Abstract
The effects of soil and foliar Zn applications on growth, yield, and fruit quality of `Rio Red' grapefruit were studied in the field for 4 years. Two annual foliar sprays applied in winter (W), spring (S), or W+S were compared to a single application of 10 or 30 g of Zn/tree applied to the soil around the tree as ZnDTPA or ZnEDTA chelate. In the first 2 years, when control trees displayed severe Zn deficiency symptoms affecting 60% to 70% of the tree foliage, the W and W+S sprays resulted in significant yield increases. Similar yield increases were obtained after a single soil application of 30 g Zn as ZnEDTA. The effects of other soil treatments were statistically insignificant. Foliar Zn deficiency symptoms were much more severe in winter than summer months irrespective of treatment. As the trees aged, however, the severity of symptoms decreased in all treatments. Corrective foliar or soil Zn applications were found to increase grapefruit yield when 15% or more of the canopy foliage showed Zn deficiency symptoms in January, ≈2 months before anthesis.
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