Response of crops to fertilizer application in volcanic soils

Potassium (K) has been considered as a non-deficient nutrient in most Ethiopian soils. However, some studies recommended K application to K sufficient soils if K/Mg ratio is < 0.7. To resolve this controversy, field experiments and laboratory soil analysis were conducted in two districts (Yilmana Densa and Dera) in north-western Ethiopia on Mollic Nitisols (aric, humic) (pH = 5.5), Pellic Vertisols (aric, gilgaic, mazic) (pH = 6.2) and Vertic Luvisols (aric, nitic) (pH = 5.2) using wheat, tef and maize, respectively as test crops. The field experiments were laid out in a randomized complete block design with 7 K fertilizer rates (0, 42, 83, 125, 166, 208, and 249 kg ha(−1) K) and four replications. KCl and DAP (200 kg ha(−1)) fertilizers were added at planting. Urea (200 kg ha(−1)) was added in split, half at planting and half at tillering for wheat and tef; and at knee-height stage for maize. Soil samples were taken two weeks after planting to determine K and Mg contents, K critical levels and optimum K/Mg ratio. The ammonium acetate extraction method was used to determine the K and Mg contents in the soil using a flame photometer and an atomic absorption spectrometer, respectively. Yield data were collected after harvest. The results of the study indicated that Mollic Nitisols (aric, humic), Pellic Vertisols (aric, gilgaic, mazic) and Vertic Luvisols (aric, nitic) had high K contents of 351, 380 and 434 mg kg(−1), respectively. Rising K fertilizer levels increased soil K contents; however, this did not significantly increase crop yields. Mg contents were also in high category (>351 mg kg(−1)). The K/Mg values ranged from 0.60 to 0.80 in Mollic Nitisols (aric, humic), 0.70 to 0.88 in Pellic Vertisols (aric, gilgaic, mazic) and 0.71 to 1.04 in Vertic Luvisols (aric, nitic), and the values increased with an increasing K rates. However, wheat, tef and maize grain yields showed an increasing trend up to K/Mg ratio of 0.71 in Mollic Nitisols (aric, humic), 0.78 in Pellic Vertisols (aric, gilgaic, mazic) and 0.88 in Vertic Luvisols (aric, nitic), respectively, and declined above these values.