Forecasting corn nitrogen fertilizer rate based on soil water fluctuation

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Abstract
The objective of this paper has been to develop a model for nitrogen(N) fertilizer rate forecasting for corn crop (Zea mays L.) based on plant available soil water (PAW) and precipitation. Crop N requirements, soil organic matter, N supplying capability of soil, or initial soil nitrate were used as the basis for N fertilizer recommendations. Often a standard average N rate is recommended for a specific area, irrespective of stored soil water and rainfall conditions of actual year. Long‐term experiments carried out at the same site (soil and field) in south central Romania have shown a wide variation in yield, optimum N rate, and N fertilizer efficiency over the years. No study was done in this area to forecast N fertilizer rate based on soil water. Since both precipitation and soil water have a paramount effect on organic matter mineralization and on mineral N behavior in soil, the PAW in March 1st, June 1st and June precipitation, at the time of N fertilizer application, were taken into account for N rate forecasting. The experiments were conducted 10 years on a deep well‐drained chernozem in a dry area, with annual precipitation of about 500 mm with wide annual and seasonal fluctuation. The data indicate that optimum N rate is more correlated with March 1st PAW from 0 to 100 cm (r = 0.90**) than with June‐July precipitation (r = 0.53). Summer precipitation affects the optimum N rate more in years with high than in years with low PAW in spring. Therefore, the dryer the year the more important the spring PAW than summer precipitation is for N fertilizer effect on crop yield. A first forecast of N rate may be done in early spring before sowing, applying 1 kg N/ha for each 1 mm for PAW exceeding 100 mm between 0 and 100 cm soil depth on March 1st. In years with March 1st PAW higher than about 150 mm between 0 and 100 cm and spring months wetter than normal, a second forecast may be done at the middle of June taking into account March 1st PAW, June 1st PAW up to 150 cm and June precipitation. With this method, forecasted N rates were very closed to the optimum N rate (r = 0.96**) and both yield increase and profit were higher than with standard long‐term average N rate. In this dry area, where water is a limiting factor of corn yield and N fertilizer efficiency, spring PAW could serve to a more accurate N fertilizer rate forecasting.