POTASSIUM DYNAMICS AND THE RESPONSE OF TEFF AND CHICKPEA TO POTASSIUM FERTILIZATION ON VERTISOLS OF EAST GOJJAM, NORTH WEST ETHIOPIA

Abstract

Vertisols cover large areas which are widely used for agriculture in Ethiopia. The chemical and physical properties of Vertisols are influenced by the type of clay minerals which appears to be mainly smectite, with different proportions of other clays such as kaolinite, illite and vermiculite. Potassium (K), a major plant nutrient with in the soil, exists in different forms. Assessing the fractions of K is important to have knowledge on the supplying power of soil K for plant nutrition. As there are different soils test methods for nutrients, it is important to assess the efficiency and suitability of extractants in predicting available K for crops as soils vary in their response to applied K under different status of soil K. The fractions and bioavailable K on ten Vertisols was extracted by chemical solutions and the extracts was measured using flame photometer. The clay mineralogy was assessed by X-ray diffraction (XRD) on Vertisols of North West Ethiopia. The availability of K for plants is governed by quantity – intensity relationships, which can be explained by adsorption isotherms. The contribution of nonexchangeable K for plant nutrition is assessed by determining the release kinetics. The adsorption characteristic of K was evaluated using the batch equilibrium technique and the release kinetics of nonexchangeable K was conducted using 0.01 M CaCl2 solution. Pot experiment was carried out to test the effect of K on chickpea in Vertisols to assess the uptake of K from soil K fractions, the K extracted by soil test methods and the K released from the nonexchangeable (NEK) forms. The pot experiment was carried out in 2016 on 10 Vertisols in a completely randomized design (CRD). The field experiments was conducted using teff and chickpea in a randomized complete block design (RCBD) replicated three times in 2015 and 2016 main cropping seasons at Gudalima and Dejen/Tik sites in East Gojjam zone. The K rates as KCl were 0, 50,100 and 150 kg ha-1 along xvii with the locally recommended dose of 100 kg ha-1 NPS fertilizer. The result showed that the fractions of K within the pool were highly correlated to each other, signifying their existence in dynamic equilibrium and the contribution of K fractions for plant nutrition. Higher proportion of water soluble K ranging from 15.8 to 32.2 mg kg-1 K was found. The quantity of K extracted by extractants increased in the following order: CaCl2 < SrCl2 < AB-DTPA < Mehlich 3 < NH4OAc. The extractant AB-DTPA better predicted the availability of K to chickpea in contrast to Mehlich 3 and NH4OAc methods on the studied Vertisols. The adsorption data was well fitted with both the Freundlich and Langmuir models. The adsorption of K was controlled by cooperative adsorption ( > 1) and a partition factor ( < 1). Adsorption parameters were related to the CEC and nature of soil minerals and the external requirement of K optimized from Freundlich isotherm was related to the adsorption capacity and adsorption maxima of soils. Based on the coefficient of determination (R2) and standard error of estimates (SE), the parabolic diffusion model conformed that release of K was diffusion controlled and the power function equation indicated the release of K from interlayer positions. The rate constants of the parabolic diffusion and the power function models were correlated (r = 0.62* and 0.66*, respectively) to the uptake of K, suggesting the contribution of nonexchangeable potassium (NEK) to plant nutrition. The application of K produced a significant (P < 0.05) dry matter and grain yield increase of chickpea and teff at a rate of 100 kg ha-1 of KCl + NPS fertilizers. The agronomic traits of teff such as plant height, and the number of pods and number of branches per plant for chickpea increased significantly (P < 0.05) with applied K during the 2015 and 2016 main cropping seasons at Gudalima and Dejen/Tik sites. Total uptake of N, P and K was enhanced and K efficiency was improved in K treated plots than those without. Hence, the study proved that 100 kg ha-1 KCl along with 100 kg ha-1 NPS fertilizer could be used to increase grain yield and improve grain quality of teff and chickpea on Vertisols of the study sites. However, under the current condition, this recommendation should not be without economic analysis to check the feasibility of using KCl by smallholder farmers. Furthermore, the optimum ratio of K: Mg and N: K needs to be evaluated for balanced crop nutrition.