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to be avoided (Clark & Ledgard 1993),
especially following large rainfall events.
High applications of N fertiliser in the
springtime may be ineffi cient both because
of low rates of root uptake activity combined
with soils close to fi eld capacity which
encourages N leaching. The increasing use
of irrigation on more drought-prone shallow
soils of the eastern BOP makes a more
considered N management strategy critical
to reduce the risk of N leaching,
The SFF trial has shown that under the
High-N treatment, the time to reach fruit
maturity appears to be delayed as measured
by dry-matter content (DM) and fruit fl esh
colour. We have found that current N
application on most kiwifruit properties is
higher than can be accounted for in the
harvested fruit. This, coupled with N
mineralisation from the soil, could lead to
an excess supply of plant-available nitrogen
that simply encourages vegetative growth
at the expense of fruit growth. It is possible
that reduced N application might not only
reduce kiwifruit vigour, but may also
encourage some improvements in fruit
quality both at harvest and during storage.
Results from the SFF trial showed a
continued high performance from those
vines where N fertilisers were withheld for
three years.
The ability of the Zero-N vines to maintain
their productivity was primarily due to the
large amount of N being mineralised from
the soil’s reserves. Our trial results suggest
an application rate of between 80-100 kg
N/ha would be suffi cient to meet the plants
needs at the experimental site (Te Puke 1,
Te Puke sandy loam) given that N removal
in the fruit was between 80-120kg N/ha
per year, on average. Values of 150kg N/
ha/y was removed in the fruit of the High-N
treatment in year three. Slightly higher N
rates may be justifi ed on other Bay of Plenty
soils that exhibit lower N mineralisation
potential compared with the soil from the
SFF trial site. However, our laboratory data
indicates that all the kiwifruit orchard soils
had a similar high potential to deliver
mineral N. Nitrogen mineralisation is an
important component of the N budget.
Orchardists considering reductions in N use
should also consider possible gains from
increased N uptake from split applications
and regular leaf nutrient analysis to monitor
N uptake.
These data provide an opportunity to
predict, using SPASMO (a research tool
developed by HortResearch), likely delivery
of plant available N in kiwifruit soils given
conditions of soil moisture and temperature.
Soil temperatures in the BOP region
between September and May range from a
minimum of 10 o C in May and a maximum
of 20 o C during January and February. The
incubation and analysis of these soils is
ongoing and will continue through to at
least 220 days. This will allow more model
validation and better predictions in future.
ACKNOWLEDGEMENTS
Funding for this study was provided by
the Sustainable Farming Fund (SFF,
administered by MAF). Researchers
would also like to thank John May for
the continued access to his orchard
and his help maintaining the trail over the
course of this study.
REFERENCES
Clark C, J. Ledgard SF 1993. Uptake of 15N
by kiwifruit vines from applications of
nitrogen fertilizer prior to budbreak. Annals
of Botany 71 (4): 311-316 1993 Ledgard
Mills TM, Boldingh H, Blattmann P, Green S,
Meekings J 2007. The sustainable use of
nitrogen on Hort16A: The impact of
modifi ed N application on fruit quality and
the accumulation of other macronutrients
in fruits and leaves. New Zealand Kiwifruit
Journal, May/June, 13-16. ■
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