G. S. P. Ritchie

The Role of Organic Matter in Soil Acidification

The pH and buffer capacity of two soils increased or remained constant after incubation with different amounts of plant material (lucerne cham at field capacity and when air dry. For both soils, the pH changes were greater at field capacity, whereas the buffer capacities were independent of the water treatments. The pH changes observed could be explained in terms of the organic anion concentration of the plant material. The results indicate that the initial soil pH and the anion concentration (i.e. the per cent dissociation of soluble organic acids when released into the soil) determine the acidifying effect of organic matter.

Phosphorus leaching in sandy soils. I. Short-term effects of fertilizer applications and environmental conditions

The consequences of previous as well as current environmental conditions and management practices on the potential for phosphorus (P) to be lost by drainage from sandy soils in the short term (< 1 year) were studied in the laboratory and the field. The potential for P losses by drainage was estimated by measuring soil solution P levels and rapidly released P. Rapidly released P was measured by determining the concentration of dissolved inorganic P contained in filtered (<0.45 pm) soil solutions after incubating soil at saturation for 15 min at ambient temperature. In the laboratory, sandy soils were incubated with ordinary superphosphate, coastal superphosphate (a granulated mixture of equal parts of superphospate, rock phosphate and elemental sulfur) or lime-superphosphate (a lime-reverted superphosphate with 18% kiln dust) and sequentially desorbed with deionized water. The effects of the extent of leaching, fertilizer type, application rate and the time of contact with the soil on soil solution P levels were investigated. The influence of annual pasture death and summer rainfall on rapidly released P in soils that had been pre-treated by leaching were also investigated. Phosphorus concentrations decreased logarithmically in the successive supernatants of the sequentially desorbed soils. More P was desorbed from soils incubated with superphosphate and lime-superphosphate than soil incubated with coastal superphosphate. At each level of pre-leaching, the P concentrations in the soil solution increased with increasing time. The level, to which the P concentration in the soil solution increased at each time, decreased with increased extent of pre-leaching. The addition of P fertilizers increased the concentration of P in the soil solution. The concentrations increased with increasing application rate and were much higher for superphosphate than for coastal superphosphate; however, there was little effect of contact time on soil solution P levels. Rapidly released P levels after leaching increased during a period of no further leaching. Additional moisture or plant material during this period of no further leaching increased the rate and extent to which rapidly released P increased. Monitoring of rapidly released P in the 0-2, 2-5, 5-10 and 10-20 cm layers of field plots, with and without applications of superphosphate, showed that sampling depth, water flow path, fertilizer management, rainfall pattern and background P levels would affect the estimate of short-term P losses. Rapidly released P in the 0-2 cm layer varied markedly with time and was higher (P < 0.05) than that in lower soil layers. Rapidly released P increased after the winter and spring rains diminished and then decreased after the rains commenced again at the end of the summer. A possible annual cycle of P in sandy soils in a mediterranean climate is postulated by considering the laboratory and field data in combination.

Calcium Chloride Extractable Cadmium as an Estimate of Cadmium Uptake by Subterranean Clover

Cadmium (Cd) may accumulate in soils which have been regularly fertilized with phosphate fertilizers which contain Cd originating in rock phosphate. Soil was taken from three sites in the wheatbelt of Western Australia which were estimated to have received different amounts of phosphate fertilizer over the past decade. The pH was adjusted with dilute HCl or CaCO3. No Cd was added experimentally. The concentration of Cd in the whole tops of Trifolium subterraneum cv Mt Barker grown in a glasshouse pot experiment increased from 0-2-0.8 µg g-1 dry wt at pH 6 -6-6.9 (1:50-01 M CaCl2) to 2-4 µg g-1 at pH 4.1-4.2. The highest concentration of Cd in the plant tops at any particular pH occurred on the soil which had the highest concentration of P in the CaCl2 extract. There was a linear relationship between the concentration of Cd in the whole tops of sub-clover and the concentration of Cd in the CaCl2 extracts which was independent of site. The concentration of Cd in the CaCl2 extracts was a function of pH and concentration of P in the CaCl2 extract.

The Influence of pH on the Forms of Cadmium in Four West Australian Soils

The forms of cadmium in soils affect its uptake by plants and hence its potential toxicity to animals and humans. We studied the effect of pH on the forms of native and added Cd in four West Australian soils which differed in their clay, hydrous oxide and organic matter content. The forms of Cd were extracted sequentially by KCl, BaCl2, NaOCl, ammonium oxalate and concentrated acids. The majority of Cd applied to a sandy soil was found in the soluble (KCl) and the exchangeable (BaCl2) forms at all pH values. In the siliceous sand, the proportion of the Cd present in the exchangeable form increased as the soil solution pH increased. However, in the peaty sand, the opposite trend was observed; at pH 5, approximately 50% of the Cd was found in the exchangeable form, while at higher pH values, 5, the majority (90%) of it was extracted in these forms. Soils, containing clay (mainly kaolinite) as the major adsorbent, retained Cd mainly in exchangeable form at all pH values and at all the rates of Cd application. At pH > 5, however, some of the Cd was also found in the residual form and bound to organic matter. This work has shown that the form of added Cd in a soil cannot be elucidated by considering the major adsorbing component alone. It is also necessary to know the pH, the presence of other adsorbing surfaces and the rates of applied Cd.

Estimates of soil solution ionic strength and the determination of pH in West Australian soils

The average ionic strength of 20 West Australian soils was found to be 0.0048. The effects of three electrolytes (deionized water, CaCl2 and KNO3), three ionic strengths (0.03, 0.005 and soil ionic strength at field capacity, Is) and two soil liquid ratios (1:5 and 1:10) on the pH of 15 soils were investigated. pH measurements in solutions of ionic strength 0.005 differed the least from measurements made at Is. The differences that occurred in comparisons with distilled water or CaCl2 of ionic strength 0.03 (0.01 M) were much greater (20.4 pH units). An extractant with an ionic strength of 0.005 may provide a more realistic measure of pH in the field than distilled water or 0.01 M CaCl2 for West Australian soils.

Changes in the Forms of Cadmium with Time in Some Western Australian Soils

Changes in the forms of Cd with time could affect its uptake by plants and hence potential toxicity to animals and humans. The effect of time on the forms of native and added Cd was studied in four West Australian soils which differed in their clay, hydrous oxide and organic matter content. Sequential extraction of soluble (KCl), exchangeable (BaCl2), bound to organic matter (NaOCl), bound to oxides/clays (ammonium oxalate) and residual (concentrated acids) forms of Cd was carried out at different time intervals after the addition of Cd. The Cd that was added to the soils transformed with time to less soluble forms; the extent depending upon the type of soil. In addition, the rate of transformation in a particular type of soil was affected by both pH and rate of Cd addition. Soluble cadmium in the sandy soil decreased with time whereas the exchangeable form increased. The extent of the changes increased with increase in pH. In the peaty sand at pH 6, the exchangeable form of Cd decreased whereas Cd bound to organic matter and residual Cd increased with time. In the yellow earth (dominated mainly by goethite), soluble Cd decreased with time at pH values 5 whereas Cd bound to oxides and residual Cd increased with time at all the pH values.

The long term fate of copper fertilizer applied to a lateritic sandy soil in Western Australia

A soil copper fractionation was carried out on soils sampled from plots in a long-term copper fertilizer trial on a lateritic sandy soil in Western Australia. At copper application rates up to 8.25 kg copper sulphate ha-1, a high proportion of the applied copper was initially associated with the soil organic matter. During the course of the trial (20 years), a substantial proportion of this copper became redistributed to a residual soil fraction, i.e. the residue remaining after extractions to remove organic matter and iron oxides. However, significant redistribution of copper with time was not detected in plots with a higher rate of copper application (19.25 kg copper sulphate ha-1). The change in distribution of copper at the lower copper application rates appeared to be only partly responsible for a corresponding decrease observed in EDTA-extractable soil copper during the trial. The changes with time in the nature of fertilizer copper applied to this soil are considered to be responsible for the previously observed decline in plant availability of such copper.

Soluble aluminium in acidic soils: Principles and practicalities

Our ability to predict toxic quantities of aluminium (Al) in acidic soils is limited by our understanding of the interactions between different solid forms of Al in solution and our lack of knowledge of which form control soluble Al. This review briefly considers each type of solid form of Al, particularly from a kinetic point of view and discusses models that have been developed to predict release of Al from individual forms. More comprehensive models (i.e. more than one source or sink of Al) are then discussed as well as the interactions between different solid sources of Al.

A Soil Test for Aluminium Toxicity in Acidic Subsoils of Yellow Earths in Western Australia

Many of the yellow earths in the Western Australian wheatbelt have naturally acidic subsoils which can reduce the yield of wheat grown on them. Current methods of assessing soil acidity cannot identify which soils have subsoil acidity severe enough to restrict wheat yields. We conducted 53 field experiments at 34 sites in 5 regions over 3 years to determine the relationship between yield of wheat and several different indices for identifying subsoils with toxic concentrations of aluminium, Al. Initially, we identified that the concentration of aluminium, [All, in the soil solution and in 1 : 5 0.005 M KCl extracts of soil from the 15-25 cm layer was responsible for the majority of the decrease in wheat yield. The concentration of Al in a 1 : 5 0.005 M KCl extract in the 15-25 cm layer was well correlated with grain yield of wheat grown on yellow earth soils in the Merredin region, provided the soils had similar fertilizer treatments. The ratio [All : [Na] in a 1 : 5 0.005 M KCl extract was a better predictor than [All alone of grain yield of wheat grown on yellow earths in different regions and with different fertilizer practices. The three seasons had little effect on the correlation between yield and different soil indices. The correlations determined were strongly affected by regional differences, which were probably due to differing water supply and availability. The choice of toxicity index depended on the uniformity of fertilizer management practices within a region and it appeared that both ionic strength and calcium were important mitigating factors.

Phosphorus Leaching in Sandy Soils. II.* Laboratory Studies of the Long-term Effects of the Phosphorus Source

Long-term phosphorus (P) losses and gains in sandy soils continuously fertilized with either ordinary superphosphate or coastal superphosphate (a granulated mixture of superphosphate, rock phosphate and elemental sulfur) or previously fertilized with superphosphate were investigated under leaching conditions in columns in the laboratory. The soils were subjected to 10 consecutive cycles designed to simulate the mediterranean weather conditions in the Harvey region of the Coastal Plain of Western Australia. Each cycle consisted of a wet phase during which the equivalent of 850 mm of rainfall was leached through the soil and a drier phase during which the soil was incubated in the presence of moisture equivalent to summer rainfall (150 mm). Dissolved inorganic P in the leachate was used as a measure of P loss. A sequential fractionation procedure (a resin extraction followed by 0.5 M sodium bicarbonate, 0.1 M sodium hydroxide and 0.1 M sulfuric acid extractions) and total inorganic and organic P were used to measure changes in P levels in the soils. Phosphorus losses from the previously fertilized soils decreased logarithmically with increasing number of cycles. Total inorganic P and resin-extractable P were able to explain >94% of the variation in P losses. Addition of either fertilizer increased the amount of P leached from the soil and 10-40% more P was leached by adding superphosphate rather than coastal superphosphate. The percentage of the cumulative P lost by leaching decreased with increasing application rate of both fertilizers when expressed as a percentage of the cumulative water plus citrate-soluble P added. Addition of either fertilizer increased the amount of acid-extractable P, but coastal superphosphate had a much greater effect than superphosphate. Leaching losses of P were influenced by fertilizer solubility in the short term (< 1 year). In the long term, however, the water plus citrate-insoluble P in the fertilizers also contributed to P losses by leaching.