Fb Mwaura

Nitrogenase activity in the papyrus swamps of Lake Naivasha, Kenya

Nitrogenase activity (acetylene reduction activity) was found to occur universally in the Cyperus papyrus swamp in Lake Naivasha. Low rates of acetylene reduction activity (0.9–104.9 nmol C2H4 g d.wt. roots-1 h-1) were associated with excised roots of C. papyrus but higher rates of activity (89.0–280.4 nmol C2H4 g d.wt. roots-1 h-1) were associated with intact root systems of the plant. It was estimated that nitrogen fixation associated with young roots alone could supply about 26% of the nitrogen requirements of growing papyrus plants. Acetylene reduction activity in the lake bottom sediments was generally low and associated with adjacent papyrus stands. Plate counts of putative aerobic and facultatively anaerobic N2-fixing bacteria associated with papyrus roots showed the presence of high numbers of diazotrophs (5.4 × 106 CFU g d.wt. roots-1). Fewer numbers of N2-fixing bacteria were detected in the sediments (1.9 × 103-3.2 × 104 CFU g d.wt. sediment-1).

Strategies for Maintenance and Improvement of Soil Productivity Under Continuous Maize and Beans Cropping System in the Sub-humid Highlands of Kenya: Case Study of the Long-Term Trial at Kabete

Inappropriate soil management practices are cited as the main causes of soil fertility decline of cultivated lands in sub-Saharan Africa (SSA). Small-scale farmers are faced with daunting challenges and limited opportunities for maintaining the productivity of their land due to low accessibility and affordability of agricultural inputs. In Kenya, a series of fertilizer trials conducted throughout the country in the 1990s, showed that nitrogen (N) and phosphorus (P) were deficient in 57% and 26% of the sites covered. The effect of continued cultivation with application of mineral and organic fertilizers on soil quality and crop yields was studied in a long-term field experiment at Kabete, in the highlands of Kenya, which was started in 1976. The area is sub-humid with an average bimodal rainfall of 980 mm and two cropping seasons per year. The treatments included: control (no-input); fertilizer nitrogen (N) and phosphorus (P) at 60 kg N/ha and 26 kg P/ha, and 120 kg N/ha and 52 kg P/ha; farmyard manure (FYM) at five (5) and ten (10) t/ha and farmyard manure (5 t/ha) combined with 60 kg N and 26 kg P/ha. For each treatment, crop residues were retained (+) or removed (−) in half of the plots. Maize and beans were planted during the long and short rain seasons, respectively. This paper presents a review of experimental results derived from the long-term experimental site at Kabete, Kenya over a period of 30 years. Results indicate that application of all inputs significantly (p = 0.05) improved maize yields over the control but there was no significant differences among the treatments. Cumulative use of FYM gave better yields than NP fertilizers but the combined use of FYM and inorganic fertilizers was the most economically and promising strategy for sustained soil crop yields A general decline in soil bio-physical properties was observed over time. Soil acidification, decline in soil organic carbon and nitrogen has been noted over time. Use of inorganic fertilizers without addition of FYM or crop residues led to loss of soil biodiversity. None of the strategies used have maintained carbon stocks at the initial level. Integrated use of farmyard manure and chemical fertilizers, combined with liming to reduce soil acidification and use of deep rooted rotational crops should be recommended to sustain soil productivity under continuous cultivation.

Long-Term Land Management Effects on Crop Yields and Soil Properties in the Sub-humid Highlands of Kenya

The effect of continuous cultivation using inorganic and organic fertilizers on crop yields and soil agro-properties was studied in a 30-year-old long-term field experiment at Kabete, near Nairobi, in the highlands of Kenya. The area is sub-humid with an average bimodal rainfall of 980 mm and two cropping seasons per year. The soil is a dark red, friable clay classified as a Humic Nitisol and is considered to be moderately fertile. The main treatments consisted of three rates of inorganic fertilizers nitrogen (N) and phosphorus (P), farmyard manure with or without stover restitution. Maize and beans were planted during the long and short rains seasons, respectively. Results indicate that the use of chemical fertilizers alone increased maize grain yields by more than 50% during the first 6 years of experimentation but declined thereafter. Application of combined chemical fertilizers and farmyard manure proved superior to inorganic fertilizers alone and maintained maize yields at 3–5 t ha–1. Farmyard manure also gave better yields than did chemical fertilizers. However, application of chemical fertilizers alone led to decreased maize yields, increased soil acidification from pH of 5.5 to 4.3 and raised bulk density from 1.04 to 10.8 g cm–3 soil. The total % N declined by 25% from 0.16%, while soil organic carbon decreased from 2 to 1.2% after 27 years. Fertilizer N utilization ranged from 25 to 33% but was higher in plots supplied with chemical fertilizers than in those with combined organic and inorganic inputs.

Isolation and characterization of pectolytic bacterial pathogens infecting potatoes in Nakuru County, Kenya

Isolation and characterization of pectolytic bacteria associated with soft rot disease of potatoes in Nakuru, Kenya, to provide the basis for the development of disease control measures.

Silicon Induces Resistance to Bacterial Blight by Altering the Physiology and Antioxidant Enzyme Activities in Cassava

Cassava bacterial blight (CBB), caused by Xanthomonas axonopodis pv. manihotis (Xam) is a devastating disease limiting cassava production. The potential effect of Si application on the physiological and biochemical mechanisms attributed to Si-mediated resistance of cassava to Xam was evaluated. The optimal concentration of Si in enhancing resistance to CBB without detrimental effects on plant growth was determined using cultivars TME14 and TMS60444 known for their susceptibility to Xam. Varied concentrations of Si (0.7 to 2.1 mM) were administered by watering the plants three times per week before and after Xam inoculation. The optimized Si concentration was used to evaluate the effect of Si supplementation on resistance to CBB disease using eight farmer-preferred cassava cultivars. The population of Xam, cultivar resistance, chlorophyll content, lipid peroxidation, H2O2 content, activity of antioxidant enzymes and total Si content in cassava cultivars were quantified 21 days post inoculation. Silicon concentration of 1.4 mM was optimal in enhancing cassava resistance to Xam. Silicon-treated plants of all cassava cultivars showed significantly (P ≤ 0.05) lower Xam population ranging from 5% to 26.7% compared to non-Si treated control plants. Activities of antioxidant enzymes, malondialdehyde, H2O2 and chlorophyll contents were significantly (P ≤ 0.05) higher in Si treated plants than non-Si treated plants. Silicon accumulation in leaves of Si treated plants was higher compared to non-Si treated control plants.

Isolation of Actinomycetes from the Soils of Menengai Crater in Kenya

This study was carried out to isolate actinomycetes from the soils of Menengai crater. The study area was divided into regions A, B, C and D. Soil samples were collected from 8 sampling points from each region. The samples were separately mixed to form composite samples. Starch casein, Luria Bertani and starch nitrate agar were used in isolating actinomycetes using spread plate technique. Prior to isolation, the soil samples were heat for 1h at 121C followed by serial dilution upto 10. The isolates were characterized using cultural, morphological and biochemical means. Region B and C produced many actinomycetes than A and D. Luria Bertani agar was better in recovering actinomycetes than starch casein and starch nitrate agar. There was significant difference in the number of actinomycetes isolated using the three media (F=3.315 P=0.04218). The number of actinomycetes isolated from region A, B C, and D also varied significantly (F= 27.50 P=0.000). A total of 152 actinomycetes were isolated. There is need to test the actinomycetes isolates for production of antimicrobials.

Some physiological studies of diazotrophic bacteria associated with maize in a temperate and a tropical soil

Previous studies on Azospirillum spp. (Day and Dobereiner, 1976) and Klebsiella oxytoca isolated from wheat roots in Turkey (Cakmacki et al., 1981) indicated little activity below 24°C and high activity at 30–40°C In this study the effect of temperature on the growth and acetylene reduction activity, (ARA) of two temperate-soil bacteria and a tropical-soil isolate is reported.