Relationships among soil test potassium forms influenced by clay mineralogy

Potassium fertilizer recommendations for optimal crop production may be improved by considering the ratio between expanding 2:1 layer silicates (smectite) with non-expanding 2:1 layer silicates (illite). However, the interactive effects between clay mineralogy and various soil tests are not well understood. This study evaluated the relationships among soil test K (STK), water-soluble K, HNO3-extractable K, pH, apparent cation exchange capacity (CECa), soil organic matter, clay content, and smectite:illite ratios. Soil samples (0-15 cm) were collected from 41 locations in central and eastern South Dakota, with textures ranging from sandy loam to clay. Data were partitioned into soils with smectite:illite ratios < 1 (illitic), >= 1 but <= 4.5 (smectitic), and > 4.5 (highly smectitic). Mean pH and CECa were lowest in illitic soils, higher in smectitic soils, and highest in highly smectitic soils, whereas STK and water-soluble K were lowest in highly smectitic soils relative to illitic and smectitic. Correlation analysis also showed that STK decreased with increasing smectite:illite ratio. These results suggest that exchangeable and water-soluble K forms are reduced when the proportion of smectite increases. There was a strong, positive relationship between STK and HNO3-extractable K across all three smectite:illite ratio groups. For soil pH, however, the relationship with STK was positive for illitic and smectitic soil groups, but negative for highly smectitic soils. Overall, these results suggest that the smectite:illite ratio influences the relationship among soil parameters and STK. This improves our understanding of the influence of clay mineralogy on plant-available K and the implications for K fertilizer recommendations.