A soil phosphorus saturation index decreasing scooped weight effect in Mehlich-3 procedure

The Mehlich-3 method extracts a quantity of phosphorus (P-M3) well correlated with crop yield in acid soils, and a quantity of aluminum (Al-M3) well correlated with P sorption capacity of mineral soils. Phosphorus fertility levels in soils are generally determined on a volume basis, while soil sorption capacity for P and P saturation of the soil sorption capacity are assessed on a weight basis. However, scooped weights vary widely among tested soils. The purpose of this paper was to test the stability of a constructed ratio of P-M3/Al-M3 across a range of soil:solution ratios using 24 soils. Twenty-four surface soils of different genetical and textural groups were extracted for P-M3 and Al-M3 without replication. Scooped weights varied between 3.21 and 4.17 g per 3-mL scooped volume. Reproducibility of volumetric test (3 mL of soil per 30 mL of Mehlich-3 solution) was within 3% for P-M3, Al-M3, and 100 P-M3/Al-M3 using two contrasting soils with two replications. Extracted P-M3 and Al-M3 expressed on a weight basis decreased across the 24 soils as sample weight increased between 1.50 and 5.00 g, by steps of 0.50 g, per 30 mL of extracting solution. The 100P(M3)/Al-M3 ratio was less variable than P-M3 or Al-M3 taken alone using a wide range of sample weights. In contrast with P-M3 and Al-M3 values taken alone, the 100P(M3)/Al-M3 ratio produced stable data across the entire range of sample weights corresponding to scooped weights between 0.50 and 1.67 g mL(-1), as shown by a power test. The 100P(M3)/Al-M3 ratio could be used simultaneously as a P saturation index for assessing environmental risk, and as a P fertility index for making fertilizer recommendations.