AG Waters

An Alfisol, a Mollisol and an Oxisol were fractionated into different particle sizes after a range of disaggregating treatments from gentle to vigorous. The Alfisol and the Mollisol appeared to break down in steps; macroaggregates >250 µm diameter breaking down to microaggregates 20-250 µm diameter before particles <20 µm were released. Vigorous disruption led to particle size distributions similar to those obtained by classical methods used to determine particle size distributions. The Oxisol was stable to rapid wetting treatments but when aggregate disruption was initiated by vigorous treatments particles <20 µm diameter were released and there was no evidence of aggregate hierarchy. Scanning electron microscopy of particles of different sizes showed distinctly single grain particles and aggregates. The microscopic studies indicated the potential role of roots and hyphae in the stabilization of larger aggregates, and for fragments of roots as nuclei for smaller aggregates. Plant debris was not visible in aggregates <20 µm but clay microstructure was evident. It is suggested that aggregate hierarchy occurs in Alfisols and Mollisols because organic materials are the dominant stabilizing agents in larger aggregates but in the Oxisol oxides are dominant stabilizing agents and prevent the expression of aggregate hierarchy caused by organic materials.

Size and density fractions separated from a red-brown earth have been analysed by high-resolution solidstate 13C nuclear magnetic resonance (n.m.r.) spectroscopy. Resonances from O-alkyl carbon (mainly carbohydrate) predominated in the spectra of the largest (250-2000 µm) fractions, whereas alkyl carbon resonances (mainly polymethylene) predominated in the spectra of clay fractions. Paramagnetics, mainly Fe3+, were found to influence the amount and type of carbon seen in 13C solid-state n.m.r. spectra of clay fractions containing more than several per cent iron oxides. Removal of iron oxides by reduction with dithionite allowed aromatics, carboxyls and carbohydrates to be detected by 13C n.m.r.

Samples were obtained from the same red-brown earth: (a) in an undisturbed state, (b) after 60 years of an exploitive wheat-fallow rotation and (c) after 40 years under a fertilized mixed grass-legume pasture. Organic materials were concentrated in various fractions which enabled comparative chemical composition of the organic materials in the three soils by 13C CPMAS n.m.r. spectroscopy. Despite more than twofold differences in the organic carbon content of the soils, the chemistry of the organic matter in the soils was similar, particularly organic matter associated with clay fractions. Most of the differences detected were associated with plant debris in particles > 20 µm which contained most of the aromatic carbon. The results indicate a rapid disappearance of phenolic-carbon which originates in lignins. The composition of sodium hydroxide extracts reflects quite well the composition of the organic matter in the soil. It is concluded that in a particular soil type, changes in amounts and nature of added photosynthate do not change the composition of the organic matter which is controlled by the microbial biomass and interactions of the biomass and its decomposition products with the soil matrix. Implications of this conclusion for the turnover of organic carbon in soil and stability of soil structure are discussed.

The organic matter in the A, Bh, Bhs and Bs horizons from a number of profiles from a chronosequence of podzols spanning some 7x105 years was studied using solid state 13C n.m.r. spectroscopy. Organic matter was effectively concentrated without chemicals using a Spex mixer and sedimentation. Acid dithionite treatment of samples containing appreciable amounts of iron significantly improved the signal to noise ratio. Acid oxalate treatment had a lesser effect. The organic matter from the A horizon was highly aromatic but was low in carboxylic acids. In the B horizons, aromaticity decreased in the order Bh-Bhs-Bs and increased with the degree of profile development. Only in the Bh horizons of the older highly developed profiles did aromatic carbon exceed alkyl carbon. The 'core' structure of these materials appeared to be aromatic rings heavily substituted (>90%) with alkyl and carboxylic acid groups. The Bhs and Bs horizons contained substantial amounts of carboxylic acid substituted alkyls with structures similar to the polymaleic or hydroxy acid models. Sixty per cent of the organic matter removed with iron on dithionite treatment was found to be alkyl. The aromatic and alkyl dominated horizons can exist in close proximity, occurring less than 10 cm apart in the case of the giant pipey podzols. None of the current theories on podzol genesis can adequately explain the arrangement of organic and inorganic components found in these profiles and a revised model based on existing theories is proposed.

A 1.2m wide conveyor belt carrying lump ore and some associated fines was sampled to determine the rate and degree of size segregation that occurred due to the vibration of particles on the belt. It was found that most of the segregation occurred within the first 40m. No noticeable increase in segregation was observed beyond that obtained after 300m. Size analyses showed that the plus 16mm lump migrates preferentially to the top of the bed of material on the conveyor belt, and minus 6mm material preferentially reports to the lower sections. Hence, a small proportion of the material on the conveyor belt would result in a substantial reduction in the level of fines. Typically, if the fines rich lower than 20% of the material on the conveyor belt was rescreened, the amount of total fines would be reduced by approximately 50%, whereas rescreening the top 35 percent of the material on the conveyor would reduce the fines content by less than 5%.