Kiwifruit accumulate large amounts of nutrients in vegetative growth during spring. Only part of this can be supplied by the limited reserves in the upper portion of the vine's perennial framework. Recognition of this has lead to the industry-wide practice of applying basal fertiliser dressings in late-winter, or early spring, so that nutrient elements are available in soil when active root uptake commences. But just when does active root absorption begin?

A field experiment was conducted at Blands Research Orchard near Hamilton to establish more precisely when roots take up nitrogen, and how quickly and effectively early-applied fertiliser is available to new season's growth. The experiment used nitrogen fertiliser enriched with ¹⁵N - a natural isotope used to trace the fertiliser - which was applied on four different occasions to mature kiwifruit vines over winter and early spring. Fertiliser treatments consisted of 50 kg N per ha (as ammonium sulphate) applied either on June 13 (1990), July 11, August 8, or September 5 (two weeks before budbreak). Thus, by analysing small plugs of tissue taken fortnightly from the roots, stem, cordon and fruiting canes of treated vines, it was possible to differentiate between, and track the movement of, nitrogen derived from the fertiliser applications, as opposed to nitrogen already present in the soil/plant system.

Findings

• Uptake of fertiliser nitrogen by roots was immediate, regardless of the time of season it was applied. By two weeks after the application of each treatment, ¹⁵N equivalent to 7% of the amount applied was recovered within the roots. There did not seem to be any additional effect the longer roots were exposed to applied nitrogen. Despite occasional fluctuations in the measurements, there were no significant differences among treatments in the concentrations of ¹⁵N present in roots at different sampling dates, nor trends in concentration over time (Fig. 1).

• Although taken up by roots there was little movement of ¹⁵N into other parts of the plant until between 4 and 6 weeks prior to budbreak - the period when vines start to bleed if pruned. In the most dramatic instances, considerable amounts of ¹⁵N were measured in the stem and cordon of treated vines just two weeks after the August and September treatments were applied (Fig. 1). Significant concentrations of ¹⁵N were not found at the ends of the fruiting canes until at least two weeks after budbreak (Fig. 1).

• Amounts of ¹⁵N were also measured in the soil, and in the leaves and fruit at harvest (9 May 1991). The July, August and September fertiliser treatments were equally effective in terms of supplying fertiliser nitrogen to new season's growth. Quantities of nitrogen from the June treatment, however, tended to be slightly smaller in leaves and fruit. In soil, movement of ¹⁵N down the soil profile (a Horotiu sandy loam) indicated that substantial leaching had occurred from both the June and July applications.

• Mean daily soil temperatures at the 10 cm depth averaged 13.3°C and ranged between 9.2 and 18.7°C over the period of the investigation (the majority of the fine root content lies within the top 30 cm of soil). Soil temperatures at 10 cm were about 2.5°C warmer than the corresponding air temperature and, because of insulating effects, would have been even warmer at depths below this. The lowest temperatures measured in this study are comparable with values reported as being the minimum necessary for root growth and nutrient uptake in grape and a number of other species. Hence nutrient uptake by kiwifruit under normal winter temperatures shouldn't be regarded as being unexpected.

Conclusions

Vines absorbed nitrogen throughout the winter, retaining the nitrogen taken up in the roots. The four- to six-week period prior to budbreak was identified as a particular time during which nitrogen moved from the roots into the cordon without the stimulus of vegetative growth to assist movement of the transpiration stream. The trigger for this event is unclear, but may be related to changes in osmotic pressure in the soil or roots.

Fertiliser applied in winter reached new season's growth at about two weeks after budbreak. Thus nutrient reserves within the perennial framework sustain new season's growth for only a limited time before being supplemented from sources external to the vine.

Fertiliser treatments in July, August and September were equally effective, and the June treatment slightly less so, in terms of suppling nitrogen for leaves and fruit. To minimise leaching losses, however, application of fertiliser closer to budbreak would be preferable from an environmental and economic standpoint.

Figure 1. Concentration of 15N in the roots, stem, cordon, and one-year-old fruiting canes of kiwifruit following application of 15N-enriched nitrogen fertiliser treatments to soil in June and August. Values for the July and September treatments (not shown) were intermediate or similar to the data presented. Arrows indicate the timing of the June and August treatments, and budbreak (BB), (19 September), respectively.