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Yield and quality maintained

After four years, scientists have found no yield or quality problems from using compost instead of inorganic fertilizer

November 14, 2007
By Helen McMenamin


6aAfter four years, scientists have found no yield or quality problems from using
compost instead of inorganic fertilizer, but the system is in transition to
a new equilibrium.

Practices that aim to build the soil ecosystem may seem impractical or too
costly to implement around high value crops like potatoes or beans, but this
may be short-sighted. An Agriculture Canada project at Lethbridge aims to assess
the long-term impact of different management practices.

"We want to be able to grow potatoes and other crops on this land 30,
50, even 100 years from now," says Frank Larney, the soil scientist who
is leading the project. "We want to know whether our conventional cropping
systems are sustainable or if we're mining the soil."

The project uses some 'sustainable practices' in intensive irrigated cropping
systems: direct seeding or reduced tillage whenever possible; seeding fall rye
as a cover crop after potato harvest, then direct seeding narrow row beans into
the residue following burnoff with glyphosate; using compost instead of all
or part of the inorganic fertilizer requirements of potatoes; and solid-seeding
beans so they can be straight cut rather than undercut.

Plots farmed this way are being compared to conventionally farmed plots in
three and four year rotations of potatoes-beans-wheat and potatoes-wheat-sugar
beet-beans. Sustainable practices are also used in five and six year rotations
of potatoes-wheat-sugar beet-wheat-beans and oats underseeded with timothy,
which is harvested the next two years and followed by sugar beet-beans-potatoes.
Continuous wheat plots provide baseline data.

The sequences, particularly the three and four year cycles, are typical for
row-crop farms in southern Alberta and timothy is a popular break from row crops.

The project was started in 2000, so it is too soon to have any definite results
yet. The scientists can say the sustainable practices do not lower potato yields
or quality, or those of any of the other crops. Even when compost replaces all
the fertilizer P, potato yield and quality matched those grown conventionally.

"We looked for differences in emergence and vigour," says Larney.
"We thought inorganic phosphate would have more of a pop-up effect, because
only about 10 percent of the nutrients in compost are available. But, we couldn't
find any differences due to phosphate source."

Because compost contains only 1.5 percent N and phosphate equivalent, application
rates are high, up to 17 tonnes per acre, Larney hopes to see clear benefits
as the organic matter increases. Soil structure should improve and higher organic
matter makes nutrients more accessible to plants and supports greater populations
of soil microflora, microfauna and arthropods (insects and similar creatures).

Compost, with its high trace elements, protects against micronutrient deficiencies.
Also, it may suppress diseases such as scab. You might expect more information
after four years of field work, but that is not the way long-term studies work,
says Bob Blackshaw, the weed scientist in the project.

"The value of a long-term project isn't evident for years," he says.
"You need patience for rotation studies. It's likely another generation
of farmers and scientists who will reap the benefit of our investment and effort.
They may find value in some quite unexpected aspect of the study, just as we
have in studies started long ago."

The scientists are gathering samples and taking measurements, but they will
not leap to any conclusions yet. "The system needs to come into a new equilibrium
to show the impact of a system," says Blackshaw. "Everything's in
transition during the first rotation cycle, so any conclusions we draw in the
early years are quite likely wrong.

"During the second cycle, we start to get an indication of what's happening,
then we hope results from the third rotation are consistent and confirm our
findings. So, in this case we won't have definitive results until 2012."

Rotation studies are very costly and labour intensive. To ensure weather that
favours a particular crop in a particular year does not skew results, every
phase of each rotation is grown every year. Then, to account for field variations,
treatments are replicated, usually four times. Plots are relatively large, 18.3
by 10 metres, so that even if the edges get blurred over the course of long-term
trials, the plots retain their integrity.

Larney's 104 plots cover about 20 acres, increasing the chances of variation
among plots. This means average yields that seem quite different may not be
statistically significant.

Staff continuity is difficult, too. Scientists and technicians may move or
retire. Changes of technicians can be especially challenging, because they know
how to get farming tasks done.

As with farming in the real world, anything that goes wrong one year can be
compounded the next. But this may add to the value of the information, because
the studies are done in the real world.

The biggest challenge to long-term research is funding. Funding agencies generally
expect results in three or four years. Long-term studies may not seem exciting,
especially in their early years, before they start to yield results.

For the moment, the scientists involved in the potato rotation study measure
and take samples of everything from soil to crops and insects. When the system
starts to stabilize, they will analyze these samples and data along with those
they collect then. You need patience for rotation studies. -30-