Field Correlation of Potassium Soil Test Methods Based on Dried and Field-Moist Soil Samples for Corn and Soybean
2013
Barbagelata, Pedro A. | Mallarino, Antonio P.
Soil K extraction with ammonium-acetate (NH₄OAc) from oven-dried samples is the most widely used K test method, but drying often increases extracted K compared with field-moist soil. This study assessed sample drying effects on soil K extracted by NH₄OAc and used field response data to correlate K tests based on dried (35–40°C) (DK) and field-moist (MK) samples for corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] based on 162 single- and multi-year response trials conducted during 6 yr (200 site-years for corn and 162 for soybean). Potassium (15-cm depth) extracted by DK was higher than for MK (on average 1.92 times higher). The ratio DK/MK decreased exponentially with increasing K (R² 0.77); increased linearly with soil clay, organic matter (OM), estimated cation exchange capacity (ECEC), and (Ca+Mg)/K ratio (r² 0.15–0.32); and increased with sample moisture but the relationship was poor (r² 0.03). The MK test correlated better than DK with grain yield response. The R² values for Cate–Nelson (CN), linear-plateau (LP), and quadratic-plateau (QP) models across crops were 0.24 to 0.27 for DK and 0.39 to 0.58 for MK. Critical concentration (CC) ranges for corn defined by these models were 144 to 301 mg K kg⁻¹ for DK and 51 to 82 mg K kg⁻¹ for MK; whereas for soybean were 136 to 283 for DK and 49 to 84 for MK. Potassium testing of field-moist samples predicts crop response to K fertilization better than the common test based on oven-dried samples.
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