For years, crop advisors – myself included – encouraged farms to incorporate manure immediately after spreading it in order to conserve nitrogen that would otherwise volatilize away as ammonia gas. Waiting even one day to incorporate manure could mean losing almost half of the nitrogen that would otherwise be available to your crop. Incorporation, however, isn’t feasible in no-till systems, where the whole point is to minimize any soil disturbance. That’s not to say that incorporation is wholly impossible; there’s equipment for injecting manure below the soil surface, but it can be costly, and it’s impractical for operations with fields located significant distances away. What’s a farm to do?
The situation may not appear so dire once you take a look at the actual numbers. Consider a typical manure slurry application of 10,000 gallons per acre to a cornfield; you apply roughly 225# of nitrogen, with about 150# as more slowly-available organic N and the remaining 75# pounds in the immediately-available-but-volatile ammonium fraction. Without incorporation immediately after spreading, you could lose up to 40# per acre leaving it on the soil surface. That’s a significant amount – it would cost about $22 per acre to replace that nitrogen with fertilizer – but it’s also a worst-case scenario. Thinner manure or spreading in the rain tends to allow more manure to infiltrate the soil and reduce losses. Also, even with that worst-case scenario, you’re still left with over 180# of nitrogen for your crop. True, most of that is in the organic form and only a portion of that will be available this year, but contributions from previous years’ applications keep your crop from getting shortchanged.
Research on manure incorporation under no-till backs this up. These studies confirm that ammonium nitrogen losses do in fact occur with surface-applied manure under no till, but they did not observe any reductions with crop yields or quality accompanying those losses. It appears that the loss is small compared to the large pool of readily available N in the soil resulting from a long manure history coupled with management practices that favor efficient mineralization of organic nitrogen. Farms that couple no-till with the use of cover crops would also benefit from having them take up any nitrogen remaining in the soil at the end of the season, keeping them in the field for another year’s crop.
None of this is to suggest that it’s OK to be cavalier about manure nutrient losses; keeping them in the field as much as possible keeps the environment healthy and helps your bottom line. In this case however, any losses are offset by the benefits – fuel and labor savings, less compaction, better water management – that no-till delivers.