Grid‐based soil sampling for Northeast Region phosphorus index assessment

Abstract The Northeast Region Phosphorus Index (NR P‐index) is a risk assessment tool that evaluates phosphorus (P) loss potential using soil test P (STP) concentrations, transport factors, and management practices. It informs whole‐farm P strategies by guiding site‐specific manure application decisions. This study aimed to (1) evaluate the implications of grid‐based versus whole‐field STP concentrations on P management implications according to the NR P‐index, and (2) examine if different grid sizes impact NR P‐index‐based management implications. Soil samples were collected from 20 corn ( Zea mays L.) fields across six farms in New York, each with varying STP concentrations, and analyzed at three grid resolutions (0.2, 0.5, and 1.0 ha). Grid sampling allowed for more precise P management in fields with moderate STP concentrations (20–80 mg kg −1 ), through identification of areas with higher or lower P‐index score compared to whole‐field assessment. For fields with STP concentrations in the agronomic range (<20 mg kg −1 Morgan P) or excessive levels (>80 mg kg −1 Morgan P), whole‐field assessments can be used to inform P recommendations. Grid sizes finer than 1.0 ha did not impact the management implications, indicating limited benefit from higher spatial resolution when STP concentrations are largely uniform or fall within a single P‐index category. These findings suggest that grid sampling for P‐index assessment was most effective for these fields with moderate STP concentrations and P‐index scores, allowing for more targeted P management.