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The Iowa Legislature last year passed a bill providing more than $270 million over 12 years to reduce nitrate and phosphorus running from farm fields and industrial sources into streams, rivers and lakes.
Critics say that’s not enough money to fix the problem of nutrients flowing into the Mississippi River and down to the Gulf of Mexico, where they have created an oxygen-deprived dead zone that wildlife must flee or die. But the state can get the best bang for its buck by funding proven conservation strategies in targeted watersheds.
“It’s so much cheaper to prevent the pollution than cleaning it up on the drinking-water side,” said Sarah Graddy, spokeswoman for the Environmental Working Group, based in Washington, D.C.
The Gazette’s Iowa Ideas magazine highlights seven farm conservation practices to explain how they work, how broadly they’re used in Iowa and their cost.
1. Cover crops
Cover crops — mostly rye and oats in Iowa — are grown over the winter to protect soil from erosion, break up compacted soil and absorb nitrate and phosphorus from fertilizer so less runs into nearby waterways. The cover crop usually is killed off with herbicide before spring planting of corn and soybeans.
• Pros: Enriches soil over time. Can be used for cattle grazing and weed control before soybeans.
• Cons: Most cover crops don’t generate income. Can be tougher to grow in cold climates if not planted immediately after harvest. Some farmers fear a short spring will mean not enough time to kill cover before planting corn.
• Extent of use in Iowa: Estimated 880,000 acres in 2018
• Estimated nutrient reduction for rye: 31% nitrate, 29% phosphorus
• Cost: $30 acre
• Sources: Iowa Learning Farms evaluation, Iowa Nutrient Reduction Strategy, “Understanding Budget Implications of Cover Crops” by the University of Illinois and Purdue University, USDA’s Conservation Practices Physical Effects matrix
2. Buffer/filter strips
Vegetated areas between traditional row crops and streams, lakes and ditches that protect water from fertilizer runoff, provide bank stabilization and animal habitat. Filter strips include mainly shrubs and/or grasses.
• Pros: Supports wildlife.
• Cons: Takes land out of production. Doesn’t filter water from underground drainage tiles.
• Extent of use in Iowa: 557,700 acres in contour buffer strips
• Estimated nutrient reduction: 91% nitrate — though but not all water is filtered — 58% phosphorus
• Cost: $233 to $330 annual cost per acre, depending on vegetative or forest strip
• Sources: Iowa DNR LIDAR mapping, Iowa Nutrient Reduction Strategy, ISU Nutrient Reduction Strategy 2016 Decision Support Tool, USDA’s Conservation Practices Physical Effects matrix
3. Saturated buffers
Buffer strips in which the water table is raised by diverting underground tile drainage into a water control structure.
• Pros: Filters water from underground tiles. Lasts 20 to 40 years with minimal upkeep.
• Cons: Relatively expensive at outset. During heavy rain, not all water is filtered.
• Extent of use of Iowa: Estimated 30 installed
• Estimated nutrient reduction: 50% nitrate
• Cost: $3,584 to $4,400 installation, $360 annual costs
• Sources: ISU Nutrient Reduction Strategy 2016 Decision Support Tool, Agricultural Drainage Management Coalition report, USDA’s Conservation Practices Physical Effects matrix
4. Wetland restoration or construction
Many historical wetlands were removed for farming, but the government now provides funding to construct or restore these areas of standing water or saturated soil that filter runoff, help control flooding and support wildlife.
• Pros: Takes relatively few acres out of production. Often built in areas that flood anyway.
• Cons: High first-year construction costs. Reduced filtration in flood events.
• Number of wetlands: 87 wetlands done, 13 more under development through Iowa Conservation Reserve Enhancement Program — 31 counties in north-central Iowa
• Estimated nutrient reduction: 68% nitrate, 43% phosphorus
• Cost: $10,000 per acre in the first year, $785 per year afterward
• Sources: Iowa Conservation Reserve Enhancement Program 2018 annual report, ISU Nutrient Reduction Strategy 2016 Decision Support Tool, USDA’s Conservation Practices Physical Effects matrix
5. Tillage management
No-till farming is when farmers do not plow their fields after harvest. Conservation tillage leaves stubble on the field, uses a chisel plow for less soil disruption.
• Pros: Saves time and fuel costs. Reduces soil erosion and nutrient runoff.
• Cons: Farmers may need different equipment, more herbicide with no-till.
• Extent of use in Iowa: 7 million no-till acres, 8.8 million acres conservation tillage, in 2012.
• Estimated nutrient reduction: 90% phosphorus with no-till; 33% phosphorus with conservation tillage
• Cost: $305 per acre average for no-till compared to $280 per acre for conventional till, although higher profitability has been reported on no-till farms in the north-central United States.
• Sources: Iowa Nutrient Reduction Strategy, 2016 Kansas State University’s Department of Agriculture Economics study, 2012 USDA Ag Census, USDA’s Conservation Practices Physical Effects matrix
Earthen structures that catch runoff on farmed slopes, dividing a long slope into a series of shorter runs. Terraces reduce runoff and allow soil to settle, letting cleaner water run into nearby waterways.
• Pros: Allows sloped fields to be farmed without extensive erosion.
• Cons: Relatively expensive to construct. While terraces stop soil movement, they don’t always decrease water pollution.
• Extent of use in Iowa: 506,100 terraces stretching 88,874 miles
• Estimated nutrient reduction: 77% phosphorus
• Cost: $100 to $250 per acre, depending on the type of terrace system
• Sources: Iowa DNR LIDAR mapping, Iowa Nutrient Reduction Strategy, USDA’s Conservation Practices Physical Effects matrix, “Choosing Terrace Systems” by University of Missouri Extension
Trenches filled with wood chips host bacteria that convert nitrate into harmless nitrogen gas. Water from tile-drained farm fields is funneled through the trench.
• Pros: Doesn’t take land out of production. Little maintenance. Lasts up to 20 years.
• Cons: During heavy rains, some water will go around bioreactors and not be filtered. Doesn’t improve soil.
• Extent of use in Iowa: Estimated 20 to 25 bioreactors
• Estimated nutrient reduction: 43% nitrate
• Cost: $10,150 the first year, about $700 a year after
• Sources: ISU Nutrient Reduction Strategy 2016 Decision Support Tool, 2013 Gazette article, Iowa Soybean Association, USDA’s Conservation Practices Physical Effects matrix
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