Flooding Has a Deep Impact on Soil

February 9, 2012 06:03 AM


The following story was written by a University of Missouri student as part of the 2011 Sonja Hillgren/Farm Journal Ag Journalism Field Reporting Institute. Learn more.

By Claire Friedrichsen
A large yellow mechanical arm scoops sediment out of a drainage ditch against the quivering carpet of vibrant soybeans in the Mississippi River Valley, near East Prairie, Mo.
Here, the New Madrid Floodway is painted forest green to neon yellow across the patterned farmland. Low-lying areas suffering from saturated soil conditions and lack of available nitrogen cause the discoloration.
"Drainage is essential here," said Robin Sitzes, Soil Conservationist for Mississippi County, Mo.
Saturated soil, lack of nitrogen and other problems were left behind this spring after the opening of the floodway, inundating130,000 acres of farmland. 
Swampy State. Not far away, Big Oak Tree State Park occupies 416 acres of the floodway. The park preserves a droplet of the hardwood bottomland swamp forest that covered this floodway 150 years ago.
Farmers came to this area for the same reason that six state champion trees are found here. This is one of the most fertile and productive soil basins in the United States, said Rose Marie Muzika, forest ecologist at the University of Missouri.
Historically the Mississippi River was wide and meandering, flowing across the river valley. The soil was formed by alluvial deposits of sediment over hundreds of thousands of years along with the decomposition of swamp vegetation.
The land became arable in 1909 after a government decree to drain southeast Missouri. The system of drainage, canals and levees, displaced more soil than the construction of the Panama Canal, according to The Little River Drainage District of Southeast Missouri.

Fast Forward and Rewind. A historic flood swept through the area in 1927, which provoked the Flood Control Act of 1928. The act federalized the levee system, creating interstate coordination for flood relief and protection for residents. As part of the flood control system, the New-Madrid Floodway was created.
At 5 a.m. on Monday, May 2, the first of three levee sections was blown away in the floodway, relieving pressure from the Mississippi River, which was swelling from heavy rainfall and snowmelt.
Digging Out. Twenty-five acres of farmland had six or more feet of sand deposited on them.
Two hundred twenty acres had between two and three feet of sand deposited.
Hundreds of miles of drainage ways were filled with silt.
Down to Dirt. Soil is comprised of six components: sand, silt, clay, air, water and organic material.
Soil has physical properties, like texture, structure and porosity. Texture is the ratio of sand, silt and clay. Soil has structure because aggregates like clay and organic matter hold the soil into units. The area between these units is porosity and can be filled with water and air.
Soil also has chemical properties, like the ability for soil to hold essential elements for plant growth like nitrogen.
The biological properties of soil include organisms that help fix nutrients into plant-available forms.
Drowning. The floodway was inundated from May 2 until June 7. Such long-term soil saturation kills aerobic microbes, allowing an increase in anaerobic microbes that can survive in the limited-oxygen environment. Instead of fixing nutrients into plant-available forms, anaerobic bacteria turn nutrients into gases like methane and nitrogen. These molecules can be toxic to plants, according to Gary Hoette, in his book Fertility Changes in Wet or Flooded Soils.
Standing water has other effects. Soil can become compacted due to dissolving aggregates in the soil, which results in a decrease in porosity, said Peter Motavalli, professor of soil fertility at the University of Missouri.
Drying Out. Lack of aggregation can mean a continued anaerobic environment after water has receded.
With the reintroduction of oxygen, phosphorous bonds into plant-unavailable forms, leading to phosphorous deficiencies in crops.
Tilling soil to incorporate air can promote aerobic soil microbe activity and decomposition.
"Microbes and mycorrhizal (activity) are limited and can be encouraged to restore using green manure," Motavalli said.
Nitrogen and phosphorous will become available slowly for plant use as organic matter decomposes. Higher quantities of phosphorous and nitrogen will need to be added in years following the flood, according to Hoette.           
Sandy Situation. Flood-deposited sand makes farms less productive. Sandy soils have smaller surface areas, retaining less water for plants and larger pores that drain quickly.
Management of Fields with Sand Deposits, by University of Missouri Extension, suggests incorporating the sand while keeping an equal ratio of sand, silt and clay.
Management techniques for the newly deposited material include increasing irrigation, choosing a more drought-resistant crop and increasing nutrient inputs.  

"Floodplains are risky to farm, but they benefit from high yields due to high productivity," Motavalli said.


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