Yield responses of wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) to additions of KCl fertilizer on high‐K soils have been documented. One of the principal effects of KCl on high‐K soils is a reduction in foliar disease. Little research has been conducted on the effects on oat (Avena sativa L.) production of KCl on high‐K soils. The objective of this study was to evaluate the effects of adding KCl to high‐K‐test soils on grain yield, quality, plant water relations, crown rust (Puccinia coronata Corda) incidence, and lodging of commonly grown oat cultivars. The experiments were conducted at four locations in eastern South Dakota. Fertilization with KCl increased oat grain yields by 4% at two locations. Although the increase in grain yield was significant, it was too small to be profitable under current economic conditions. A significant increase in 1000‐kernel weight (4–7% relative to the control) occurred at locations with a yield response. Additions of KCl did not affect crown rust infection, protein, or oil content, relative water content, or stomatal conductance; however, KCl addition decreased solute potential adjusted for full turgor during the boot stage at two locations by 0.07 MPa. The decrease in adjusted solute potential was not consistently associated with a grain yield response.