This research paper describes a study that used models of phosphorus flows in Vermont agriculture between 1925 and 2012. The study finds that intensification has offset any increases in efficiency, and that Vermont soils continue to accrue legacy phosphorus, with implications for water quality remediation efforts. Key points:
- Vermont’s farms accumulated >1000 tonnes of legacy phosphorus each year from 1925 to 2012
- Since 1982, feed imports have been a larger source of P inputs than fertilizer
- Regulatory accountability for feed inputs can help reduce imbalances and legacy P
Phosphorus (P) is a scarce but critical input for agriculture, yet its overuse can lead to water quality degradation. Most P applied as fertilizer and manure binds to soils, accumulating over time, constituting a legacy source with implications for mitigating nutrient pollution. To investigate how the flows and balance of P evolved over a period of rapidly changing technology, agricultural practices, and land cover, we modeled P flows in Vermont’s dairy-dominated agricultural system at county- and state-levels from 1925 to 2012. An important dairy exporter, Vermont faces water quality challenges complicated by a mismatch between the scale of the market and that of policymaking, a common occurrence in export-oriented agricultural regions. Over the period analyzed, agricultural soils accumulated at >1000 tonnes of P annually, accruing a legacy stock >230,000 tonnes. The peak surplus of 4439 tonnes occurred in 1950, declining to 1493 tonnes per annum in 2012. Legacy P accumulation at the state-level ranged from <1 to> 16 kg ha−1, depending on year and measurement method. The decline in total P surplus reflects an 82% decline in fertilizer use that was partly offset by an increase in animal feed imports, the largest source of P entering Vermont since 1982. Despite declining inputs, milk output doubled, evidence of increased P use efficiency. Simultaneously, animal unit density increased by >250%, enabled by rising feed imports. While feed is imported and milk exported, manure remains in Vermont; hence, Vermont soils continue to accrue legacy P at rates > 5 kg ha−1, undermining efforts to reduce P runoff and achieve water quality targets. We discuss the governance, management, and policy implications, outlining opportunities to improve input accountability to address the persistent P imbalance. We highlight constraints facing regional policymakers due to increased embeddedness in commodity trade networks.