PhD ThesisMaize (Zea mays) is the most important food crop in Kenya. However, its productivity is greately constrained across the Kenya highlands. Soil acidity and low available phosphorus (P) are among the major hindrances to maize production in acid soils of the Kenya highlands. A survey, greenhouse experiment and field trials were carried out to assess the extent of soil acidity, soil nutrient levels and common soil management practices by farmers that contribute to the poor growth performance of maize in the Kenya highlands. The study also sought to evaluate the interactive effects of tillage, P and lime on maize yields in the acid soils of the Kenyan Highlands. The survey was carried out in nine counties across the Kenya highlands namely: Meru, Embu, Kirinyaga, Nyeri, Kiambu, Nyandarua, Siaya, Busia and Eldoret. Soil acidity and nutrient levels were determined on soil samples taken from the study sites, while farmers’ practices were established by administration of a structured questionnaire. Phosphorus adsorption rates and lime requirements of the soils were also evaluated. The adsorption data were fitted into the Langmuir and Freundlich adsorption equations and the relationships between sorbed P and soil properties determined by correlations. Evaluation of the interactive effects of lime, phosphorus and aluminium was carried out by establishment of a greenhouse pot experiment at Waruhiu Farmers Training Centre, Githunguri - Kiambu. Extremely acidic (pH 4.48) and strongly acidic (pH 4.59) soils were used for the study. Four lime (CaO) rates and phosphorus (Ca (H2PO4)2 rates were used. The liming rates were: 0, 2.2, 5.2 and 7.4 tonnes ha-1 for the extremely acidic soil and 0, 1.4, 3.2, and 4.5 tonnes ha-1 for the strongly acidic soil. Phosphorus application rates were: 0, 0.15, 0.30 and 0.59 g P kg-1 soil for the extremely acidic soil and 0, 0.13, 0.26, and 0.51 g P kg-1 for strongly acidic soil. The experiments were a 42 factorial laid down in a Randomized Complete Block Design (RCBD) and replicated three times. After the greenhouse experiment, field experiments were carried out during the 2014 long rains and 2014/15 short rains in Waruhiu Farmers Training Centre, Githunguri, Kiambu County. The objective of the study was to assess the interactive effects of tillage, P and lime in the management of soil acidity, P-uptake and maize growth performance. Tillage practices evaluated included: ploughing, strip tillage and use of hand hoe while fertilizer lime combinations included: DAP + lime (DAPL), TSP + lime (LP), TSP alone (P) and lime alone. The experiment was laid down in the Randomized Complete Block Design (RCBD) with split plot arrangement. Lime and P rates applied were 3.2 tonnes ha-1 and 52kg P ha-1, respectively. Data collected included: maize plant height, number of leaves, internode interval, P-uptake at 30 days after emergence and at tasseling stage, grain yield, and soil chemical characteristics. Information gathered through personal interview via questionnaires revealed that most farmers (>96%) were not aware of the soil acidity problems and management strategies. The farmers also lacked sources of information about new technologies or innovations and < 37% were members of farmers’ training groups. Additionally, among the 4% who were aware of soil acidity, only less than 8% had ever tested their soils for soil acidity. Farmers who had applied lime on their farms at least once were also < 3% in all sites. On the basis of overall soil fertility manangement, >80% of the farmers in the research area applied both fertilizer and manure in their farms, with the majority using diammonium phosphate (DAP), calcium ammonium nitrate (CAN) and farmyard manure. However, a negative balance was identified between livestock keeping and fertility manangement, with <8% of the farmers returning crop residues back to the farm. Correlation analysis showed that there were significant relationships (P ≤ 0.05) between farmer education level, farmer training groups, fertilizer usage and crop yields. It is, therefore, concluded that more farmers’ awareness is critical for effective manangement of acid soils in the Kenya Highlands. Soils in all the study sites were strongly to extremely acidic (pH 4.83 - 3.76), had high exchangeable Al (>2 cmol Al kg-1), and high Al saturation (> 20% Al), which most maize germplasms grown in Kenya are sensitive to. The cation exchange capacity was low (CEC < 15 cmol kg-1) in Siaya, Busia and Eldoret while available P was low (<10 mg P kg-1 Bray 1) for all sites. The P adsorption data showed that average maximal adsorbed P of the soils were high (1 383- 2 238 mg kg -1 soil) while SPR varied from 189.43 mg P kg-1 in Eldoret to 529.76 mg P kg-1 in Nyeri. Freundlich P adsorption isotherms were found to have the best fit for all the tested soils (r2 = 0.882 - 0.990). Additionally, analysis of the interaction between P adsorption maxima and soil properties revealed that there was a positive relationship between P adsorption maxima and clay content, extractable P, exchangeable acidity and aluminium saturation. Conversely, organic matter and electrical conductivity negatively correlated with P adsorption maxima. Greenhouse pot experiments revealed that there existed good lime-Al-P interactions in acid soils of the Kenya Highlands. The interactions significantly (P ≤ 0.05) increased soil pH, extractable P, Ca, plant P uptake and maize yields. The interactions were also found to negatively affect exchangeable Al, oxalate Al, dithionate Al and P adsorption levels in the soils. Use of 7.4 tonnes ha-1 lime for the extremely acidic soils and 4.5 tonnes ha-1 or the strongly acidic soils significantly reduced exchangeable Al and standard phosphate requirement (SPR) by > 70% compared to lower lime rates. Similary, use of 0.59 mg P kg-1 soil reduced SPR of the soil by 40% while 0.51 g P kg-1 reduced SPR by > 73% in strongly acidic soils. It was, however, observed that there was no significant difference between combination of 7.4 tonnes ha-1 with either 0.59 g P kg-1 or 0.3 g P kg-1 in extremely acidic soils or combination of 4.5 tonnes ha-1 with either 0.51 g P kg-1 or 0.26 g P kg-1 in strongly acidic soils. This shows that the lower lime and P rates can be efficient in the management of soil P in these soils. Analysis of the relationship between lime and P availability showed that liming significantly (P ≤ 0.05) and positively correlated with soil pH, available P, exchangeable Ca, Langmuir maximum adsorption constant and Langmuir constant related to binding energy. Lime also negatively correlated with SPR and exchangeable Al. It was, therefore, concluded that lime and P positively interact to reduce Al and P adsorption in acid soils in the Kenya highlands. Further research is, however, required to evaluate the short and long term effects of the lime-P interaction on crop yields, availability of other plant nutrients and the soil environment under field conditions. Results from field experiment revealed that tillage-P- lime interactions significantly (P ≤ 0.05) increased soil pH, available P, CEC, Ca, maize growth, maize plant P-uptake and yields, and reduced soil exchangeable Al. Plough-LP interactions significantly (P ≤ 0.05) led to > 90% increase in soil pH, extractable P, CEC, Ca, and > 90% reduction of exchangeable Al as compared to the control. Plough-DAPL interactions on the other hand also led to > 96% increase in soil CEC and extractable P. Similary; plough-DAPL and plough-LP interactions significantly (P ≤ 0.05) increased maize plant height, P- uptake, drymatter yields and grain yields. Plough-DAPL or LP interactions significantly increased grain yields by > 70% as compared to the control treatment. Integrating plough with either DAPL or LP was, therefore, found to be effective in the manangement of soil acidity for increased maize production in Kiambu in the Kenya highlands. Further research is, however, required to ascertain the longterm effects of the integration on maize production, soil acidity and overall soil fertility status of the soils in the Kenya highlands.