A New Decay Series for Organic Crop Production.

For organic farmers who apply manure and/or compost to maintain soil fertility, a pressing question is how much should be used.  When a cheap or even free source of organic material is available, many farmers apply manures/composts based on achieving maximum yield, as has been the paradigm for conventional fertilizer recommendations. On the other hand, high costs of organic inputs may lead a farmer to apply manure/compost based on the expected economic returns in any given year. An important factor in this decision, however, is that manure and compost continue to fertilize for many years after being applied.  If these carry-over effects are not considered, the maximum yield strategy will lead to over-fertilization, while the profit-maximizing strategy will underestimate the economically optimal manure/compost rate.  In conventional agriculture, the carry-over effects of manure have traditionally been quantified in terms of nitrogen fertilizer equivalents, i.e., through a decay series.  Nitrogen decay series may also be used by organic farmers trying to meet nitrogen fertilizer recommendations with manure or compost.  However, this creates an unsustainable paradigm for organic fertility management since nitrogen fertilizer equivalency experiments cannot be conducted on organic farmland.  We propose a new type of decay series that can be measured under organic regulations.  This is possible because the carry-over effects are measured, not against nitrogen fertilizer, but against prospective applications of the amendment.  This change of basis, from nitrogen fertilizer equivalents to manure/compost equivalents, also quantifies non-nutritive effects in a more meaningful way.   Using literature data from a continuous corn system amended with manure slurry, we explore the methodological challenges involved in estimating decay series when the response to N fertilizer is not available.