An alfalfa variety that takes its nitrogen from the soil rather than the atmosphere could help environmental engineers know when soil is no longer chemically contaminated. Researchers developed "ineffective" alfalfa for use as a high-nitrogen forage and a type of biological chemical detector. Typically, alfalfa takes nitrogen from the atmosphere and transforms it into a form the plant can use for growth. By contrast, ineffective alfalfa receives all its nitrogen from the soil. When nitrogen levels in the soil are low or depleted, ineffective alfalfa turns yellow. Scientists are studying the use of ineffective alfalfa on chemical spill sites as a scrubbing agent to draw contaminants out of the soil and as a biological indicator that the site is clean. The alfalfa can also be used as high-quality animal feed.
Plant Science Research, St. Paul, MN
Carroll Vance, (612) 625-5715

Grasses and sedges clipped short are as effective as taller stubble vegetation in trapping sediment being washed into streams. Researchers say the key is the width of the grass barrier, not stubble height. A grass barrier 30 feet wide will reduce sediments in overland runoff by 98%, according to results from a cooperative 2-year study by ARS and Colorado State University. The scientists used a large rotating irrigation boom to simulate rainfall over two test sites along Sheep Creek in northern Colorado's Roosevelt National Forest. They discovered stubble as short as one inch could slow water flow and trap sediment as effectively as stubble up to six inches tall. That's important news for land management agencies seeking information on how to rehabilitate overused areas--such as areas damaged by recreation or livestock grazing along rivers and streams.
Crops Research Laboratory, Fort Collins, CO
Gary W. Frasier, (970) 498-4232, gfrasier@lamar.colostate.edu

Rising carbon dioxide levels in the atmosphere could prompt a future adjustment in fertilizer recommendations for soybean and other crops. A study of grain sorghum and soybean shows that a carbon boost may enable plants to yield more on less nitrogen. But it also can make soybeans appear nitrogen deficient by a standard leaf test. Researchers grew the crops in air containing twice the current CO2 concentration--now about 360 parts per million--because CO2 levels are expected to double during the next century if they continue increasing at the current rate. Soybean leaf nitrogen fell below a critical threshold that, by today's standards, should have caused at least a 10-percent drop in yield. Instead, yield was up 35 percent without any additional fertilizer. That's because the increase in plant growth and leaf size caused by additional CO2 diluted nitrogen levels. Sorghum yields were up 18 percent, not enough to affect the nitrogen level in the plants. If yields of soybeans, sorghum and other major crops go up in step with CO2 levels over the next century, scientists will have to alter the estimated critical nitrogen threshold for maximum yields.
Soil Dynamics Research Laboratory, Auburn, AL
D. Wayne Reeves, (334) 844-4741 Ext.138, wreeves@acesag.auburn.edu

Tropic Sun, a variety of sunn hemp, is a super cover crop that grows to its full 6-foot height in 10 weeks, compared to seven months for other crop rivals such as hairy vetch and crimson clover. Tests show that sunn hemp, Crotalaria juncea--not to be confused with hemp, or marijuana, Cannabis sativa--can be planted in the southern United States immediately following a corn harvest, quickly providing almost total ground protection against fall and winter rains. It takes seven months for other cover crops such as hairy vetch and crimson clover to provide similar protection. Tropic Sun hemp also has proven useful as a forage for livestock, providing high levels of protein in its leaves when other pastures are normally in a "lull." And, applied at 120 pounds per acre, it makes as much nitrogen as slower growing legume cover crops. That leaves some for next year's corn crop but also increases the possibility of nitrogen losses to groundwater over the winter. Scientists are trying to work winter wheat into the corn-hemp rotation to use up the excess nitrogen, perhaps giving wheat a free ride, with no additional fertilizer. USDA's Natural Resources Conservation Service released Tropic Sun seed for potential development as a green manure/cover crop variety. The sunn hemp it was bred from has been used for centuries as a green manure crop elsewhere, primarily Southeast Asia. People in India make cloth from its fibers. ARS researchers in Weslaco, Texas, are investigating using sunn hemp as a fiber crop similar to kenaf, which can be used to make a variety of products including paper. The Weslaco researchers are testing the possibility of producing sunn hemp for paper and for a supplement for peat moss used to grow nursery plants.
Soil Dynamics Research Laboratory, Auburn, AL
D. Wayne Reeves, (334) 844-4741 Ext. 138, wreeves@acesag.auburn.edu

Giving each plant just the right amount of water and nutrients is the goal behind a new irrigation system being developed by ARS scientists. It's part of an emerging high-tech science called precision agriculture--providing water and fertilizer to plants based on their relative need, instead of applying uniform quantities across the whole field. Scientists started with a commercial center pivot irrigation system, which pivots in a giant circle, sprinkling plants underneath. Irrigation from these pipes is controlled by how fast it moves around the circle. But in two years of field tests, scientists modified the sprinkler system by dividing its length into several sections. Then they added to each section three manifolds that allow the water to be sprayed from nozzles at eight different rates. On a 15-acre field, researchers can precisely apply water and nutrients on up to 500 separate areas, each about the size of a two-car garage. The long-term goal: to allow farmers to precisely irrigate each part of a field, based on the type of soil and crop present in that area. This will allow farmers to make sure plants get enough water in sandy areas, while avoiding flooding and runoff in areas with different slopes, clay content, or compaction. Researchers are working cooperatively with Valmont Industries, Inc., a large manufacturer of commercial irrigation systems, to further refine the system.
Coastal Plain Soil, Water and Plant Research, Florence, SC
Carl R. Camp, Jr./E. John Sadler, (803) 669-5203

Cotton plants in Southern California can produce more cotton fiber if they take smaller, more frequent sips of the same amount of irrigation water they would normally get during July. ARS scientists developed and tested the new approach in California's Imperial Valley. In their three-year test, they applied just over 1-1/2 inches of water every five days during July, resulting in increased cotton yields by five to 11 percent. The new technique used no more water than traditional July regimens of three inches every 10 days, or 5 inches every 15 days. Some growers and others have believed the traditional regimen is necessary to flush away salt that can damage crops if it accumulates in the soil. But the scientists found that no salt accumulated in the top 6 inches of soil during the study. Why irrigate more often in July? The scientists say July is the peak time when cotton buds open up in the valley, and stresses such as inadequate water can strike yields especially hard. The scientists found that small, frequent irrigations keep plants in peak condition with lower leaf temperatures, high leaf moisture content and higher leaf transpiration rate.
Irrigated Desert Research Station, Brawley, CA
Chang Chi Chu, (619) 344-4184

A commonplace cultivator tool can help Pacific Northwest winter wheat growers "sweep" away about two-thirds of their potential downy-brome problem--a year before this costly weed could otherwise take over their wheat fields, ARS field studies have shown. The sweep cultivator is a broad, flat tool attached to a thin vertical shaft. Pulled by tractor, it increases seed contact with the soil and makes weeds germinate quickly. Weeds then can readily be killed by herbicide or cultivation in the fallow year. If growers don't act, however, the seeds stay dormant through winter and germinate the next fall, when wheat is again planted. Downy brome--also called cheatgrass--and other bromes infest 14 million of the 50 million acres of western winter wheat. The weeds cost growers $300 million a year in lost yield. The sweep cultivator is ideal for wheat growers who use conservation tillage to cut erosion. That's because the sweep does not invert soil or chop straw left to shield the soil. For two wheat-fallow rotations, scientists measured survival of downy brome seed using five cultivation techniques. These were no-till, sweep, light harrow, shallow disc and skew-treader. In years with a dry fall, survival of downy brome seeds was only 13 percent with the sweep--and 40 percent with no-till. In years with a wet fall, 3 percent of the seed survived with the sweep, and 9 percent with no-till. Harrowing was largely ineffective. Shallow discing cut seed survival dramatically, but buried much of the soil-protecting residue.
Nonirrigated Agriculture Weed Science Research Unit, Pullman, WA
Alex Ogg, (509) 335-1551

Last Updated: January 28, 1997
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