What we’re seeing from continuous-crop research

jack dewitt


Recently, I received a surprise package in the mail from my friend Richard Smiley, Professor Emeritus of Plant Pathology at Oregon State University. It was a large book — 444 pages and 8½ inches by 11 inches. Titled Dryland Crops Research Stations in Eastern Oregon, Moro and Pendleton: 1909-2020, it was researched and authored by Smiley. It is a well-written history of the research and the people who did it at those stations. Not a page-turning novel, but open it anywhere, and I guarantee that people in agriculture will find something of interest to read.

I have always had a special interest in long-term plots and what they can tell us about our soils and where we are going with our cropping practices. Some plots the Pendleton Station have been in continuous wheat since 1932, without adding nutrients of any kind. Yield averages about 30 bushels per acre. This tells us that natural processes will supply approximately 70 pounds of nitrogen per year. These natural processes are nitrogen in rainfall (and some sulfur), decomposition of organic matter, and nitrogen fixation by soil bacteria. Minerals are supplied by weathering of soil particles and decomposition of organic matter.

There are seven long-term experiments at the Pendleton Station. Crop residue management plots in a wheat-fallow rotation were stated in 1931. Annual plots of winter wheat, spring wheat and spring barley were started in 1932. Tillage-fertility plots (wheat-fallow rotation) began in 1940, starting with ammonium sulphate as the nitrogen source and changing to ammonium nitrate in 1963. From 1988 to the present the nitrogen source has been solution 32. Rates go from 0 to 160 pounds of N per acre. A wheat-pea rotation experiment has been on-going since 1963. Annual cereal crop no-till plots were begun in 1997, and no-till wheat-chemfallow plots were started in 1982.

Since 2010, winter pea cover crop plots have been superimposed on the 1997 long-term no-till plots. Research results on these plots have not yet been published, but it appears growing the pea crop and terminating it in late June incurs no yield penalty and can result in up to a 6 bushel bonus yield in the 16-inch rainfall zone at the Pendleton Station. If statistically proven, this would be good news for wheat-fallow growers trying to reverse organic matter losses inherent in a wheat-fallow system, with the additional benefit of nitrogen input from the peas.

The crop residue management plots include several plots in which the stubble is burned in either the fall or spring. After 58 years of this treatment (1989) the infiltration rate had deteriorated so much that lister furrows were installed between the plots each fall to prevent runoff from the burn plots onto neighboring plots. This is something to think about when burning a field or baling the straw. Significant changes in soil texture may not be evident for a generation or more of continuous burning or baling, but my conscience won’t let me burn or bale even one year if I can help it. Sustainability requires putting as much residue as possible back into the soil every year. Farmers that can grow a cover crop should do it. Big yields are no excuse to burn or bale. With modern equipment my neighbors routinely return all residue from 130 bushel-plus wheat crops. Even with equipment 30 to 40 years old I have found ways to deal with straw from a 135-bushel crop without moldboarding.

The most interesting plots to me are the long-term continuous cereal plots. Monoculture is the word environmentalists and pundits like to use to pejoratively describe this kind of continuous cropping. They even use it to describe corn-soybean rotations, a perfectly sound cereal-legume rotation. Continuously cropping to the same crop, they claim, ruins the soil. Long-term plots indicate otherwise when crop refuse is returned and nutrients added.

Continuous wheat plots at the Pendleton Station that have received adequate nutrients continue to produce normal yields for the area year after year. Other experiment stations around the U.S. get the same result. The Agricultural Experiment Station at the University of Illinois at Urbana-Champaign has plots, named the Morrow Plots, that have been in continuous corn since 1876. One plot has never received any supplemental nutrients. In the beginning, yields were in the high 40s (bushels per acre), but as organic matter levels dropped, yields dropped into the 20s. In 1955 the plot was split and lime, nitrogen, phosphorus and potassium were added yearly to one portion. Yield more than doubled in the first year, and by the third decade of treatment yield had tripled to a respectable 121 bushels. Organic matter, which had fallen to 2.3 percent by 1955, rose steadily with bigger crops, and was 2.9 percent by 1973.

The Magruder plots at Oklahoma State University have been in continuous wheat since 1892. After 112 years without fertilization (2004) the yield was 18.5 bushels per acre. Plots receiving 238 pounds of nitrogen as manure every four years (equivalent to 60 pounds per acre per year) yielded 39.5 bushels in 2004. Plots receiving 60 pounds of inorganic nitrogen per year yielded 41.7 bushels per acre.

Another set of long-term plots at the Rothamstead Research Station in England have been growing wheat continuously since 1843 on land that had been farmed for centuries before that. Plots now receiving high amounts of inorganic nitrogen exceed yields of plots that have received 16 tons of manure yearly, in spite of the fact that manure treated plots have double the organic matter of the inorganically fertilized plots. The best yields on these plots now exceed 166 bushels per acre and organic matter is increasing.

Closer to home, I have a field that has been in continuous wheat for 45 years. Located on the foothill slopes of the Blue Mountains near Walla Walla, Washington, the rain-fed silt loam soil ranges from deep to very shallow. Rainfall was generous this year and the yield was the best ever — 130 bushels per acre. I don’t mean to be an advocate for monoculture, just saying it is not the soil-killing practice pundits say it is and any negative effects can be quite quickly reversed.


Jack DeWitt is a farmer-agronomist with farming experience that spans the decades since the end of horse farming to the age of GPS and precision farming. He recounts all and predicts how we can have a future world with abundant food in his book “World Food Unlimited.” A version of this article was republished from Agri-Times Northwest with permission.

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