Changes in soil boron fractions, dry weights of biomass components (needles, branches, stem bark, stem wood) and uptake of nitrogen, phosphorus, potassium, calcium, magnesium, boron, copper, and zinc were studied in a 4-year-old Pinus radiata
D.Don stand in Ashley Forest, North Canterbury, 1 year after application of urea at 0 and 400 kg N/ha and/or ulexite at 0 and 7.4 kg B/ha.
Significant responses to both the applied nitrogen and boron were measured in total above-ground tree biomass. The needle, branch, and stem components were significantly heavier in the trees fertilised with boron alone, but only the needle component was significantly heavier where nitrogen fertiliser had been added alone. Increased tree growth was associated with increased total uptake of all nutrients except phosphorus and potassium which increased only in trees treated with ulexite. Increased nutrient uptake was measured mainly in needles of trees treated with urea, but in needles and branches of those to which ulexite was applied. Nutrient concentrations of non-fertiliser elements remained similar or declined in trees to which fertiliser was applied. This was attributed to dilution effects and/or internal translocation between biomass components.
Boron application significantly increased boron concentrations in the current and 1-year-old needles whereas nitrogen application had no effect on nitrogen concentrations. Some of the added boron was retained in plant-available fractions in the top 20 cm of the soil 1 year after application and this, together with the potential retranslocation of boron within the ulexite-treated trees, will provide a future supply of boron for tree growth.
The results confirmed that ulexite is a suitable long-term supplier of fertiliser boron to P. radiata growing in low rainfall areas.