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FERTILIZER MANAGEMENT

The Balance Sheet Method as a Conceptual Framework for Nitrogen Fertilization of Wheat in a Pampean Agroecosystem

^a Facultad de Agronomía, Univ. de Buenos Aires, Av. San Martín 4453 (1417), Buenos Aires, Argentina
^b EEA Pergamino INTA, CC 31 (2700), Pergamino, Argentina

^* Corresponding author (ralvarez{at}agro.uba.ar).

Received for publication March 5, 2004. 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.

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