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A model based on a form-quadrant representation of N fertilizer experiments is presented. Calculations showed that spatial variability of mineral N in soil led to higher 'economic optimum' fertilizer rates and rates of Environmentally Acceptable Production (EAP) decreased. Without spatial variability a positive difference of 13 kg/ha was found between the N fertilizer rates for EAP and Maximum Economic Yield (MEY) even when the strictest environmental standard was applied (soil mineral N content at harvest not exceeding 34 kg/ha). At standard levels of variability there was a negative difference of 16 kg/ha, and at doubled variability the difference was 156 kg/ha. An N residue rate of 45 kg/ha at harvest could be met at standard varibility but not at doubled variability. The model showed that higher degrees of 'luxury consumption' and improved synchronization of N mineralization and crop demand had a positive effect on the difference between EAP and MEY. It was concluded that the degree of spatial variability in N supply must be taken into account for environmental and production targets. (Abstract retrieved from CAB Abstracts by CABI’s permission)

In the period [1983][1984][1985] 18 potato experiments with poultry slurry were conducted on soils reclaimed from cut-over peat and sandy soils in the Netherlands. The experiments consisted of four main treatments: no slurry, autumn-applied slurry, autumn-applied slurry combined with the nitrification inhibitor dicyandiamide (DCD), and spring-applied slurry. As calculated from the amount of soil mineral nitrogen present at the start of the growing season (June), average nitrogen recoveries of 47, 62 and 51% were found for autumn-applied, autumn-applied with DCD and spring-applied slurry respectively. A close relationship was found between the amount of soil mineral nitrogen in June and the amount of nitrogen contained in the tubers of the potatoes at harvest. Soil mineral nitrogen at harvest was increased by even moderate amounts of nitrogen supplied. At the optimum nitrogen supply for crop growth, about 110 kg of mineral nitrogen per ha remained in the soil profile (0-100 cm) at harvest. This could lead to considerable losses of nitrogen in the following winter.

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