The Effect of Foliar-applied Acidified Iron Sulfate on Chlorophyll and Iron Contents in Leaves of 'Chandler' Walnut Trees

Walnut production in Chile has expanded to areas where the crop was not previously grown. The central zone of Chile has climatic conditions that are conducive to walnut production. However, in the calcareous soils (pH > 8) of this region, lack of availability of ferrous iron (Fe²⁺) can result in tree iron deficiency. To prevent tree iron deficiency in these soils, very expensive chelated iron can be applied to the soil or leaves. Recent research with other fruit crop species in Florida showed that foliar applications of iron sulfate plus ascorbic acid and a surfactant was nearly as effective as chelated iron for preventing symptoms of iron deficiency of trees in calcareous soil. Based on those results, we tested the effectiveness of foliar-applied iron sulfate plus ascorbic acid and a surfactant on preventing iron deficiency in walnut (Juglans regia cv. Chandler) trees in calcareous soil (pH = 8.2) in Chile. Three treatments were applied to 6-year-old trees: T0 (Control): application of well water adjusted to pH 4 with ascorbic acid (50%); T1: application of chelated iron to the soil (19 g/tree, 3 times during the growing season, at 30-day intervals); T2: foliar application of iron sulfate (500 mg/L) plus LI-700^® surfactant (2 mg/L) in well water adjusted to pH 4 with ascorbic acid. For T0 and T2, applications were made 5 times during the summer season; applications were made at 15-day intervals, from the third week of January to the third week of March. Tree responses were evaluated by measuring leaf re-greening (with a chlorophyll index meter), leaf chlorophyll content, and leaf iron content (Fe²⁺ and total Fe). There was a significant treatment difference (P < 0.05) for leaf chlorophyll index only on the fourth week of March, where plants in T2 had the highest chlorophyll index. Leaf chlorophyll index and total chlorophyll content were highly correlated (R² = 0.94). Total leaf iron content was significantly different among treatments only on the last application date, when trees in T2 had higher total leaf iron content than trees in T0.  In April (one month after the last treatment application), total Fe and Fe²⁺ concentrations were higher in T2 than T0 or T1. The results suggest that foliar application of iron sulfate plus ascorbic acid is a viable method of preventing iron deficiency in ‘Chandler’ walnut trees planted in high pH calcareous soils.