With relatively 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 animal manures can be applied without any detectable adverse effects on water quality. Application of liquid animal manures to soils with subsurface drainage has been linked to contamination with nutrients organic, estrogens and antibiotics.
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, suggests 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 contamination potential of the 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. Use caution not to back-up water where it may impair the functioning of an adjacent subsurface drainage system. Develop a contingency plan to handle situations when liquid manure discharges to ditches or streams.
The available water holding capacity of soil is determined by the upper eight inches of soil. Varies types of soil have differing AWC (available water capacity). The upper eight inches of the soil provide 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. Manure application rates may need to be adjusted the day of application to avoid reaching the available water holding capacity of the soil and is one factor determining the maximum volume that should be applied. Application rates should not exceed the lower of the nutrient restriction, available holding capacity of the soil or 13,000 gallon/acre. Smaller multiple low applications allow the soil to absorb liquid animal manures better than one large application. The manure application rates should be less than AWC to reduce the potential for contaminated runoff.
Liquid manure should not to be applied on soils that are prone to flooding during the period when flooding is expected. Manure can be applied if incorporated immediately or injected below the soil surface during periods when flooding is not expected.
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 animal 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.
Liquid manure should be applied in a manner that will 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.
Shallow injection is recommended for liquid manure. Till the soil 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.
Liquid manure should not be applied to subsurface drained cropland if the drains are flowing. Generally, flowing subsurface drain 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.
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