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A number of nutrient management practices are used to enhance fertilizer use efficiency and reduce nutrient losses into the environment. These practices include:

• Assessing nutrient need through annual or regular soil and plant tissue testing before applying nutrients, in contrast to limited or no testing before applying nutrients. Soil testing identifies the amount of nutrients already available for plant uptake, and is used to identify the additional amounts of nutrients needed to meet a realistic yield goal. A plant tissue nitrogen test uses chlorophyll (or greenness) sensing to detect nitrogen deficiency during the growing season to assist in assessing the need for additional commercial fertilizer applications. Correction of any nitrogen deficiency is then made through chemigation or other foliar application.
• Timing nutrient application to tailor feeding to plant-growth needs, for example, split application of nitrogen fertilizer into at planting and after planting, in contrast to fall and early spring applications of nitrogen before planting.
• Applying nutrients close to the root zone so they are more readily accessible to the plant, through banded and injected applications and chemigation, in contrast to ground and air broadcast and application in the furrow. With side-dressing or banded application, granule or liquid nitrogen fertilizer is placed to one side of the plant or placed every other row at planting or during the growing season.
• Selecting the nutrient product according to the chemical stability in the soil, in order to minimize nutrient loss to the environment. For example, use an ammonia-based fertilizer on fields with high leaching soils, and a nitrate-based fertilizer on fields where ammonia volatilization is a problem.
• Rotating nitrogen-using with nitrogen-fixing crops. Cover crops are planted between crop seasons to tie up and preserve nutrients, in contrast to continuous planting of the same nitrogen-using crop and not planting any cover crops.
• Applying manure and organic waste based on manure and waste test results and nutrient management plan. Adequate storage is available for manure so that applications will mesh with plant nutrient needs and applications are injected or incorporated into the soil.
• Using nitrogen inhibitors and other products to slow the release of nitrates from ammonium fertilizers until later in the growing season, by delaying the conversion of ammonium nitrogen into nitrate nitrogen, which is susceptible to leaching. N-inhibitors can also be used with manure and other forms of organic nitrogen fertilizer.
  

  • Urease inhibitors—Chemical compounds that can be added to urea to slow the conversion of urea to ammonium and therefore to slow nitrate leaching.
  • Slow-release nitrogen fertilizer—Fertilizer coated with chemicals that can retard release of nitrogen from applied fertilizer and prolong the supply of nitrogen for plant uptake.

• Refraining from broadcasting nitrogen fertilizer, or if broadcast, incorporating the product into the soil, which reduces the losses of nitrogen to the atmosphere. Certain nitrogen products, especially urea, are subject to extensive volatilization when broadcast. Certain nitrogen products are injected or knifed-in, usually 12-24 cm below the soil surface. Nitrogen can also be incorporated into the soil by tillage. High-pressure liquid nitrogen such as anhydrous ammonia is the most common form of nitrogen injected into the soil. Nitrogen solutions in low-pressure liquid form are also injected into the soil.

• Applying all nitrogen at and/or after planting, when the demand by the crop is greatest, which reduces the risk of nitrogen loss through leaching. Conversely, applying all nitrogen in the fall can increase the risk of leaching, under certain soil and weather conditions.

Results from the 1996 USDA Agricultural Resources Management Study survey of corn farmers indicate a modest utilization of nutrient testing techniques on corn acreage. Soil tests were the most extensively used (on 44 percent of the corn acreage). Nitrogen tests, nitrogen inhibitors, and broadcast applications with incorporation were used to lesser degrees. Nitrogen management on those acres receiving the nitrogen test followed recommendations closely, with 82 percent receiving nitrogen at rates exactly as recommended or lower. Nitrogen fertilizer was applied to corn several times during the year, with the largest acreage receiving it before planting of corn, either in the fall, spring, or both. The second largest bloc of acreage had nitrogen applied at or after planting time, followed by all the nitrogen applied in the fall.

Potential Contribution of Precision Agricultural Technologies Contribute to Nutrient Management

Precision agriculture is typically characterized as a suite of information technologies used to monitor and manage sub-field spatial variability. Variable rate application of seeds, fertilizers, pesticides, and irrigation water has the potential to enhance producers’ profits and reduce the risk to the environment from agricultural production through the tailoring of input use and application more closely to ideal plant growth and management needs.

Precision agriculture developments reflect innovations during the last decade in the computer, telecommunications, and satellite industries which have made more detailed spatial and temporal management of nutrients and other inputs within fields technically feasible. The application of these information technologies, known as precision farming or site-specific farming, enables producers to monitor and differentially manage small areas of a field that have similar soil or plant characteristics. Components of a comprehensive precision farming system typically include:

• Intensively testing soils or plant tissues within a field
• Equipment for locating position within a field via the Global Positioning System (GPS)
• Ayield monitor
• A computer to store and manipulate spatial data using some form of Geographic Information System (GIS) software
• A variable-rate applicator.

More involved systems may also use remote sensing from satellite, aerial, or near-ground imaging platforms during the growing season to detect and treat areas of a field that may be experiencing nutrient stress.

Precision farming has the potential to improve net farm income by: (1) identifying places in a field where additional nutrient use will increase yield, and thus farm income, by more than the added cost; and (2) identifying places where reduced input use will reduce costs while maintaining yield. One preliminary estimate of additional fixed and variable costs of precision farming for corn is about $7-$8 per acre (Lowenberg-DeBoer and Swinton, 1995). Precision farming also has the potential to reduce off-site transport of agricultural chemicals with surface runoff, subsurface drainage, and leaching (Baker and others, 1997; Watkins, Lu, and Huang, 1998). Two years of Kansas field data indicate less total nitrogen fertilizer use with precision farming than with conventional nitrogen management (Snyder and others, 1997).

Corn production represents a potentially large market for precision agriculture technologies as corn producers are the largest users of cropland and agri-chemicals in U.S. agriculture. USDA surveys in 1996 and 1997 indicate that about 10 percent of all corn farms in the United States are using some aspect of precision farming (Daberkow and McBride, 1998). Among precision agriculture adopters, 70 percent used some aspect of precision agriculture—grid soil sampling, variable-rate technology (VRT) for lime or fertilizer application, or yield monitors.

Results from 1996 USDA Agricultural Resources Management Study found precision agriculture adopters more likely than non-adopters to:

• Operate larger farms with greater assets and sales
• Farm in the central Corn Belt (IL, IN, or IA)
• Have more corn acres and achieve higher yields
• Earn greater cash farm income
• Have completed college
• Use a computerized farm record system
• Be less than 50 years of age
• Rely on crop consultants for information on precision farming

Additional information is available from precision agriculture technology web sites.

Extent of Animal Manure Use in U.S. Corn Production

Animal manure contains nutrients and organic matter that can contribute to plant growth. However, its availability and content variability often limits its use in crop production. In the case of corn, data from the 1996 ARMS survey of U.S. corn growers in the 16 major corn producing States show that 8.4 million acres, 12 percent of the corn acres, received 50 million tons of manure. The average application rate was 5.9 tons per acre.

Pennsylvania had the largest percent of corn acres treated with manure, 58 percent, followed by Minnesota and Michigan. Dairy manure was the most common source of manure, applied to 42 percent of the corn acres on which manure was applied. Next in importance were cattle manure, applied to 27 percent of corn acres receiving manure, and hog manure, applied to 23 percent of the manure-receiving corn acres. Sixteen percent of corn acres on which manure was applied were soil tested for nitrogen.

The method of spreading and incorporating (or not incorporating) manure on corn acres influences the amount of nitrogen available for the crop. Typical application methods include surface broadcast using a manure spreader, irrigation systems, a tank wagon followed by incorporation by plow or discing, or injection (knifing) under the soil surface. For the States surveyed, manure was applied in a solid form to 72 percent of the corn acreage with the remaining acreage share receiving it in liquid form. Incorporation at time of application occurred on 68 percent of the manured acres, with 32 percent not incorporated.

In general, the closer to planting time that manure is applied, the greater the availability of nutrients for plant growth. Early spring application of manure before planting, thus allowing for the mineralization of organic nitrogen, is considered the best for corn production. Fall application of manure, even with incorporation, can result in large nitrogen losses because of the time between application and the next growing season. In the surveyed States, 60 percent of the manured acres received applications in the spring before planting, and 39 percent received the manure in the fall. Fifty percent of the manured acres received one application, and the remainder received two and three applications.

For more information refer to Chapter 4.4, Agricultural Resources and Environmental Indicators.

Trends in Precision Agriculture for Major Crops