Comparison of Soil Extractants and Spectral Reflectance Measurement for Estimation of Available Soil Potassium in Some Ethiopian Soils

A study was conducted with the purpose of comparing the efficiency of ammonium acetate (NH4OAc), Mehlich 3 (M-3), Calcium Chloride (CaCl2) and alpha MIR spectroscopy measurement, for the determination of available potassium (K) on 58 Ethiopian agricultural soils. Four soil reference groups were sampled for the study. The NH4OAc extractant was used as standard method against which K values estimated by other methods were compared. Results showed that generally highly significant correlations existed among all the methods used for available K extraction. The coefficients of determination (R 2 ) values between NH4OAc method and the other methods were 0.90 (M-3), 0.70 (CaCl2), and 0.37(spectral). A statistically poor relationship (R 2 = 0.07) was found between CaCl2 and spectral methods. On an average, the K extracted by M-3 and CaCl2 amounted to 106 and 49% of NH4OAc K, respectively while the spectral method detected 196% of the NH4OAc K. The highly significant correlation between different soil extraction methods indicated that any of the methods can be used to accurately predict the concentration of available K in the soil. The correlations between K concentration estimated by different methods and plant uptake (product of plant K concentration and dry matter yield) of K were the highest with M-3 and the lowest with spectral methods with R 2 values of 0.65, 0.64, 0.54 and 0.16 for M-3, NH4OAc, CaCl2 and spectral methods, respectively. It can, therefore, be generalized that the M- 3 is a suitable extractant for K in Ethiopian soils, but further study is recommended to determine how these relationships could be translated to plant K uptake under field condition. Besides, the spectral measurement of K as a soil test method for heterogeneous group of soils warrants further investigation and refinement.