David Halliwell

A review of the effects of wastewater sodium on soil physical properties and their implications for irrigation systems

This paper reviews the effects of wastewater sodium on soil physical properties, particularly with respect to irrigation systems. Fundamental sodicity concepts are examined including (i) sodicity definitions, (ii) the effects of sodium on soil properties, (iii) a discussion of factors that impede the infiltration rate and hydraulic conductivity, (iv) the changes that occur in ionic strength of percolating water in soil, and (v) consideration of the wastewater and soil constituents that modify the effective sodium adsorption ratio. Importantly, the ability for soils to assimilate wastewater over time changes, but these changes are not often considered prior to the planning of such irrigation systems, or after the irrigation systems are operating. The general lack of understanding of sodicity is in part due to the considerable variation in sodicity definitions. Exchangeable sodium percentage (ESP) values that are reported to pose a sodicity problem vary around the world due to the different mineralogy of the soils investigated, but variations in threshold ESP values have also been caused by a lack of consideration of the solution electrolyte concentration when determining ESP. In practice, the effects of sodicity may be evident in soils that are well under reported threshold values. When the effects of sodicity are identified, the landholder at least has the opportunity to implement remediation practices. However, more often than not, the effects of sodium from irrigation water are latent, leading to considerable problems following the cessation of effluent irrigation and changed land use.

Fertilisers and phosphorus loss from productive grazing systems

This paper reviews phosphorus loss from productive high rainfall grazing systems. In particular it describes the processes occurring when phosphatic fertilisers are added to soil, the different pathways through which fertiliser and other nutrient sources may contribute to phosphorus losses, and an evaluation of the management strategies aimed at minimising phosphorus loss. It is now generally accepted that soil is not an endless sink for phosphorus uptake and that at the landscape scale the highest concentrations of phosphorus loss occur in surface runoff, followed by macropore flow and vertical matrix flow. However, loads of phosphorus lost through these pathways are unknown. The development of an understanding of the transport mechanisms and phosphorus species being transported is fundamental to developing management strategies that are effective in decreasing phosphorus losses from grazing systems.

The orthophosphate content of bicarbonate soil extracts

The Olsen P and Colwell P bicarbonate extraction procedures are empirically derived tests that provide an estimate of the soil P that is available for plant uptake. This paper examines each procedure using high performance liquid chromatography with flow injection analysis detection (HPLC-FIA) to specifically measure orthophosphate in bicarbonate extracts. Extract solutions from 3 soils of contrasting plant-available soil P contents were analysed for orthophosphate, total filtered ( 0.05), suggesting the Olsen P test was a good measure of orthophosphate, the most immediately plant-available form of P. However, the average amount of P extracted by the Colwell procedure was 14% higher (P < 0.01) than the corresponding average orthophosphate concentration, presumably due to the presence of labile organic/condensed P, colloidal associated orthophosphate, or high molecular weight reactive P. These results suggest that there is a pool of non-orthophosphate P present in the molybdate reactive Colwell P extract that is potentially plant available. Future work should focus on specific identification of P compounds extracted from soils and soil solutions to examine their role as a source of P to plants.

A field study of phosphorus mobilisation from commercial fertilisers

Fertilisers contribute to phosphorus (P) exported from agricultural catchments in south-eastern Australia. Phosphorus concentrations were initially measured in overland flow caused by rainfall after broadcasting either single superphosphate [SSP, Ca(H2PO4)2] or diammonium phosphate [DAP, (NH4)2HPO4] blends to pastures. In addition, P concentrations in overland flow were measured at intervals down border irrigation bay before and after fertiliser application. The period between fertiliser application and irrigation varied from 1 to 10 days. For the rainfall-induced overland flow, total dissolved P (TDP) concentrations were higher where DAP rather than SSP had been applied. For the irrigation study, sampling position behind the wetting front, irrigation pre and post fertiliser application, and irrigation number post fertiliser application explained 49.7, 20.5, and 15.2% of the total sum of squares, respectively. TDP concentrations were highest in the wetting front and diminished with distance behind the wetting front. For the irrigation before, and 2 irrigations following, fertiliser application, concentrations in the wetting front were 2.3, 17.6, and 6.5 mg TDP/L, respectively. In general, wetting front concentrations were c. 4 times the mean concentrations for the bays. As most P is exported when the wetting front enters the drainage network, sampling behind the wetting front would appear to underestimate P exports. The TDP concentration decreased as the time between fertiliser application and irrigation increased but the effects were variable between farms and fertilisers. Contrary to the rainfall induced overland flow study, in the irrigation study higher TDP concentrations were measured where SSP rather than DAP had been applied. This finding is explained in terms of differing rates of P mobilisation from the 2 fertiliser blends and an interaction with soil hydrology. The rapidly infiltrating water at the wetting front of irrigation-induced overland flow is likely to carry with it P mobilised at, or near, the soil surface and P infiltration will be proportional to mobilisation rates. It is suggested that higher rates of P mobilisation from DAP than SSP would reduce P exports in border irrigation systems where DAP is applied to the soil.

Separation and detection of condensed phosphates in waste waters by ion chromatography coupled with flow injection

A method for the separation and detection of condensed phosphates in domestic waste waters using unsuppressed ion chromatography coupled with post-column flow injection detection of phosphorus is reported. The advantages of this technique over previously reported methods are its short analysis time (approximately 12 min per sample) and high sensitivity. The method was linear over the concentration range 10–1000 µg l–1 P for orthophosphate and 20–2000 µg l–1 P for pyrophosphate and triphosphate using a 500 µl sample loop. Detection limits for PO3–4, P2O4–7 and P3O5–10 were 10, 20 and 20 µg l–1 P, respectively. Recoveries of pyrophosphate and triphosphate for spiked filtered (0.22 µm) raw sewage samples were between 95 and 102% over the concentration range 50–1000 µg l–1.

A laboratory study of phosphorus mobilisation from commercial fertilisers

Phosphorus (P) exported from pastures following fertiliser application contributes to the nutrients and associated problems in the streams and rivers of south-eastern Australia. This laboratory study examined whether attributes of P fertilisers may affect P exports soon after their application to field soils; 3 commercial fertilisers [diammonium phosphate (DAP), single superphosphate (SSP), and sulfur-coated single superphosphate (CSSP)] were applied to 2 repacked soils (Arawata and Ellinbank) at 5 moisture contents. Soil type was the most important factor affecting water-extractable P (expressed as a percentage of the P added as fertiliser), accounting for 30% of the total variation. The majority of this variation is explained by the water-extractable P concentrations in the Arawata low moisture treatments. These treatments [7, 6, and 6% soil moisture when equilibrated at 99, 95, and 86.5% relative humidity (RH), respectively] contained water-extractable P concentrations c. 3 times higher than the high moisture (c. 20 and 25% soil moisture) or the Ellinbank treatments. This result probably reflects differences in soil properties including the extent of water repellency and P adsorption. Fertiliser type explained only 6.9% (P CSSP > SSP if overland flow occurs soon after their application.

Hydrolysis of triphosphate from detergents in a rural waste water system

The concentrations of detergent phosphates in raw sewage entering a small, predominantly domestic waste water treatment facility were determined using an ion chromatographic-flow injection analysis technique. Hourly loads of detergent phosphates were measured between 0600 and 2300 hrs (the major flow period in the plant) on days of both low and high phosphorus loads. The calculated loads of detergent phosphorus entering the plant on low and high load days were 260 g P/day and 350 g P/day, respectively. The half-life of detergent phosphates (triphosphate) in waste waters was measured to be 7.3 hours at 15 degrees C and 3.0 h at 20 degrees C. The major factor contributing to triphosphate degradation in waste water was shown to be biological in nature, with the most likely mechanism being enzymatic hydrolysis.

Inorganic Monophosphate Determination in Overland Flow from Irrigated Grazing Systems

Abstract This paper presents a novel technique for measuring soluble inorganic monophosphate in water based on high performance liquid chromatography and flow injection detection (IC-FI). The IC-FI technology was applied to irrigation water from the Macalister Research Farm in South Eastern Australia. Overland flow was measured exiting two irrigation bays. Inorganic monophosphate constituted over 97% of the DRP component and between 86–93% of the total phosphorus. Similar results were obtained from a channel that drained the excess irrigation water from the farm. This work supports the findings of others that show DRP contributes a large portion of the P in runoff from grazing systems. Additionally, it confirms this fraction is dominated by immediately bioavailable inorganic monophosphate, something that previously could only be inferred due to the limitations of the chemistry applied.

Phosphorus and selected metals mobilized from two commercial fertilizers into overland flow during border irrigation

Metals, present as impurities, may be used to trace phosphorus (P) derived directly from fertilizers in field studies. This paper reports a study that compared P to strontium, copper, zinc, cadmium, lead, chromium and nickel concentrations in overland flow from border irrigated fields, before and after fertilization with commercial diammonium phosphate (DAP), or single superphosphate (SSP) blends. The concentrations of P and various metal species in overland flow were within the guidelines for potable water. However, with the possible exception of chromium, all samples, including pre-fertilization samples, were outside the trigger values for Australian and New Zealand streams. Changes in the cadmium and strontium concentrations were related to changes in the total dissolved P (TDP) concentrations following fertilization. This suggests that these metals may be used for tracing P derived directly from fertilizers. However, the background concentrations of cadmium and strontium in overland flow varied between farms and the decay in P concentrations in the two irrigations following fertilizer application were not reflected in cadmium and strontium concentrations. These appear to be major limitations on the use of cadmium and strontium for tracing P derived directly from fertilizers.

Controlling soil water content in fertiliser dissolution experiments

Phosphorus lost in runoff from agricultural land leads to the enrichment of surface waters and contributes to algal blooms. Fertilisers are one source of this P. To compare the water available P of different fertiliser formulations in the laboratory it is necessary to control environmental conditions, temperature, relative humidity and soil water content, prior to simulating rainfall. Two chambers were designed in which relative humidity and soil water content were controlled using salt solutions. An initial design comprising a sealed chamber with three layers of soil samples over a salt bath was found to be inferior to a single layer design. The changes in water content of soil samples were used to test the single layer chamber in a constant temperature environment (15 °C) using a saturated KCl solution (90% relative humidity). Based on the final soil water content of the samples, the spatial variation within the chamber was within tolerable limits. The single layer chamber was used for a simulation experiment comparing the water available P of two commercial fertilisers. Using a saturated resorcinol solution (95% relative humidity) soil samples were equilibrated at 15 °C for 21 days, fertiliser added, and the water available P measured up to 600 h after fertiliser application. The results indicate that the amount of water available P was related to the fertiliser compound and exponentially related to the time since fertiliser application. It was concluded that the single layer chamber is suitable for controlling relative humidity and soil water content in trials such as these where the water available P of fertilisers are being compared.