•
POTATONUTRITION
•
THE
VEGETABLE
FARMER •
MARCH 2017
the soil are on an upward
trend.
“While one third of seed and
maincrop receive organic
manure, the nutrient value is
often not fully accounted for
by reductions in fertiliser
application rates,” said Dr
Allison. “This is amissed
opportunity in regard to cost
savings and impacts on yield
and quality.”
According to the British
Survey of Fertiliser Practice,
compared to the 80’s, N
efficiency is improving.
Encouragingly, Dr Allison
assured us there is still plenty
to go for in regard to driving
down the amount used and
improving yield.
Moving on to precision
farming he saidmapping soils
properly, in regard to pH etc,
could be used to target
applications. While applying
varying rates can lead to
improvements in accuracy and
cost effectiveness, he again
urged caution; “Growers often
takeminimum soil samples to
keep the cost down and as a
result maps are not often that
reliable – inwhich case you
should be skeptical of such
data/information.”
Nmanagement
Moving on to the principles
of Nmanagement, we learnt
that apply too little and there
is a risk of yield loss. However,
apply toomuch and there is
certainty of increased costs
and possible implications to
quality and yield. For each
nutrient we were told it is a
case of looking first at the
crop requirement and
subtractingwhat is supplied
from the soils and any
application of organic
manures, to arrive at the crop
requirement.
N has the largest impact on
yield and quality. Research
work exploring the effect of
different application rates on
canopy persistence shows little
impact early in the season. It
is only towards the later end
that effects are seenwith
greater canopy persistence.
However, by September /
October day length is
decreasing and the benefit of
a large canopy is less
important raising the question
– dowe get value from
persistent canopy?
Dr Allison explained the
answer lies in the
‘determinacy’ of the potato
variety. Indeterminate varieties
- such as Cara andMarkies -
have the potential to grow on
and on. “Determinate and
indeterminate varieties have
very different N requirements,”
he said. “Indeterminate
varieties need relatively little N
– they move resources in to
the canopy rather than the
tubers.”
Season length also plays a
part – as data from CUF
showed. Short season, 60-90
day crops, do not needmuch
N – in fact it can decrease
yield if toomuch is applied –
this is particularly true with
indeterminate varieties.
Moving on tomaking actual
N fertiliser recommendation in
practice, course delegates
were given two exercises to
complete using the RB 209
Fertilizer Manual. *
The first stepwas to
calculate soil nitrogen supply
(SNS) looking at factors
including the previous crop in
the rotation, soil type and
rainfall. The second step
involved identifying the
determinancy group of the
variety before calculating the
season length in days. Step
four required calculating the
total N uptake requirement of
the crop, while step five
looked at calculating the
supply from organic manures
which finally led to step six –
the fertiliser requirement.
However, Dr Allison
explained even then it is not
simple as other factors need
to be taken in to account such
as irrigation (irrigated crops
use Nmore efficiently),
changes in the cost of N
fertilizers, the value of the
potatoes, the level of soil
compaction and planting date.
An estimated 50% of the UK
potato crop is grown on
compacted soil restricting root
growth and N andwater
uptake. “Under applying N on
compacted soil gives the
biggest yield hit,” he warned.
Split N applications?
According to Dr Allison the
benefits of splitting N
applications is quite weak – an
opinion shared by Cooke et al
following research in 1957.
30
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Indeterminate varieties - such as Cara andMarkies (pictured) - have the potential to grow on and on and need
relatively little nitrogen, said Dr Allison.
