Photo Credit: Sjoerd Duiker, Penn State Extension.
‘Incidental transfer’ is the loss of sediment, nutrients, pesticides, and dissolved carbon from the soil during infrequent runoff events. These events may be due to saturated soil conditions during certain times of the year, or due to high intensity rainfall events. Anything laying at the surface at that time is under high risk to be carried away in runoff. Incidental transfer of nutrients from surface applied manure is receiving much attention with the widespread adoption of no-tillage in Pennsylvania. However, it can even be worse in fields managed with tillage if manure is not incorporated immediately as is often the case. On well-drained soils the use of continuous no-tillage leads to great infiltration improvements, and one would typically not expect high losses of nutrients in surface runoff. However, if the soil is near saturation, the surface protection of crop residue and improved aggregation in long-term no-tillage do not increase infiltration very much because infiltration is now governed by the capacity of the subsoil to take in water. This situation is common in the spring in Pennsylvania. If manure is laying at the surface it is very likely to run off and may end up in streams. The issue has been termed ‘incidental transfer’ of manure nutrients in surface runoff because it is usually only a threat for short periods of time after surface manure application. After some time the manure nutrients and dissolved organic carbon infiltrate in the soil profile and are incorporated by biological organisms.
So what can be done about incidental transfer? Here are a few suggestions:
- Avoid spreading manure on snow.
- Spread manure when soils have had the chance to dry out.
- Plant cover crops that slow down runoff giving manure and water a chance to slowly soak into the soil.
- Increase infiltration by avoiding soil compaction.
- Speed incorporation by promoting soil organisms with high biological activity.
- Consider manure injection. This application method keeps manure from the surface which has been shown to reduce nutrient losses in runoff, but perhaps increases sub-soil leaching losses. By using low-disturbance manure injection most benefits of no-tillage for soil health can still be maintained.