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¶ Equation Format
If the soil test potassium is less than or equal to the critical level (STK < = K_crit), then:
¶ K_rec = [Potash_A * exp(Potash_B * STK)] + [(YG - BYG) * K_adj]
If the soil test potassium is greater than the critical level (STK > K_crit), then:
¶ K_rec = [(Potash_A * exp(Potash_B * K_crit)) + ((YG - BYG) * K_adj)] * [(K_col - STK)/(K_col - K_crit)]
¶ Equation terms
¶ K_rec ........ Final recommended rate for broadcast application expressed as "lb K2O/ac". Adjust this rate for fertilizer analysis or manure/biosolid analysis (as N-P2O5-K2O).
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¶ STK .......... Soil test potassium value as "parts per million (ppm K)"; equivalent to milligrams per kilogram (mg/kg)".
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¶ Potash_A ..... Maximum recommended fertilizer potassium rate when soil test value is 0 ppm K; also known as "y axis intercept"; expressed as "lb K2O/ac"
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¶ Potash_B ..... Rate adjustment factor from 0 ppm to K_crit value; also known as "equation slope".
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¶ K_crit ....... Critical soil test value as "ppm K"; K_crit value is beginning of the optimum category, the soil test range to maintain long-term profitable crop production. Recommended rate below K_crit value is to correct agronomic deficiency (i.e., "sufficiency") plus buildup and maintenance to move soil test potassium gradually to K_crit value. Recommended rate for soil test level above K_crit value is primarily for soil test maintenance.
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¶ K_col ........ Cut-off value for maintenance; recommended rate begins to gradually decrease with soil test levels above K_crit value, finally reaching zero at K_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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¶ K_adj ........ Full or partial crop removal of potassium in harvested yield unit. Applied to maintain soil test. K_adj value multiplied by positive or negative difference between base yield goal (BYG) and target yield goal (YG), then expressed as "lb K2O/ac".
Also see the following Crop Files:
- 1.02.033 Nutrient Removal Values Potassium K2O
- 1.04.410 Potassium (K) in the Soil and Plant
- 1.04.411 Potassium Transformations in the Soil