| Arable
Long-term phosphorus research
yields ‘unexpected’ results
Can soil phosphorus (P) levels be reduced in a way that maintains or even enhances yields? Can the efficiency of P
fertilisers be increased? And can crop P sufficiency be monitored more reliably?
hese key questions are all
being considered by a long-
term collaborative research
effort – which is providing
answers but also its fair share
of new questions too.
At the 2017 AHDB Agronomists’ Conference,
ADAS’ Roger Sylvester-Bradley outlined
intriguing findings from P research and asked
growers to participate in a free Grain Nutrition
Benchmarking survey.
Patience begins with ‘P’
Compared with the fast-paced world of plant
protection, the P world moves relatively slowly,
according to Roger. Soil P levels run down
relatively slowly and this means
experimentation on this essential crop nutrient
requires a great deal of patience.
Best practice for P is detailed in RB209 and
the guidance is relatively straightforward – for
cereals and oilseeds, soil should be analysed
every three to five years and maintained at P
Index 2.
The key thing to observe in RB209 is the
guidance treats all soil as the same but it is
known that soils differ in behaviour – ‘especially
calcareous soils needing annual P,’ said Roger.
Although there must be an interaction
between P and the soil environment, creating
the robust data sets to prove this is somewhat
difficult.
What is clear, however, is more P runs off the
land at Index 2, or higher, than at Index 1, and
P is the largest cause of failure, when it comes
to achieving the targets set by the Water
Framework Directive.
With around 75 per cent of UK arable land at
P Index 2 or more, it goes without saying that
the current blanket recommendation results in
costly inputs going down the drain, rather than
being taken up by the crop.
Can soil P levels be reduced in a way that
maintains or even enhances yields?
At the conference, Roger provided a
comprehensive summary of a large
collaborative research effort that investigates
ways to reduce the soil P index – to save
money, enhance/maintain yields and protect
the environment.
Data from SOYL showed the scale of the
challenge. The data (around 6000 data points)
was from 36 farms with a known history. Based
on soil P analysis (mg/l), these farms had a
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range of starting P values. The farms, which did
not use organic manures, followed an RB209
strategy to home in on Index 2. The results
were unexpected: P levels increased on most
farms.
Roger said that the data behind the averages
‘shows a lot of variability and this highlights
how much care is needed in managing and
interpreting soil P analyses’.
Other long-term experiments, at four arable
sites (two in the East and two in the West),
where no P had been applied for six to seven
years, also showed very little run-down in soil P,
whereas the half-life of P is meant to be nine
years. Clearly, further work is required to get to
the bottom of the P puzzle.
Can the efficiency of P fertilisers be increased?
Looking at the efficiency of P fertilisers,
Roger said research, conducted on ten sites at
soil P Index 1 with above-average yielding
crops, showed, on average, that 80 per cent of
crop P requirements were met from soil P, with
the rest coming from fertiliser. The average
recovery from fresh broadcast Triple
Superphosphate (TSP), however, was just four
per cent, which is clearly not good enough, if
farmers are to work at lower soil P indices.
Placement of P fertilisers had no effect on P
recovery by the crop, although it seemed to
help crops establish resulting in some small
yield benefits (four times out of ten) in the
spring crops (barley and potatoes) trialled. The
yield benefit, however, was not observed in
winter crops (wheat and oilseed rape).
AVAIL ®-treated TSP, which provides a
slower release of P gave no consistent benefits.
Struvite, an inorganic by-product of sewage
treatment, which also releases P more slowly,
only gave a small increase in P recovery –
‘recoveries were still below ten per cent’.
Can crop P sufficiency be monitored more
reliably?
Careful soil sampling and analysis can
provide good information on the soil availability
of nutrients for crops and leaf analysis can
reveal whether the uptake of nutrients is
sufficient at one point in time. But monitoring
grain P could provide a complementary nutrient
monitoring technique, particularly, according to
Roger, as it provides information on whether
crops got enough nutrient through the season,
as well as providing more accurate information
on nutrient offtakes.
The relationship between Grain P (mg/kg)
and grain yield has been investigated.
Evidence suggests a grain P of around
3200mg/kg is a good ‘critical threshold’, as
yields stop increasing above it. Interestingly,
this offtake is less than RB209 suggests
(4000mg/kg, equivalent to 7.8kg/t of moist
grain) and could, at least in part, explain why
the rundown of soil P is less than expected.
Data from the Yield Enhancement Network
(YEN), revealed that only 17 per cent of grain
samples had more P than the RB209 value,
whereas 24 per cent had less P than the
possible critical value – suggesting insufficient
P