Fertigation of apples with nitrate or ammonium nitrogen under drip irrigation. I. tree performance

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Abstract
The effect of fertigation with 128 kg/ha of N of various fertilizer sources, containing 0, 50 or 100% ammonium ions, on apple (cv. ‘Anna') performance was studied in a field experiment under drip irrigation. Growth, yield, leaf and fruit mineral analysis and fruit size and maturity were measured throughout the experiment, which lasted for 7 years. The fertilizer source containing nitrate only (0% NH4 +) increased yield by 15% and growth by 10–14% as compared with fertilizers containing 50 or 100% NH4 +. The effect of the N form on yield was thought to be mediated through enhanced growth by higher nitrate concentration in the soil, rather than via growth inhibition by the ammonium ions, since only traces of NH4 + could be detected in the soil solution. The N form or the phosphate carrier of the 100% NH4 + source fertilizer, had no consistent effect on the mineral composition of the leaf and fruit, or on acidity, total soluble solids (TSS) and pressure of the fruit.