R. Rhodes

Extractable Silicon in Soils of the South African Sugar Industry and Relationships with Crop Uptake

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 the Si 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 (CaCl2) and 0.02 N sulfuric acid (H2SO4). Silicon extracted with 0.02 N H2SO4 ranged from 2 to 293 mg kg−1, whereas with 0.01 M CaCl2 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 cmolcL−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 CaCl2 soil test proved markedly superior to H2SO4 as a predictive test for leaf Si levels.

Evaluation of Desmodium uncinatum, Fagopyrum esculentum and Brachiaria humidicola as potential green manure crops for nematode management in sugarcane

Plant-parasitic nematodes remain a challenge to sugarcane growers in South Africa, and alternatives to chemical control are desirable. This project aimed to determine the impact of green manure crops Desmodium uncinatum, Fagopyrum esculentum and Brachiaria humidicola on nematodes and subsequent sugarcane yield. A pot trial examined the growth of these crops and their effect on plant-parasitic nematodes and subsequent sugarcane growth. Brachiaria produced the greatest biomass (to the detriment of the succeeding sugarcane crop) and significantly reduced the numbers of Pratylenchus nematodes in soil and roots. In a subsequent field trial, plots were planted to these same three green manure crops, with control plots either planted to sugarcane or left as a bare fallow. Four months later, sugarcane was replanted over the whole area. There were no significant differences between treatments in sugarcane growth or yield after growing these crops, and changes in selected nematode genera at different stages during the crop cycle were inconsistent across nematode genera and treatment. Further work is needed to investigate the nematode control potential of these crops before the system can be adopted at the field scale.