Seasonal Variation in Nutrient Concentration in Unagitated Manure Storage Lagoons.

The EPA revised Concentrated Animal Feeding Operation (CAFO) rule and the Natural Resource Conservation Service (NRCS) Agronomy Standard (590) require annual testing of manure storages as part of the nutrient management planning process.  We sampled three un-agitated manure storage lagoons (two swine and one dairy) approximately monthly from February 2003 to December 2005.  Manure was subsample in 12 randomly selected points around each lagoon by dipping a sampler to remove liquid from the top 25- to 30-cm of the lagoon approximately two meters from the edge of the lagoon.  The lagoon was not agitated before sampling and care was taken not to sample the floating scum layer on top of the lagoon, when present.  Samples were transported on ice, frozen and submitted for analysis of total nitrogen, ammonium nitrogen, total phosphorus, total potassium, dry matter, electric conductivity and other selected nutrients.  All analyses were completed by a commercial laboratory along with standard control samples.

Total nitrogen and ammonium nitrogen concentration varied throughout the year.  Highest concentrations were obtained in 90 to 150^th day of the year and reached their lowest concentration after the 240^th day of the year.  Total nitrogen value decreased 30% or more from the maximum to the minimum measured values.  Phosphorus and potassium values were much more consistent throughout the year.

These results imply that producers would benefit from sampling these types of manure storages at least seasonally when they apply manure multiple times per year.  Using spring manure test results for fall application is likely to lead to applying significantly less nitrogen than predicted by the spring manure test.