Microelement nutrition of sugarcane 3 Critical microelement levels immature leaf sheaths

The critical levels for 7 essential micro-nutrients, and for silicon and the manganese/silicate ratio, have been determined for three cultivars of sugar-cane in Hawaii. Two yield parameters were used to measure the critical levels, tons cane per hectare (TCH) and tons sugar per hectare (TSH). Immature, elongating leaf sheaths were used as the index tissue in every case. The critical levels for both yield criteria were as follows: for Ca 0.20- 0.25 wt %, dry sugar-free; Mg, 0.10 wt %, dry, sugar-free; S, 0.30 wt %, dry, sugar-free; B, 2 ?g g-1 dry wt, sugar-free; Mn, 10 ?g g-1 dry wt, sugar-free. There are no significant differences among the three cultivars as far as these critical levels are concerned. The critical level for Cu depends on the yield parameter, with 5 ?g g-1 dry wt, sugar-free, being optimum if TCH is used. If TSH is used, the critical level for Cu is 8 ?g g-1 dry wt, sugar-free. The critical level for Zn is independent of yield parameters but varies with cultivar. The critical level for Zn in H53-263 is 7 ?g g-1 dry wt, sugar free, whereas it is 10 ?g g-1 dry wt, sugar-free, in H59-3775 and H49·5. Silicate concentration in the immature sheaths per se is not a meaningful determinant in sugarcane growth and sugar yield. It is the Mn/SiO2 ratio that is important with a ratio value in the range 50-75 being optimum.