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Nitrogen credit:

You grew a pulse crop last year. Can you cut back on nitrogen fertilizer this year? It depends.


November 19, 2007
By Bruce Barker

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In an era of high nitrogen (N) fertilizer prices, every penny counts, so many
growers are looking for ways to cut back on N fertilizer inputs after a pulse
crop. However, many factors and interpretations go into the decision, including
N credits already accounted for by soil test laboratories, the type of pulse
crop and even the farm's location.

"Whether you allocate a N credit to your pulse crop is a good question.
It's hard to answer without considering several factors," says Don Flaten,
a professor of soil science at the University of Manitoba. "There has been
a lot of research on the N benefit of pulses, but you have to separate out the
N benefit from the rotational benefit. The other consideration is when does
the N benefit show up in the soil test? Perhaps it has already been accounted
for and further deductions shouldn't be made."

Over the years, researchers have developed a rule of thumb for the amount of
N released from a pulse residue. Many soil test laboratories deduct these credits
from the N rates when establishing fertilizer recommendations. Since this practice
varies from laboratory to laboratory, growers should ask their agronomist or
soil test laboratory if they have provided a N credit to their fertilizer recommendation.

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Sample calculation:
3000lb/ac field peas x 5 divided by 1000 = 15lb.ac N credit.

For years, researchers have tried to separate out N credits from rotational
benefits. Research in Manitoba by David Przednowek and Martin Entz at the University
of Manitoba evaluated N contributions from preceding crops of field peas, soybeans,
chickpeas and dry beans in southern Manitoba. They found that apparent N credits
are small (on average 10lb/ac N or less) for a crop following peas compared
to following a non-pulse crop of flax.

The University of Manitoba study also showed the rapid increase in soil test
N values after pea crops. Soil samples collected in early spring showed that
soil nitrate-N concentrations were significantly higher where field peas were
grown, compared to flax. However, during the growing season for the subsequent
wheat crop, more N appeared to be released by flax stubble than by pea stubble,
especially later in the growing season, between flowering and harvest.

Other research in northeastern Saskatchewan found that fababeans, field peas
and lentils improved barley yield by 21 percent in the first year and wheat
yield by 12 percent in the second year following the pulse crop. However, the
study found that even at rates of 180lb/ac, the barley yields on barley stubble
were not equal to the barley yields of those on pulse residues. This would indicate
the yield benefits were not just due to the N credit from the previous pulse
crop but rotational benefits were also at play.

Indeed, other research at the University of Saskatchewan by Stevenson and Van
Kessel found that the additional N credit determined in a peas-wheat rotation
only explained eight percent of the increase in wheat yield following a pulse
crop. The other 92 percent of the yield advantage was attributed to non-N rotational
benefits.

That non-rotational benefit was supported by research done by Agriculture and
Agri-Food Canada's Hugh Beckie at Saskatoon. In his study, field pea rotations
were compared with mono-cereal, mono-oilseed, as well as mixed cereal-oilseed
rotations. The results indicated that crops grown on field pea stubble generally
yielded the highest. However, the non-N benefit/ rotational effect of field
peas to the following crop was similar to that of an oilseed crop following
a cereal crop. Beckie notes that, as in previous studies, the non-N benefit
of field peas to the following crop was large when compared with mono-cereal
or mono-oilseed rotations – but equally large was the non-N benefit of
a cereal crop to a following oilseed crop or oilseed crop to a following cereal
crop. He also found the N credit was higher for the Black soil zone than the
Dark Brown soil zone.

If N credits are small, should they be deducted from
fertilizer rates?

Flaten says several more considerations come up when looking at soil tests and
N fertilizer rates after a pulse crop. If conditions after the field pea harvest
are warm and moist, rapid mineralization of N occurs from the pea stubble, resulting
in higher levels of nitrate-N detected by the late fall nitrate soil test. In
this case, fertilizer recommendations based on the soil test would not change.
Under these circumstances, field peas provide a nitrogen credit, but since the
soil test would already have picked up the additional mineralized N, the field
should be fertilized as per soil test without an additional N credit in order
to optimize the yield potential of the following crop.

However, predicting mineralization of pulse residues in the fall is somewhat
hit-and-miss, and growers need to use some common sense when looking at whether
N credits should be deducted from fertilizer recommendations. Ross McKenzie,
an agronomy research scientist with Alberta Agriculture, Food and Rural Development
(AAFRD) at Lethbridge, says consideration must be given to the type of pulse
crop when it was harvested and the fall environmental conditions. He agrees
that early harvested peas will have higher levels of mineralized N if the late
summer and fall weather conditions are warm with moist soils. Conversely, if
the soil is dry, the weather cool, or a crop like chickpeas is harvested late,
little if any N will be mineralized from crop residue and the soil test will
not pick up the N credit.

"How you approach N fertilizer rates based on pulse N credits is hard
to figure out and takes a bit of guess work," says McKenzie. "It really
depends on the crop and weather conditions after harvest."

Flaten says the other consideration is whether a N credit should be adjusted
at all, even if a grower knows for certain what it is and if it had been accounted
for in the soil test recommendations. He explains that with the non-rotational
benefit producing higher cereal yields after a pulse crop, perhaps growers should
be setting higher target yields to take advantage of the pulse crop's N credit.
Rather than cutting N fertilizer rates, perhaps they should be left the same
as if a non-pulse crop was grown.

Table 1. Average N released from pulse
residue.
Crop lb/ac N released per 1000lb/ac pulse yield Minimum yield of pulse crop used Example N credit
Chickpea 2.5 2000 2500lb yield (41bu/ac) = 6.25lb/ac N credit
Fababean 5.0 1500 3000lb yield (44bu/ac) = 15lb/ac N credit
Field bean 2.5 800 1000lb yield (16lb/ac) = 2.5lb/ac N credit
Field pea 5.0 2000 3000lb yield (50bu/ac) = 15lb/ac N credit
Lentil 4.0 1000 1500lb yield (25bu/ac) = 6.0lb/ac N credit
Soybean 4.0 2000 2400lb yield (40bu/ac) = 9.6lb/ac N credit
Source: Adapted from Pulse Agronomy Update #26; 2005.

For growers, then, the key is to work with their soil test laboratories and
agronomists to factor in all these considerations when establishing fertilizer
N rates. Yes, there are N credits attributed to pulse crops, but they are generally
small. Understanding that they may or may not show up in a fall or early spring
soil test will further refine the recommendation. Also, to take advantage of
the non-N rotational benefits, growers may want to consider setting higher target
yields as a way to ensure their fertilizer recommendation is appropriate.

Finally, the best way for producers to 'audit' their N fertilization program
after pulse crops is to test the soil for N after harvesting the subsequent
crop. If the yield of the subsequent crop was good and residual N is higher
than expected, the N fertilizer rate can be trimmed back a little further. -30-