Stanford's Equation As a Framework for Making Nitrogen Recommendations and for Improving Nitrogen Recommendations.

The complexity and dynamic nature of the nitrogen cycle makes it very difficult to accurately make a site specific N recommendation for a given growing season. Developing N recommendations for a specific situation is an exercise in adaptive management where information and experience guide ongoing, iterative, and collaborative decision making. To develop a more rigorous approach for making N recommendations, it is proposed that the Stanford Nitrogen Mass Balance Equation would be a good framework.  An expanded form of the Stanford Equation accounts for the majority of the soil, crop, environmental, and management factors that determine the N application necessary to meet the internal N requirement of a crop:

N_fert =  ([Yld x N_crop]  – [N_SIN  x R_SIN x e_SIN] –  [N_SON x R_SON x M_SON x e_SON]  –  [N_MRON x R_MRON x M_MRON x e_MRON]   –  [N_MIN x R_MIN x e_MIN]  –  [N_MON x R_MON x M_MON x e_MON] -  [N_LN  x R_LN x M_LN  x e_LN]) / e_fert

where: N_fert  =  Total fertilizer N required, Yld=Dry Matter Yield of the above-ground plant portion, N = Nitrogen pool, R=Resilience Factor, M=Mineralization Factor, e=Recovery Efficiency Factor,  and the subscripts: crop=above-ground plant portion, fert=fertilizer N, SIN=Soil Inorganic N, SON=Soil Organic N, MRON=Manure Residual Organic N, MIN=Manure Inorganic N, MON=Manure Organic N, and LN=Legume N. While this equation is theoretically sound, most of the factors are either not known or only roughly estimated for any given situation; however, the equation does show the important mechanisms and connections to field conditions and management that can be useful in guiding adaptive management decisions. It suggests where better inputs from measurements or models could improve the prediction of the actual N recommendation. The challenge becomes how to use this conceptual framework to develop a practical approach to N recommendations using the principles of adaptive management.