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Mineral N (mg/kg)
Figure 2. Changes in mineral nitrogen (N) content of fi ve kiwifruit soils (solid lines and fi lled symbols) and
two vineyard soils (open symbols and broken lines) following incubation at 20°C and soil moisture at 80 per
cent of fi eld capacity. Symbols represent laboratory data and the solid lines are model output from SPASMO.
cumulative increase in the mineral N
content of the soil, noting that there are no
plant roots, or leaching processes to remove
N from the soil sample. In the fi eld the
mineral N content of the soil is expected to
be much more dynamic, with continuous
inputs and losses occurring from the soil.
The mineral N content of the other soils has
also been evaluated in incubation
experiments, and modelled using SPASMO
(Figure 2). These soils were evaluated for
up to 60 days (Bay of Plenty (BOP) x two
(Opotiki and Katikati), Northland x one
(Kerikeri), Hawke’s Bay x two (Omahu and
Poporangi)) or 118 days (one x BOP (Te
Puke two), one x Nelson). Interestingly, we
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observed similar mineralisation rates from
all the kiwifruit orchard soils (solid lines and
symbols), and they were consistently higher
compared with the two vineyard soils (open
symbols and dashed line).
The total mineral N present at day zero is
marginally different among soils. These
differences are predominantly due to
differences in the labile (readily degraded)
fraction of the soils organic matter. Soils
with higher levels of labile carbon are
capable of delivering more mineral N to the
soil system, all other factors being equal. In
contrast, mineralisation rates from the
vineyard soils are markedly lower. The
Omahu soil is a shallow gravely sand with
low organic matter that produces low-vigour
vines that typically have small berries with
intense fl avour. The Omahu soil form part
of the Gimblett Gravels wine growing
region, near Hastings, that has become
synonymous with high quality wine grape
production. In contrast, the Poporangi soil is
more fertile (shown at the top of Figure 2)
as shown by the higher initial mineral-N
content of the soil. Grapevines on the
Poporangi soil tend to be more vigorous
(cf. vines on Omahu soil), although the
greater vegetative vigour may reduce wine
grape quality.
In the case of the Omahu vineyard soil, all
stones larger than two mm were also
removed (noting these accounted for 65
per cent V/V of the soil profi le!). Because of
the high stone fraction in this soil in situ, the
rate of mineralisation presented here, for
the fi ne-earth fraction, is much higher than
would otherwise occur naturally in the fi eld
under optimum moisture and temperature.
Low rainfall in the Hawkes Bay over the
summer period means both the Poporangi
and the Omahu soils have periodic moisture
limitations that will reduce the actual
mineralisation rates. This contrasts with
many of the soils from the Bay of Plenty
region, which are typically deep and free
draining with regular rainfall. Increasing use
of irrigation will help maintain these soils at
close to optimum moisture conditions for N
mineralisation.
CONCLUSIONS
Under ideal conditions in the laboratory, the
rate of N mineralisation is maximised and
we are able to determine the soil’s potential
to deliver plant-available nitrogen from the
soil’s reserves of organic nitrogen. Because
moisture and temperature are optimised in
these measurements, we expect that in situ
values of mineral N would be less than
these reported here. The initial mineral N
content of the kiwifruit orchard soils ranged
between 10 and 35kg N/ha, as calculated
for the top 15cm of the soil. The initial
mineral N content of soil from the SFF trial
site was higher (at about 75kg N/ha)
compared with the other Bay of Plenty soils
(which ranged between 30-50kg N/ha).
For the purpose of calculation we have
assumed a soil bulk density of 1.0kg/L,
although we recognise that most pumicederived
volcanic soils have a bulk density
between 0.85 and 1.0kg/L.
Following 60 days of incubation at 20 o C,
the equivalent of an additional 50 to 75kg /
ha of N is released from the soil via
mineralisation. All soils from the kiwifruit
orchards showed similar rates of
mineralisation, as evidenced by the parallel
lines from Figure 2. Differences in mineral
N contents after 60 days are largely
explained by the different initial mineral N
contents which are dependent on the labile
carbon content of the soil. Overall, our
results show that soils from kiwifruit orchards
have a high potential to deliver signifi cant
levels of mineral N. It is important to
consider the implications of soil-N
mineralisation in combination with the
application of N fertiliser under high rainfall
and on the free draining soils of the Bay of
Plenty region.
Nitrate-nitrogen is readily–soluble. It is also
highly-mobile and will move downwards
through the soil profi le, as a passenger
chemical that travels with the drainage
water that quits the root-zone. Timing of N
application is therefore critical if wastage is