It has been a challenging fall to get manure to the field and as a result, some farmers had had to consider switching manure application from injection to surface application. While that may be necessary, there are a few additional considerations you should make in your nutrient application.
In terms of nutrient management planning, look at updating the volatilization correction factor. Based on Table 2., of PMR 1003, "Using Manure Nutrients for Crop Production," a correction factor of 0.75-0.90 is recommended for not incorporated surface applied liquid manure and a factor of 0.70-0.85 is recommended for not incorporated surface applied solid manure, as compared to 0.95-1.00 for immediate incorporation and injection. There is a wide range for ammonia volatilization because there is considerable uncertainty about the process – the weather conditions we face, the characteristics of the manure, and how quickly it infiltrates into the soil all make a big difference on how much of that nitrogen is actually lost. The cooler temperatures we have this time of the year will slow ammonia loss. As long as the manure is infiltrating into the soil relatively quickly we should be on the lower end of the scale, probably losing around 10-15% of the nitrogen we apply to surface application.
The second thing to consider is setback distance requirements. With injection/incorporation, the required setback distances are often 0, but when switching to surface application setbacks of 750 feet from residences and public use areas, 200 feet from water sources and other designated areas, and 800 feet from high quality water resources will be required for liquid manure. If soils are wet, consider increasing setback distances to provide some insurance that no manure moves out of the field.
Finally, the last thing to remember is the soil hydraulic properties and weather conditions have a much greater impact on surface manure application. In some cases, if the soils are wet or we are on sloping topography we may have to adjust manure application rates down to ensure that no runoff occurs. When we inject we get immediate mixing of manure with soil and that can help to hold the manure in place. With surface application, we rely on the soil’s ability to infiltrate the manure, which can take more time for higher application rates. While applying watch from pass-to-pass to make sure the manure you are applying is soaking in and not moving over the field.
Figure 1. What happens when we can no longer inject and have to switch to surface application?