bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural decline of soils, assessment and prevention
Authors: Emerson WW
Published: April 2005

Two extreme textural types of cultivated surface soils are mainly considered here, non-shrinking red-brown earths and highly shrinking cracking clays. Total porosity is used to assess the structural status of the former. Values are compared with the highest and lowest values found in the field. For the latter, the criterion used is the porosity of dry aggregates or clods. Values here are taken from the literature. To find out why inter-particle bonding in soil aggregates is insufficient to stop structural decline, a scheme has been developed which includes a modified version of Emerson's (1967) classification of soil aggregates. Slaking is carefully assessed. The bulk density of a cube made from soil at 'field capacity' is measured as well as testing another for dispersion. Class 3 is now divided into 3a and 3b, according to the degree of dispersion of remoulded soil in water. Also apart from soils which disperse spontaneously from dry, classes 1 and 2, the dispersion of all soils is assessed after remoulding at 'field capacity'. It has been found that the red-brown earth site which had the best visual structure also had the largest total porosity and aggregates in class 4. At the worst site, aggregates were in class 3a and the porosity had been reduced to that of the soil cube. For cracking clays, porosity is appreciably higher where the aggregates are in class 4 rather than class 3a. Water content/dispersion curves are presented for the clays showing the extent of the increase in OD apparently associated with the presence of carbonate. Dispersion of sheared, class 3a soil immersed in water is only an outward sign of the structural damage caused when the soil is sheared too wet. If the soil is dried instead, porosity is still lost. Mechanisms are suggested by which the structure of class 3a clay soils are improved by adding carbonate. The slumping of red-brown earths and the use of surface dressings of gypsum to prevent severe dispersion after cultivation wet are discussed. The structural stability of aggregates in the other five classes is briefly considered. Classes 1 and 2 require an ameliorant to be added, the rest pose few problems.
Drainage and soil structure - a review
Authors: Collis-George N
Published: April 2005

Definitions of soil structure and drainage are proposed that would allow a more useful interaction between the two areas of interest that appear to have developed independently. Drainage phenomena in three structural systems are quantitatively described. Firstly a simple uniform profile is described; then wormholes and stones are added to the upper layer of this soil; finally the upper layer is given a 23% stable macropore space and its depth is proportionately increased by biotic activity. Numerical solutions of these geometries (supported by experimental work) show that if the structure enhancements are confined to the upper horizons, the drainage behaviour of the matrix is unaffected and only the early stages of the drainage hydrograph are affected. More complex structures are considered qualitatively. The influence of entrapped and encapsulated air within the soil pore space is outlined. It is concluded that a major problem in correlating drainage phenomena with soil structure is that the structural description of wet soils is rarely attempted. In particular, the descriptions of structures, naturally occurring and those enhanced by cultivation, which change with time and with wetting and drying, are presently only described by soil surveyors in qualitative terms.
Sodicity and soil structure
Authors: Rengasamy P , Olsson KA
Published: April 2005

Sodic soils are widespread in Australia reflecting the predominance of sodium chloride in groundwaters and soil solutions. Sodic soils are subject to severe structural degradation and restrict plant performance through poor soil-water and soil-air relations. Sodicity is shown to be a latent problem in saline-sodic soils where deleterious effects are evident only after leaching profiles free of salts. A classification of sodic soils based on sodium adsorption ratio, pH and electrolyte conductivity is outlined. Current understanding of the processes and the component mechanisms of sodic soil behaviour are integrated to form the necessary bases for practical solutions in the long term and to define areas for research. The principles of organic and biological amelioration of sodicity, as alternatives to costly inorganic amendments, are discussed.
Surface structure in gray clays of northwestern New South Wales in relation to micromorphology, cation suite and particle size attributes
Authors: Little IP , Ringrose-Voase AJ , Ward WT
Published: April 2005

Considerable differences in surface structure (0-100 mm) were observed in the field in two adjacent areas of grey clays near Narrabri, N.S.W. The absence of any differences in clay mineralogy and granulometry of the sand fraction supported the field assessment that both types of soil were similar in provenance. A transect of soil profiles including seven with poor structure and five with well-structured surface horizons was examined. The field observations of structure were supported by photographs of the surface, and water entry after rain. Micromorphological examination showed that closely spaced porphyric to adporphyric fabric in the poorly structured soils contributed to poor structure, highlighting the importance of textural attributes. The well-structured soils had a more widely spaced porphyric fabric. A measure of dispersibility depending on clay content and exchangeable plus soluble Na, Ca and Mg tallied very well with the field assessment of soil structure. Five groups were obtained from a euclidean distance/flexible sort strategy on the basis of cation suite, carbon content and particle size attributes. The groups identified areas of poor structure very well and the groups appear to be discriminated mainly on the basis of differences in Na, Ca and clay content. Treating the transect as a continuum of soils of very poor structure at site 1 grading to very good at site 12 showed that greater values for Ca, K, and clay were associated with good structure and greater values for Mg, C and silt were associated with poor structure. The sodium adsorption ratio and ionic strength of the soil solution were not on their own good predictors of structural behaviour possibly due to the independent contribution of Ca and Mg in this respect.
The effects of fire on the soil in a degraded semi-arid woodland .III. Nutrient pool sizes, biological activity and herbage response
Authors: Tongway DJ , Hodgkinson KC
Published: April 2005

The effect of experimental fire regimes on soil nutrient pool values, surface condition status and herbage growth was studied in a shrub-invaded woodland on massive red earths near Coolabah, New South Wales. Artificial fuel was burnt in spring on plots to achieve intervals of 1, 2, 3, 4, 5 and 10 years between fires. No differences between fire regimes were detected below 1 cm for any of the measured soil variables. Soil respiration declined linearly with increasing number of fires and increasing total fuel load. There were no differences or trend for available or total soil nitrogen, organic carbon or soil wettability in the 0-1 cm horizon, but relatively refractory elements such as P and K, derived from the ash from the artificial fuel, built up in the soil proportionately with increased fire number and total fuel load. The surface condition status of the soil as assessed by a monitoring method was strongly inversely related to the number of fires and total fuel load. Differences in herbage growth between fire treatments were not significant; however, where plots had been burnt five times, herbage growth was greater with increased variability (35.8 g me2, s.e. 19.0) compared with unburnt controls (10.7 g m-2, s.e. 1.7). Plots burnt seven times annually had similar means and variance to controls. Low herbage yields in control plots were attributed to the high biomass of mature shrubs, whilst increasing variability with number of fires was caused by the combination of patchy decreased shrub biomass and small-scale erosion-deposition cells which differentially distribute seeds across the soil surface. Plots burnt annually seven times had deposition beyond the plot area.
Soil physical properties of three geomorphic zones in a semiarid mulga woodland
Authors: Greene RSB
Published: April 2005

Soil physical properties were measured in three contiguous geomorphic zones of a patterned sequence of alternating groves and intergroves in a semi-arid mulga (Acacia aneura) woodland : (1) a runoff zone of stony, severely sealed, surface soil, (2) an interception zone at the bottom of the runoff zone, and adjoining (3) a runon zone of mulga groves. Infiltration was measured in the field under unsaturated and saturated conditions using a disc permeameter at water supply potentials of -40 and +10 mm respectively. Under unsaturated flow conditions, there were no significant differences in the sorptivity, three-dimensional infiltration rate and hydraulic conductivity between the three zones. However, under saturated flow conditions, the soils in the mulga groves had infiltration rates 5-10 times higher than the soils in the runoff and interception zones. This difference was explained by the presence of stable macropores >0.75 mm diameter in the mulga grove soils. Surface soil (0-10 mm) aggregates from mulga groves were also particularly stable to rapid wetting, measured by wet-sieving. Volumetric water contents (measured over a range of matric potentials from 0 to -5.0 kPa) of the 0-50 mm layer of soil from the mulga grove and interception zone were significantly (P = 0.05) higher than the 0-50 mm layer from the runoff zone. Micromorphological and scanning electron microscope (SEM) examination indicated that the total porosity of the soil surface from the mulga groves and interception zones was greater than that of the runoff zones. Measurement of soil-water content following a major rainfall event indicated that water had flowed off the runoff zones and accumulated in the mulga groves. These findings are consistent with the higher herbage production and biotic activity that is found to occur in the mulga grove and interception zone compared with the runoff zone following adequate rainfall. They also reveal part of the delicate balance of runoff redistribution in grove/intergrove areas and the potential for management to alter this balance.
Organic carbon and associated soil properties of a red earth after 10 years of rotation under different stubble and tillage practices
Authors: Chan KY , Roberts WP , Heenan DP
Published: April 2005

Differences in soil organic carbon level as a result of different tillage and stubble management practices under continuous cropping were studied in a 10 years old wheat/lupin rotation experiment on a red earth at Wagga Wagga, New South Wales. Stubble burning and tillage had a similar impact in reducing the total amounts of organic carbon in the top 0-2 m of soil. There was no significant difference between the conventional cultivation (3 cultivations) and reduced cultivation (1 cultivation) systems. A 31% difference in organic carbon in the top 0.1 m (2.42% v. 1.68%) was found between the extreme management practices, i.e. direct drill /stubble retained treatment and the conventional/stubble burnt treatment. These results highlight the important effect of management practice on soil organic carbon level under continuous cropping. Tillage had the additional effect of changing the distribution of organic carbon resulting in higher level in the 0.10-0.15 m layer. The reduction in organic carbon was accompanied by significant losses in total nitrogen, exchangeable calcium and magnesim, as well as reduction in biological activity and aggregate stability. Loss of 1% organic carbon resulted in a loss of 2-97 cmole(+) kg soil-1 of negative charge. However, C/N ratio remained constant at 12-1 under different tillage and stubble treatments. Finally, while stubble burning resulted in pH increase, tillage led to a significant reduction in soil pH (5.38 to 4.98) in the 0 - 0.05 m layer due to increased exchangeable A1 brought to the soil surface as a result of an inversion action.
Effects of plot size on size distributions of water stable material at the soil surface under simulated rain
Authors: Loch RJ , Foley JL
Published: April 2005

When small samples are wetted by simulated rain, a significant proportion of the sample may be lost over the plot boundary as splash, and the relative importance of this edge effect will increase as plot size is reduced. Because there are considerable practical advantages in using plots as small as possible, it is important to determine the smallest plot sizes that will give results unaffected by splash losses. Therefore, this study considered effects of the size of plot exposed to simulated rain on size distributions of particles (aggregates and sand grains, with no distinction made between them) in the surface seal layer. Reducing the areas of square plots from 0.81 to 0.36 m2 had no discernible effect on size distributions in the surface seal layer, but for one of the three soils for which it was tested, a reduction of plot area to 0.09 m2 gave a significant (P < 0.01) increase in the proportion of particles <125 �m in the surface seal layer, and a corresponding significant reduction in the Mean Weight Diameter of particles in the same layer. Therefore, the use of plots of 0.09 m2 is considered to be only marginal in its acceptability, and a minimum plot area of approximately 0.36 m2 is recommended, and is generally preferable as it allows sufficient plot area for both sampling of the surface and any other measurement of surface properties that is required.
Changes in soil phosphate fractions in the rhizosphere of semi-arid pasture grasses
Authors: Armstrong RD , Helyar KR
Published: April 2005

The comparative ability of several species of grass common to the semi-arid mulga shrublands of south-western Queensland, namely Cenchrus ciliaris cv. U.S.A., Aristida armata, Digitaria ammophilla and Thyridolepis mitchelliana, to deplete various fractions of soil inorganic/organic phosphorus was assessed using a rhizosphere thin section methodology. All species significantly depleted inorganic phosphorus extractable with 0.5 M NaHCO3, 0.1 M NaOH, and 1.0 M HCl. The grasses were able to deplete 39, 32, and 29%, respectively, of the inorganic component of each of these extracts. In contrast, the organic phosphorus fraction of all three extracts either remained unchanged or increased in the presence of the grasses compared with unplanted control sections. Depletion of sparingly soluble soil phosphorus coincided with rises in rhizosphere pH. This result contrasts with previous studies that attributed plant depletions of phosphorus pools that are sparingly soluble to an acid hydrolysis process mediated by the root. The liming of the soil had only a minor effect on the distribution of added phosphate between the various soil inorganic phosphorus fractions but resulted in a depletion in the organic fraction of the 0.5 M NaHCO3 and 1.0 M HCl extracts. There was no difference between the grasses in ability to extract the various soil phosphorus pools. These results confirm earlier studies using radioactively labelled compounds of mineral phosphate that these grasses have the ability to utilize sparingly soluble pools of soil phosphorus that are not traditionally considered to be 'plant available'.
Adsorption of cadmium by an aquent New Zealand soil and Its components
Authors: Kim ND , Fergusson JE
Published: April 2005

Cadmium adsorption by a New Zealand soil, three of its particle size fractions, the soil with organic matter removed, and the soil with iron and manganese oxides removed, was studied at the soil's natural pH (5.70) and at cadmium concentrations (0.01 to 0.23 �g mL-1) typical of those found in the environment. Under these conditions, the whole soil had a strong affinity for trace amounts of cadmium, adsorbing about nine-tenths of that added. Relative conditional equilibrium constants for cadmium adsorption by the soil's components decrease in the following order: clay (1.59) > [whole soil (1.00)] > soil without iron and manganese oxides (0.55) > silt (0.54) > sand (0.52) > soil without organic material (0.08) > silica sand (0.001). It is estimated that the <3.9 �m particle size fraction (22% of the soil) and the 3-9-563 pm particle size fraction (78% of the soil), contributed about 45% and 55%, respectively, to the overall capacity of the soil to adsorb cadmium. Most data obtained gives good fits to both linear and Freundlich adsorption isotherms, but is not consistent with the Langmuir adsorption model. Thus it is likely that, under the conditions studied, the soil and its components supply a range of cadmium adsorption sites with varying adsorption energies.
Changes in effective cation exchange capacity and exchangeable aluminum with soil pH in lime-amended field soils
Authors: Hochman Z , Edmeades DC , White E
Published: April 2005

Eleven acidic soils from northern N.S.W., having a wide range of values for ECEC, A1 and soil organic carbon (%C), were treated in the field with five rates of lime. The relationships between soil pH and the effective cation exchange capacity (ECEC), and between pH and exchangeable aluminium (Al), were investigated for the top 10 cm of these soils. Increases in the total exchangeable cations (TEC) calculated as ECEC-Al, were shown to be madelup almost entirely by increases in exchangeable calcium. There were no consistent changes in the amount of exchangeable magnesium, potassium or sodium due to liming these acidic soils. Formulae used to predict changes in A1 and ECEC with pH in the 'Lime-it' model were tested and modified on the 11 soils from northern N.S.W. A strong linear relationship was observed in each soil between Al and pH (transformed to hydrogen ion concentration x 103). The slope of this relationship (SALs) can be predicted from the pH and A1 values of unlimed soils. Strong linear relationships were also observed between pH and TEC, for each of the 11 soils. The SL, (the slope of the linear relationship TEC/pH for any soil 's') was shown by multiple regression analysis to be a function of TECi/pHi (where TECi is the sum of exchangeable cations of unlimed soil 's'; and pHi is the pH value of unlimed soil 's'), %C of the unlimed soil, and SALs. By using the measured values of pH, ECEC, Al and %C of unlimed soils, the values of Al, and TEB can be predicted for any pH value that may be measured (or predicted) after liming. The predictive relationships developed on N.S.W. soils were tested against independent data from New Zealand. The results confirmed the Al/pH predictions (R2 = 0.955), while the TEC/pH predictions were less well matched (R2= 0.62) possibly due to unusual clay mineralogy or organic matter fractions of 3 of the 18 soils tested.
Long term trends in total nitrogen of a vertisol subjected to zero-tillage, nitrogen application and stubble retention
Authors: Dalal RC
Published: April 2005

The effects of conservation practices, zero-tillage and stubble retention, on long-term trends in total N (0-0.1 m depth) of a Vertisol used mainly for wheat cropping were studied in a semi-arid subtropical environment (28�12'S. and 152�06' E.) in Queensland. Trends in total N content of a Vertisoi (65% clay, pH 7.2) were discerned during a 22-year period of management practices including: zero-tillage (ZT) and conventional tillage (CT); stubble retention (SR) and stubble burning (SB); and fertilizer N application of nil (Nl), 23 kg N ha-1 yr-1 (N2) and 69 kg N ha-1 yr-1 (N3). Soil total N (0-0.1 m) declined under all treatments at an overall rate of 25f 2 kg N ha-1 yr-1 although after 22 years soil under ZT, SR and N3 treatments still contained higher soil total N than under CT, SB and N1 treatments. Apparent fertilizer N recovery in the soil-plant system was poor (34 64%) under CTSB, CTSR and ZTSB and ZTSR treatments, because N removed by the wheat crop was equivalent to less than 20% of fertilizer N in the first 12 years of management practices, due mainly to disease. Deep leaching losses of NO3-N was the likely factor for poor recovery of N. The ZTSR treatment showed better apparent N recovery than the CTSB treatment, most likely due to greater immobilization of fertilizer N, more N uptake in grain due to additional available soil water and hence less leaching losses of NO3-N. Under the current cultural practices, soil total N (0-0.1 m) may decline further to reach a steady state (about 1000 kg N ha-1). However, the apparent N recovery in the soil-plant system can be increased by disease control (for example, resistant cultivars and winter-summer crop rotations) and optimum utilisation of soil water (opportunity cropping) to minimize NO3-N leaching losses and to maximise production of crop biomass.
Recent developments regarding the influence of soil surface characteristics on overland flow and erosion
Authors: Hairsine PB , Moran CJ , Rose CW
Published: April 2005

Recent developments in modelling overland flow and soil erosion, which permit the consilidation of soil surface characteristics, are reviewed. The models of overland flow are found to be sensitive to soil surface micro-topography and surface pore structure. The former affects depressional storage and hydraulic routing and the latter soil hydraulic properties, notably infiltration rate. The models of soil erosion by water are found to be sensitive to surface roughness, strength and the aggregate size distribution, and the deposition characteristics, of the eroded sediment. Methods for describing the roughness of a soil surface are described and the links to the above processes discussed. The implications of new techniques of roughness measurement for this area of research are briefly examined. It is concluded that the new models of erosion and overland flow provide a more realistic description of the influence of soil surface characteristics than previous semi-empirical models. The review identified the case of large form roughness and shallow flow as a poorly described regime. It was also concluded that the relationship between structural degradation and surface roughness warranted investigation.
Soil structure, soil hydraulic properties and the soil water balance
Authors: Cresswell HP , Smiles DE , Williams J
Published: April 2005

We review the influence of soil structural change on the fundamental soil hydraulic properties (unsaturated hydraulic conductivity and the soil moisture characteristic) and utilize deterministic modelling to assess subsequent effects on the soil water balance. Soil structure is reflected in the 0 to -100 kPa matric potential section of the soil moisture characteristic with marked changes often occurring in light to medium textured soils' (sands, sandy-loam, loams and clay-loams). The effect of long-term tillage on soil structure may decrease hydraulic conductivity within this matric potential range. The 'SWIM' (Soil Water Infiltration and Movement) simulation model was used to illustrate the effects of long-term conventional tillage and direct drilling systems on the water balance. The effects of plough pans, surface crusts and decreasing surface detention were also investigated. Significant structural deterioration, as evidenced by substantially reduced hydraulic conductivity, is necessary before significant runoff is generated in the low intensity rainfall regime of the Southern Tablelands (6 min rainfall intensity <45 mm h-1). A 10 mm thick plough pan (at a depth of 100 mm) in the A-horizon of a long-term conventionally tilled soil required a saturated hydraulic conductivity (K,) of less than 2.5 mm h-1 before runoff exceeded 10% of incident rainfall in this rainfall regime. Similarly, a crust K, of less than 2.5 mm h-1 was necessary before runoff exceeded 10% of incident rainfall (provided that surface detention was 2 or more). As the crust K, approached the rainfall rate, small decreases in Ks resulted in large increases in runoff. An increase in surface detention of 1 to 3 mm resulted in a large reduction in runoff where crust K, was less than 2-5 mm h-1. Deterministic simulation models incorporating well established physical laws are effective tools in the study of soil structural effects on the field water regime. Their application, however, is constrained by insufficient knowledge of the fundamental hydraulic properties of Australian soils and how they are changing in response to our land management.
A sensitivity analysis of the Jackson method of predicting unsaturated hydraulic conductivity
Authors: Cresswell HP
Published: April 2005

An assessment is made of the sensitivity of the unsaturated hydraulic conductivity predictions from the Jackson model to changes in the measured moisture characteristic and matching factor hydraulic conductivity inputs. The model is shown to be sensitive to the volumetric water content corresponding to the matching factor hydraulic conductivity as well as to the 0 to -1.0 kPa matric potential section of the moisture characteristic input. The significance of this sensitivity is dependent on intended use of the data. Where accurate conductivity prediction is required over a small range of water contents near saturation, the moisture characteristic input used with this model should include measured points between 0 and 1.0 kPa matric potential.
The analysis of fracture profiles of soil using fractal geometry
Authors: Young IM , Crawford JW
Published: April 2005

The fracture profiles of three soils (taken from Dexter and Horn 1988) are analysed according to their fractal dimensions D. D, which is a measure of how the 'apparent' length of a fracture path increases with decreasing ruler size, is found to be a good quantifier of the tortuousity of fracture paths. The fractal analysis is compared with a more traditional statistical method of analysing fracture profiles. It is shown that the latter method, unlike fractal analysis, can omit a significant proportion of a tortuous fracture path and therefore leads to an underestimate of any roughness parameter.
Phosphorus sorption by gravels in lateritic soils
Authors: Weaver DM , Ritchie GSP , Gilkes RJ
Published: April 2005

The effect of sesquioxidic gravels (>2 mm size fraction) on phosphorus sorption by two gravelly lateritic soils was investigated by phosphorus sorption experiments, X-ray diffraction, autoradiography and electron microscopy.In one soil the abundance and size of gravels decreased downslope. As the gravel content of both soils increased (13-61%), the proportion of large (>8 mm) gravels increased (0-70%). Phosphorus sorption increased in the order: (>2 mm fraction) < (whole soils) < (<2 mm fraction). Phosphorus sorption on lateritic gravels (>2 mm) decreased with increasing gravel size and increased with increasing time. Phosphorus sorption by whole soils decreased with increasing gravel content. Phosphorus sorption by the >2 mm fraction was principally by the external surfaces of the gravels. The estimation of phosphorus application rates for lateritic soils depended on the amount of phosphorus required by the plant and the gravel content of the soil. The former is the important factor at low soil gravel contents whereas the latter becomes more important as the gravel content increases. The implications for fertilizer management on the soil with consistent relationships between landscape position, gravel content and subsequent P sorption properties of whole soils are discussed.
Influence of intensity/quantity characteristics of soil phosphorus tests on their relationships to phosphorus responsiveness of wheat under field conditions
Authors: Holford ICR , Doyle AD
Published: April 2005

Six soil phosphorus tests (lactate, Brayl, Bray2, neutral fluoride, Olsen and Colwell) were regressed against potassium chloride-soluble phosphorus (intensity) and isotopically exchangeable phosphorus (quantity) measured in 59 soils of the northern and central wheat belts of New South Wales. Wheat nutrition experiments on these soils during 1986-89 measured yield responses to phosphate and nitrogen fertilizers. Soil tests varied widely in their correlations with yield responsiveness to phosphate, with the lactate and Bray2 tests accounting for more than twice the variance accounted for by other soil tests. The intensity parameter was also highly correlated but the quantity parameter was not. All soil tests, except Bray1, were very highly correlated with the intensity parameter, so this relationship did not differentiate the relative efficacies of the soil tests. Soil tests were less correlated with the quantity parameter, but those soil tests (neutral fluoride, Olsen and Colwell) that were most highly correlated (r2 > 0.62) with this parameter were most weakly correlated (r2 < 0.29) with yield response. It was concluded therefore that exchangeable phosphorus is not a satisfactory measure of the quantity factor and that an effective soil test for wheat-growing soils will be highly correlated with intensity but not necessarily with exchangeable phosphorus. The critical value of the lactate test was the same (17 mg/kg) as in previous studies with wheat but was lower (14 mg/kg) in 1989 when very low in-crop rainfall occurred. With deeper sampling (15 cm rather than 10 cm) the lactate test was slightly less accurate and the critical value was lower (11 mg/kg).
Amounts and sources of phosphate in hill country rocks, south-eastern North Island, New Zealand
Authors: Eden DN , Parfitt RL
Published: April 2005

The phosphate (P) present in a range of parent rocks for hill country soils in Wairarapa region, New Zealand, has been studied. In general the highest P concentrations occur in the mudstones which contain 50% more P than the fine to medium sandstones, and at least 10 times more P than the coarse sandstones. Most of the P is in the 20-2 �m and <2 �m size fractions of the rocks. The P is present both in apatite and calcite mineral grains. Although the highest P concentrations occur in the heavy fraction (containing apatite), most of the total P is held in the light fraction in which calcite is abundant and where some apatite also occurs within aggregates. Since both calcite and apatite are relatively soluble in this soil environment, and since both have large reactive surfaces, they may readily release P for plant growth.
Evaporation from a salt encrusted sediment surface - Field and laboratory studies
Authors: Chen XY
Published: April 2005

Estimates of hydrologic budgets from arid zones are constrained by difficulties in evaluating evaporation loss from groundwater discharge areas, especially playa surfaces. Evaporation from a salt-encrusted playa surface (Lake Amadeus, central Australia) is estimated by field measurement of moisture loss from sediment blocks in plastic receptacles set into the playa. The evaporation process consists of two distinctively different evaporative patterns. E1 is a very low rate (70 mm/year, 2.4% of pan evaporation) from the salt-encrusted surface. E2 is a much higher rate which occurs after rain dissolves the surface salt crust. The total E2 evaporation is lower than the rainfall, indicating that a portion of rainfall recharges the playa brine. Therefore, the total E1(70 mm/year) can only be used as an upper limit of the net evaporation and the actual value may be significantly lower. In a laboratory analogue experiment, a very thin (2 mm) salt crust diminishes the evaporation to about 2% of that from a fresh water surface, even though the sediments underlying the crust remain saturated. When distilled water was added to the salt crust, the evaporation rate increased by nearly 20 times for a short period, then returned to the previous low rate. However, a portion of the distilled water infiltrated to the watertable, and became part of the brine supply to the sediments. Both the salt crusts of Lake Amadeus and those formed in the laboratory experiment are porous and buckled, and significantly drier than the underlying sediments. The significant reduction of evaporation from salt-encrusted sediment surface seems to be mainly due to the porous, buckled and dry nature of the crust which inhibits the removal of the vapour from the underlying sediments. The vapour pressure decrease of the brine has relatively less effects.
Influence of soil structure on sulfate leaching from a silt loam
Authors: Mclay CDA , Cameron KC , Mclaren RG
Published: April 2005

The influence of soil structure on sulfate leaching from a silt loam soil was assessed by comparing the results of leaching experiments conducted using undisturbed soil monolith lysimeters and re-packed soil columns. A pulse application of sulfate solution was leached through both soil systems under non-ponded infiltration conditions. Breakthrough curves (BTCs) from the undisturbed soil lysimeters were poly-modal compared with the uni-modal curves observed in re-packed column experiments. The poly-modal BTC is considered to result from sulfate leached through isolated porosity regions within the undisturbed soil monolith. Sulfate leaching rates were faster through the undisturbed soil lysimeters and this was attributed to solute transport through the natural soil macropore system, which is retained in the undisturbed soil but lost in the repacked soil columns. Leaching through soil macropores therefore can result in considerable losses of sulfur from silt loam textured soils.
Corrigenda - A collection system for measuring runoff and soil erosion with a mobile rainfall simulator on sealed and stony red earth soils
Authors: Greene RSB , Sawtell GR
Published: April 2005

A collection system was developed for use with a Grierson and Oades rainfall simulator. The collection system enabled measurements of runoff and soil erosion to be made on massive red earth soils with severe seals (or crusts) and stones at their surface. In our design, the collection flume was separated from the other three sides of the plot frame. This allowed the collection flume to be initially inserted with minimum disturbance to the soil hydraulic properties and then connected to the other three sides of the plot frame. Runoff data obtained with this collection system, from a stony, sealed, surface soil, are presented.
Changes in microbial biomass and structural stability at the surface of a duplex soil under direct drilling and stubble retention in north-eastern Victoria
Authors: Carter MR , Mele PM
Published: April 2005

Changes and relationships for organic C, microbial biomass C and N, and soil structural stability indices were determined at the soil surface after 10 years of direct drilling stubble retained (DDR) and stubble burnt (DDB), and cultivation with stubble burnt (CCB) for cropping systems on a sandy clay loam, duplex soil (calcic luvisol) in south-eastern Australia. Direct drilling caused a slight but significant increase in soil organic C at the 0-25 mm soil depth compared to the cultivated treatment. Microbial biomass C and N increases over the 0-100 mm soil depths were seasonal and generally greater for the DDR in comparison with DDB and CCB systems. Use of short duration wet sieving for the 0-25 mm soil depth showed a significant increase in aggregate stability for the DDR, especially for 2-10 mm sized aggregates, compared with the other tillage treatments. Such differences were reduced by standard wet sieving or use of a dispersion test illustrating the fragile nature of these unstable aggregates developed under cropping systems. Soil structural indices (water stable aggregates >2.00 mm, and >0.25 mm; mean weight diameter) were weakly correlated with increases in microbial biomass (r = 0.45, P < 0.01) and to total organic C (r = 0.35, P < 0.05). For these tillage systems, microbial biomass tended to be a poor predictor of changes in soil organic C. Overall, the long term effect of direct drilling and stubble retention in these cropping systems provided only relatively minor increases in organic C and, consequently, aggregate stability.
The effects of direct drilling and stubble retention on hydraulic properties at the surface of duplex soils in north-eastern Victoria
Authors: Carter MR , Steed GR
Published: April 2005

Soil hydraulic properties in the sandy clay loam surface horizon of duplex soils were assessed under cultivated, stubble burnt (CCB); direct drilling, stubble burnt (DDB); and direct drilling, stubble retained (DDR) systems for three wheat-lupin rotations, established from 7 to 10 years, in north-eastern Victoria. Disc permeameters were used to determine sorptivity and steady state infiltration in each rotation. A rainfall simulator was used on the 10 year rotation to characterize saturated infiltration and surface soil stability. Cultivation caused an increase in soil bulk density and decreased organic C at the soil surface. Application of a -40 mm water supply potential removed macropore flow from the infiltration process and mainly characterized water flow in the soil matrix. Differences in sorptivity among tillage treatments at this potential mainly reflected initial soil moisture, rather than soil structure. Sorptivity measured on soil cores was related (T = 0.74, P = 0.01)= to sorptivity measured in the field. Under Rainfall simulation , DDR increased sorptivity, wetting depth and time to runoff, and decreased runoff rate and sediment loss, compared with DDB and CCB. Overall, steady state infiltration rate was controlled over time by the permeability of lower soil horizons. The combination of disc permeameter and rainfall simulation measurements provided a useful description of unsaturated and saturated infiltration under field conditions. The DDR system improved the potential for saturated infiltration, maximized rainfall storage in the surface horizon, and increased the stability of macroporous infiltration.
Soil erosion of agricultural land in Western Australia estimated by cesium-137
Authors: Mcfarlane DJ , Loughran RJ , Campbell BL
Published: April 2005

The caesium-137 technique was used to estimate net soil loss from 10 hillslopes in the agricultural area of Western Australia. The gravel fraction of the soil was found to have approximately 56% of the total 137Cs activity found on the <2 mm fraction of the soil on three slopes where it was measured. In the lower rainfall zones, 137Cs appeared not to have uniformly labelled the soils in uncleared areas, possibly due to the redistribution of rainfall in the canopy and above the soil surface, and the water repellence of some soils. A previously established calibration curve was used to calculate net soil losses from the 10 hillslopes. Potato-growing land and slopes below rock outcrops in the Wheatbelt appear to have experienced the most severe erosion since the soils were first labelled with 137Cs in the mid 1950s (19-27 t ha-1 yr-1). Hillslopes below rock outcrops had appreciably less 137Cs than those below lateritic breakaways, indicating the presence of sheet erosion.
Prediction of unsaturated hydraulic conductivity changes of a loamy soil in different salt solutions by using the equivalent salt solutions concept
Authors: Jayawardane NS
Published: April 2005

Equivalent salt solutions series have been previously defined as solutions with combinations of sodium absorption ratio (SAR) and electrolyte concentration (E,) producing the same extent of clay swelling in a given soil. These equivalent salt solutions series values have yielded satisfactory predictions of changes in saturated hydraulic conductivity, with changes in salt solution composition and concentrations. In the present study, previously published data on changes in saturated and unsaturated hydraulic conductivities of Gilat soil in salt solutions of cationic ratio 0-50 (mmol dm-3)1/2 and electrolyte concentration 2-50 (m.e. dm-3) were used to compare the equivalent salt solution series values for hydraulic conductivities at different water contents. The equivalent salt solution series causing a given change in saturated hydraulic conductivity of a loamy Gilat soil were derived. These equivalent salt solution values were used to predict the unsaturated hydraulic conductivities of this soil at low water contents. Predictions of unsaturated conductivity at relative water contents (�) ranging from 0.80 to 0.20 agreed closely with the measured values. Coefficients log a1 and b1 for Gilat soil, in the equation log Ec = log a1+b1 log SAR, relating the Ec and SAR values of each equivalent salt solutions series were determined at � values between 1.00 and 0.20. The relationship between log a1 and bl was similar at all water contents, in agreement with the equivalent salt solutions concept. Therefore, equivalent salt solution parameters derived from saturated hydraulic conductivity measurements could be used to predict changes in unsaturated conductivities and hence flow rates of saline water under specified boundary conditions.
A hydrofluoric acid dissolution method for determining volcanic glass content of tephra derived soils (andisols)
Authors: Lowe DJ , Green BE
Published: April 2005

The determination of volcanic glass content is essential for the classification of many Andisols in Soil Taxonomy. Previously, polarizing microscope and heavy liquid methods have been used to estimate glass content, but we suggest here an alternative method based on gravimetric analysis of glass-bearing samples before and after hydrofluoric acid (HF) dissolution treatment. A series of Bw and C horizons of weakly developed buried paleosols derived from post-c. 15 ka rhyolitic tephra deposits in North Island, New Zealand, was subjected to the HF treatment and glass contents calculated. Our results, corroborated by qualitative examination using a polarizing microscope, indicate that the HF dissolution method for these materials is simple, fast, and accurate, and, with further testing, could be applicable to all types of glass. Slight modifications to the method are probably needed where glass compositions or primary mineralogical assemblages vary significantly between samples. As well as being potentially useful for characterizing and defining Andisols, the method is likely to be applicable wherever estimates of the volcanic glass content of soils, paleosols, or tephra deposits are required.
Genesis of podzols on coastal dunes in southern Queensland .III. The role of aluminum organic complexes in profile development
Authors: Skjemstad JO
Published: April 2005

Organic matter was extracted sequentially with 0.1 M HC1, 0.5 M HCl and 0.5 M NaOH from three soil samples consisting of the dark brown organic nodules and matrix material from the Bhs2 horizon and the untreated Bhs3 horizon material of a freely drained podzol from Cooloola. The NaOH extract was further divided into fulvic and humic acid. After extracts were separated on G-50 Sephadex gel, titration data demonstrated that carboxyl groups in the collected fractions ranged from 8.8% to 61.3% of the total carbon although some overestimate of the carboxyl content may result from the chromic acid method used. Fractions excluded by the gel made up >95% of each extract and the first fraction collected in each case was the lowest in carboxyl content. These fractions from the 0-1 M HC1 extracts were generally low in carboxyl content (8.8%-24.9%) as were those from the humic acids (10.7-11.2%) although the major remaining humic acid fractions were extremely high (58.6-61.3510). The 0.5 M HCl and fulvic acid extracts appeared similar in chemical properties with carboxyl content of the separated fractions ranging from 16.4 to 44.0%. Molar absorptivities were in the order 0.1 M HCl < 0.5 M HC1< fulvic acid < humic acid. The pKa values of the acids were found to increase in the order humic acid < fulvic acid = 0.5 M HCl < 0.1 M HCl as well as in the order Bhs2 (dark brown nodules) < Bhs2 (yellow brown bulk) < Bhs3 for each fraction. Ease of flocculation of the extracts by aluminium counter ions decreased in the order humic acid > fulvic acid > 0.5 M HCl > 0.1 M HCl. It was demonstrated that other organic extracts low in aluminium content could be used to remobilize flocculated humic acid through redistribution of the aluminium bound to the precipitated phase into the solution phase. From these and other data, a hypothesis explaining the specific manner in which organic matter is arrested during podzolization is proposed. Some major factors appear to be (a) the pH of the horizon, (6) pKa and aromaticity of the organic acids and (c) availability of aluminium to the organic fractions. Remobilization of precipitated organic matter requires the presence of organic acids of low aluminium content in the percolating soil solution. This hypothesis adequately describes the process by which distinct Bh, Bhs and Bs horizons are formed and evolve during profile genesis.
Genesis of podzols on coastal dunes in southern Queensland .IV . Nature of the organic fraction as seen by 13C nuclear magnetic resonance spectroscopy
Authors: Skjemstad JO , Waters AG , Hanna JV , Oades JM
Published: April 2005

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.