Determining Plant-Available Potassium in Representative Maize Soils of Thailand.

Plant available K is determined using ammonium acetate, which underestimates the amount of plant available K in the soil of Thailand. Ammonium acetate extracts only exchangeable K, whereas, plant available K is consist of exchangeable and non-exchangeable K pools. Consequently, ammonium acetate will probably not be adequate to determine the availability of non-exchangeable K released from the soil minerals. Therefore, the quantify amount of plant available K which include both exchangeable and non-exchangeable by different extractants was studied. The objectives of this study were (i) to determine plant-available K using exhaustive cropping with maize in a greenhouse experiment, (ii) to compare the efficiency of selected extraction methods to estimate the plant available K in selected maize soils. Eight different minerals soils were amended with two K rates (0 and 200 mg K kg-1) and exhaustively cropped with maize until the soils became deficient in plant-available K. The results revealed that total plant available K in kaolinitic soils ranged from 0.17 to 0.43 cmolc K kg-1 in control soils and 0.50 to 0.93 cmolc K kg-1 in K added soils. In smectitic soils plant extracted K ranged from 0.52 to 1.44 cmolc K kg-1 in control soils and 0.88 to 1.92 cmolc K kg-1 in K added soils. The total plant available K of each soil and the initial extractable K by different extractants (NH4OAc, HNO3, H2SO4, mixed acid, and Ca-resin) was described by linear regression and did not showed significantly vary in all soils (AdjR2 = 0.885, 0.915, 0.819, 0.883 and 0.939, respectively). The 1:1 line relationship indicated that mixed acid and Ca-resin extractable K, which can extract both exchangeable and non-exchangeable K, were the extractants which most nearly extracted the amount of plant-available K in both kaolinitic and smectitic soils.