It is proposed that microbial polysaccharides behave anion-exclusively, permitting the transport of cations, but excluding the diffusion of anions. This hypothesis has been investigated in the context of polysaccharides produced by microorganisms in the rhizosphere. The anion-exclusive behaviour of exopolysaccharides, extracted from broth cultures of a range of rhizosphere microorganisms together with several commercial polysaccharides (i.e., xanthan, scleroglucan, dextran, guar gum) was investigated by measuring the electrochemical potential which developed as a result of the diffusion of KC1 across a polymer layer. Considering xanthan as a 'model' microbial polysaccharide, polymer concentration, layer thickness and the presence of either O-acetyl or pyruvyl groups were found to positively affect the degree of anion exclusion. The anion-exclusive behaviour of xanthan was verified by direct ion analysis of solutions either side of the polymer layer. It was found that in a range of ionic environments, the diffusion of anions was reduced by ~70% by the presence of a 3% w/w xanthan layer. The influence of xanthan on the diffusion of cations was studied using magnetic resonance imaging. In contrast to the proposed theory of anion exclusion, the rate of cation (Mn2+) diffusion through a 3% w/w xanthan layer was found not to be greater than that through free aqueous solution. The possible occurrence of anion exclusion and consequences thereof in the rhizosphere were assessed by studying the effect of substitution of the water films in soil with a layer of 3% xanthan on the growth of wheat seedlings. The phosphate, but not potassium content of those plants grown in soil with xanthan was ~20% lower than in the control plants. Those plants grown in xanthan-amended soil produced ~30% more biomass by dry weight. The anion-exclusive properties of polysaccharides produced naturally in bulk soil, the rhizosphere and root surface of pea was studied by the measurement of diffusion potentials. All three were found to show a high level of anion exclusion. The influence of O-acetyl groups in xanthan on the rate of water transport and degree of water binding was studied using stray field NMR methods. It was found that removal of O-acetyl groups reduces the rate of water transport and increases the rate of water binding at any given xanthan concentration. It is proposed that microoganisms produce anion-exclusive polysaccharides in the rhizosphere to protect themselves against the potentially lethal effects of water stress.

Dissertation submitted in compliance with the requirements for the Masters Degree in Technology: Civil Engineering, Technikon Natal, 1997. / The strength and durability of any soil structure is dependent on the quality of the compaction of the soil. This quality is measured by employing a standard compaction test, which provides a standard with which density may be compared, called the maximum dry density, and the moisture content of the soil at which this is achieved, called the optimum moisture content. As a matter of routine during quality control, the particle size distribution, plasticity index and liquid limit of the soil are determined at the same time as its maximum dry density and optimum moisture content. / M

No description available.

No description available.

No description available.

No description available.

No description available.

No description available.

No description available.

No description available.