THE USE OF NUTRIENT BIOASSAYS TO ASSESS THE RESPONSE OF EUCALYPTUS-GRANDIS TO FERTILIZER APPLICATION .2. A FIELD EXPERIMENT

The nutrient status of a Eucalyptus grandis Hill ex Maiden field fertilizer experiment in Natal, South Africa, was assessed at 6 and 18 months by a bioassay test in which the rate of influx of N-15, P-32, and Rb-86 in roots excised from trees was measured. Fertilizer treatments consisted of all combinations of two levels of nitrogen (N), phosphorus (P), potassium (K), and trace elements, applied at planting and at either 3 or 6 months. There were also untreated control plots. At 6 months, bioassay influx rates of the three isotopes by the roots were all higher when a nutrient was applied at a lower level than when it was applied at a higher level. Foliar analysis did not indicate that any of the trees were deficient in N, P, or K, although control plants were significantly smaller than fertilized ones. There was no difference in the foliar P or K content between the treatments, although percent N increased with increasing application of N. The greatest difference in the root response between fertilizer levels was in the influx of N-15. Height of the trees was significantly increased by increasing N levels. There was no effect of P or K levels, but there was a negative effect of the combination of high levels of K and trace elements. From the significant drop in root N-15 influx following the 6 month fertilizer addition, growth of the trees in response to fertilizer was predicted. At 18 months of age trees receiving their second application of fertilizer at 6 months were as large as those receiving fertilizer at 3 months, confirming growth predictions based on root bioassay data. The bioassay results at 18 months suggested that P may be supplanting N as the major growth limiting factor at this later stage in the development of the stand.