AbstractMagnesium is an essential plant nutrient but magnesium deficiency occurs fairly widely in Europe and North America, especially on light soils. In Great Britain this is known to be so for many horticultural crops, although information on deficiencies in agricultural crops is limited.Magnesium sulphate applied to the seedbed for potatoes increased yield in some but not all experiments on light soils, while in a few on both early and main crop potatoes it depressed yield.Deficiency symptoms often seen in cereals and sugar beet are usually transient, and in almost all experiments on these crops applying Mg has not increased yield.There is a well established interaction between Mg and K, and high rates of the latter can intensify deficiency symptoms and reduce the Mg content of the crop. Field experiments strongly suggest that the induced Mg‐deficiency from heavy rates of applied potash seldom reduces the yield of potatoes, cereals or sugar beet.Methods of controlling or averting Mg deficiency are discussed.