Livestock farmers and commercial manure applicators need to be extra cautious applying liquid manure to dry farm fields this summer. While it may seem that dry fields can absorb larger amounts of liquid manure, there are other soil conditions that must be taken into consideration. Dry soils often crack and deep cracks can create a pathway directly to subsurface drainage. Worm holes can also have the same effect as night crawlers and even crayfish tend to burrow directly down to field tile.
Manure reaching field tile through soil cracks, worm holes and other direct conduits is referred to as preferential flow. Taking measures to prevent manure from reaching tile outlets is crucial to avoid contamination of water bodies.
Any measure of disturbing the soil will aid in keeping manure nutrients in the root zone. This can be tillage equipment that works the ground 3 to 5 inches deep prior to application, or injection equipment that disturbs the soil below the line of manure injection. With tillage, be sure the equipment leaves at least a 90% residue cover.
During and after application, tile outlets should be monitored for manure flow. Sometimes it can only take seconds for manure to reach tile lines. Checking immediately as well as throughout the next 24 hours is recommended depending on how dry the tile is and how far the outlet is from the site of application. Tile plugs and control structures can be used to ensure manure does not exit outlets or they can be used in an emergency situation to stop manure that is already flowing.
All soils are rated for their available water holding capacity (AWC). AWC is exactly what it sounds like; it is the maximum amount of moisture soils can hold before runoff begins. NRCS Practice Standard 633 (available online or from local SWCD and NRCS offices) provides an easy guide to determining what that moisture level is by soil type.
If more manure is applied than the soil can hold, there is an increased risk that the manure will move offsite and contamination of water bodies could occur. When injecting manure on 30 inch centers, remember that manure is being applied at much greater volume directly behind the injectors than the soil may be capable of absorbing. Using lower rates and covering the fields twice might be advisable.
One final suggestion for summer manure application is to plant a cover crop ahead of time. Oats, cereal rye and radishes have all been successfully used. Each can capture the ammonia nitrogen portion of the manure and convert it to plant tissue, which eventually gets released after the plant dies as organic nitrogen in future crop seasons.
For more details on preferential flow, click here.