A New Decay Series for Organic Crop Production

Abstract

For organic farmers who use manure or compost to maintain soil fertility, a pressing question is how much should be applied. Although many factors influence this decision, an important consideration is that manure and compost continue to fertilize for many years after their application. In conventional agriculture, the carry‐over effects of manure have traditionally been quantified in terms of N fertilizer equivalents, that is, through a decay series. Nitrogen decay series may also be used by organic farmers trying to meet N fertilizer recommendations with manure and/or compost. However, this creates an unsustainable paradigm for organic fertility management because N fertilizer equivalency experiments cannot be conducted on organically certified land. We propose a new type of decay series compatible with organic regulations in which the carry‐over effects of manure/compost are measured, not against N fertilizer, but against new applications of the amendment. The result is a decay series in manure/compost equivalents, which, although conceptually straightforward, is not as readily estimated as a N decay series. The methodological challenges of this approach were explored using data from a continuous corn (Zea mays L.) system fertilized with manure slurry. Regression analysis revealed that 1 unit of manure slurry had the same effect on yield as 0.21, 0.16, and 0.13 units of new slurry after 1, 2, and 3 yr, respectively. By quantifying the nutritive and non‐nutritive carry‐over effects of manure/compost, this new decay series has the potential to fully credit organic matter in fertility planning.