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SOIL FERTILITY & PLANT NUTRITION

Potassium Release in Representative Maize-Producing Soils of Thailand

^a Dep. of Soil Science, Kasetsart Univ., 50 Phaholyothin Road, Bangkok 10900, Thailand
^b Dep. of Computer Engineering, Kasetsart Univ., 50 Phaholyothin Road, Bangkok 10900, Thailand
^c Dep. of Tropical Plant and Soil Sciences, Univ. of Hawaii at Manoa, 3190 Maile Way, Honolulu, HI
^d Water Resources Research Center, Univ. of Hawaii at Manoa, 3190 Maile Way, Honolulu, HI

^* Corresponding author (rsyost{at}hawaii.edu).

Ammonium acetate K does not adequately measure available K in the mineralogically diverse maize (Zea mays L.)-producing soils of Thailand. The objective of this study was to understand the causes of this problem and propose a solution using laboratory and greenhouse experiments to examine the release patterns and the availability of exchangeable and nonexchangeable K. Eight kaolinitic and smectitic soils were examined for the release of K in each soil fraction (sand, silt, and clay) using the Ca-resin successive extraction method. The data were fitted using parabolic diffusion, power function, and segmented straight line regression models. The power function and the segmented regression model fitted the data well. All soils were exhaustively cropped with maize in the greenhouse until the soils became K deficient. The results indicated that a segmented regression model described nonexchangeable K release to the Ca resin and to plants in a way that seemed to correspond with earlier predictions. According to the model, there were two fractions of nonexchangeable K, which were released at distinctly different rates. The greenhouse study yielded fast and slow K release rates from the nonexchangeable K pool of 0.45 to 0.85 and 0 mg kg^–1 d^–1, respectively, in kaolinitic soils and 1.60 to 1.98 and 0.27 to 0.52 mg kg^–1 d^–1, respectively, in smectitic soils. Our results suggested that NH₄OAc-extractable K was suitable and sufficient to determine plant-available K in kaolinitic soils. In contrast, a successive Ca-resin extraction characterization plus NH₄OAc-extractable K was required to determine plant-available K in smectitic soils.

Abbreviations: Cd, Chai Badan • CEC, cation exchange capacity • K_nonex, nonexchangeable K • Lb, Lob Buri • Ln, Lam Narai • Pc, Pak Chong • Ps, Phu Sana • RMS, residual mean square • Suk, Satuk • Tk, Takhli • Wn, Warin