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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.

In the period of 1983-1985, 18 field experiments with potatoes grown for industrial starch production were set up in the Netherlands to investigate the effects of poultry-slurry application on tuber yield and on soil mineral nitrogen. Slurry was applied in autumn with and without the nitrification inhibitor dicyandiamide (DCD) and in spring without DCD. Control treatments without slurry or DCD were included. Various nitrogen fertilizer rates were applied to all slurry treatments. In autumn, following slurry application without DCD, slurry-derived nitrate moved to the 0.3-0.6 and 0.6-1 m soil layers. Following DCD-application, most of the slurry-derived nitrate remained in the 0-0.3 m soil layer. Maximum yields as estimated from a nitrogen fertilizer response function were slightly increased by the slurry application. Nitrogen supplied from the slurry decreased the amount of fertilizer nitrogen needed for maximum yield. Increasing the amounts of soil mineral nitrogen in June from slurry or applied inorganic nitrogen fertilizer increased residual soil mineral nitrogen at harvest.