No abstract

Reports of sugarcane yield responses to silicon (Si), coupled with mounting evidence that elevated crop Si levels reduce both biotic and abiotic stresses, account for the interest in theSi nutrition of this crop. In terms of managing Si supplies to sugarcane in South Africa, uncertainties exist regarding, first, the reserves of plant-available Si in soils, and second, the reliability of soil-test methods for predicting Si availability. In this study, extractable Si was measured in 112 soils collected from sugarcane-producing fields in South Africa. Soils were selected on the basis of dominant soil types and included Inceptisols, Alfisols, Mollisols, Vertisols, Oxisols, Entisols, and Ultisols, varying widely in chemical properties, texture, and extent of weathering. Extractants employed were 0.01 M calcium chloride (CaCl 2 ) and 0.02 N sulfuric acid (H 2 SO 4 ). Silicon extracted with 0.02 N H 2 SO 4 ranged from 2 to 293 mg kg −1 , whereas with 0.01 M CaCl 2 the range was 5 to 123 mg kg −1 . With both extractants, extractable Si decreased significantly with decreasing pH, exchangeable calcium (Ca), and total cations. In soils with potassium chloride (KCl)-extractable Al+H levels of greater than 0.5 cmol c L −1 , extractable Si levels were consistently low, suggesting that soluble Al is implicated in reducing plant-available Si levels. Extractable Si levels were not related to the Bache and Williams P-sorption indices of soils. In the second part of the investigation, sugarcane leaf Si concentrations from 28 sites were related to soil extractable Si levels. The CaCl 2 soil test proved markedly superior to H 2 SO 4 as a predictive test for leaf Si levels.

Although no‐till (NT) is now practiced in many countries of the world, for most smallholders, the crop residues are of such a value that they cannot be left on the soil surfaces to promote soil protection, thus potentially limiting NT benefits and adoption. In this study our main objective was to evaluate runoff, soil, and soil organic carbon (SOC) losses from traditional small‐scale maize (Zea mays) field under conventional tillage (T) and NT, with crop residues cover of less than 10% during the rainy season, in South Africa. Six runoff plots of 22.5 m2 (2.25 × 10 m) under NT and T since 2002 were considered. At each plot, soil bulk density (ρb) and SOC content of the 0–0.02 m layer were estimated at nine pits. Top‐soil SOC stocks were 26% higher under NT than under T (P = 0.001). The NT reduced soil losses by 68% (96.8 vs. 301.5 g m−2 yr−1, P = 0.001) and SOC losses by 52% (7.7 vs. 16.2 g C m−2 yr−1, P = 0.001), and differences in runoff were not significant. Dissolved organic carbon accounted for about 10% of total SOC losses and showed significantly higher concentrations under T than NT (1.49 versus 0.86 mg C m−2 yr−1). The less erosion in NT compared to T was explained by a greater occurrence under NT of indurated crusts, less prone to soil losses. These results showed the potential of NT even with low crop residue cover (<10%) to significantly reduce soil and SOC losses by water under small‐scale agriculture.

No abstract

Fire is a pervasive feature of the tropical savannas of northern Australia. Increasingly extensive and intensive fires have had an adverse effect on grass layer diversity. Reduced grass species diversity and abundance are important correlates of the decline of granivores in these tropical savannas. The Gouldian finch (Erythrura gouldiae), an endangered species that is endemic to northern Australia, is particularly vulnerable to changes to the grass layer as its diet comprises only grass seed, and it relies mostly on Sorghum stipoideum during the breeding season. Although this annual grass species is abundant at breeding sites, the finches do not always choose to breed at these sites, raising the possibility that seed quality may vary from year to year. This study examines the effect of fire (time since last fire; fire frequency) on soil fertility and seed nutritional quality. We hypothesise that recently burnt sites produce a flush of soil nutrients and Sorghum stipoideum seed at these sites is of higher nutritional quality. Furthermore, we posit that frequently burnt sites become depleted of soil nutrients and their seeds are of lower nutritional quality. There was a significant increase in inorganic nitrogen in soils following a fire, but no notable change in other soil nutrients. Contingent on this increase in soil inorganic nitrogen, seed nutrient levels, particularly essential proteins, were greater at sites that were recently, but infrequently burnt. Fires appear to affect soil nitrogen and in turn seed nutrition, providing a plausible explanation for why Gouldian finches choose recently, but infrequently burnt breeding sites.