Manuel Marcaida III1, Kirsten Workman1,2, Karl Czymmek2, and Quirine M. Ketterings1
1Cornell University Nutrient Management Spear Program (NMSP) and 2PRO-DAIRY
Introduction
The New York Phosphorus Index 2.0 (NY P-Index) and the Northeast Region Phosphorus Index (NR P-Index) help farmers assess the relative risk of phosphorus (P) loss from their fields and make informed decisions about manure and fertilizer P applications. Both P-indices combine soil test phosphorus (STP) in four categories (<40, 40-100, 101-160, and >160 lbs Morgan P/acre) with a field P-Index score derived from field features such as soil type, flow distance to streams, and flooding frequency, and management practices such as incorporation or injection of manure, to derive a management implication (Figure 1).
Farmers and advisors have increasingly looked at grid sampling to better management soil fertility for improved crop production. To address questions on how to use P-Index in grid sampling context where STP information are more spatially granular than traditional whole-field samples, we analyzed soil data from 20 corn fields across six New York farms. We compared P-Index results and management recommendations based on whole-field composite samples and grid-based STP data at three grid sizes (0.5, 1.0, and 2.5 acres).
Key Findings
Homogeneous field conditions lessen the need for grid sampling
Most fields sampled were relatively homogeneous in terms of STP categories, with STP levels consistently classified as either A (<40 lbs Morgan P/acre) or D (>160 lbs Morgan P/acre) regardless of grid size (Figure 2). In such cases, whole-field composite sampling was sufficient for P-Index assessment and P management planning. Three fields showed noticeable variability across STP classes, with their whole-field averages in the STP range B (40-100 lbs Morgan P/acre). For these fields continued grid-based or zone sampling can help refine P management as well as address other fertility goals of the farm manager.
Grid sampling is valuable for fields with variable STP levels
For the three fields whose STP levels were predominantly in class B (40-100 lbs Morgan P/acre), grid sampling revealed meaningful within-field differences (Figure 2) that would not be apparent from a whole-field composite sample. The finer scale information can help farmers and crop advisors designate P management zones, identifying whether portions of a field may warrant more conservative manure applications, or whether lower-P areas justify continuing N-based manure rates. The value of grid sampling is not in calculating grid-level P-Index scores, but in using the spatial pattern of STP to determine whether a single field-level P-Index assessment is adequate or whether zone-based management could enhance nutrient-use efficiency and advance environmental stewardship.
Coarser grid sizes provide comparable P management insights
Analysis on Fields E3, D1, and C1 showed that fields with STP levels in 40-100 lbs Morgan P/acre (class B) range gave similar management results whether sampled on 0.5-, 1.0-, or 2.5-acre grids at different P-loss risk scenarios (Figure 3). This finding suggests that coarser grids, such as 2.5 acres, can still capture the key spatial patterns needed to guide P management decisions. For farmers and crop advisors, this means sampling and testing can be streamlined without losing the detail needed to understand whether P-risk varies meaningfully across the field. While the P-Index is not intended for grid-level application, an initial grid sample can reveal where distinct P zones exist, allowing those zones to serve as the basis for future P-Index assessments.
Conclusions
Grid sampling for P management is most useful in fields with 40–100 lbs/acre STP levels, where nutrient variability within the field can change management recommendations. For fields with uniformly low or excessively high STP levels, whole-field composite sampling provides adequate information for nutrient management planning. When grid sampling is beneficial, a 2.5-acre resolution captures meaningful variability without the added cost of finer grid sampling. Using the New York P-Index 2.0 or Northeast Region P-Index together with grid-based data helps farmers make informed decisions that balance productivity, nutrient efficiency, and water quality protection.
Full Citation
This article is summarized from our peer-reviewed publication: Marcaida, M. III., K. Workman, K. J. Czymmek, and Q.M. Ketterings (2025). Grid-based soil sampling for Northeast Region phosphorus index assessment. Soil Science Society of America Journal, 89, e70156. https://doi.org/10.1002/saj2.70156
Acknowledgments
The authors would like to thank the staff of Champlain Valley Agronomics, Western New York Crop Management Association, and participating farmers. Funding came from the Northern New York Agricultural Development Program, the New York Corn and Soybean Growers Association via the New York Farm Viability Institute, the New York State Department of Environmental Conservation, the New York State Department of Agriculture, and the intramural research program of the U.S. Department of Agriculture, National Institute of Food and Agriculture, Hatch 2021-22-210. The findings and conclusions in this publication have not been formally disseminated by the U.S. Department of Agriculture and should not be construed to represent agency determination or policy. For questions about these results contact Quirine M. Ketterings at 607-255-3061 
or qmk2@cornell.edu, and/or visit the Cornell Nutrient Management Spear Program website at: http://nmsp.cals.cornell.edu/.
