Abstract
Macadamia seedlings were grown in a soil-sponge rock mixture which was leached frequently with solutions of NO3 and SO4 of different concentrations. Growth and mineral nutrient composition of leaf tissues were determined. The external and requirements were estimated from graphs of plant weight plotted against solution concentrations of NO3-N and SO4-S. The internal and requirements were estimated from plant weight plotted against composition. The external and requirements for seedling macadamia are estimated at 30p.p.m. and 5p.p.m. respectively. The internal N requirement (95 per cent maximum growth) is about 1·5 per cent. At maximum growth, percentages of leaves of various ages were nearly constant at 1·54 per cent. The internal requirement is increased with increasing age of the leaves sampled. It is about 0·15 per cent for leaves of the current or most recent Rush, 0·20 per cent for leaves of the previous Rush and 0·25 per cent for old leaves. Macadamia growth was usually near maximum when the N:S ratio was in the range 9 to 14. The appropriate ratio of N:S in the soil solution was about 6. Since rainwater in the tropics is usually very low in S, the use of S in fertilizers for macadamia growing on leached soils should anticipated.
