About 20 elemental nutrients are essential for plant growth. Some of these nutrients are supplied naturally by the air, water, and soil. Fertilizers and manures are used to supplement the natural supplies for optimum crop growth. When nutrients are used correctly they are very beneficial, but when they are used in the wrong place at the wrong time they become pollutants. Both groundwater and surface water are very vulnerable to pollution. Water is one of our most valuable resources, and protecting it is an important concern.
Because they stimulate unwanted algae growth, nitrogen and phosphorus are the nutrients most often blamed for degradation of surface water in North Carolina. They promote excessive algae growth, which can cause fish kills, taste and odor problems, and reduce recreational uses of water resources. Sediment can also cause problems and is the number one pollutant of water resources in North Carolina.
Nitrogen dissolves in water and therefore is readily carried from application areas to surface and groundwater. Nitrogen in the nitrate form is the main threat to groundwater quality because it is not held by the soil and can leach through the soil and into the groundwater where it can contaminate well (drinking) water and eventually move into streams. Excessive nitrates in drinking water are a health hazard, especially to infants. Phosphorus is held tightly by soil particles and is transported mainly on eroded soil particles, but soluble (dissolved) forms of phosphorus can also enter water resources. Agricultural production can add nitrogen, phosphorus and sediment to water resources. This publication outlines ways to minimize the movement of nutrients and sediment to surface and ground waters while maintaining healthy, productive crops.
Best Management Practices (BMPs) are farming methods that are designed to minimize adverse environmental effects while maintaining agricultural production. Nutrient BMPs, referred to as the 4Rs—Right rate, Right timing, Right source, and Right placement—should be used on all cropping systems and is the first line of defense. Additional BMPs should be used to control nutrients as they move from application area to the water resource. Put together, these BMPs form a system to avoid, control, and trap nutrients.
The first line of defense to control nitrogen and phosphorus is the use of nutrient management. Managing nutrients is referred to as the 4Rs: Right rate, Right timing, Right source, and Right placement. Consistent use of the 4Rs will help prevent excess nutrient loss from agricultural fields into surface and ground water resources.
1. Right Rate: A soil test report indicates the amount of nutrients (except nitrogen) that the soil can supply and recommends the amount, if any, needed from other sources to produce the indicated crop. Soil testing is no better than the quality of the sampling, so ensure that a random sample is taken to the correct depth. (Additional information is available in Careful Soil Sampling—The Key to Reliable Soil Test Information, AG-439-30.)
It is critical to test soil every two to three years to assure sufficient nutrients. Many soils in North Carolina have been so well fertilized that they do not need any additional phosphorus. Soil test recommendations also include the amount and kind of lime to apply if the pH is too low. All the recommendations should be followed completely because a deficiency of one nutrient or an undesirable soil pH will limit crop response to the other nutrients. In North Carolina, a reliable soil test for nitrogen has not been developed, but nitrogen application rates are available from the North Carolina Realistic Yield database.
2. Right Source: Nitrogen and Phosphorus. It is important that nitrogen remain in the root zone long enough for it to be used by the growing crop. Although ammonium nitrogen can be held by the soil, it is generally nitrified (changed from ammonium to nitrate) very rapidly (three to five days) under North Carolina conditions. Nitrification is a natural process in which ammonium is converted to nitrate by soil microorganisms. Soil temperature, moisture, pH, and aeration determine the rate of nitrification.
Most farmers use liquid nitrogen as urea ammonium nitrate although there are other sources, such as urea and organic sources. Nitrogen that is structurally part of manures and other organic materials is less prone to short-term loss by leaching because it only becomes available via decomposition. Most farmers in North Carolina use diammonium phosphate fertilizer, which contains a small amount of nitrogen along with the phosphorus, primarily because it is produced and sold in North Carolina.
3. Right Placement: Apply Nitrogen and Phosphorus Correctly. Nitrogen and phosphorus are less likely to be lost by erosion or runoff if they are banded directly into the soil or incorporated. However, incorporation disturbs the soil, which promotes soil erosion. Although surface application of nitrogen and phosphorus without incorporation is the least desirable fertilizer application method, it is often used for pastures, lawns, turf, other perennial crops, and in conservation-tilled fields because it is the easiest and least disruptive method. Nitrogen tends to move down into the soil as rainwater infiltrates, so the application method (surface applied or banded) has little effect on losses of nitrogen by leaching. However, nitrogen and particularly phosphorus applied to the soil surface are more susceptible to being transported by runoff and less likely to become attached to soil particles or be taken up by plant roots. For this reason, nitrogen and phosphorus should be incorporated into the soil before crops are planted/established or banded, even if using conservation tillage. Where surface application is unavoidable, minimize the use of phosphorus.
Do not allow fertilizer to be applied outside crop areas. The poorest and most common method of broadcasting fertilizers is by spinner spreader. These spreaders apply fertilizers unevenly because of their tapered pattern. Field edges either do not receive enough of the nutrient or the nutrients are applied outside the field boundary. Use spinner spreaders only where vegetative field borders are provided to trap misapplied nutrients. Full-width or boom spreaders (including gravity-flow boxes, auger booms, drag-chain booms, pneumatic booms, and spray booms) are capable of applying fertilizers very evenly within field boundaries and are far superior to spinner spreaders. The main limitations of these spreaders are their higher initial cost and more complex operation. All fertilizer spreaders should be calibrated for the proper application rate and distribution pattern.
Applying nitrogen or other nutrients in irrigation water (fertigation) has some advantages and disadvantages. The main advantage is timeliness of application—that is, the nutrient can be applied in small amounts that are matched to the plants' needs. Precision placement is possible with drip-tube irrigation, but sprinkler irrigation has the same limitations as spinner spreaders. Nutrients should not be applied through sprinkler irrigation systems unless vegetative buffers are provided.
4. Right Timing. Apply Nitrogen and Phosphorus When Needed. The timing of application is more important with nitrogen than with any other nutrient because nitrogen is applied in large amounts to many crops and is highly mobile. Phosphorus is stable when it is mixed into the soil and can be applied when most convenient.
Ideally, nitrogen should be applied frequently in small amounts that are tailored to the plants' immediate needs. This is usually feasible only where fertigation is used or with high-value crops. For most crops, nitrogen should be applied in split applications that coincide as closely as possible with the uptake pattern of the crop. For example, corn requires relatively little nitrogen early in the growth cycle, but the need increases considerably when the plant begins to elongate. Therefore, most of the nitrogen required by corn should be applied as sidedressing after the plants are established. Fall application of nitrogen for spring-planted crops is never recommended in North Carolina. Improper amounts or placement of fertilizers or animal waste can lead to water pollution and poor crop growth. Guidelines on proper use of animal waste can be found in Extension Service publications Swine Manure as a Fertilizer Source, AG-439-4 and Poultry Manure as a Fertilizer Source, AG-439-5.
All nutrients can be lost when soil is eroded, but phosphorus is especially vulnerable. The primary way to prevent phosphorus loss is to control erosion. If no sediments leave the land, sediment-attached phosphorus does not leave, although soluble phosphorus may be lost. Many erosion control BMPs can be used in various cropping systems across North Carolina. A conservation farm plan providing for erosion control should be developed with assistance from the USDA Natural Resources Conservation Service or your county Soil and Water Conservation District. Some specific practices are:
- Maintain a soil cover. Managing fields for maximum yield, also encourages high residue, which helps protect the soil from erosion and can reduce potential compaction. Leave crop residues on the soil surface during the winter. Do not till too early in the spring and, where feasible, use conservation tillage methods, which leave 30% residue on the soil surface. With highly erodible land, this is especially important. On soils that are subject to erosion or leaching, a winter cover crop will reduce erosion and take up nitrogen to reduce leaching.
- Slope field roads toward the field; seed roads with a permanent grass cover. Water erosion and dust from traffic on field roads contribute significantly to soil loss and potential pollution from farms. Do not plow field roads when preparing land. Shape roads for good drainage, and seed them with a perennial grass where possible. Direct field road runoff toward the field or into a sodded waterway and away from any bordering ditch or canal.
- Shape and seed field edges to filter runoff as much as possible. Do not plow up to the edge of the field, especially along ditches or canals. Leave a buffer strip along drainage ways, and establish a perennial sod. Shape and seed hoe drain outlets to filter runoff.
- Use windbreaks and conservation tillage to control wind erosion. Wind erosion can be minimized by leaving the soil surface rough, maintaining crop residue on the soil surface, bedding to trap wind-blown sediments, keeping the soil wet, or maintaining a cover crop.
- Slow water flow. Use contour tillage, diversions, terraces, sediment ponds, and other methods to slow and trap runoff. The carrying capacity of running water is directly proportional to the flow rate. When water is still, sediments can settle out. Production practices such as installing water control structures (also known as flashboard risers) on field ditches in poorly drained soils benefit water quality significantly by reducing nitrogen, phosphorus, and sediment losses. Reductions occur when sediment and associated phosphorus settle out of the drainage water, and nitrogen is converted to its gaseous form by the process of denitrification or used by the crop or instream vegetation.
- Buffer strips. Leave buffer areas between farmland and water resources. The amount of buffer needed varies with the farming activity and the nature of the adjacent area (see Agricultural Riparian Buffers, AG-439-38). In some cases, buffers are mandated by law.
- Maintain vegetation on ditch banks and in drainage channels. Keep vegetation in drainage channels such as ditches and sod waterways. If necessary, construct ditches larger than needed so the bottoms can be left vegetated to trap sediment and other possible pollutants. Seed ditch banks, and prevent ditch bank erosion by proper sloping and diversion of field runoff water.
The first step in manure nutrient management is to control where the manure is deposited. Most swine and poultry are confined, and their manure is concentrated and managed. Recommended practices for handling and using swine and poultry manure are given in the Extension publications noted earlier.
Most cattle, horses, sheep, and other large animals are pastured. Manure is therefore deposited at random, making nutrient management more difficult. Cattle and other livestock should not be allowed free access to streams and ponds as waste will be deposited directly in the water resources, and animal traffic can cause soil disturbance and accelerated erosion of stream banks. After fencing the livestock out of the water, alternate water supplies should be provided by diverting or pumping water to livestock, preferably using watering tanks. Clean water sources benefit animal health and rate-ofgain as well as water quality. Feed, water, and lounge areas where animals congregate should be located so that runoff is filtered through vegetative buffer strips before entering streams and ponds.
We are becoming increasingly aware that almost everything we do may have some potential negative effect on the environment. Conservation practices or BMPs are designed to reduce the negative effects of agricultural production on surface and ground water resources. In some especially sensitive areas, the acceptable level of production may be minimal, especially with respect to fertilization. In other places, fertilizers may be used along with BMPs. Fertilizers and other nutrient sources should never be applied haphazardly.
No single set of BMPs applies in all situations. The BMPs presented here are for nutrient management on a wide variety of agricultural lands across the state. The best set of practices for a specific cropping situation will depend on individual circumstances; however, it is always recommended to use a combination of BMPs to avoid, control, and trap nitrogen and phosphorus.
Publication date: Aug. 24, 2017
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