Overall Summary of the Research Background, Description of Study Area and Findings

cid soil infertility is a major limitation to crop production on highly weathered and leached soils of the world. Likewise, about 30% of the highly weathered soils of Ethiopia have been reported to be acidic and it is becoming a serious threat to cereal crops in general and that of wheat crop production in particular. Because of this vast soil acidity coverage, concerted efforts are undergoing to demonstrate soil acidity ameliorating effects of liming materials. However, even though farmers have already started utilization of lime to ameliorate these acidic soils, scientific information regarding the nature of the soil acidity, the relative effectiveness of the available liming materials, the combined effects of lime and fertilizer application and residual and over-liming effects on wheat crop production is very limited. This limitation is more significant specifically in the recently investigated acidic soils of the Tsegede highlands, Tigray National Regional State, Ethiopia. Accordingly, five independent studies to characterize and produce soil acidity map of the Tsegede Highlands, evaluate the response of wheat crop to the applications of lime and nitrogen (N) and phosphorus (P) fertilizers, evaluate wheat crop response to the applications of two different liming materials (Wukro and Sheba limes) and N and P fertilizers, evaluate wheat crop response to different rates of sulfur (S) fertilizers applied in the forms of gypsum and potassium sulfate with and without lime and determine the effects of increasing lime levels on selected chemical and biological soil properties were conducted in the acidic soils of Tsegede highlands from 2011-2012. A randomized complete block design was employed in the field and greenhouse pot experiments whereas the incubation experiment was arranged in a completely randomized design. Their respective treatments were at least replicated thrice. Data were statistically analyzed using the GLM procedures of the SAS computer package. Results showed that site, land use and soil depth differences affected significantly (P
0.05) the physical and chemical properties of the acidic soils. At the 0-30 cm depth, soil pH values were 4.87, 5.06 and 5.47 in the grazing, cultivated and forest land soils, respectively, and its average (5.07) showed a negative correlation with exchangeable acidity (r = -0.77**) and Al (r = -0.86**). High average soil organic matter (OM) accumulation (8.87%) was exhibited while available P status was low (10.60 mg kg- 1). Quartz, tridymite, limonite, nacrite, kaolinite, muscovite, vermiculite and labradorite clay minerals were identified. The distribution of sand, clay and OM contents was also observed to be significantly influenced by depth. Soil acidity map was developed and about 50% of the area falls under very strongly acidic soils with more of its coverage in the grazing and lower in the forest land soils. Furthermore, the results of the field experiment revealed that wheat yield and yield components, N and P nutrient uptake efficiency and some soil chemical properties were significantly (P
0.05) affected by the main effects of fertilizer, lime, and residual lime. Higher biomass yield, grain yield, N and P uptakes, agronomic efficiency and apparent recovery efficiency of N and P fertilizers were documented in the plots that received combined NP fertilizers + full recommended lime followed by NP fertilizers + half recommended lime and only P + full recommended lime while the lower values were recorded in the control and in the plots that received only N fertilizer. Moreover, the plots supplemented with NP fertilizers + residual of the full recommended lime rate applied in the preceding year gave significantly (P
0.05) higher grain yield than the control by about 437%. In the cropping season of year two, average wheat grain yield was reduced by 34% as compared to the preceding cropping season. Likewise, soil pH, available P, exchangeable Ca and Mg were reduced by 0.15 units, 20%, 30% and 28% while soil OM and exchangeable acidity increased by 27% and 2.01 units, respectively. Moreover, in the greenhouse experiment carried out to determine the effectiveness of liming materials; the soils which received only recommended N + recommended P fertilizers (NP), Wukro lime and Sheba lime showed significant grain yield increment by about 78, 76 and 96% over the control, respectively. However, the applications of NP + Wukro lime and NP + Sheba lime augmented grain yield by 239 and 233%, respectively, over the control. Likewise, wheat N and P uptakes showed similar increment trends by the application of only NP, lime, and the NP along with the limes over the control. The sulfate status assessment and greenhouse experiment also revealed that soil SO4-S was significantly (P
0.01) affected by site and land use type differences. Its content in the forest land soils was significantly higher by about 162 and 160% over the cultivated and grazing land soils. Grain yield and S use efficiency of the wheat crop were considerably (P
0.05) affected by the application of S in the form of potassium sulphate (K2SO4). Successive increasing of S fertilizers in the form of K2SO4 up to the level of 50 kg ha-1 S has been observed to increase biomass and grain yield of wheat considerably. Furthermore, soils treated with S fertilizer along with lime attained significantly higher yields than those without lime. Furthermore, the results of the lime incubation experiment showed that acidic soils incubated with the 7, 14 and 21 t ha-1 lime showed significant increment in average soil pH by 0.42, 1.00 and 1.59 units over the control. However, soil pH was observed to decline appreciably with increasing the incubation periods. Following the 90 days incubation period, soil available P increased by about 80,130 and 170%; extractable Fe decreased by about 26, 31 and 34% and Mn by about 55, 71 and 77% over the control, with the application of the 7, 14 and 21 t ha-1 lime, respectively. Similarly, bacterial growth significantly rose by about 42, 53 and 63%, accordingly. Hence, to achieve sustainable wheat crop production in the Tsegede highlands; scaling up of liming practices, combined application of NP fertilizers and lime, introduction of optimum grazing and agro-forestry practices, due considerations to agro-ecosystem transformations, application of S fertilizers in combination with lime and avoiding of over liming practices are recommended.