Not the Leaves
University of Minnesota guidance advises boron for potatoes only on soils testing 0.4 ppm or less, applied to the soil. Florida advises 1.5 lb of boron per acre on deficient soils and testing every two years.
Micronutrients are needed in small amounts, and both too little and too much can hurt a potato crop. Extension guidance from the University of Minnesota and the University of Florida gives clear limits for boron, zinc and other micronutrients.
The University of Minnesota Extension guide for irrigated potatoes, by Carl Rosen and reviewed in 2024, sets these boron classes:
- Soil test 0.0 to 0.4 ppm (low): 1 lb of boron per acre broadcast, or 0.5 lb banded.
- 0.5 to 0.9 ppm (medium) and 1.0 ppm or more (high): no boron.
- Sufficiency range in petioles and whole leaves: 20 to 40 ppm.
The guide recommends soil application, because boron applied to the leaves is not readily transported to the tuber. It also warns that excessive boron can be toxic. For zinc, soils testing 0.0 to 0.5 ppm are low and get 10 lb per acre broadcast or 2 lb in the row. Soils of 0.6 to 1.0 ppm get 5 lb broadcast or 1 lb in the row. A 5-year Minnesota study on irrigated sandy soil found yield increases from boron and zinc. The same work found no yield increase from manganese or copper.
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The guide adds that pesticide sprays often contain enough copper and zinc for the crop. Manganese toxicity may occur in extremely acid soils below pH 4.8. Tissue manganese above 1,000 ppm is often linked to stem streak necrosis. Iron deficiency depends more on soil pH than on soil test levels.
The University of Florida IFAS recommends 1.5 lb of boron per acre on soils with no vegetable production in the past two years or with known deficiency. For deficient soils it lists 5 lb manganese, 3 lb zinc, 4 lb iron and 3 lb copper per acre. It advises soil tests every two years to avoid toxicity, because some micronutrients build up in the soil. Growers should also count micronutrients supplied by fungicides.
Why it matters
Boron and zinc are the micronutrients most likely to pay on sandy potato soils. Other micronutrients often give no yield gain. Foliar boron sprays are a common habit, but Minnesota advice says they do little for the tuber.
The gap between enough and too much is narrow. Blanket micronutrient mixes can add cost and build up in the soil. A soil test and a tissue test are a cheaper guide than routine application.
Photo: Soil sampling in a field (illustrative). T. Hengl / Wikimedia Commons, CC BY-SA 4.0
