Simulated potato yield, and crop and soil nitrogen dynamics under different organic nitrogen management strategies in The Netherlands

In order to improve crop yields but limit phosphorus (P) accumulation in organic farming systems, insight into the effects of organic nitrogen (N) management on crop yield is required. After validation, an adapted model was used to explore N uptake, tuber yield and residual soil mineral N (RSMN) of a potato crop (Solanum tuberosum L.) for 30 years of historical weather data, as influenced by: (1) the time of slurry application; (2) cultivar maturity; (3) the N/P ratio of the manure; and (4) historical N use. All these variations were compared to a model scenario with cattle slurry (N/P 6.2), applied in spring to an early cultivar which was harvested 1 September, on farms with a stable soil N and P content. Average simulated tuber yield increased from 8.0 to 11.2 Mg ha−1 for 490 kg slurry N ha-1 when spring-applied (SA) and up to 10.0 Mg ha-1 when autumn-applied (AA), but year-to-year variation was considerable. Results indicated that a spring-applied slurry is to be preferred over an autumn-applied slurry in order to avoid over-winter N losses. Tuber yield for a mid-late cultivar was larger than for an early cultivar when harvested on 1 September, but was generally smaller when harvested on 1 August. The RSMN of the mid-late cultivar was smaller than of the early one at both harvest dates due to a larger crop N uptake. Tuber yield was at most 5% smaller with application of manure having an N/P ratio of 4.0 than with a ratio of 6.2. On farms recently converted from conventional farming, simulated yields were about 5% larger (with large historical N inputs) or smaller (with small historical N inputs) that on farms with a stable soil N content. Patterns of N uptake suggest that organic N with a large proportion of mineral N and applied shortly after emergence, could improve potato yields in organic farming.