D.N. Mugendi

Determinants of the decision to adopt integrated soil fertility management practices by smallholder farmers in the central highlands of Kenya

SUMMARY Declining soil fertility is a major cause of low per capita food production on smallholder farms of sub-Saharan Africa. This study attempted to provide an empirical explanation of the factors associated with farmers’ decisions to adopt or not to adopt newly introduced integrated soil fertility management (ISFM) technologies consisting of combinations of organics and mineral fertilizer in Meru South district of the central highlands of Kenya. Out of 106 households interviewed, 46% were ‘adopters’ while 54% were ‘non-adopters’. A logistic regression model showed that the factors that significantly influenced adoption positively were farm management, ability to hire labour and months in a year households bought food for their families, while age of household head and number of mature cattle negatively influenced adoption. The implication of these results is that the adoption of ISFM practices could be enhanced through targeting of younger families where both spouses work on the farm full-time and food insecure households. It is also important to target farmers that lack access to other sources of soil fertility improvement. Examples include farmers that do not own cattle or those owning few and who, therefore, have limited access to animal manure.

Alley cropping of maize with calliandra and leucaena in the subhumid highlands of Kenya: Part 2. Soil-fertility changes and maize yield

Although N-rich leaf biomass of multipurpose trees is known to be a good source of N to crops, integrating such trees into crop production systems is a major challenge in the development of viable agroforestry systems. An approach to integrating calliandra (Calliandra calothyrsus Meissner) and leucaena (Leucaena leucocephala (Lam.) de Wit), two promising agroforestry tree species, into maize (Zea mays L.) production system was investigated in the subhumid highlands of central Kenya during four maize-growing seasons from 1994 to 1996. The experiment consisted of maize plots to which tree prunings obtained from hedgerows grown either in situ (alley cropping) or ex situ (biomass transfer from outside) were applied. When alley-cropped with leucaena, maize produced significantly higher yields compared to maize monoculture (both non-fertilized and fertilized) treatments, but when alley-cropped with calliandra, the yield of maize was less than that of the monocropped unfertilized control. Application of ex situ grown calliandra and leucaena prunings with or without fertilizer resulted in higher maize grain yield than in the nonfertilized and fertilized treatments. Yields of calliandra alley- cropped maize were 11% to 51% lower than those of nonalley-cropped treatments receiving calliandra prunings from ex situ grown trees; the decrease was 2% to 17% with leucaena, indicating that calliandra hedges were more competitive than leucaena hedges. The alley-cropped prunings-removed treatments produced the lowest maize yields. The study showed that, in the subhumid tropical highlands of Kenya, inclusion of calliandra hedges on cropland adversely affected maize yields. On the other hand, alley cropping with leucaena was advantageous.

Predicting the Decomposition Patterns of Tree Biomass in Tropical Highland Microregions of Kenya

Decomposition- and nitrogen-release patterns of biomass from three agroforestry multipurpose trees (Calliandra calothyrsus, Cordia africana and Grevillea robusta) were investigated in four contrasting environments (microregions) in the Kenyan tropical highlands during two cropping seasons. Dried leafy biomass was placed in 2-mm litter bags, buried at 15-cm depth and recovered after 2, 4, 7, 10, 15 and 20 weeks. Decomposition patterns were best described by first-order exponential decline curves. The decomposition rate constants ranged from 2.1 to 8.2 yr−1, and the rates of decomposition among the species were in the order: calliandra ≥ cordia > grevillea. There was a species-by-environment interaction during both seasons, but the nitrogen released did not follow such a pattern. Among the three tree species, calliandra released the highest amount of cumulative N, followed by cordia and grevillea. Using multiple regression techniques, decomposition pattern was described as a function of three groups of factors: biomass quality (N, C, lignin and polyphenol), climate (soil temperature and rainfall), and soil conditions (pH, soil organic C, total N and P). For all the species and factors combined, the adjusted R2 values were 0.88 and 0.91 for seasons 1 and 2, respectively. Among the three groups of factors, climate and biomass quality had the most influence on decomposition rates. Climatic factors accounted for 75% of the total rate of decomposition in season 1 (‘irregular’ season with less rainfall and more soil temperature fluctuations), whereas biomass quality factors were more influential in season 2 (‘regular’ season), accounting for 65% of the total variability.

Nitrogen fertilizer equivalencies of organics of differing quality and optimum combination with inorganic nitrogen source in Central Kenya

Decline in crop yields is a major problem facing smallholder farmers in Kenya and the entire Sub-Saharan region. This is attributed mainly to the mining of major nutrients due to continuous cropping without addition of adequate external nutrients. In most cases inorganic fertilizers are expensive, hence unaffordable to most smallholder farmers. Although organic nutrient sources are available, information about their potential use is scanty. A field experiment was set up in the sub-humid highlands of Kenya to establish the chemical fertilizer equivalency values of different organic materials based on their quality. The experiment consisted of maize plots to which freshly collected leaves of Tithonia diversifolia (tithonia), Senna spectabilis (senna) and Calliandra calothyrsus (calliandra) (all with %N>3) obtained from hedgerows grown ex situ (biomass transfer) and urea (inorganic nitrogen source) were applied. Results obtained for the cumulative above ground biomass yield for three seasons indicated that a combination of both organic and inorganic nutrient source gave higher maize biomass yield than when each was applied separately. Above ground biomass yield production in maize (t ha−1) from organic and inorganic fertilization was in the order of senna+urea (31.2), tithonia+urea (29.4), calliandra+urea (29.3), tithonia (28.6), senna (27.9), urea (27.4), calliandra (25.9), and control (22.5) for three cumulative seasons. On average, the three organic materials (calliandra, senna and tithonia) gave fertilizer equivalency values for the nitrogen contained in them of 50, 87 and 118%, respectively. It is therefore recommended that tithonia biomass be used in place of mineral fertilizer as a source of nitrogen. The high equivalency values can be attributed to the synergetic effects of nutrient supply, and improved moisture and soil physical conditions of the mulch. However, for sustainable agricultural production, combination with mineral fertilizer would be the best option.

Maize yields Response to application of organic and inorganic input under on-station and on-farm experiments in Central Kenya

SUMMARY This study investigated the feasibility of using sole organics or a combination of organics with inorganic fertilizer to improve maize production in on-station and on-farm experiments in central Kenya. In the on-station experiment, combined application of Calliandra calothyrsus, Leucaena trichandra and Tithonia diversifolia at 30 kg N ha −1 plus inorganic fertilizer (30 kg N ha −1 ) consistently gave significantly higher maize grain yields than the recommended rate of inorganic fertilizer (60 kg N ha −1 ). Sole application of calliandra, leucaena and tithonia also increased maize yields more than the recommended rate of inorganic fertilizer. In the on-farm experiment, calliandra, leucaena, tithonia and cattle manure either alone or combined with inorganic fertilizer increased maize yields with a similar magnitude to that of inorganic fertilizer. These organic resources could therefore be used to supplement inorganic fertilizer as a whole or in part. There was a yield gap between on-station and on-farm trials with on-station yields having, on average, 65% greater yields than the on-farm yields. There is therefore potential for increasing yields at the farm level by closing the yield gap.

Rainfall Variability, Drought Characterization, and Efficacy of Rainfall Data Reconstruction: Case of Eastern Kenya

This study examined the extent of seasonal rainfall variability, drought occurrence, and the efficacy of interpolation techniques in eastern Kenya. Analyses of rainfall variability utilized rainfall anomaly index, coefficients of variance, and probability analyses. Spline, Kriging, and inverse distance weighting interpolation techniques were assessed using daily rainfall data and digital elevation model using ArcGIS. Validation of these interpolation methods was evaluated by comparing the modelled/generated rainfall values and the observed daily rainfall data using root mean square errors and mean absolute errors statistics. Results showed 90% chance of below cropping threshold rainfall (500 mm) exceeding 258.1 mm during short rains in Embu for one year return period. Rainfall variability was found to be high in seasonal amounts (CV = 0.56, 0.47, and 0.59) and in number of rainy days (CV = 0.88, 0.49, and 0.53) in Machang’a, Kiritiri, and Kindaruma, respectively. Monthly rainfall variability was found to be equally high during April and November (CV = 0.48, 0.49, and 0.76) with high probabilities (0.67) of droughts exceeding 15 days in Machang’a and Kindaruma. Dry-spell probabilities within growing months were high, (91%, 93%, 81%, and 60%) in Kiambere, Kindaruma, Machang’a, and Embu, respectively. Kriging interpolation method emerged as the most appropriate geostatistical interpolation technique suitable for spatial rainfall maps generation for the study region.

Nitrogen recovery by alley-cropped maize and trees from 15N-labeled tree biomass in the subhumid highlands of Kenya

Abstract The effectiveness of tree-leaf biomass as a source of N to crops in agroforestry systems depends on the rate at which crops can obtain N from the biomass. A study was conducted to determine the fate of 15N labeled, soil-applied biomass of two hedgerow species, Calliandra calothyrsus Meissner (calliandra) and Leucaena leucocephala (Lam.) de Wit (leucaena), in the subhumid highlands of Kenya. Labeled biomass obtained from 15N fertilized trees was applied to microplots in an alley cropping field and maize planted. N uptake and recovery by maize and hedgerow trees was periodically determined over a 20-week period during the short rain (1995) and the long rain (1996) growing seasons. In maize crop from treatments that received leucaena biomass, higher N uptake and recovery were recorded than in maize from the plots that received calliandra biomass. However, N uptake and recovery were higher in calliandra tree hedges than in leucaena hedges, indicating differences in N uptake by the two tree species. The largest fraction (55–69%) of N in the applied tree biomass was left in the soil N pool, 8–13% recovered by maize, 2–3% by tree hedges, and 20–30% could not be accounted for. Some of the unaccounted for N may have been left in the wood and root portions of the tree hedges and in the bulk soil below the 20-cm depth. The study shows that only a small fraction of the N contained in the N-rich biomass that is applied to the soil is taken up by the current seasons crop, suggesting that a major benefit may be in the build-up of the soil N store.

ENHANCING MAIZE PRODUCTIVITY AND PROFITABILITY USING ORGANIC INPUTS AND MINERAL FERTILIZER IN CENTRAL KENYA SMALL-HOLD FARMS

SUMMARY Declining land productivity is a major problem facing smallholder farmers today in Sub-Saharan Africa, and as a result increase in maize grain yield has historically staggered behind yield gains that have been achieved elsewhere in the world. This decline primarily results from reduction in soil fertility caused by continuous cultivation without adequate addition of external nutrient inputs. Improved soil fertility management practices, which combine organic and mineral fertilizer inputs, can enable efficient use of inputs applied, and can increase overall system’s productivity. The trials were established at two sites with different soil fertility status to determine the effects of various organic sources (Tithonia diversifolia, Mucuna pruriens, Calliandra calothyrsus and cattle manure) and their combinations with mineral fertilizer on maize grain yield, economic return and soil chemical properties. Drought spells were common during the peak water requirement periods, and during all the seasons most (90%) of the rainfall was received before 50% flowering. In good and poor sites, there was a significant (p < 0.001) effect of season on maize grain yield. Tithonia diversifolia recorded the highest (4.2 t ha −1 ) average maize grain yield in the poor site, while Calliandra calothyrsus gave the highest (4.8 t ha −1 ) average maize grain yield in the good site. Maize grain yields were lower in treatments with sole fertilizer compared with treatments that included organic fertilizers. The maize grain yields were higher with sole organics compared with treatments integrating organic and inorganic fertilizers. Soil pH increment was statistically significant in the sole manure treatment in good and poor sites (t-test, p = 0.036 and 0.013), respectively. In the poor site, magnesium increased significantly in the sole manure and manure + 30 kg N ha−1 treatments with t-test p = 0.006 and 0.027, respectively. Soil potassium was significant in the sole manure treatment (t-test, p = 0.03). Generally the economic returns were low, with negative net benefits and benefit cost ratio of less than 1. Inorganic fertilizer recorded the highest net benefit and return to labour (p < 0.001 and <0.01, respectively) in the good site. The treatments that had very high maize grain yields did not lead to improved soil fertility, thus there is need for tradeoffs between yield gains and soil fertility management when selecting agricultural production technologies.

Improving food production using ‘best bet’ soil fertility technologies in the Central highlands of Kenya

Declining crop productivity is a major challenge facing smallholder farmers in central highlands of Kenya. This decline is caused by continuous cultivation of soils without adequate addition of external inputs in form of manures and fertilizers. With this background, an on-station trial was initiated at Embu in 1992 to evaluate the feasibility of using two leguminous shrubs; Calliandra calothyrsus and Leucaena leucocephala for improving food production. In 2000, an off-station farmers’ participatory trial aimed at offering farmers soil enhancing technologies for replenishing soil fertility was established in Meru South District. The results from the Embu on-station trial indicate that, over the 11 years of study, calliandra and leucaena biomass transfer with half recommended rate of inorganic fertilizer treatments gave the best average maize grain yields of 3.3 Mg ha-1. Treatment where calliandra was alley cropped with maize but the prunings removed recorded the lowest maize yield of 1.2 Mg ha-1. Treatments with calliandra and leucana biomass transfer had similar yields but treatments that were alley cropped with leucaena did better than those that were alley cropped with calliandra. On the other hand, results from the off-station trial in Meru South indicate that, on average, across the seven seasons, sole tithonia gave the highest maize grain yield followed closely by tithonia with half recommended rate of inorganic fertilizer with 6.4 and 6.3 Mg ha-1 respectively. Control gave the lowest yield of 2.2 Mg ha-1 across the seasons. On average, integration of organic an inorganic sources of nutrients gave higher yields compared to all the other treatments.

Using Apsim-Model as a decision-support-tool for long-term integrated-nitrogen-management and maize productivity under semi-arid conditions in Kenya

This study was part of a research conducted through funding from the Regional Universities Forum for Capacity Building in Agriculture (RUFORUM), Call ID RU/CGC/GRG/15/10/109.