| On Topic
N
itrogen
ightmares?
2016 Soil Nitrogen Survey highlights the
need for regular N testing.
his, the company emphasises, highlights the
importance of testing soils for Nitrogen content every
spring to ensure that only the exact amount of
additional N needed to achieve optimum yield, quality
and economic return is applied.
"Most farms still rely on the standard Soil Nitrogen
Supply index system in the Defra Fertiliser Manual (RB209), but these
figures are simply averages and cannot account for individual field
and/or crop variations," states CF Fertilisers agronomist Allison Grundy.
"Growers who rely on them will therefore only ever get their Nitrogen
application right around 30 per cent of the time. To ensure that they
apply the optimum level they will almost certainly need to tailor these to
their own conditions and take account of seasonal variations.
"Without effective testing it is impossible to know for certain how
much Nitrogen is present in the soil and available to the crop. This
becomes particularly important following extremes of weather, such as
the record rainfall and higher-than-average temperatures recorded in
many parts of the country this winter, which could have moved Nitrogen
and other nutrients further down the soil profile and, critically, out of the
rooting zone.
"At the start of the year it seemed that everyone was suggesting that
soil Nitrogen levels would be low this spring and it would have been
easy to assume, based on rainfall, that your farm had moved into a
different category in terms of the RB209-index system.
“In fact, our data shows that 2016 and 2015 were actually very
similar and that more samples measured lower for soil Nitrogen in 2014
than in 2015 or 2016. However, it was quite comforting to see that the
general profile of the distribution graphs for soil Nitrogen (see below)
were quite similar, despite substantial differences in seasonal weather
patterns."
The figures highlight the importance of annual soil testing but the
amount of N already taken up by the crop must not be forgotten, as this
contributes to the overall SNS calculation, Allison Grundy emphasises.
"Fertiliser represents the largest single arable crop input, but also
offers the greatest return on investment. However, our experience
T
www.farmingmonthly.co.uk
shows that most farming businesses are 20kg to 30kg/ha out in terms
of the Nitrogen that they apply, either above or below the optimum level,
while those that have never checked their soil Nitrogen could be out by
up to 50kg/ha.
“Either they are applying too much Nitrogen, thereby risking the local
environment and wasting money, or using too little and missing out on
additional crop performance, either of which will make a big difference
to profitability.
"At a time when margins are under extreme pressure because of low
crops prices the key is to fertilise for what the soil is actually capable of
producing, not what you think it can produce. To do that you have to
know how much N is in the soil, how much N will become available to
the crop during the growing period and how much has already been
taken up by the crop.
“To calculate that we need to know plant populations and tiller
numbers, together with the farm's yield and quality aspirations. We then
use CF Fertilisers' N-Calc system to calculate the optimum level of
Nitrogen to apply to cereal and oilseed rape crops.
"The key to optimising crop performance is never to over-estimate
Nitrogen supply, so you have to be certain that the fields which are
sampled are representative of the farm as a whole. Typically, on a 1000acre farm you would take four samples and block fields that represent
the whole area.
The best time to take N-Min soil samples is in the spring before the
first fertiliser application and at least six weeks after the last manure has
been applied, Allison Grundy says.
"N-Min testing is even more important where nitrogen-containing
products such livestock manures, sewage sludge, digestate from AD
plants and paper crumb, are applied. Although these are a potentially
valuable source of Nitrogen, their contribution varies significantly,
especially as most will be applied in the autumn and lose Nitrogen over
the winter. Poultry manure, for example, is high in Nitrogen which is very
available, but prone to losses.
"Applying products such as these results in greater variation in soil N,
and without testing it is impossible to know how much additional
Nitrogen will be required. This season, for example, the majority of
manures were applied in the autumn, so over the mild, wet winter it is
inevitable that some of the Nitrogen they contained was mineralised
and either lost to the environment or taken up by the crop. N-Min®
enables you to identify precisely what the N contribution from these
sources will be.
"All this supports what industrial businesses have been saying for
years, namely that 'if you don't measure you can't manage'. Certainly,
the results of our tests show that to be true when it comes to assessing
how much Nitrogen to apply.
“Every season is different, so the key is not to assume that your farm
will be in the same SNS category each year, especially if organic
manures have been applied or if it has rained heavily over the winter,
due to likely leaching loss of soil Nitrogen. Instead, the nutrient status of
your soils should be tested by taking representative samples during the
spring so that any shortfall in recommended levels can be made up
through spring application of N, P, K and S."
June 2016 | Farming Monthly | 13