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 (Triticumaestivum)/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.