Phosphorus Budget and Soil Extractable Dynamics in Field Crop Rotations in Mollisols

A better understanding of the P cycle is necessary to provide insights into management strategies that could enhance P bioavailability for crops. Our objectives were to evaluate the effects of continuous P fertilization on (i) the P budget, (ii) soil‐extractable P (Pe) dynamics, and (iii) the relationship between these variables. Two crop rotations during a 6‐yr period were analyzed in seven on‐farm experiments: corn ( Zea mays L.)–double cropped wheat ( Triticum aestivum )/soybean [ Glycine max (L.) Merr.] (C‐W/S), and corn–soybean–double cropped wheat/soybean (C‐S‐W/S). Calculations of net P input considered only P removal (output) and fertilization (input). Initial soil‐test P was 13.4 mg P kg −1 (average for the four sites) for the C‐W/S rotation and 37.9 mg P kg −1 for the C‐S‐W/S rotation (average for the three sites). Treatment combinations were two annual rates: unfertilized and grain P removal estimation +10% (average 34 kg P ha −1 yr −1 ). Not applying P to soils testing 45 to 68 mg kg −1 of initial Pe resulted in a steep Pe decline (slope 0.2 mg kg −1 per unit of negative P budget). Nevertheless, soils with <25 mg kg −1 of initial Pe levels resulted in a very small and gradual decline (slope 0.02 mg kg −1 ). Use of current P rates applied by local farmers probably results in a small soil‐test P buildup with time. The P required to increase Pe by 1 mg P kg −1 was 11 and 8 kg P ha −1 yr −1 for the C‐W/S and C‐S‐W/S rotations, respectively.