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Identifying cost-effective soil sampling schemes for variable-rate fertilization and liming.

Within-field nutrient variability causes some areas of a field to be more or less responsive to fertilization. The best soil sampling and fertilization strategies are those that best estimate and apply economic optimum fertilizer rates across a field. Although current site-specific management practices could achieve this goal, questions remain concerning the cost-effectiveness of alternative sampling strategies. This study compared various soil sampling schemes in eight fields using soil test P, K, pH, and organic matter. The schemes were grid sampling, sampling by digitized and detailed soil survey map units, sampling by elevation, and a targeted sampling based on various layers of information. The soil-test variability patterns varied markedly across the fields. The schemes varied greatly in reducing the within-unit variability and in the recommended fertilizer rate, but no scheme was superior across all fields and nutrients. The efficacy of all sampling schemes was lower for P and K, probably because of the larger impact of fertilization on small-scale variability. Over all fields, only schemes based on field averages or digitized soil survey maps resulted in significantly lower correctly fertilized areas than other schemes. For P, the most highly variable nutrient, the grid and targeted schemes usually were similarly effective, although the latter sometimes required fewer sampling units. Consideration of costs, field fertilization history, and likely response to fertilization is needed to select among various similarly effective sampling strategies.