Impact of Sampling Depth on Soil Phosphorus and Fertilizer Recommendation.

Phosphorus (P) is one of the essential primary macronutrients that is inherently deficient in most of Alabama soils. Databases compiled for more than half a century at Auburn University Soil Testing lab show that more than 50% of Alabama soil samples analyzed by the lab each year needed P fertilizer applications for optimum plant growth. Conventionally, soil samples are taken at 2-3 inches depth for established pastures, hayfields, and lawns and, 6-8 inches for field and row crops. Our objective was to determine the effect of sampling depth on soil extractable P. Soil samples were taken at 0-3, 0-4, 0-6, 0-8 and 0-12 inch depths from fields under different crops, P treatments and farming cultural practices. The samples were extracted by Mehlich I solution and P in the extractant was analyzed by inductively coupled plasma (ICP). Preliminary results of the study indicated that, (1) in general, extractable P in the 0-3 inches was higher than lower depths, (2) extractable P values were affected by soil type, pH, P treatments, crop rotation and cultural practices, (3) for soils that had “low P” rating, extractable P decreased with increasing sampling depth, and (4) for “high P” rated soils, depth has no significance impact on extractable P. Revisiting and making necessary adjustments to the traditional static soil sampling depths used to determine extractable P for different farming cultural practices will enhance P use efficiency and productivity.