Nitrogen and potassium nutrition of Australian waxflowers grown in siliceous sands. 2. Effect on leaf colour, vase life, and soil pH and conductance

Abstract

The effects of nitrogen (N) and potassium (K) on leaf colour, vase life of flowering stems, and soil pH and electrical conductivity (conductance) were investigated for Australian waxflowers. Experiments were conducted on commercial plantings of Chamelaucium uncinatum cvv. Alba and Purple Pride, and a Chamelaucium hybrid (C. floriferum x C. uncinatum) known locally as Walpole wax, at 3 sites in South Australia. Nitrogen (as NH4NO3) and K (as K2SO4) were applied at rates up to 160 g N/plant and 80 g K/plant over several side dressings during the growing season. Application of N significantly (P<0.05) increased leaf colour ratings for the cv. Alba and Walpole wax. At the higher N rates leaves were dark green. Differences between years were small compared with the effect of applied N. The N x year interaction, and the effect of applied K, were not significant (P>0.05) at any site. For cv. Alba, application of N significantly increased vase life by 5 days in 1992 and 3 days in 1993. For Walpole wax, the effect of N was not consistent between years. Each year, the vase life of flowering stems from cv. Alba were consistently greater compared with stems from Walpole wax. Application of K did not significantly (P>0.05) effect vase life at any site. Annual applications of 80 or 160 g N/plant, as ammonium nitrate, significantly decreased soil pH by 0.3-1.4 units after 2-3 years, whereas application of K, as potassium sulfate, did not affect soil pH. The effect of applied N on soil conductance, although significant, was not consistent between sites. For example, at site 1, increasing the rate of applied N from 0 to 80 g N/plant increased conductance from 0.04 to 0.08 mS/cm in 1992. However, in 1993 it decreased from 0.04 to 0.02 mS/cm. The low conductance values (0.02-0.09 mS/cm) in the 0-60 cm soils, show that fertiliser salts did not accumulate over the course of the study to concentrations which adversely affect plant growth and yield. We conclude that N stress was a significant factor in the occurrence of poor leaf colour, and may be a major factor in the occurrence of defoliation during the flowering period. Optimising N nutrition improved leaf colour, vase life and, depending on the rate, did not significantly increase soil acidity or conductance after 2-3 years.