High Threat of ‘Incidental Transfer’ of Surface-applied Nutrients to Streams

By Sjoerd Willem Duiker

Consider the ‘incidental transfer’ of surface applied nutrients and dissolved organic carbon from surface applied manure and fertilizer on saturated winter soils.

Soil profiles are fully recharged with water and it is likely runoff will occur from farm fields over the winter, even if soil health is very good. There is therefore a high threat of ‘incidental transfer’ of nutrients and dissolved organic carbon from surface applied manure and fertilizer to streams and sinkholes.

‘Incidental transfer’ is the loss of sediment, nutrients, pesticides and dissolved carbon from the soil during infrequent runoff events. These events occur primarily when soil is saturated, or during 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 be worse in fields managed with tillage if manure is not incorporated immediately. The infiltration rate in tilled soil decreases with time after tillage as the surface soil slumps back together and a seal forms. On well-drained soils the use of continuous no-tillage leads to great infiltration improvements, and one would typically not expect much 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 late winter/early spring in Pennsylvania. If manure or fertilizer 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 nutrients in surface runoff because it is usually only a threat for short periods of time after surface application. After some time, the manure and fertilizer nutrients and dissolved organic carbon infiltrate in the soil and are incorporated by biological organisms. Here are some suggestions to limit the chance of incidental transfer:

• Do not spread manure or fertilizer on snow.
• Spread manure and fertilizers when soils have had the chance to dry out.
• Use low application rates in winter.
• Spread manure and fertilizers on actively growing cover crops that slow down runoff giving manure and water a chance to slowly soak into the soil.
• Avoid soil compaction and improve soil structure using soil health improving practices.
• Promote high soil biological activity so that manure gets incorporated quickly by soil organisms.

Inject manure and fertilizer, removing manure from the surface. By using low-disturbance injection the benefits of no-tillage for soil health can still be maintained.