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¶ Equation Format
If the soil test phosphorus is less than or equal to the critical level (STP < = P_crit), then:
¶ P_rec = [Phos_A * exp(PhosB * STP)] + [(YG - BYG) * P_adj]
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If the soil test phosphorus is greater than the critical level (STP > P_crit), then:
¶ P_rec = [(Phos_A * exp(PhosB * P_crit)) + ((YG - BYG) * P_adj)] * [(P_col - STP)/(P_col - P_crit)]
¶ Equation terms
¶ P_rec ........ Final recommended rate for broadcast application expressed as "lb P2O5/ac". Adjust this rate for fertilizer analysis or manure/biosolid analysis (as N-P2O5-K2O)
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¶ STP .......... Soil test phosphorus value as "parts per million (ppm P)"; equivalent to milligrams per kilogram (mg/kg)"
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¶ Phos_A ....... Maximum recommended fertilizer phosphorus rate when soil test value is 0 ppm P; also known as "y axis intercept"; expressed as "lb P2O5/ac"
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¶ PhosB ........ Rate adjustment factor from 0 ppm to P_crit value; also known as "equation slope"
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¶ P_crit ....... Critical soil test value as "ppm P"; P_crit value is beginning of the optimum category, the soil test range to maintain long-term profitable crop production. Recommended rate below P_crit value is to correct agronomic deficiency (i.e., "sufficiency") plus buildup and maintenance to move soil test phosphorus gradually to P_crit value. Recommended rate for soil test level above P_crit value is primarily for soil test maintenance
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¶ P_col ........ Cut-off value for maintenance; recommended rate begins to gradually decrease with soil test levels above P_crit value, finally reaching zero at P_col value
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¶ BYG .......... Base yield goal for calculation; final maintenance/crop removal rates are adjusted up or down according to difference between BYG and YG (target yield goal)
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¶ YG ........... Target or desired yield goal; yield units expressed as "lb/ac", "ton/ac", etc
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¶ P_adj ........ Full or partial crop removal of phosphorus in harvested yield unit. Applied to maintain soil test. P_adj value multiplied by positive or negative difference between base yield goal (BYG) and target yield goal (YG); expressed as "lb P2O5/ac"
Also see the following Crop Files:
- 1.02.032 Nutrient Removal Values Phosphorus P2O5
- 1.04.310 Phosphorus (P) in the Soil and Plant
- 1.04.311 Phosphorus Transformations in the Soil
- 1.01.231 Soil Test Phosphorus Methods