S. Mpepereki

Pigeonpea rhizobia prevalence and crop response to inoculation in Zimbabwean smallholder-managed soils.

A study was conducted to determine the population sizes of indigenous pigeonpea (Cajanus cajan)-nodulating rhizobia and responses of the crop to rhizobial inoculation in soils under smallholder management. Rhizobia populations were determined in 21 soils from three different agro-ecological regions of Zimbabwe using the plant infection most-probable-number technique. Pigeonpea response to rhizobial inoculation was tested in five soils representative of low, medium and high rhizobia populations. Pigeonpea rhizobia ranged from undetectable to 121 cells per g soil compared with 16 to 159 cells per g soil for cowpea (Vigna unguiculata) which was used for reference. Soils with high cowpea rhizobia counts had relatively low counts of pigeonpea rhizobia and vice versa, showing that the two legumes associate with different subgroups of rhizobia. Poor soil organic matter, low soil moisture at sampling, low pH and low clay content of the soils had a significant negative effect on rhizobial counts. Organic matter appeared critical for maintenance of high populations of indigenous rhizobia in the mostly sandy soils sampled. Lack of pigeonpea response to inoculation in all the soils tested despite the low initial rhizobial populations could be the result of within-season proliferation of indigenous populations which are competitive and effective. There was evidence of rapid build-up of pigeonpea-compatible rhizobia within one growing season when the crop was first introduced. It was concluded that effective pigeonpea rhizobia occur in many arable soils of Zimbabwe. However, to fully exploit biological nitrogen fixation and maximize yields of pigeonpea, highly efficient, adapted and competitive indigenous rhizobial isolates must be identified and evaluated.

Diversity in symbiotic specificity of cowpea rhizobia indigenous to Zimbabwean soils

Tropical cowpea rhizobia are often presumed to be generally promiscuous but poor N fixers. This study was conducted to evaluate symbiotic interactions of 59 indigenous rhizobia isolates (49 of them from cowpea (Vigna unguiculata)), with up to 13 other (mostly tropical) legume species. Host ranges averaged 2.4 and 2.3 legume species each for fast- and slow-growing isolates respectively compared to 4.3 for slow-growing reference cowpea strains. An average of 22% and 19% of fast- and slow-growing cowpea isolates respectively were effective on each of 12 legume species tested. We conclude that the indigenous cowpea rhizobia studied have relatively narrow host ranges. The ready nodulation of different legumes in tropical soils appears due to the diversity of indigenous symbiotic genotypes, each consisting of subgroups compatible with a limited number of legume species.

Persistence and impact on microorganisms of Bacillus thuringiensis proteins in some Zimbabwean soils

The persistence of the Bacillus thuringiensis subsp. kurstaki (Btk) toxin (Cry1Ab protein) from Bt maize (MON810, Yieldgard®) residues incorporated in a vertisol (739 g clay kg−1) was investigated. The maize residues were incubated in the soil for 4 weeks, and activity of the toxin in the residues was bioassayed using larvae of the diamondback moth, Plutella xylostella (Lepidoptera: Yponomeutidae). Corrected mortality of P. xylostella in the bioassays decreased from 76% to 30% in less than a week of incubation in the soil. In addition to the above observations, the effects of Btk, Bt subsp. israelensis (Bti), and Bt subsp. tenebrionis (Btt) proteins on the soil microbiota were examined using a vertisol, an alfisol, and an oxisol. The pre-incubated soils (7 days after moisture adjustment) were treated with crystal proteins of Btk, Bti, and Btt and incubated for further a 7-day period. Microbial biomass carbon (MBC) and counts of culturable bacteria and fungi were determined. The proteins did not show effects on MBC or bacterial and fungal counts, possibly as a result of adsorption of the proteins on soil particles, which could have rendered the proteins inaccessible for microbial utilization. Microbial biomass carbon and counts arranged in decreasing order were vertisol>oxisol>alfisol, similar to the amounts of organic C and clay in the soils. However, bacteria and fungi counts were higher in the vertisol than in the alfisol and the oxisol soils. Our observations suggest that larvicidal proteins produced by different subspecies of Bt and Bt maize could persist in tropical soils as a result of adsorption on soil clays but that there were no observable effect on the soil microbiota.

Soyabeans and sustainable agriculture in southern Africa

Maize is the dominant staple crop across most of southern Africa—it is so dominant in some areas that more than 80 per cent of the smallholder land area is planted with maize. Soyabean was identified as the crop with a potential to address the need for diversifying the cropping systems, which could assist in overcoming the pervading soil fertility constraints and could provide smallholder farmers with an opportunity to earn income while also addressing the nutritional security of households. An initiative was launched in the 1996/97 cropping season in Zimbabwe, to test soyabean as a potential smallholder crop. From an initial 55 farmers in the first year, soyabean production expanded rapidly to an estimated 10,000 farmers three years later. Since then, soyabean has diffused spontaneously to most smallholder farming areas in the higher rainfall zones of Zimbabwe. Thus, the initiative has assisted a large number of smallholders to grow soyabean, and exploded a long-held belief in Zimbabwe that soyabean is not a suitable crop for smallholders.

Adsorption of the insecticidal toxin from Bacillus thuringiensis subspecies tenebrionis to clay fractions of tropical soils.

Publisher Summary In the study discussed in the chapter, Bacillus thuringiensis toxins were found to adsorb onto pure clay minerals and onto humic acids extracted from soils, and the complexes formed from these interactions were less available for microbial degradation. They retained their pesticidal activity longer than free forms. The aim of this study was to analyze the adsorption of the toxin from Bacillus thuringiensis subsp. tenebrionis (Btt) to surface active soil constituents. Clay-sized fractions were separated from a vertisol, an alfisol, and an oxisol after complete dispersion of the soils by mechanical means and the toxin was purified from a Btt commercial preparation. Adsorption isotherms were established by the difference method in deionized water or in phosphate buffer (pH 6.8). Adsorption of Btt toxin was in the order vertisol > alfisol > oxisol, and it was mostly irreversible, except for the alfisol. Adsorption could be explained by physicochemical conditions and the nature of clay constituents—that is, the dominant clay mineralogy: montmorillonite (vertisol) and kaolinite (alfisol, oxisol). The results show that Bt toxins released into the soil may be retained by soil colloids.

Soil Fertility Variability in Sandy Soils and Implications for Nutrient Management by Smallholder Farmers in Zimbabwe

ABSTRACT In-field variability of soil properties creates niches that have been perceived by smallholder farmers, especially in hostile environments, as constituting an essential part of their subsistence farming. They can exploit niche variability as a risk minimisation strategy for crop production. Smallholder farmers largely base their nutrient management strategies on their perception of niche fertility. The study physically and chemically characterised soil samples up to the 130 cm depth of predominant arable niches from a representative sample of nine smallholder farms with fields cropped for over 70 years. The niches were homestead surroundings, termitaria environments, areas under Parinari curatellifolia and open sandy patches. The data were analysed using discriminant analysis, a statistical method that investigated niche differentiation based on simultaneous analysis of soil nutrient variables. The analysis of variance complemented the discriminant analysis. Results showed that the first two discriminant functions contributed significantly (p < 0.001) to niche differences and accounted for 95% of the total variation. The discrimination was most powerful in classifying termitaria environments and around homestead niches, indicated by greater than 81% aggregation of cases. Termitaria environments had the lowest significant (p < 0.05) bulk density of 1.47 g/cm3 while the remaining niches had bulk densities greater than 1.57 g/cm3. The texture of termitaria environments significantly differed (p < 0.001) from the rest of the niches and consisted of sandy loams and loamy sands. Homestead surroundings, areas under P. curatellifolia and open sandy patches had sandy soils with greater than 90% sand. Macronutrients generally declined with depth. Total C in the 0–20 cm was relatively high in under P. curatellifolia (14.8 Mg/ha) and lowest in the open sandy patches (7.2 Mg/ha). Total N for all niches was low (<1.4 Mg/ha), indicating possible limitation to crop growth. Available P in the active rooting zone of 0–40 cm was adequate only in homestead surroundings (30–50 ppm) and marginal (15–30 ppm) in termitaria environments and under P. curatellifolia. However, it was deficient in the open sandy patches. Homestead surroundings and termitaria environments were the niches with relatively high soil pH (>6.5) and exchangeable base content of Ca, Mg and K. Areas under P. curatellifolia, perceived by farmers to be more fertile, had no comparative advantage over the open sandy patches in texture and macronutrient levels. Farm management practices of applying less nutrient inputs to termitaria environments and areas under P. curatellifolia were misguided and perpetuated low productivity in smallholder cropping environments. We recommend site-specific nutrient management of niches where limited purchased nutrients are applied to relatively fertile niches, for example, homestead surroundings and termitaria environments while open sandy patches are left to recover under natural or improved fallow.

Response of Soyabean (Glycine max) to Co-Inoculation with Rhizobia and Mycorrhizae

Legumes do not nodulate unless their P content is at least 0.15%. Colonisation of legume roots by VAM enhances nodulation and N-fixation by rhizobia (Smith et al., 1979) due to improved P nutrition. This study determined the effect of co-inoculating soyabean with VAM and rhizobia. Sorghum plants were grown for 28 days in potted soils collected from under Eucalyptus, Acacia and Brachystagia tree sp. and the roots dried, cut and mixed to make the inoculum. Soyabean (G. max) var. “SCS-1” was inoculated with 1.0 ml of rhizobium culture and 0.5 g VAM per pot and grown to physiological maturity in non-sterile granitic sandy soil. Treatments were: non-inoculated control, inoculated with rhizobia only, inoculated with VAM only, inoculated with VAM and rhizobia, +50 kg P and +60 kg N ha−1. Seed yield was determined.