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UW studies how cover crops, high rates of compost affect soil health

A long-term experiment by the University of Wyoming near Lingle is studying if dryland wheat farmers can become organically certified through use of compost and cover crops to improve soil health.

Starting in 2015, researchers from the ecosystem science and management and plant sciences departments in the College of Agriculture and Natural Resources looked into how soil health and wheat are affected by applying a high rate of compost once every 10 years – as many as 18 tons per acre, followed with cover crops.

The study is at the James C. Hageman Sustainable Agriculture Research and Extension Center and collaborating farms.

“The purpose of planting the cover crops is attenuating nitrogen through the cover crop biomass and perhaps create additional benefits to winter wheat by returning cover crop organic matter to the soil,” said Urszula Norton, an associate professor of agroecology in the plant sciences department.

The cover crops are:

  1. Pure stand of Lacy phacelia
  2. Cold-season nitrogen-fixer mix of spring pea, vetch, lentils, chick peas and oats
  3. A mycorrhizal mix of vetch, bean, oats, barley, Flax.
  4. Cool-season soil-builder mix of barley, oats, spring pea, lentil, sunflower.

The cover crops are expected to provide many benefits. A combination of the nitrogen taken up by the plants from soil as a nutrient and atmospheric nitrogen fixed by the leguminous plant species in the cover crop mixture (peas, lentils), all eventually converted into cover crop biomass, will break down and decompose in the soil, providing winter wheat a more useable form of plant nutrients.

Weed Control

Organic producers can’t use conventional weed control to retain certification and have to devise other methods. How the use of cover crops will compete with weeds is part of this project, said Norton.

Cover crops planted shortly following compost may be able to smother weeds due to the rapid growth of the cover crops and biomass production before weeds emerge.

Inadequate soil moisture for the subsequent winter wheat planting and precipitation in the early stage of wheat growth are the biggest concerns for using cover crops in an area with low precipitation. Cover crops need to be killed early but allow enough time to establish enough biomass to smother the weeds.

The first year of the project showed reduced weed biomass and weed species diversity that would normally be present in the bare fallow stage, according to Norton.

“Also, in the first and second year of monitoring, we observed a reduced population of weeds in the winter wheat,” Norton said. “So, I think it’s kind of a win-win situation.”

So far, water stress on winter wheat has not been reported; however, it is still a question of how many years can this be sustained before water stress becomes an issue.

Plant Performance and Soil Indices

To track the soil indices, the researchers are looking at trace gas emissions, like carbon dioxide, methane and nitrous oxide. These indices are sensitive to change. Too much compost might cause a high amount of mineralization, creating more carbon dioxide and other gases.

Planting cover crops following high rates of manure application will allow incorporation of all the nitrogen from compost into the biomass, improve soil health and increase sustainability of the system.

Collaborating Farmers

While some components of this project are at SAREC, some collaborating farmers are growing cover crops and allowing researchers to collect additional variables.

Some farmers are looking at the benefits of cover crops by planting in skipped rows to see if more soil moisture can be retained.

They are also looking at different seed mixtures to determine which cover crop is best suited for the environment. Norton said her team will monitor growth and performance of winter wheat following the cover crop.

Next Phase

Norton said the project will continue monitoring soil health and moisture content over the next several years to determine which cover crop is most-suited for the environment, the best times to plant that crop and at what point should the crop be killed to allow enough biomass to smoother the weeds but conserve soil moisture.

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