Assessing the effectiveness of manure export plus intensive silage cropping for lowering the Olsen-P status of P-enriched grassland

A substantial proportion of farmed grassland soils in Northern Ireland (NI) are overly enriched with P and pose a risk to water quality. To address this problem, manure could be exported rather than recycled to P-enriched land and the latter intensively cropped with grass silage to deplete soil P. To assess the efficacy of such a strategy, a P- and K-enriched grassland site was intensively cropped over a 6-yr period with fertilizer N alone supplied to support silage growth. By year 6, soil P had declined from index 5 to index 3, and it was estimated that two more years of this management may bring it into the target index 2 range. Soil K, however, declined rapidly from index 4 to index 1 in just 4 yr, with the result that grass production became limited by K deficiency. It was concluded that nonrecycling of manure to P-enriched grassland under silage management is probably the most effective strategy for lowering soil P status, but care must be taken to prevent K deficiency occurring.