Liquid manure is a valuable source of nutrients and organic matter for crop production and may be applied by a variety of methods including irrigation, surface spreading and shallow subsurface injection. With a low nutrient concentration, liquid animal manure may be applied at relatively high volumes, but it is generally recommended that it not be applied at rates that exceed the soil infiltration rate, nor exceed the amount needed to bring the soil to field water-holding capacity. Liquid manure discharges from agricultural drains have been reported in soils with subsurface (tile) drainage due to macropore flow.
However, liquid manures can be applied without any detectable adverse effects on water quality. For instance, Randall et al. (2000) noted no difference in nitrogen, phosphorus, or fecal bacteria losses in drainage effluent when they compared plots fertilized with liquid dairy manure and mineral fertilizer. The fact that liquid animal manure nutrients can be safely land-recycled in some instances, but are discharged in subsurface drainage water under different circumstances, suggest a complex system that needs to be managed. Soil texture, available water-holding capacity, tillage history, as well as the type and quantity of manure applied, application method, and timeliness of rainfall after application may all play a role in determining the fate of manure.
The available water-holding capacity of the upper 8 inches of the soil provides an estimate of the maximum volume of water that can be applied before additional water, manure and nutrients may begin to move through the soil profile. Manure application rates may need to be adjusted to avoid reaching the available water-holding capacity of the soil and is one factor determining the maximum volume applied. North Central Region guidelines suggest that application rates should not exceed the lower of the nutrient restriction, available holding capacity of the soil or 13,000 gallons per acre. Smaller, multiple low applications allow the soil to absorb liquid animal manures better than one large application.
Guidelines to minimize the downward movement of liquid manure:
• Identify subsurface drain outlets, and control or regulate discharge prior to application, or have on-site means of stopping the discharge from subsurface drains. Subsurface drainage outlets should be monitored before, during and after application for potential liquid manure discharge. Drainage control structures and inline tile stops are recommended control practices to reduce the risk of a discharge, while tile plugs may be used in emergency situations but have been known to fail. Develop a contingency plan to handle situations when liquid manure discharges to surface water.
• Liquid manure should not be applied on soils that are prone to flooding. Manure may be applied if incorporated immediately or injected below the soil surface during periods when flooding is not expected (subject to your state guidelines).
• Avoid applying manure when rainfall is predicted, eminent or directly after a rainfall event. After a significant rainfall event, the site should be allowed to drain to below field capacity, so that the soil has the capacity to absorb additional water or liquid manure. As part of the manure application recordkeeping, maintain a log of weather forecasts and actual weather conditions 24 hours before and after a manure application event.
• Repair broken drains and blowholes prior to application, and follow recommended or required minimum setback requirements (setback distances vary from state to state) for surface inlets.
• Liquid manure should not be applied to subsurface drained cropland if the drains are flowing. Generally, flowing subsurface drains indicate soil moisture levels that are near or exceeding the soil water-holding capacity. The addition of liquid manure under these conditions will increase the probability of manure moving downward and discharging through these drains or moving overland as surface runoff.
• Application rates should be closely tied to nutrient requirements and available holding capacity of the soil. The method of application can influence application rates. For example, with an injection toolbar with four nozzles on 30-inch spacing, each knife and nozzle produces a concentrated application to a small area. Under these concentrated flow conditions, the effective rate differs considerably from an average application rate. The effective rate is calculated as the volume of manure applied per unit area per nozzle. For example, assume a 10,000 gallon-per-acre application rate, an injection toolbar with 30-inch spacing and 6-inch sweeps, the effective rate is 50,000 gallons per acre, five times greater than a uniform even distribution over an acre.
• Liquid manure application should not result in ponding, or runoff to adjacent property, drainage ditches or surface water regardless of crop nutrient need, and should be uniformly applied at a known rate. Liquid animal manure applications using irrigation or surface application equipment tend to have a greater risk of ponding.
• Fields with a history of downward movement of manure or bare/crusted soils may require some tillage to improve infiltration and absorption of the applied liquid. Prior to manure application, use shallow tillage to disrupt the continuity of worm holes, macropores and root channels (preferential pathways) to reduce the risk of manure reaching drain lines, or till the surface of the soil 3-5 inches deep to enhance liquid manure absorption. Pre-application tillage should leave as much residue as possible on the soil surface to minimize soil erosion.
• Clay soils with a high shrink swell capacity tend to have larger deeper cracks during dry conditions. These soils may require tillage to disrupt the cracks and macropores, and a lower initial application rate applied to help close the cracks. Application rate should be based on the field conditions and nutrient limitations.
• Shallow injection is recommended for liquid manure. The soil should be tilled at least 3 inches below the depth of injection prior to application, and/or control outflow from all drain outlets prior, during and after manure application.
• For perennial crops (hay or pasture) or continuous no-till fields where tillage is not recommended or with cover crops, all subsurface drain outlets from the application area should be monitored and if manure-laden flow should occur, all effluent should be captured. Cover crops may be planted to improve soil structure and absorb available manure nutrients. Crops with deep tap root systems (alfalfa) tend to have more problems than hay crops with fibrous roots (grasses) because liquid animal manures may flow along the tap roots to subsurface drains and outlet to surface water.
Improved management is a key issue in greatly reducing the potential of liquid manure reaching surface water. While climate and some environmental conditions cannot be controlled, producers may better manage and control when and how they apply liquid manure. These recommended practices are intended to help producers apply liquid manure in a manner that minimizes the potential to impact water resources through the downward movement of manure into subsurface (tile) drains by incorporating the best available knowledge. See the fact sheet on “Guidelines for Applying Liquid Manure to Cropland with Subsurface (Tile) Drains” at http://ohioline.osu.edu/anr-fact/pdf/0021.pdf for more information. PD
James J. Hoorman