Magnesium Fertilization of Potatoes as Related to Liming and Potassium

The effects of lime and soil K levels on yield and levels of Ca, Mg, and K in petioles of potatoes ( Solanum tuberosum , var. Sebago) fertilized with Mg were determined in field experiments on Karlin loamy sand. Yield response to Mg fertilizer was related to both level of exchangeable soil Mg and to soil Mg/K ratio. Above 0.3 meq/100 g of soil Mg, yields were not affected by either level of soil Mg or Mg/K ratio, and below 0.3 meq Mg, yields decreased markedly when the soil Mg/K ratio fell below 0.8 (equivalent basis). Yields were increased by liming when soil pH was 5.0, but these differences were not statistically significant. Soil test levels after cropping indicated that considerable amounts of Ca, K, and Mg probably leached below the plow layer. Yields decreased when Mg in petioles was less than 0.15% after 6 weeks. Increasing the level of K in petioles decreased the level of Mg. In Mg‐deficient plants, Mg in petioles decreased as the season progressed, while Mg increased in nondeficient plants.