The influence of magnesium nutrition on fruit quality of sweet orange was investigated in sand culture, 'and the relationships between Mg and fruit quality were explored in the field.
In the glasshouse, fruit produced over 3 seasons by cuttings, cv. 'Washington Navel' grown witk high and low Mg nutrient solutions were compared With controls (5.29, 0.37, and 1.62 milliequivalents Mg++/L). High Mg resulted in fruit with a much greater juice acid content than the control, other effects on fruit quality attributes were .small and mostly non-significant. High Mg depressed uptake of Ca, but not K indicating a Ca-Mg antagonism. Phosphorus uptake was depressed by both high and low Mg compared to the controls. Leaf concentrations of Mg were: 0.595, 0.178, and 0.392 per cent (means over 3 seasons) in the high, low Mg and controIs.
In the field, fruit from 'Washington Navel' trees on Rough Lemon rootstock had acid levels that correlated well with leaf K, also with (K + Mg) : Ca ratio.
Leaf levels of Mg were below 0.4 per cent. 'Late Valencia' on Poncirus tri]oliata rootstock produced fruit With higher acid levels which correlated with leaf Mg (mostly > 0.4 per cent), also with K + Mg: Ca ratio. Therefore, Mg was a significant predictor of juice acidity when Mg levels were high.