The balance sheet method as a conceptual framework for nitrogen fertilization of wheat in a pampean agroecosystem

Yield response curves to fertilizer are needed to determine N rates for wheat (Triticum aestivum L.) in the Argentine Rolling Pampa. Relationships between yield and available N have not been developed for this region. The balance sheet method can be use to define N fertilization strategies when yield data are not available. The shortcoming of this method is that profitability cannot be assessed. This study was conducted to: (i) determine N requirements for wheat in the Rolling Pampa, (ii) develop algorithms for predicting crop available N, and (iii) develop a model for making N fertilizer recommendations. Fifty-eight field plots were selected with contrasting soil and management conditions. Soil mineral N content and N in residues of preceding crop were determined at initial and harvest stages of wheat from 1997 to 1999. Plant N accumulation was measured, and N mineralized from organic soil pools was calculated using a mass balance approach. Nitrogen available in the mineral and organic soil pools limited N uptake and yield. As available N increased, crop use efficiency and agronomic efficiency decreased. The model prediction agreed well with measured plant N uptake (R2 = 0.82) and explained 51% of yield variability. The model estimated the impact of increasing fertilizer N on yield in soils differing in N mineralization capacity. The economic optimum N requirement in the Rolling Pampa is 140 kg ha(-1) for soil having an average capability to supply N from organic pools and at a mean fertilizer price/grain price ratio. This optimum increased or decreased 30 kg ha(-1) across the range of N mineralization rates observed. Despite the empirical nature of the algorithms developed, the concepts may be used in other regions where response functions to N are not available.