Soil boron application for the alleviation of boron deficiency of avocado (Persea americana Mill.) in the KwaZulu-Natal Midlands.
The avocado tree's requirement for additional boron in B deficient soils has traditionally been met solely by foliar sprays in South Africa. Since boron is regarded as poorly phloem translocated in most plants including avocado, foliar applications are unlikely to cater for the requirement of the entire tree. Foliar sprays are made prior to leaf analysis so that artificially high readings are likely. A survey of the boron status of four KwaZulu-Natal avocado orchards showed all soils to be in the deficient range, viz. <1 mg kg(-1). Leaf analysis records on these estates appeared inflated with more than occasional spurious results. Despite marginally adequate leaf boron concentrations, widespread deficiency symptoms were noted in all orchards. For foliar application, leaf analysis of spring flush leaves does not provide a true indication of orchard boron status. Soil applications of borax (11 % B) in the range 0 to 60 g m(-2) (soil canopy area) year(-1) split into three applications, succeeded in increasing orchard B levels to above the recommended optimum of 40 mg kg(-1) without any deleterious effects visible on the feeder roots or tree, except at the highest rates. Initial uptake of soil B was slow, particularly in older orchards and with standard rates as developed in Australia (typically between 5 and 20 g m(-2) year(-1), split into at least 3 applications). Higher application rates (40 and 60 g m(-2) year(-1) showed greater effectiveness at raising leaf boron concentrations, particularly in the second season. Toxicity occurred with 40 and 60 g m(-2) year(-1) rates, 18 months after initial applications were made. High application rates indicated the tolerance of established avocado orchards to very high soil B concentrations. Soil applications increased fruit yield through increased fruit size in younger 'Hass' trees. Older, more deficient orchards did not show increased fruit size within the experimental timespan. Glasshouse trials supported findings in that soil B applications significantly increased leaf B concentrations (P < 0.001) proportional to soil application rate. Recently grafted young potted trees were extremely sensitive to soil boron applications which were not split, with toxicities occurring at low application rates. 'Edranol' seedling, a rootstock of Guatemalan origin was shown to be ca. 40 % more efficient in boron uptake than clonal 'Duke 7', the widely used rootstock in South Africa. Results indicate that boron deficiency is primarily the result of soil deficiency rather than poor rootstock uptake and translocation. On the Inanda soil type used and under the conditions of the experiments, it is suggested that application rates do not exceed 20 g borax m(-2) year(-1) (split into 3 applications) in severely deficient trees (10-30 mg kg(-1) B leaf analysis), and rates of ca. 10 g borax m(-2) year(-1) would be adequate in marginally deficient trees.