Eighteen field experiments, between 1971 and 1973, compared the effects of 100kg Mg/ha as kieserite (MgSO4.H2O) with two forms of calcined magnesite (both mainly MgO) on yield and chemical composition of sugar beet, carrots or cereals. Fields were chosen where the soil contained less than 50 μg exchangeable Mg/ml and with pH values between 6·4 and 8·1. Seven of the experiments also compared autumn pre-ploughing application of magnesium with a spring application.
Autumn or spring dressings of kieserite increased sugar yield by 0·32 t/ha, a 5% increase compared with plots given no magnesium. Seed-bed applications of the magnesite, calcined at about 850 °C, had little effect on sugar yield. However, when the magnesite ore was burnt at 800 °C and the resultant particles screened to > 1 and < 4 mm, autumn and spring applications gave 0·21 and 0·08 t/ha more sugar respectively. The magnesium fertilizers did not increase yield of carrots or cereals.
Changes in the magnesium concentration in the plants and the percentage of plants with deficiency symptoms indicated that on soils of pH greater than seven, magnesium was readily available from kieserite, less available from the lightly burnt magnesite and unavailable from the over-burnt magnesite. A glasshouse experiment confirmed these effects so it was concluded that on neutral and alkaline soils, kieserite was the best form for immediate use by sugar beet. At present prices, however, on average autumn applications of the lightly burnt calcined magnesite gave the largest economic return. As further improvements in the availability of magnesium from calcined magnesite seem likely, this is now being investigated.