P. Nyamugafata

EFFECT OF ON-FARM SEED PRIMING ON EMERGENCE, GROWTH AND YIELD OF COTTON AND MAIZE IN A SEMI-ARID AREA OF ZIMBABWE

The effects of on-farm seed priming (i.e. seed soaking) on the emergence, growth and yield of cotton and maize were studied in the field in the south-eastern lowveld of Zimbabwe. Experiments were conducted on both crops in the 1999/2000 and 2000/2001 seasons and, in the 2001 winter season, on maize only. The interaction of priming with tillage (ox-drawn ploughing or hand-hoeing) and simulated sowing rainfall regimes (irrigations of 15 mm, 30 mm or 45 mm at planting) was studied. Priming usually increased the rate of emergence in maize, but always decreased final percent emergence in cotton. In the 2000/2001 season, there was an interaction between priming and simulated sowing rainfall regimes such that the 15 mm treatment gave a smaller adverse effect of priming in cotton than the 30 and 45 mm treatments. In maize, however, the 15 mm treatment gave an adverse rather than a positive effect of priming on emergence. There was little effect of tillage on emergence or growth. Priming did not affect the relative growth rate of cotton or maize, although plants grown from primed maize seed were consistently larger at any given date throughout the 2001 winter season. Plants from primed seed also flowered and matured earlier in the winter 2001 season. There were no significant effects of priming on yield, except in the 1999/2000 season, where priming decreased yield in cotton. It was concluded that the effect of priming can depend on crop 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.

Characterization of cohesion, friction and sensitivity of two hardsetting soils from Zimbabwe

Abstract Hardsetting soils are defined as those which develop very high strength with little observable structure when they dry, but lose much of their strength when wet. Sandy loam soils (haplic lixisol) which showed typical hardsetting behaviour in the field were identified in a small-scale farming are in Zimbabwe. They were too hard to cultivate when dry, and produced a cloudy structure when plowed by a tractor in a slightly moist state. Samples of two sandy loam topsoils were collected, prepared at different water contents varying from saturated to field-dry and tested for stress-deformation and shear strength behavior in a direct shear box. For both soils at water contents above 10 g 100 g−1, the stress-deformation curves are of the plastic material type with continually increasing shear stress with deformation. At water contents less than 10 g 100 g−1, curves associated with more brittle material behavior resulted, with a peak shear stress reached at 3–4 mm deformation followed by a considerable loss in strength. At nearly all of the water contents, the angle of friction was 34–37° for both soils, but cohesion changed from nearly zero at saturation to well over 100 kPa in the field-dry state. The contribution of matric tension alone to soil cohesion is more than enough to account for the observed increases in strength on soil drying, and the potential role of soluble silicate cementing agents does not appear to be a factor in the case of these two soils.

Effects of seed priming and water potential on germination of cotton (Gossypium hirsutum L.) and maize (Zea mays L.) in laboratory assays

Sub-optimal crop emergence and establishment is a problem in crop production owing to low soil moisture. The effects of seed priming (seed soaking) on cotton and maize germination at different water potentials (0, −10 kPa, −100kPa, −200 kPa, −500 kPa and −1500 kPa) were determined in laboratory experiments. The interaction of these treatments with cultivar was studied in cotton (CY889 and SZ93–14), and with seed size in maize (small, medium and large seeds of CG4141). Germination of cotton decreased progressively as water potential was lowered, but non-primed cotton seed was more sensitive to moisture stress than primed cotton seed. For maize, there was significant (P < 0.01) water potential × seed size and water potential × seed treatment interaction. Smaller seeds were less sensitive to low water potential than large and medium seeds with respect to germination, but they may have less food reserves to emerge from deteriorating seedbeds. Final germination percent of cotton and maize seed decreased as the water potential was lowered, but non-primed seed was much more sensitive to moisture stress than primed seed.

Analysis of porosity in a tilled crusting soil' in Zimbabwe

Abstract Samples were collected in a tilled “crusting soil” from zones with different structures and their porosity was analysed with mercury porosimetry. The results showed three types of elementary pore volumes that differ in both the size and origin of the pores. The first type of elementary pore volume results from the packing of clay particles, and the second one mainly arises from the packing of silt and sand particles with the porous clay fraction. Their variations are mainly related to the clay content. The third elementary pore volume is related to tillage and biological activity, but there is no close relationship between its variation and the type of structure. These results obtained with mercury porosimetry are consistent with observations on thin-section images, using scanning-electron microscopy with backscattered electrons.

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.

Management of cattle and goat manure in Wedza smallholder farming area, Zimbabwe

Cattle and goat manure are widely used as soil fertility amendments in the smallholder farming sector of Zimbabwe but their fertilizer value is often reduced due to poor handling and management. Management of goat and cattle manure in Wedza smallholder farming area in Zimbabwe was studied from 1996 to 1998. The survey showed that farmers removed manure from kraals between May and August and heaped or composted it for between 8 to 16 weeks and then applied it to fields in October just before the commencement of the rainy season. Goat and cattle manure collected from kraals and heaps outside kraals and finally, a few days after application to the field but before incorporation were monitored for total N, P, K and organic C. Goat manure sampled after application to the field had mean percentage (%) total N, P, K and organic C of 1.37 ± 0.097, 0.26 ± 0.028, 0.66 ± 0.048 and 18.51 ± 1.610, respectively. Cattle manure collected after application to the field had a mean % total N, P, K and organic C of 1.01 ± 0.043, 0.20 ± 0.009, 0.40 ± 0.019 and 18.12 ± 0.869, respectively. Goat manure managed in the same way as cattle manure was found to be of better quality when applied to the field than cattle manure for the macronutrient N, P and K (P<0.05).

Burning, biomass removal and tillage effects on soil organic carbon and nutrients in seasonal wetlands (Dambos) of Chiota smallholder farming area, Zimbabwe

Seasonal wetland (dambo) cultivation in smallholder farming areas is important because it improves household food security. However, most farming practices, such as burning of vegetation and conventional tillage in dambo gardens, may reduce soil organic carbon (SOC) and nutrient dynamics. We evaluated the effects of simulated burning, vegetation clearing and clipping, and conventional tillage in dambo gardens on SOC, nutrient contents and biomass production over a 3-year period. The results showed that clearing and clipping of vegetation and conventional tillage reduced SOC, soil nutrient contents and biomass yields, while burning increased SOC and soil nutrient contents. For the 0–10 cm depth, conventional tillage, clearing and clipping resulted in a 37%, 34% and 18% decrease in SOC, respectively, after three seasons, burning resulted in a 25% increase in SOC, while there were no changes in the control after 3 years. For the 0–40 cm depth, the average change in SOC was 32%, 25% and 16% for conventional tillage, clearing and clipping, respectively. Locally and regionally, conventional tillage, clearing and clipping reduce SOC, nutrient contents and biomass production in dambos. Though annual burning increased SOC and nutrient contents in the short term, the long-term effects are uncertain, hence there is a need for long-term studies.

Soil organic carbon and nitrogen stocks along a seasonal wetland (dambo) transect in central Zimbabwe

Ecosystems of central and southern Africa are occupied by some of the largest seasonal wetlands commonly called dambos. Dambos are likely to store huge stocks of soil organic carbon (SOC) because of their saturated conditions. However, most available literature report average SOC concentrations while ignoring pedological and hydrological variations. The objectives of the study were to quantify effects of catena position and hydrology on SOC and nitrogen stocks along a dambo transect in Chiota, Zimbabwe. Soil organic carbon stocks varied significantly with catena and were 7.3, 9.5, 30.4, 12.9 and 7.2 Mg ha−1 for upland, margin, middle slope (midslope), lower slope and bottom, respectively, for the 0–40 cm depth. Corresponding nitrogen stocks were 0.6, 0.8, 2.1, 1.1 and 0.7 Mg ha−1 for upland, margin, midslope, lower slope and bottom, respectively. The dry-season water table was 8, 1.5, 1.0, 1.3 and 1.5 m, whereas the wet-season water table was 6, 0.5, 0, 0 and 0.9 m below the surface for the upland, margin, midslope, lower slope and bottom, respectively. Biomass stand was highest in the midslope and least in the bottom. It was concluded that SOC and nitrogen stocks varied significantly with catena and this was attributed to differences in wetness.