Evaluation of the soil nitrogen supply (SNS) system - opportunities for further improvements to the nitrogen economy of the GB potato crop

Theoretically, the optimum fertiliser application rate could be calculated from the target green leaf area, an estimate of existing soil mineral nitrogen (SMN) and the amount already in the crop in early spring. However, data from the project showed that analysis of SMN, as currently used, is less reliable than the field assessment method used before 2000. Experiments to understand this discrepancy revealed that mineralization of soil organic matter is more variable and more significant than previously thought, even in soils with a modest organic matter content. Cultivations increase the rate of mineralization, but this increase is greater if cultivations are done when the soil is warm and moist. The timing of soil sampling in relation to cultivations and environmental conditions will therefore affect the amount of N measured.

Although more reliable than SMN analysis, the field assessment method was found to consistently overestimate the amount of nitrogen required. The smaller value of the range of recommendations given in the 7th edition of RB209 should be used and this can result in a saving of 20-30kg N/ha without loss in marketable yield.

Experiments into the canopy management approach showed that the amount of nitrogen associated with production of leaf area is variable, so nitrogen requirement cannot readily be calculated from target canopy area. Analysis of existing data from Potato Marketing Board experiments in 1992-95 revealed that nitrogen uptake by the crop is initially rapid but then slows. Only in the initial phase is N uptake proportional to canopy area, suggesting that canopy persistence and yield potential may be determined relatively early in the season.