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Managing lentil fertility

Lentils show good response to granular inoculants, but nitrogen and phosphate response are variable.


November 19, 2007
By Bruce Barker

Topics

Research by Agriculture and Agri-Food Canada at Swift Current, Saskatchewan,
is firming up lentil fertility recommendations. Conducted by research scientist
Yantai Gan, of AAFC Swift Current, it shows that formulation and placement can
have a dramatic effect on yield. However, soil type, soil fertility and inoculant
type also have large impacts on final yield.

"The use of inoculants increased lentil yield by 45 percent averaged across
the six site years. In four of those six site years, the increased seed yield
was due to increased seed size," explains Gan. "Between the two formulations,
the granular soil inoculants increased seed yields by three to 38 percent in
five of the six site years compared to seed-applied inoculants."

The field experiments were conducted on a silt loam soil at Swift Current and
on a heavy clay soil at Stewart Valley. Plots were direct seeded into wheat
stubble using a hoe press drill. Peat powder and granular inoculants were compared.
Granular inoculant placement was either seedrow placed or side-banded. Starter
N and P, when applied, were applied at 15kg/ha. The fields had no legume crops
grown in the previous five years.

The positive influence of inoculation on seed yield and seed mass in lentils
was partly due to the inoculation promoting nodule formation. Measurements at
flowering showed that the inoculated lentils produced 9.6 nodules per plant
with nodule dry mass of 9.2mg per plant, which were, respectively, 92 and 76
percent greater than those measured on lentils that was not inoculated.

Granular formulation usually better
Gan says that between the two formulations of inoculants, the lentil plants
with granular soil inoculants produced 12.6 nodules per plant with nodule dry
mass of 11.9mg per plant, which were 47 and 45 percent, respectively, greater
than those measured on lentils with seed-applied inoculants.

"Granular inoculants applied to the soil allow rhizobium to become more
uniformly distributed in the rooting zone, encouraging more nodules to be formed
on the lateral roots," explains Gan.

Responses of lentils to inoculation were stronger and more consistent on the
heavy clay at Stewart Valley than those on the silt loam at Swift Current in
Gan's study. The use of inoculants increased the seed yield of lentils by an
average of 15 percent at Swift Current, while it was 70 percent at Stewart Valley.
Similarly, on the clay soil, soil inoculation increased lentil seed yield by
26 percent over seed inoculation, while on the silt loam, soil inoculation performed
similarly to seed-applied inoculation with only two percent yield difference.
Gan explains that the greater response to soil inoculation on the heavy clay
was probably due to the greater water-holding capacity of the clay, that had
permitted better root development and rhizobium colonization.

The research also found that placement of granular inoculant in either the
seedrow or side-band resulted in similar seed yield and seed mass. As a result,
this provides lentil producers with options to apply granular inoculants in
either the seedrow or side-banded, depending on equipment configuration.

Gan says that a basic economic calculation found that the extra net income
obtained from increased yield with granular soil inoculants is far in excess
of the added cost associated with the use of granular inoculants compared to
peat-based inoculants.

Starter N sometimes useful
The use of 15kg/ha starter fertilizer N increased lentil seed yield by an average
of 13 percent at Stewart Valley. The highest response of yield to starter N
was in 2002 when there was a 24 percent yield advantage with N fertilization.
The large portion of the increased yield was due to the increased seed dry mass.
Although the addition of starter N reduced the number of nodules by 67 percent
and nodule dry mass by 150 percent in lentils, the strong response of lentils
to fertilizer N over-shadowed the negative impact on nodulation. By comparison,
there was no effect of starter fertilizer N on the yield of inoculated lentils
at Swift Current.

"If the stubble crop previously grown was a high yielding wheat or canola
crop and most of the N was used up, then starter N might be beneficial to the
lentil crop," says Gan. "But if the soil has 20 pounds per acre of
available N, then you should be okay without it."

In this study, starter P at 15kg/ha did not significantly affect plant growth
or yield. In four of six site years, there was a small four percent increase
in yield, but it was not statistically significant. Pre-seeding soil tests found
that bicarbonate extractable P was between nine and 28kg/ha. Gan says that starter
P may not be needed on soils testing at these levels.

"Phosphorus is very important in the nitrogen fixing process. Insufficient
amounts of P in the soil will negatively affect nodule formation and the functionality
of the nodules. Therefore, growers should soil test to ensure that phosphorus
fertility is adequate," says Gan. "If a soil test is not available,
growers could make some guess based on the previous years' crops and the size
of the yields. Starter P may provide some response to plant growth when soils
are low in residual P."

Putting a seeding system together
For air-seeders with a two tank delivery system, the use of granular inoculants
will preclude application of fertilizers, while low doses of fertilizer N are
somewhat beneficial to a lentil crop particularly on the heavy clay. In this
case, use of granular soil inoculants is the priority over N fertilization,
since greater economic benefit can be derived from inoculants. Do not mix granular
inoculant with N fertilizer in the same tank because rhizobium may be partly
killed by toxin released from the N fertilizer. Another alternative is to use
a peat-based inoculant applied to the seed, should fertility levels call for
N-P starter fertilizer.

In a three tank seeding system, more options are available to accommodate multiple
requirements, so granular inoculants can be coupled with starter N and P. The
use of granular soil inoculants may also result in a more uniform distribution
of rhizobium in the root zone, a more effective nodulation and increased seed
yield. -30-