Fertility and Nutrients
Giving field peas an extended crop production credit
Field trials and tissue testing confirm greater than expected N benefits.
November 19, 2007 By John Harapiak
Including a legume crop such as field peas within a crop rotation has benefits
that extend beyond the amount of peas harvested. However, pinning down the dollar
value of this benefit on the following crop has been difficult. For this reason
Kaun's Seed Farm, located at Penhold in west-central Alberta, and its crop consultant
are attempting to quantify the value of the 'pulse benefit' for this farm. The
information they have gathered has been a real eye opener for them, as well
as the neighbouring growers who have seen this information.
Making use of historical yield records
Based on a review of field records alone, David Kaun was able to start quantifying
the impact of growing a crop of peas on the following crop. For instance, his
detailed cropping records indicate that the yield boost to a cereal crop following
peas in a rotation was in the range of 20 percent. On the other hand, the yield
gain that accrued from growing a cereal crop following a crop of canola was
only about 10 percent. This information helped convince Kaun and brother Mark
that they had been under-estimating the value of including peas in their cropping
Evidence in tissue analysis of excess N
David and Mark feel that the ability of pulse crops to fix their own N is a
real plus for growing peas. However, due to the rising cost of fertilizers,
the Kaun brothers felt there was an additional aspect resulting from the residual
benefits of pea production that they wanted to quantify; namely the impact of
a pea crop reducing the N fertilizer requirements for the following crop. Their
suspicions were supported by the fact that the analysis of tissue samples collected
by their crop consultant from crops following peas tested higher than required
in N content.
Field strips test fertilizer recommendations
Kaun and his crop consultant set out strip trials that were designed to assess
the impact of the previous crop on profitability resulting directly from reducing
the rates of fertilizer application. In these trials, they compared the full
or recommended rate of fertilizer to the performance of the 1/3, 2/3 and 4/3
rates. They selected a uniform field area and laid out replicated strips over
the full length of the field. The strips were the width of a single pass of
the air-seeder, which exceeded the width of their swather. The swaths were then
combined and the yield of each strip was determined using a weigh wagon. More
recently, they have switched to straight combining some of these test strips.
Fertilizer rate correct on barley stubble
In 2005, in a fertility strip trial where canola was grown on barley stubble,
the yields ranged from a high of 62bu/ac of canola for the full fertilizer rate
(i.e. 80-35-40-20), to 55 and 48 bushels per acre respectively, for the 2/3
and 1/3 fertilizer rates. The canola yield for the 4/3 rate of fertilizer was
61bu/ac. For these trials, the recommended rate of fertilizer proved to be the
most economical, thereby confirming the value of following the soil test recommendation
made by Green Key. Similar trends were also observed when wheat was grown on
Test fertilizer recommendations on pea stubble
They found that the results from strip trials of the recommended fertilizer
rates for barley grown on pea stubble were quite different. Over a period of
three years, four separate barley trials were established on pea stubble. The
average fertilizer N, P2O5, K2O
and S recommendations for these four trials was 70-30-40-10 pounds per acre
respectively. The average barley yields were 124, 123 and 120 bushels per acre
respectively for the full, 2/3 and 1/3 rates of fertilizer.
Reducing fertilizer improves economics
Compared to the full rate of fertilizer, the average economic returns were increased
by $15.90 per acre by reducing the fertilizer application to 2/3 of the recommended
rate. By further reducing the fertilizer application to 1/3 of the recommended
rate, the economic returns were increased by an average of $34.20 per acre.
Kaun and his crop consultant suggest that cutting back on the N component of
the fertilizer blend had the greatest impact on improving the economic returns
in these trials.
Residual benefits improve pea economics
Kaun states, "In addition to the peas providing their own N, when you factor
in these rotational benefits, it makes the economics of growing peas look a
lot better." He suggests, "Other farmers should also be assessing
this rotational benefit to the crop following peas. Subtracting the revenue
gained from the extra 10 to 20 percent yield increase of the following cereal
crop and adding it back to the revenue of the pea crop gives a clearer picture
of the total benefit of growing peas." Kaun feels that for their farm,
"This additional credit to the pea crop ranges between $30 to $55 per acre.
These rotational credits can change the economics of pea production quite significantly."
Many practical benefits of including peas
For Mark, one of the important benefits of growing peas is that it helps to
spread the harvest workload for their various crops. He states, "By starting
to harvest our peas in August, that means less crop is harvested later in the
year when harvest conditions can be less than ideal." Since they are in
seed production, they cannot use Roundup to speed crop maturation. Mark points
out that they can seed peas earlier, thereby also spreading the spring workload.
Pea 'rotational benefit' often under-estimated
Prairie crop consultants have gradually been learning that there can be a significant
yield boost following a crop of peas, especially on some of the more fertile
soils located within the Black soil zone. The Kauns' fields have a long history
of aggressive fertilization, especially with respect to applications of N fertilizer.
This action would have undoubtedly created a more dynamic pool of soil organic
matter that would release more crop-available N earlier during the growing season.
Potential pea benefit varies with location
Should the Kauns' results be expected in all areas of the prairies? Certainly
not in all cropping situations. Researchers suggest that gains in wheat yield
due to the rotational benefits of the previous pea crop have been reported to
be 20 to 30 percent in the higher rainfall areas of the prairies. In the drier
regions, researchers suggest a yield boost of zero to 10 percent. However, some
growers located within the drier regions have felt that the benefit exceeded
Conflicting explanations for pea benefit
Prairie based researchers have suggested that the pulse rotational benefit following
peas should primarily be attributed to suppression of soilborne diseases. However,
as demonstrated in the case of Kaun's Seed Farm, it is obvious that an added
release of N to the crop following peas also contributed quite significantly
to the benefit that was observed. The boost in soil N supply following peas
could partially be attributed to the residual benefit of their long-term, soil-building
N fertilization program. In other words, their fertilizer management had enabled
them to create a more fertile and more productive soil. Perhaps the pea crop
helped to further stimulate a more rapid recycling of the residual N fertilizer
that was present within the soil.
Strip trial results change practices
Previously, the Kauns had applied significant amounts of fertilizer to their
pea crops. Now as a direct result of the information learned from their strip
trials and in recognition of the tremendous ability of peas to fix N, the brothers
have now completely eliminated the application of fertilizer to peas. In addition,
for cereal crops grown on pea stubble, 30lb/ac is the maximum amount of fertilizer
N that they will consider applying. It is obvious that the information learned
from conducting strip trials on their farm has resulted in considerable savings
in input costs.
John Harapiak has more than 40 years of western Canadian based
fertilizer related experience. He will continue to contribute stories to
Top Crop Manager. He can be contacted by e-mail at: firstname.lastname@example.org.