Abstract.The tolerance of wheat plants to manganese is shown to depend on the magnesium concentration in shoot tissue, such that growth is a function of the ratio of the two ions Mg: Mn. This ratio in the plant (Rp) and in the soil solution (Rs) was related by the equation:ln Rp= 1.45+0.31 ln Rs.Values of Rpnot limiting to growth need to be determined for different crops, but we conclude that corresponding values of Rscan be predicted using solution culture trials. They can then be used to identify remedial treatments in soils where manganese toxicity occurs. In the Eutric Cambisol examined, small amounts of calcium carbonate decreased manganese concentrations in the soil solution such that amounts of magnesium which could easily be applied to a field gave appropriate values of Rsfor wheat.