Prior to application of manure, the following parameters should be assessed in order to determine if nutrient application is appropriate. The pre-application field assessment will walk you through applying these steps to the application process. An ARM Field Assesesment Worksheet should be filled out for each field you are considering applying manure to prior to application. Failure to assess any of the following parameters could result in application under unfavorable conditions, which could lead to resource pollution event.
- Water Table
- Available Water Holding Capacity (Soil Moisture)
- Field Characteristics
- Field Cover
- Application Method
- Current field Conditions
- Application Setback Distances
- Vegetative Buffers
Forecast. In order to apply manure at appropriate times throughout the year, it is vital to have an understanding of the climate trends in your area, as well as seasonal weather patterns and current conditions. The forecast is one of the most important parameters to check prior to application of manure or grazing events. In order to avoid negative soil and water quality impacts (i.e., runoff, soil saturation, compaction, etc.) and properly manage crop irrigation needs, the critical forecast characteristic that should be observed on a daily basis is precipitation.
In general, runoff from applied manure has a lower risk if there has been no significant rain prior to application to saturate soils, and limited (<0.20 inches) rain forecasted within three days after application. These guidelines will vary with soil type and time of year. If rain has occurred prior to application, check soil conditions thoroughly prior to application. A high soil moisture content will tolerate less rain and be more prone to runoff than a low soil moisture content.
In general, if less than 0.25 inches of rain is forecasted in any one event in the 72 hours following application, you may continue with the application process, but be cautious. If more than 0.5 inches of cumulative rain is expected in the 72 hours following application, consider postponing application until more favorable conditions arise. Do not surface apply if a hard, sustained freeze is expected the same night, or if more than 1 inch of snow is predicted. These events will increase the risk of surface runoff.
Water Table. The water table under your field will depend on soil type, season, and recent weather conditions. If the water table is high, manure has a greater opportunity to leach into groundwater and/or be transferred to surface waters. If the water table is greater than 24 inches from the surface, the water table will not be the limiting risk factor for application; if it is between 12 and 24 inches, application will be limited by this parameter; if less than 12 inches from the surface, no application is permitted. Maintain your water table by making sure your drainage infrastructure (i.e., tiles, ditches, etc) is in good working order. If the watertable is within 24 inches of the surface, thoroughly monitor tiles after applying manure. If any discharge of manure occurs, address immediately by capping tile.
Available Water Holding Capacity (AWC)/ Soil Moisture. The available water holding capacity (AWC) is a measure of the saturation of your soil. The lower the AWC (or the higher the soil moisture %), the less likely your soil will be able to hold and infiltrate applied manure into the soil and the greater the runoff possibility will be. All soil types will have different thresholds of saturation. In genera, if the moisture content of the soil is greater than 90%, application is not recommended. Between 70-90%, application is permitted, but will be limited by recommended application volume and equipment limitations. Below 70% moisture, no soil saturation restrictions are in place.
Field Characteristics. The characteristics of your field are very important to consider prior to application. Some characteristics may limit application on specific areas of a field (i.e., ponding, tiles, low spots, etc). You must take into consideration slope (prefer to apply to <3% slope), swales (you may need to observe setbacks from swales certain times of the year), soil type (if the field is mixed, apply only to the appropriate soil types), low spots (areas where ponding may occur), and tiles. If tiles are present in your field, know where they are, where they empty to, and have a way to shut them down in case a manure discharge occurs. Do not apply over tiles with less than 24 inches of cover and always observe tiles after application. If you don’t know these things, do not apply closer than 80’ from tiles. Observe seasonal manure setback distances from areas that are conduits to surface waters (i.e., swales, tiles, ditches, streams, etc.).
Field Cover. Field cover is an important consideration for application of manure. Application to bare or sparsely vegetated fields (<50% cover) will have a much greater risk of runoff than dense fields and should be avoided during high risk times. Only established, dense (>70% cover) grass or cover/relay crops grown for harvest are permitted for application during higher risk times of the year. Sparse vegetation (<50% cover) have limited application rates and seasonal restrictions. Established vegetative buffers adjacent to waterways are recommended for application to bare and/or sparsely vegetated fields to prevent and/or filter runoff. Vegetation should be greater than 3 inches high at application to reduce runoff transport.
Application Method. Not all methods of liquid manure application are appropriate all year. In general, methods that inject or incorporate manure into the top layer to the soil are best for limiting transport. If manure is surface applied (i.e., big gun sprinkler, splash plate, wagon), it is recommended that it be applied in a way that minimizes clumping and delivers liquids and/or solids below the grass canopy. If using a big gun, only apply when transport via wind drift is low and stay back the recommended setback distances at all times (at least 40 feet year round). Make sure any type of manure application equipment is properly calibrated prior to use.
Current Field Conditions. Current field conditions need to be visually evaluated prior to application. If there are any of the following conditions present, application may be limited or not permitted: ponding or standing water, compacted soils, saturated soils, frozen ground, snow cover, vegetation cover less than 3 inches in length, unmonitored tiles, and/or sparse or dead vegetation. All of these field conditions are evaluated prior to application on the ARM Worksheet.
Application Setback Distances. If your field is adjacent to a waterway, swale, wet area, or other area of concern identified by your planner, you must adhere to all setbacks specified. For instance, if ponding is noted on the field, investigate why and find out if it is running off into a waterway. If it is just a low spot and not flowing into a ditch or other waterway, you may apply manure near the ponded area. If it is running off into a waterway, you must exercise the required manure setback distance from the area. Setbacks will change throughout the year based on seasonal considerations and should be assessed on a daily basis.
Vegetative Buffers. Any areas that have the possibility of runoff to a neighbor, waterway, swale, or water body and cannot be managed with setback distances alone need to be contained with a vegetative filter strip or field boarder to reduce the risk of runoff contamination. Vegetative buffers need to be maintained to function properly. If buffers are well established, have dense vegitation (>70% cover), and grass height is more than 3 inches in length, runoff pollution from application is reduced. If buffers are not maintained properly, an additional manure setback distance may by needed.
Each of these parameters will determine the ability to apply or not to apply to your individual fields. The Manure Spreading Advisory and Application Risk Management Worksheet will guide you through the process of evaluating if ambient and field conditions are appropriate for manure application and at what rate. It is your responsibility to apply only under appropriate conditions to avoid having a pollution event.