A new conservation practice reduces cropland erosion to sustainable levels even on moderately sloping land: contoured strips within corn and bean fields, planted to native prairie grasses. The deep rooted grasses slow runoff, trapping suspended soil and nutrients. They also provide habitat for insects and wildlife. No more than 10% of a field need be put in prairie strips to gain full benefits from the practice.
Researchers in the STRIPS project (Science-based Trials of Row-crops Integrated with Prairie Strips) developed the "prairie strips" practice in an ongoing experiment using flume-metered test fields. The test fields lie on slopes of 6-10%, in soils moderately damaged by previous erosion.
Each test field is cultivated by the "no-till" method — no plowing and minimal cultivation. At harvest time, a combine chops cornstalks into a mulch, spreading it back onto the soil. Spring planting is done directly into this mulch layer, without disturbing the soil.
Many farmers have adopted no-till methods, as much to reduce expenses as to protect topsoil. But no-till alone may not reduce erosion to a sustainable level in a particular field. During a year with heavy midsummer rains, the test field without prairie strips lost 11 tons per acre — over twenty times the estimated rate of soil formation (half a ton per acre per year), and much more than would be predicted by standard estimation methods.
Test fields with prairie strips lost approximately half a ton per acre that year — 95% less — putting them approximately in balance with soil formation. In a year with milder rains, the prairie strip fields lost approximately 100 pounds per acre, suggesting that they added topsoil during the year. The field without prairie strips lost nearly four tons per acre.
From STRIPS brochure, permission requested.
Many soil conservation practices use permanent grasses with soft stems, such as brome or fescue, to provide a slippery surface which lets rainwater flow rapidly without disturbing the soil underneath. Prairie grasses, in contrast, provide a barrier that slows runoff, allowing it to sink into the ground, trapping suspended soil and dissolved nitrogen. (One farmer has successfully converted some of his grassed filter strips to prairie grasses.)
Prairie strips enhance the biodiversity of a field out of proportion to their small land area. Species counts on the test fields showed twice as many bird species and five times as many plant species on the stripped fields. The strips support 70 species of bees as well as some insect predators of corn and soybean pests.
The STRIPS staff is working with farmers to introduce the practice on working farms, targeting those on highly erodible land. A number of farmers have installed the strips, some featured in the STRIPS documentary. USDA will probably provide cost sharing for installation and rent, for a limited term, on land taken out of production.
These subsidies help all taxpayers to some extent, not just farmers and landowners. NRCS policy has long aimed at improving water quality as well as protecting topsoil. The effects of cleaner water extend beyond local streams and lakes, right down to the Gulf of Mexico.
The "dead zone" in the Gulf is created by soil and nutrients washing into the Mississippi from Midwestern farms. Algal blooms, feeding on the nitrogen and phosphorus, reduce dissolved oxygen in the water to a level where fish can’t breath. Prairie strips and other soil conservation measures, supported by tax dollars, are helping to clarify Mississippi waters and ultimately to shrink the dead zone.
Mississippi-Missouri watershed and the Dead Zone. From Marine Science Today, permission requested.
Prairie strips provide one more soil conservation option for farmers and landowners. USDA subsidizes all conservation practices at some level, depending on the "farm bill" currently in force and the risk of erosion on a particular field. The funds available are always directed to the most vulnerable land.
The risk of erosion varies widely from field to field, depending on soil type, slope, even the size and shape of the field. Landowners tailor their conservation practices — if any — to the risk level of a particular field, and also to the level of government support available. The STRIPS test fields fall somewhere in the middle of the risk spectrum, between nearly flat farmland on good soil, and steeply sloping land with soils that wash easily.
Many conservation practices are available, often with USDA support, but only the land owner can take positive action to prevent soil depletion. On average, it’s not being done. Iowa is losing about five tons of topsoil per cropland acre per year, ten times the theoretically sustainable rate.
But hidden in this average are many carefully managed farms using the best conservation practices, and stretches of level land on good soils that have a minimal risk of erosion. With new options such as prairie strips and growing public awareness of the nationwide impact of farm runoff, there are more opportunities to turn this trend around.
Everything I’ve said about "erosion" in this and previous posts applies only to water-borne soil erosion, essentially the only soil conservation concern in the midwestern corn belt. Further west, in the high plains, wind erosion becomes the overriding concern. It’s a different world.