Timing of 15N fertiliser application, partitioning to reproductive and vegetative tissue, and nutrient removal by field-grown low-chill peaches in the subtropics

Abstract

The effect of timing of nitrogen (N) application as ¹⁵N-enriched ammonium sulfate (50 kg N/ha) on the growth response and N uptake by vegetative and reproductive tissues was investigated in the low-chill peach cv. Flordagem growing on a krasnozem soil at Alstonville. Nitrogen was applied in late August, late September, late October, mid February, and early May. Tree parts were sampled for ¹⁵N at 4 and 8 weeks after application and after fruit harvest in December the following season. After fruit yield was measured, trees were excavated and divided into parts for dry weight and nutrient concentration determinations, and fertiliser N recovery and to estimate tree nutrient removal.

Nitrogen enrichment was detected in all plant parts within 4 weeks of N application, irrespective of timing, and was greatest in rapidly growing tissues such as laterals, leaves, and fruit. The most rapid (P < 0.05) ¹⁵N enrichment in vegetative tissues resulted from September, October, and February N applications and for fruit from a September application. The level of enrichment 4 weeks after fertiliser N application was similar for vegetative and reproductive tissues. The timing of N application in the first season had no effect on fruit yield and vegetative growth the following season.

At tree removal, the recovery of fertiliser N in most tree parts increased (P < 0.05) as fertiliser N application was delayed from October to May the previous season. Maximum contribution of absorbed N to whole tree N was 10–11% for laterals, leaf, and fruit. Data from this study indicate that vegetative and reproductive growth have similar demand for absorbed N, and that uptake of fertiliser N is most rapid when an application precedes a period of rapid growth. Over 2 seasons, recovery of applied fertiliser N was 14.9–18.0% in the tree, confirming that stored N and the soil N pool are the dominant sources of tree N. The recovery of fertiliser N from the May application was 18% even though uptake in all tree parts including roots at 4 weeks after application was very low, indicating that tree fertiliser N uptake occurred when growth resumed after the dormant winter period. The low proportion and recovery of fertiliser N in the tree confirm the lack of immediate influence of applied N to vegetative growth and yield.

Annual crop nutrient removal is a sound basis for fertiliser recommendations, and for the Flordagem orchard (1000 trees/ha), it consisted of fruit plus 70% of laterals (removed at pruning) plus 20% of leaf. Removal in vegetative tissues was relatively low at (kg/ha) 14 N, 1 P, 12 K, 13 Ca, and 2 Mg. The addition of fruit at a yield of 25 t/ha increased total nutrient removal to (kg/ha) 46 N, 5 P, 54 K, 14 Ca, and 5 Mg.