FIELD EXPERIMENTS WITH SLURRY AND DICYANDIAMIDE - RESPONSE OF POTATOES AND EFFECTS ON SOIL MINERAL NITROGEN

In the period 1983-1985, 18 field experiments with potatoes grown for industrial starch production were set up to investigate the effects of poultry-slurry application on tuber yield and on soil mineral nitroge n. Slurry was applied in autumn with and without the nitrification inh ibitor dicyandiamide (DCD) and in spring without DCD. Control treatmen ts without slurry or DCD were included. Various nitrogen fertilizer ra tes were applied to all slurry treatments. In autumn, following slurry application without DCD, slurry-derived nitrate moved to the 0.3-0.6 and to the 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 yi elds as estimated from a nitrogen fertilizer response function were sl ightly increased by the slurry. Nitrogen supplied from the slurry decr eased the amount of fertilizer nitrogen needed for maximum yield. Incr easing the amounts of soil mineral nitrogen in June from slurry or app lied inorganic nitrogen fertilizer increased residual soil mineral nit rogen at harvest.