Soil productivity evaluation under different soil conservation measures in the Harerge highlands of Ethiopia
This study aimed at evaluating the different soil conservation measures namely, level and graded bunds, level and graded fanya juus and grass strips in soil conservation, nutrient retention and crop yields in the Harerge highlands of Ethiopia. The experiment was conducted on 6 m by 30 m soil conservation experimental plots. Runoff, soil loss and crop yield data which had been collected since the establ ishment of the soil conservation measures in 1986 and which had not been ana lysed were used in this study. In addition, soil samples were collected in 1097 in order to evaluate changes in soil properties, since the establishment of the trials. The soil conservation measures showed varied performance in controlling soil loss, retention of diflerent soil properties and crop yields. Level bund and level fanya juu were significantly different (ANOVA; p=O.05) from the control in controlling runoff and soil loss. On the average, highest values of % clay (56 %), available phosphorous (P) (11.79 ppm), and. available potassium (K) (86.48 ppm) were recorded Irom level fanya juu, grass strips and level bund respectively and that of total nitrogen (TN) (0.212 %) and organic carbon (OC) (1.54 %) from graded fanya juu. All soil properties tested except % clay' were found to be lower at the upper positions of all the structural measures tested with an increasing trend down the terrace positions. In particular, total nitrogen, organic carbon and available phosphorous were significantly higher (ANOY A; p=O.05) in the lower positions when compared with the middle and upper positions of the interstructural spaces. An average increase of '0.28 % and 0.16 % in OC and TN respecti.vely was observed in 1997 as compared to the initial nutrient content determined at the site in 1983. Available K, % TN, % clay, grain and biomass yield are negatively correlated with soil loss with correlation coefficients (r) or -0.9, - 0.4, -0.9, -0.4 and -0.6 respectively. Level blind, level fanya juu, graded bund, graded fanya juu and grass strips reduced soil loss by 99.5 %,100 %, 67 %, 54 % and 60 % respectively as compared to the control plot. Grain yield was increased by 12.3 %, 4.2 %, 14.9 %, and 6.5 % on level bund, level fanya j L1~1" graded bund and graded fanya j ULI respectively as compared to the control plot. Level fanya juu, grass strips and graded bund ranked highest in soil loss reduction, nutrient retention and grain yield increase respectively. In general, all techniques tested at the research site reduced soil' loss and all of them except the grass strip (J1ll1 graded bund increased grain yield and nutrient retention respectively as compared to the control plot. Level bund showed a relatively better overall performance than other techniques tested in the area in terms of all parameters considered i.e. soil conservation, nutrient retention and crop yield.