Between 1995 and 1999, eight response experiments tested the effects of magnesium (Mg) fertilizers on the yield of potato crops grown in East Anglia, the Midlands, the West and Southwest of England. In addition, a further six experiments tested the effects of varying nitrogen (N), phosphorus (P), potassium (K) and calcium (Ca) supply on the tuber concentrations and uptake of Mg by potato crops. The experiments were done on soils that contained varying amounts of exchangeable Mg and K but were still typical of soils used for potato production.
In the eight response experiments, use of Mg fertilizer had no effect on total tuber fresh weight yield even though yields were often much larger than the national average yield. Increasing the N supply to the crop was often associated with an increase in the concentration of Mg in leaves and stems. This may have been due to N facilitating Mg uptake or a consequence of N delaying canopy senescence and, thus, delaying the translocation of Mg from haulm to tubers. Compared with the effects of N, varying the Mg and K supply to the crop had small and inconsistent effects on crop Mg uptake. Since the experiments also showed that Ca supply and soil K[ratio ]Mg ratio had no effect on crop yield and erratic effects on tissue Mg concentration, fertilizer recommendation systems based on ratios of nutrients in the soil cannot be endorsed. When these current experiments and older, published experiments are taken into account there is little justification for applying Mg fertilizer to soils with Mg Indices > 0 and on soils with Mg Index 0 an application of c. 50 kg Mg/ha would be sufficient.