THE USE OF NUTRIENT BIOASSAYS TO ASSESS THE RESPONSE OF EUCALYPTUS-GRANDIS TO FERTILIZER APPLICATION .1. INTERACTIONS BETWEEN NITROGEN, PHOSPHORUS, AND POTASSIUM IN SEEDLING NUTRITION

The nutritional requirements of Eucalyptus grandis Hill ex Maiden seedlings were studied in glasshouse pot experiments. Nitrogen (N), phosphorus (P), and potassium (K) demand was assessed by bioassays, in which the uptake of tracers (N-15, P-32, or Rb-86) by roots excised from the seedlings grown at different levels of nutrient supply was measured. A pilot study showed that there was an inverse relationship between nutrient supply and influx. The measured influx was affected by the length of time that the roots were stored before the bioassay was applied. In a 3(3) N, P, and K factorial nutrient experiment, growth was primarily influenced by N. Maximum growth occurred at the highest level of supply of each of the three nutrients. N and K influx in the root bioassay was inversely related to foliar N and K concentration, but P uptake was not related to foliar P concentration. These results indicate that the root bioassays are more sensitive determinants of nutrient limitation than foliar analysis. Interactions between N and P and between N and K alter the stem:leaf ratio, which may be of importance in optimizing wood production.