Effects of selected limiting nutrients on growth and yield of maize and their variability in smallholder farms of Kandara, Murang’a County.
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Nutrient mining and mismanagement of farmers’ fields has led to a decline in soil fertility in smallholder farms. In order to increase the productivity and subsequently improve food security in Sub-Saharan Africa (SSA), there is a need to identify soil nutrients limiting maize growth and production. The objectives of the study were; (i) to determine the most limiting nutrients for maize production in Kandara and establish the effect of selected soil amendments (CaMgS, manure and lime) on growth and yield of maize, (ii) to establish the influence of farm management history on soil variability in nitrogen, phosphorus and potassium levels. This study was carried in 2013 during the long rains season (March- May) and the short rains season (October- December) in Kandara Sub-County, Murang’a County in Kenya. Twenty three farmers were randomly selected for the trials. The experiment was laid out in randomized complete block design (RCBD) with 8 treatments (control, NPK+ CaMgS, NPK+ Manure, NPK+ Lime, NPK, NP, NK, and PK) in 23 farmers’ fields, which were considered as replicates. Soil was sampled at a depth 0 – 20 cm in all the fields before the establishment of the trials and analyzed for pH, total carbon, macro, and secondary nutrients. Treatment inputs were applied at rates equivalent to 60 kg ha-1N, 30 kg ha-1 P, 60 kg ha-1 K, 10 kg ha-1 Ca, 10 kg ha-1 Mg, 5 kg ha-1 S, 10t ha-1 manure and 1t ha-1 lime in all the 23 replicates. Data on maize growth parameters; plant height, leaf number, and basal diameter was collected at intervals of two weeks at 14, 28, 42, 56 and 70 days after planting (DAP). Plant height and basal area were used to determine crop bio-volume. Grain and stover yield data was collected at physiological maturity. A semi-structured questionnaire was used to collect data on the farmers’ management history of the 23 fields. Information on the number of seasons of manure use, the number of seasons of fertilizer use, field positions, fallow type, and land conversion history was sought. Maize growth and yield data were subjected to Analysis of Variance (ANOVA) using SAS 9.2 software. Fisher’s LSD test was used to separate maize growth and yield means at 5% significance level (P< 0.05). Plant growth results showed that control, PK and NK treatment achieved means that were significantly different (p<0.05) for leaf number and bio-volume in both seasons of growth. The grain and stover yields for, control, NK and PK showed significant differences (p<0.05) during the two seasons of growth. Nutrient response analysis indicated that omission of N and P led to a yield loss of 1.8 Mg ha-1 and 1 Mg ha-1, respectively. The regression models indicated that manure use and land conversion history influenced N, P, and K variability in the soils of Kandara area. The regression tree for nitrogen indicated that the number of seasons of manure use (NSMA) was the predictor variable, which achieved the highest improvement value (0.10) among root competitor splits. The regression tree for phosphorus and potassium had LCH as the predictor variable, which achieved the highest improvement value 1.06 and 11.22, respectively. The regression tree indicated that the number of seasons of manure use (NSMA) had the greatest influence on soil nitrogen stocks. Soil P and K stocks were greatly influenced by land conversion history. Results from nutrient omission trials indicated that N and P are the most limiting nutrients for maize production in Kandara area. Additionally, manure use and land conversion history are the major management factors influencing nitrogen, potassium and phosphorus variability in farmers’ fields. Farmers are therefore advised to use N and P fertilizers to replenish those specific limiting nutrients.