Extension Ag Update
May/June 2006
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Grain-Forage Crop Rotations Seen Boosting Soil Quality

Luis Pons, ARS News Service, ARS-USDA, 301-504-1628, lpons@ars.usda.gov

Farmers looking to maintain soil quality may want to get back to planting extended rotations of grain and forage crops. The study--headed by soil scientist Douglas Karlen of ARS' National Soil Tilth Laboratory in Ames, Iowa--found that crop rotations covering a minimum of five years, including at least three years of forage crops interspersed with corn and soybean, resulted in higher soil-quality ratings than either continuous corn or a two-year corn-soybean sequence.  The longer-term rotations also had an additional benefit: They were more profitable than continuous corn production.

According to Karlen, the study shows the need to create new markets and new uses for forage crops so that producers will have financial incentives to diversify their crop rotations.  Larger farm size, specialization, and separation of agricultural crop and animal enterprises--along with pressure to maximize short-term profit throughout the nation's corn and soybean belt--have decreased implementation of long-term crop rotations over the past 50 years. The result, according to Karlen, has been crop rotations that leave land bare for nearly six months each year, spurring organic-matter decomposition and erosion if the soils are tilled.

The researchers collected soil samples from three long-term crop rotation studies and one long-term organic study in Iowa and Wisconsin. They analyzed the samples for several physical, chemical and biological soil-quality indicators that were then used to develop an overall soil-quality index (SQI). Soil samples from extended rotations that included at least three years of forage crops such as alfalfa or oats scored the highest SQI values. The lowest SQI values were associated with continuous corn.

Deep Rooted Alfalfa

Alfalfa has long been known for its drought tolerance.  There are few plants that match alfalfa’s deep roots.  Gary Lacefield, University of Kentucky forage specialist, found that in a good growing season, alfalfa roots could grow as deep as five feet in six months.  He also looked at historical studies to confirm his findings.  The literature often discussed active roots penetrating between four to twelve feet.  The most interesting report was from the early 1900’s.  During an investigation of a mining tunnel in Nevada, a USDA scientist observed alfalfa roots from the alfalfa field 129 feet above the tunnel. 

Higher Yields with Manure Applied Before Alfalfa

Research, done by Bruce Anderson, University of Nebraska forage specialist and reported in the University of Nebraska Crop Watch Newsletter, shows that applying manure to alfalfa fields prior to planting. Manure is rich in nutrients that alfalfa needs such as phosphorus, potassium, sulfur and many micronutrients. Manure can also improve soil tilth and encourage soil microbial activities that could increase nitrogen availability.   Up to 12,000 gallons of liquid manure or 50 tons of dry manure per acre resulted in higher alfalfa yields than commercial fertilizers at the same nutrient levels. Alfalfa yields were higher on both low- and high-fertility soils. Manure should be tested to know how much to apply.  Anderson cautions producers to also keep an eye out for weeds since manure can increase weed growth as well. 

Pig Feeding Strategies Impact Manure Gases

O. Grant Clark, Soenke Moehn, Ike Edeogu, Jason Price, and Jeremy Leonard

Pig manure emits odorous (hydrogen sulfide) and greenhouse gases (carbon dioxide, methane and nitrous oxide). Changing pigs’ diets can alter manure gases. One way to slow gas is to closely match feed nutrients to pigs’ requirements. A study, published in Journal of Environmental Quality, tested feeding strategies.  When pigs were fed less protein, manure emission of carbon dioxide and methane increased. Greenhouse gases decreased when enzyme was added to high-protein diets, but increased with low-protein diets. High-protein diets resulted in manure with more sulfur, but odor wasn’t impacted. Manure from diets with more nonstarch polysaccharides contained less nitrogen; manure odor didn’t change.

Locations of Soil Organic Carbon Dramatically Alter Soil Structure Strength

Eun-Jin Park and Alvin J. M. Smucker

The more stable soil aggregates are, the less soil erosion will occur.  No-till alters soil structural components of porosity, stability and their spatial distributions of carbon contents that in turn alter carbon sequestration and soil erosion. In the Soil Science Society of America Journal, researchers identified twice as much soil organic carbon and increased porosity within soil aggregates that had not been tilled for 35 years. These changes significantly improved aggregate stability.  The overall mechanical strength of the aggregates was affected by the type and quantity of clay.