Response of Pinus radiata D. Don to Boron Fertilization in a Glasshouse Study

Limited information is available on the effect of slow-release boron (B) fertilizer on Pinus radiata growth and physiological properties and soil microbiological activities. A 7-month-long pot experiment was carried out under glasshouse conditions to investigate the response of Pinus radiata to different rates (0.0222, 0.0446, 0.089, and 0.178 mg B g(-1) soil), equivalent to 0, 4, 8 16, and 32 kg B ha(-1) of ulexite, a slow-release B fertilizer. Hot 0.02 M calcium chloride (CaCl2)-extractable soil B, soil dehydrogenase activity, plant B concentration, growth, and photosynthesis were measured at the time of harvest. The B concentrations in the soil and plant organs (needles, stem, and roots) significantly increased with increasing rates of B fertilizer. The optimum B fertilizer rates of 4-8 kg B ha(-1) produced the greatest plant growth and net photosynthetic rate. However, the B rates of 16 and 32 kg B ha(-1) significantly reduced net photosynthetic rate, and the rate of 32 kg B ha(-1) significantly reduced stem diameter growth when compared to the optimum B rates. Soil dehydrogenase activity, an indicator of soil microbiological activities, was significantly reduced by B application at the rates of 16 and 32 kg ha(-1). This study confirms the narrow range between B deficiency and toxicity in a tree crop and stresses the need for selection of the optimum rate of B fertilizer application.