A Methodology to Estimate Net Proton: Phosphorus Co-Adsorption Ratios for Acidic Soils
- School of Earth, Atmospheric and Life Sciences, University of Wollongong, NSW 2525, Australia
- Hawkesbury Institute for the Environment, Hawkesbury Campus, Western Sydney University, Locked Bag 1797, Penrith, NSW 2751, Australia
- School of Science, Hawkesbury Campus, Western Sydney University, Locked Bag 1797, Penrith, NSW 2751, Australia
Academic Editors: Bill Maher and Victoria Samanidou
Special Issue: Environmental Analytical Chemistry
Received: November 07, 2019 | Accepted: April 07, 2020 | Published: April 13, 2020
Advances in Chemical Research 2020, Volume 2, Issue 2, doi:10.21926/acr.2002005
Recommended citation: Taglieri P, Milham P, Holford P, Morrison RJ. A Methodology to Estimate Net Proton: Phosphorus Co-Adsorption Ratios for Acidic Soils. Advances in Chemical Research 2020;2(2):16; doi:10.21926/acr.2002005.
© 2020 by the authors. This is an open access article distributed under the conditions of the Creative Commons by Attribution License, which permits unrestricted use, distribution, and reproduction in any medium or format, provided the original work is correctly cited.
Phosphorus (P) is a critical element for all living systems, being widely involved in both terrestrial and marine biogeochemical cycles. P is an essential element for all biota, and is involved in cellular energy transport, structural components such as bones, teeth and plant tissues, and reproduction. P is an important component of global food production systems , and most global soils do not naturally contain sufficient P to support more than a few years of intensive crop production. Long-term, high-intensity crop production, needed to support a bourgeoning human population, requires P inputs, usually as manures or fertilisers. Fertilisers are mainly prepared (for the P component) from phosphate rock, a finite resource [2,3,4].
Soils are complex natural combinations of solids which themselves are intimate mixtures of organic and inorganic substances, a soil solution and a soil atmosphere . The solids are bound together into structural units (peds) that vary in size, shape and stability in different soils, and even within any given soil profile. The soil atmosphere and solution fill the voids between the solids (pores), with the relative proportions varying widely over short time frames (rainfall will enter the soil pores to add to the soil solution, displacing the atmosphere; when the rain stops, water will gradually drain or evaporate allowing the soil atmosphere proportion to increase) . Soils also have a living component, adding and removing substances from the solids, solution and atmosphere.
Phosphorus occurs in soils in a number of different forms in the solids, solution and living components of the soil. In the context of this study, most interest is in the behaviour of solution phase orthophosphate species (e.g., H2PO4-), and interactions with the soil solids. Understanding how additions and losses of these species occur is vital for efficient and effective fertiliser use. The processes involved are summarised in the P cycle (Figure 1).
The addition of P materials to soils is aimed at enhancing the soil solution concentration of a soluble form of P that can be readily taken up by plants (mainly H2PO4- and HPO42- ). When P fertilisers are added to soils, a complex set of interactions occurs such that only a proportion of the added P is taken up by the plants. Soluble P entering the soil solution as a phosphate ion, can be:
- precipitated by cations, especially Ca, Al and Fe ions, in the soil solution, as highly insoluble species ;
- adsorbed on the surfaces of soil minerals, especially clay-sized oxyhydroxides of Al and Fe (e.g., gibbsite, goethite, ferrihydrite) and aluminosilicate clays (e.g., allophane, imogolite, kaolinite, smectite), which have a high affinity for phosphate . P adsorption leads to the adsorbed ions contributing to the surface charge of the adsorbing particles ;
- absorbed (taken in) by plant roots ;
- retained in the soil solution as phosphate ions in low concentration.
Figure 1 The soil phosphorus cycle.
The effectiveness of P fertiliser additions depends on these interactions, which are determined by soil properties (e.g., particle size distribution, soil structure, moisture content, mineralogy, organic matter content, pH), the nature of the added P [11,12], and factors like temperature and redox conditions. The presence of several phosphate ion species adds to the complexity of the situation, and their behaviour is also influenced by involvement in a series of chemical equilibria (Figure 2), which are closely linked to the solution pH . Soil solution P is usually a small proportion of the total P content (~0.01%); much more P exists in the soil solids .
Figure 2 (left) pH dependent speciation of orthophosphate in solution; the Y axis indicates relative activity , (right) phosphate equilibrium constants at STP . The phosphate equilibrium system also acts as a buffer of pH, shifting the degree of protonation to resist changes in acidity through consuming or releasing protons following Le Chatelier’s Principle . In natural water bodies, the aqueous concentration of phosphate (5--100 µg.L-1 total P)  is generally too low for this buffering capacity to be significant. However, the initial P concentrations used in the net H+:P co-adsorption study were approximately 1000 times greater. The impact of this buffering is highly relevant to the pH measurements undertaken.
Although P-soil interactions have been studied for over 100 years, we still do not have a sufficient understanding of all the processes occurring to accurately determine the quantity of fertiliser to be used in a given field (soil) for a selected plant and yield. Understanding of the P-soil interactions is also important to avoid adding too much P, since some of the surplus P will remain mobile in the soil and may be moved into adjacent surface waters leading to eutrophication of these waters through processes including algal blooms [16,17,18]. Eutrophication is a widespread global challenge; the annual economic losses from algal blooms in Australia are believed to be between AUD$180--240 million (US$125--175 million), with occurrences in coastal environments , agriculturally significant rivers such as the Murray-Darling , and world heritage habitats such as the Great Barrier Reef .
The bioavailable nutrient concentration of a soil is one of the crucial factors governing its agricultural productivity. In their natural state, Australian soils are heavily weathered due to their age and climate, and are comparatively deficient in nutrients when compared to younger soils of other continents . Australian soil fertility has been greatly improved through the implementation of an extensive annual fertiliser regimes with as much as 5 million tonnes of fertiliser being applied on 50 million hectares of agricultural land in Australia in 2016-17 . The annual additions of P to agricultural soils in Australia have increased dramatically since the 1950s, and are now in a relatively steady state varying only with seasonal fluctuations . For most agricultural soils, P added through fertilisers exceeded the annual removal of P, creating a positive net balance of P and accumulating P in the soil .
While many factors influence the amount of P lost from the soil-plant system, and thus, the efficiency of applied fertiliser, the P adsorption capacity of the soil is one of the most important . This property describes the ability of a soil to remove soluble P from the soil solution and retain it in a bound condition on the surfaces of soil components. Many P adsorption studies have tended towards understanding how sorption capacities vary with changing soil properties (e.g., mineralogical composition [9,27], the effect of organic matter ), and using isotherm equations to create models which accurately quantify the process [17,29].
Many studies have identified a variety of physico-chemical factors affecting the adsorption of phosphate; these include soil moisture content , soil mineralogy, especially oxyhydroxide (Al and Fe) content , pH , soil organic matter content [28,32], and anion competition .
Soil particles are charged, because of ionic substitutions within the (mainly aluminosilicate) minerals forming part of the soil solids (these are usually permanent charges) and variable charges occurring as a result of interactions between surfaces of soil mineral matter and the ions in the soil solution. Positive and negative overall charges are found, with the majority being overall negative charges . As PO43- groups adsorb onto soil particle surfaces, the surfaces will tend to become more negative (Figure 3) unless co-adsorption of positive ions occurs.
Figure 3 Sorption equilibrium of phosphate onto iron oxyhydroxide surfaces.
Prior to the present study, proton (H+) co-adsorption (co-sorption) with P in soils has not been investigated, although Rahnemaie et al.  observed that a pH decrease occurred that corresponded with the quantity of added P for goethite at pH 3.98. The net H+:P co-adsorption ratio for H2PO4- was ~1.6:1, i.e., less than the stoichiometric ratio for the H2PO4- ion itself of 2:1, excess H+ being left in solution causing a pH decrease. If similar behaviour occurred in field soils, this would lead to the addition of P fertilisers causing a decrease in soil pH. It is not accurately known whether inorganic P additions to soils using certain types of fertilisers changed the pH of soil over time, although Schumann  reported that from limited field trials, non-nitrogenous P fertilisers, such as superphosphate, appeared to produce no measureable effect on soil pH.
Both the chemical complexity and biological activity in soils present challenges in producing experimentally valid data for H+:P co-adsorption. Therefore, a significant portion of this study was dedicated to the development of a credible methodology using samples of three soils. The methods were then applied to soil samples from six sites under pasture, two of which had undergone varying degrees of P fertiliser treatment. We also explored other factors potentially impacting estimates of net H+:P co-adsorption.
2. Materials and Methods
The sample of goethite was prepared in-house for general laboratory investigations . It was not synthesised under CO2-free conditions. Samples of nine topsoils were used: three (EMAI, TG1, ULL2) in method development and six for the H+:P co-adsorption studies (KA, FL, MO, MA, D, RO) (Table 1). The six sites were located in New South Wales and South Australia, and bulk topsoil samples (0–15 cm) had been taken prior to this study and amended with varied amounts of triple superphosphate (Ca(H2PO4)2) equivalent to 0–2158 mg P kg-1 . We subsequently sampled a subset of the soil × Ca(H2PO4)2 combinations, cleaned them of macroscopic organic materials, and ground and sieved them to < 1 mm for storage. We further subsampled, ground and sieved them to < 250 µm. The properties are presented in Table 1, and other properties are described in  and .
Soil pH protocols vary, but ratios of 1:5 or 1:10 are common , and we measured the pH of soil suspensions (5 g : 25 mL) shaken with 10 mM CaCl2 solution for 1 h. All pH measurements were made to at least 2 decimal places (Dual Star, Orion Pacific Pty Ltd, Australia). The Ag/AgCl/Cl reference electrode was refillable with ~4 M KCl and fitted with a ring junction (Orion 91578N, Thermo Scientific). Water was ultrapure (MilliQ HX7000, Merck Millipore) and the pH was close to 7. Equipment was soaked in 10% HCl and rinsed until acid-free using pure water before use. Suspension temperatures were maintained at 20.8-21.3oC during pH measurements and batch H+:P co-adsorption studies. Key soil parameters for Al and Fe relating to P adsorption are the quantities extracted by acid oxalate solution . The oxalate extractions were performed in the dark, and the co-extracted Al and Fe were measured by inductively coupled plasma emission spectroscopy .
We used measures of acid:base stoichiometry and of P adsorption to estimate net H+:P co-adsorption ratios for suspensions of goethite, because this material has been studied previously though under strictly CO2-free conditions —and of soils, for which no data appear to exist.
Supernatants from some co-adsorption studies were analysed for potentially competitive species, e.g., dissolved organic carbon (DOC), and sulfur (S), assuming that the S was predominantly present as sulfate. In addition, the cations NH3, Ca, Na and Mg were measured to evaluate the degree to which cation exchange may be buffering H+ release. The results were inconclusive and are not presented here, although they are described in detail elsewhere .
2.2.1 Acid/Base Stoichiometry
Microbial activity is a major threat  to pH stability of soil suspensions, and the effect of the potent antimicrobial, HgCl2, was investigated. Subsamples (4 g) of ULL2 were shaken with 20 mL of 0.01 M KCl background electrolyte with varying additions of HgCl2 for 16 h. When the HgCl2 concentration reached 6.6 mM, the suspension pH before and after 16 h of shaking was within measurement uncertainty, i.e., 4.84 ± 0.01. We consequently used 6.6 mM HgCl2 as part of the background electrolyte throughout the remainder of the study. This concentration of HgCl2 stabilised the pH of suspensions of goethite and soil during shaking for 16 h, whether or not P was added (data not presented).
Acid/base stoichiometry was examined for the background electrolyte and for goethite and soils suspended in it. For goethite, 1 g aliquots were suspended in 19 mL of 10 mM KCl containing 6.6 mM HgCl2 and the pH measured. Then 1 mL of 0.0375 M HCl was added, the suspensions shaken for 16 h, and pH was back titrated to the original value using 0.04 M NaOH. There were four replicates. We also suspended varying masses (0.5–2 g) of ULL2 soil in 19 mL of the same electrolyte and followed the acid:base neutralisation procedure described for goethite. There were eight replicates. Five replicate electrolyte solutions were also taken through the same procedure.
Assessment of acid/base stoichiometry might also be affected by imbalance of the CO2/H2CO3/HCO3- buffer system. To achieve CO2-free conditions, for suspensions of goethite or soil, this requires lowering the suspension pH to (say) 5 to convert bicarbonate to H2CO3 and exhaustive purging, for which CO2-free N2 was used . This procedure is acceptable for synthetic substrates such as goethite, but may affect the properties of the less well-characterised P sorbents in soils; consequently, the acid/base stoichiometry and net H+:P co-adsorption ratio were tested without stripping the HCO3-/CO2, i.e., at the current ambient atmospheric CO2 concentration of ~400 µL L-1 .
We also used initial and final pH measurements, and a measure of pH buffering for the particular adsorbent, to calculate the H+ release resulting from P adsorption, but this proved unreliable and no data are presented.
2.2.2 H+:P Co-Adsorption Protocol
Optimising the estimate of the H+:P molar sorption ratio requires accurate values of both terms in the ratio. For KH2PO4 additions and net H+:P co-adsorption ratios of ~2, this is a conflicted situation. The concentration of adsorbed P is more accurately measured when most of the added P is sorbed; however, this limits the accuracy of back-titration of a potentially small excess of H+. A compromise between these two conflicting demands was found by trial and error . Unsurprisingly then, we added larger amounts of P than would be applied as fertilisers in the field. In addition, the amount of KH2PO4 added was varied both for the one soil depending on the P-fertiliser pre-treatment, and also between soils. In this regard, the RO soil (Red Ferrosol) contained the greatest concentrations of Alox and Feox, and was the strongest P- sorbing of the soils.
To provide information on the effect of the P added species on the measured net H+: P co-adsorption ratio, we also tested additions of H3PO4. The high P adsorption capacity of goethite  eased the preceding conflict. Data for net H+ release and sorbed P are presented where the experimental uncertainty was deemed acceptable.
For goethite, 1 g aliquots were suspended in 20 mL of 10 mM KCl containing 6.6 mM HgCl2 in 25 mL polypropylene, screw capped centrifuge tubes, and the pH measured. Aliquots of 1 M KH2PO4 were added, the tubes were closed and the suspensions shaken end-over-end at 5 revolutions min-1 for 17 h. The final pH was measured and the suspensions back-titrated to the initial pH using carefully standardised 0.04 M NaOH solution. The suspensions were centrifuged at 7000 revolutions min-1 (rcf 5800 x g) and an aliquot of the supernatant was put through a 0.45 µm filter before P measurement (see below). There were five replicates.
For soils, 0.5--2.0 g of soil were suspended in 20 mL of 10 mM KCl solution containing 6.6 mM HgCl2 in 25 mL capacity polypropylene centrifuge tubes, and conditioned by shaking the suspension end-over-end at 5 revolutions min-1 for 1 h. The pH was measured then aliquots of 1 M KH2PO4 in 10 mM KCl were added to different tubes and the suspensions were shaken for a further 16 h, and the final pH was measured. The suspensions were restored to their initial pH by titration using 0.04 M NaOH and centrifuged as described for goethite to provide clear supernatants for P analysis (see below). The number of replicates varied from 2–5.
To test whether the estimated net H+:P co-adsorption ratio was impacted by the species of added phosphate, an additional H+:P co-adsorption test was applied to the RO soil using the preceding protocol except that 7.3 M H3PO4 was substituted for KH2PO4.
Aqueous P concentrations were measured on the clear supernatants using a flow injection analyser (QuikChem 8500, Lachat Instruments, USA) employing a modified version of the Murphy and Riley  molybdenum blue method (molybdate reactive P) measuring absorbance at 880 nm. Calibration standards were prepared in the background electrolyte to minimise matrix effects. Any colloids present would have caused positive bias due to light scattering and soils may release soluble organic phosphate species into solution  some of which may react with molybdate. The extent to which these effects may have occurred was not quantified.
Sorbed P was estimated from the difference between the initial and final measured P concentrations, and the mass: volume ratio. The H+sorbed values measured by titration were adjusted for the separately measured acid:base stoichiometry for goethite and soils, to calculate the net H+:P co-adsorption ratios. The pH changes and the estimated net H+:P sorption ratios are reported.
3. Results and Discussion
3.1 Acid / Base Stoichiometry
The acid:base stoichiometry for 10 mM KCl containing 6.6 mM HgCl2 was 1.02:1 (Figure 4), i.e., there is a ~2% overestimate of H+. This stoichiometric difference may have been due to buffering by bicarbonate. Whatever the cause, the bias was consistent within a system, and we inferred that it would not be unreasonable to adjust measured H+:P co-adsorption ratios on that basis. The corresponding values for goethite and the soil of 1.23:1 and 1.07:1 respectively (Figure 4), were reduced to 1.21:1 and 1.05:1 to adjust for the overestimation.
Figure 4 Acid-base stoichiometry of 10 mM KCl containing 6.6 mM HgCl2, and of soil ULL2 and goethite suspensions in the same electrolyte. The boxes represent the 25th and 75th percentiles, the lines within the boxes, the median and the whiskers represent the 10th and 90th percentiles.
3.2 Net H+:P Co-Adsorption Ratio for KH2PO4 and Goethite
The initial pH of the goethite suspensions in the background electrolyte was 3.91–3.94. After reaction with added P for 16 h, the net co-adsorbed quantities of H+ and P were linearly related (P < 0.001) with a slope of 1.74 (standard error ± 0.03, Figure 5). Adjustment for the net 1.21 acid: base stoichiometry for goethite suspensions (Section 3.1) gives a value of 1.44:1 for the adjusted H+:P co-adsorption ratio, which is in reasonable agreement with the value of 1.6:1 obtained in a CO2-free environment at pH 3.98 . The relatively small disparity between the studies indicates that our H+:P co-adsorption protocol may also produce useful estimates of the net H+:P co-adsorption ratio for soils.
Figure 5 Molar sorption of H+ and P (µ moles) by goethite for additions of KH2PO4.
3.2.1 Net H+:P Co-Adsorption Ratios for KH2PO4 and Soils
The measured H+:P co-adsorption ratio for soils was 2.02 (± 0.02):1 for the 26 soil × P fertiliser combinations tested (Figure 6). The relation was linear and very strong (r2 = 0.997, P = 0.001). Adjustment of the slope for the measured acid: base stoichiometry of 1.05:1 (Figure 4) gave a H+:P co-adsorption ratio of 1.92:1. That is, where sufficient acid was released to provide confidence in the accuracy of the titration using 0.04 M NaOH, the H+:P co-adsorption ratio appears independent of the soil × P fertiliser combinations. The value of 1.92:1 for soils is well in excess of the similarly adjusted ratio of 1.44:1 for goethite (Section 3.2).
Figure 6 Sorption of H+ and P by 26 soil × P fertiliser combinations for additions of KH2PO4.
3.2.2 Net H+:P Co-Adsorption Ratio for H3PO4 and the RO Soil
An operational benefit in substituting H3PO4 for KH2PO4 is that for a H+:P co-adsorption ratio near 2:1, there was a more substantial balance of H+, and a correspondingly larger and more accurate back titration using 0.04 M NaOH. When applied to the RO soil, the measured net H+:P co-adsorption ratio was 2.06 (±0.09):1 (Figure 7). Adjustment for acid:base stoichiometry of 1.05, (Section 3.2) gives a ratio of 1.96:1. We consequently infer that, within experimental limits, addition of P as either H2PO4 or H3PO4 did not affect the net H+:P co-adsorption ratio. Therefore, the use of H3PO4 might provide a means to improve the typical measurement accuracy of the H+ balance and thereby increase the accuracy of H+:P co-adsorption ratios for acidic soils.
Figure 7 Sorption of H+ and P by soil RO for additions of H3PO4.
4. General Discussion and Recommendations
The protocol we developed gives a credible (adjusted) net H+:P co-adsorption ratio of 1.44:1 for goethite; the published value is 1.6:1 . By analogy, we argue that the ratio of 1.92–1.96:1 we obtained for acidic soils is also likely to be credible. This ratio appears independent of the diverse soil × P-fertiliser combinations of our samples (Table 1) and of the form of added P (KH2PO4 or H3PO4). That is, the results indicate that similar P-ligand soil surface complexes were formed under a wide variety of conditions.
Soil system chemistry is extremely complex, and soils contain a selection of P-sorbents, including a variety of clay minerals as well as organic materials . Goethite is only one of the many P-sorbents in soils, e.g. Al substituted goethites occur  and are strong P sorbents . In acidic soils, dissolution of Al (mostly as Al(OH)2+ and Al(OH)2+) can occur and sorb or precipitate P sufficiently to influence its rapid adsorption by soils . Consequently, it is unsurprising that the (adjusted) net H+:P co-adsorption ratio for soils differs from that for goethite.
The developed protocol is a credible first step towards quantitatively estimating net H+:P co-adsorption ratios for acidic soils; nonetheless it may benefit from refinements. For example, substitution of H3PO4 for KH2PO4 increased the size of the back-titration, and therefore presumably its accuracy and, consequently, the accuracy of the estimated net H+:P co-adsorption ratios. HgCl2 is an effective microbial suppressant, but a less toxic antimicrobial would be desirable. It is possible that less potent antimicrobials may suffice in combination with pasteurisation of the soil suspensions [42,50]. Other factors worthy of investigation include: 1) eliminating HCO3- and CO2 from the system, which may interact with suspension pH to affect acid:base stoichiometry; 2) the effect of added K+ (as KH2PO4)—though the apparent indifference of the measured net H+:P co-adsorption ratios to additions of KH2PO4 and H3PO4—argues that any such effect may be relatively small; 3) our samples contained small amounts of organic fragments whose impact on soil pH  was unknown ; 4) measuring anions competitively desorbed by the P additions, e.g., sulfate [53,54,55]; and, 5) H3PO4 additions may allow ΔpH values to conveniently access net H+ balance in soil suspensions.
Soil acidity is a critical issue pertaining to agriculture within an Australian context  and also more widely [56,57]. However, in applying the knowledge gained from this study, several limitations need consideration. Firstly, key differences include the fine grinding required for replication in the laboratory , which not only destroys the natural soil structure exposing a greater surface area, but also disperses residual plant fragments. Secondly, sufficiently accurate acid/base measurement necessitated greater additions of P than used to fertilise soils, and the chemical forms of P added in the laboratory are unlike those typically used as fertilisers ; nonetheless, any P solubilised from these materials is likely to be as orthophosphate . That said, prior amendment of the RO soil with large additions of Ca(H2PO4), or triple superphosphate, had little if any effect on pH (Table 1). Lastly, soil atmospheres can have instantaneous changes in % concentrations of CO2.
The authors thank the NSW Department of Primary Industries for access to field soils and soil samples with varying P contents. The Department also provided some general data on soil properties. The GeoQuEST Research Centre from the University of Wollongong provided funding for chemicals, transport and some external laboratory analyses. We thank Anthony Haigh (Western Sydney University) for useful discussions about the manuscript.
Paul Taglieri contributed to project design, carried out most of the experimental work and made a significant contribution to writing the first draft of this paper.
Paul Milham played a major role in the concept development, project design, and data interpretation, and contributed to the manuscript writing.
Paul Holford contributed to the data interpretation and the manuscript writing.
John Morrison contributed to the project design and laboratory work planning and coordinated the preparation and writing of the manuscript.
Funding for parts of this project (limited travel, chemicals and some external laboratory analyses) was provided by the University of Wollongong GeoQuEST Research Centre and the School of Earth and Environmental Sciences (now incorporated into the School of Earth, Atmospheric and Life Sciences).
The authors have declared that no competing interests exist.
- Gilbert N. The disappearing nutrient. Nature 2009; 461: 716-718. [CrossRef]
- Esler JJ, Bennett EM. A broken biogeochemical cycle. Nature. 2011; 478: 29-31. [CrossRef]
- Filipellli GM. Phosphate rock formation and marine phosphorus geochemisty: The deep time perspective. Chemosphere. 2011; 84: 759-766. [CrossRef]
- George TS, Hinsinger P, Turner BL. Phosphorus in soils and plants—facing phosphorus scarcity. Plant Soil. 2016; 401: 1-6. [CrossRef]
- Tan KH. Environmental Soil Science. 3rd ed. New York: Marcel Dekker Inc.; 2000. 557 p.
- Buol SW, Southard RJ, Graham, RC, McDaniel PA. Soil Genesis and Classification. 5th ed. New Jersey, USA: John Wiley & Sons; 2011. pp. 543. [CrossRef]
- Mengel K, Kirkby EA. Principles of Plant Nutrition, Bern, International Potash Institute: 1987.
- Semple EC, Roper RJ, Racz GJ. Reactions of phosphate fertilisers in soils. Madison: ASA-CSA-SSSA; 1980. pp. 263-311.
- Gerard F. Clay minerals, iron/aluminium oxides, and their contribution to phosphate sorption in soils—a myth revisited. Geoderma. 2016; 262: 213-226. [CrossRef]
- Gangaiya P, Morrison RJ. A review of the problems associated with applying the terms surface charge and zero point of charge to soils. Comm Soil Sci Plant Anal. 1987; 18; 1431-1451. [CrossRef]
- Nash D, Hannah M, Clemow L, Halliwell D, Webb B, Chapman D. A laboratory study of phosphorus mobilisation from commercial fertilisers. Aust J Soil Res. 2003; 41: 1201-1212. [CrossRef]
- Nash D, Hannah M, Clemow L, Halliwell D, Webb B, Chapman D. A field study of phosphorus mobilisation from commercial fertilisers. Aust J Soil Res. 2004; 42: 313-320. [CrossRef]
- Barrow NJ. The effects of pH on phosphate uptake from the soil. Plant Soil. 2017; 410: 401-410. [CrossRef]
- Krasicka-Cydzik E. Anodic layer formation on titanium and its alloys for biomedical applications. Rijeka, Croatia: InTech; 2012. pp. 175-200. [CrossRef]
VanLoon GW, Duffy SJ. Environmental chemistry: A global perspective. 4th ed.
Oxford: Oxford University Press; 2017. pp. 585.
- Correll DL. The role of phosphorus in the eutrophication of receiving waters: A review. J Environ Qual. 1998; 27: 261-266. [CrossRef]
- Carlson-Perret NL. The effect of previous phosphorus additions to soils on phosphorus sorption and related parameters. MEnvSc Thesis. Wollongong: University of Wollongong; 2012.
- Jiao P, Xu D, Wang S, Zhang T. Phosphorus loss by surface runoff from agricultural field plots with different cropping systems. Nutr Cycling Agroecosyst. 2011; 90: 23-32. [CrossRef]
- Atech Group. Cost of Algal Blooms. Canberra, ACT: Land and Water Resources Research and Development Corporation; LWRRDC Occasional Paper 26/99; 42 pp.
- Davis JR, Koop K. Eutrophication in Australian rivers, reservoirs and estuaries - a southern hemisphere perspective on the science and its implications. Hydrobiologia. 2006; 559: 23-76. [CrossRef]
- Brodie JE, Devlin M, Haynes D, Waterhouse J. Assessment of the eutrophication status of the Great Barrier Reef lagoon (Australia). Biogeochemistry. 2011; 106: 281-302. [CrossRef]
- Baldock JA, Balks MR, Camps Arbestain M, Condron LM, Elder-Ratutokarua M, Grundy MJ, et al. Regional Assessment of Soil Change in the Southwest Pacific; Chapter 15 in Status of the World’s Soil Resources Report. Rome; FAO and Intergovernmental Technical Panel on Soils; 2015. pp. 478-519.
- ABS. Land Management and Farming in Australia, 2016-17. Canberra, ACT; Catalogue No. 4627.0. [Internet] Australian Bureau of Statistics; 2018. [2019 08 20]. Available from: http://www.abs.gov.au/ausstats/abs@.nsf/Lookup/4627.0main%20features82016-17.
- McLaughlin MJ, McBeath TM, Smernik R. Stacey SP, Ajiboye B, Guppy C. Increasing P use efficiency by fertiliser management and design. Agric Sci. 2013; 25: 40-46.
- Weaver DM, Wong MTF. Scope to improve phosphorus (P) management and balance efficiency of crop and pasture soils with contrasting P status and buffering indices. Plant Soil. 2011; 349: 37-54. [CrossRef]
- Milham P, Dougherty WJ, Morrison RJ, Clark R, Smernik R, Doolette A, et al. Understanding phosphorus in the environment. Chem Aust. 2012; 26-29.
- Daly K, Styles D, Lalor S, Wall DP. Phosphorus sorption, supply potential and availability in soils with contrasting parent material and soil chemical properties. Eur J Soil Sci. 2015; 66: 792-801. [CrossRef]
- Yu W, Ding X, Xue S, Li S, Liao X, Wang R. Effects of organic-matter application on phosphorus adsorption of three soil parent materials. J Soil Sci Plant Nutr. 2013; 13: 1003-1017. [CrossRef]
- Dandy AJ, Morrison RJ. Phosphate sorption isotherms of some South Pacific soils. NZ J Sci. 1980; 23: 399-406.
- Baskaran S, Bolan NS, Rahman A, Tillman RW, Macgregor AN. Effect of drying of soils on the adsorption and leaching of phosphate and 2,4-dichlorophenoxyacetic acid. Aust J Soil Res. 1994; 32: 491-502. [CrossRef]
- Milham PJ, Dougherty WJ, Vimpany I, Bradley J, Burkitt LL, Nicholls PJ, et al. Soil Res. 2020.
- Shi Z, Li F, Yao S. Effect of small organic acid anions on the adsorption of phosphate anions onto synthetic goethite from aqueous solution. Adsorpt Sci Technol. 2010; 28: 885-893. [CrossRef]
- Lindegren M, Persson P. Competitive adsorption between phosphate and carboxylic acids: quantitative effects and molecular mechanisms. Eur J Soil Sci. 2009; 60: 982-993. [CrossRef]
- Rahnemaie R, Hiemstra T, van Riemsdijk WH. Geometry, charge distribution, and surface speciation of phosphate on goethite. Langmuir. 2007; 23: 3680-3689. [CrossRef]
- Schumann B. The causes of soil acidity. Sydney. NSW Agriculture: 1999. http:// www.dpi.nsw.gov.au/data/assets/pdf_file/0009/167175/acidity-causes.pdf.
- Atkinson RJ, Posner AM, Quirk JP. Adsorption of potential-determining ions at the at the ferric oxide-aqueous electroltte interface. J Phys Chem. 1967; 71: 550-558. [CrossRef]
- Dougherty WJ, Mason SD, Burkitt LL, Milham PJ. Relationship between phosphorus concentration in surface runoff and a novel soil phosphorus test procedure (DGT) under simulated rainfall. Soil Res. 2011; 49: 523-528. [CrossRef]
- Choruk K, Morrison RJ. unpublished data.
- Isbell RF. The Australian soil classification. 2nd ed. Canberra: Australian Soil and Land Survey Series, Division of Soils, CSIRO; 2016. pp. 152.
- Rayment GE, Lyons DL. Soil chemical methods. Australasia, Collingwood, Victoria, Australia: CSIRO Publishing; 2011. pp. 495. [CrossRef]
- Taglieri P. Soil Phosphorus Co-adsorption Studies and the Implications for Fertiliser Recommendations. BEnvSc Honours Thesis, University of Wollongong, 2014. pp. 102.
- Alexander M. Soil Microbiology. 2nd ed. New York: John Wiley and Sons; 1977. pp. 475.
- ESRL. Trends in Atmospheric Carbon Dioxide. Earth System Research Laboratory Global Monitoring Division https://www.esrl.noaa.gov/gmd/ccgg/trends/. Last viewed October 7, 2019.
- Murphy J, Riley JP. A modified single solution method for the determination of phosphate in natural waters. Anal Chim Acta. 1962; 27: 31-36. [CrossRef]
- Hart MR, Cornish PA. Comparison of bicarbonate-extractable soil phosphorus measured by ICP-AES and colorimetry in soils of south-eastern New South Wales. Soil Res. 2009; 47: 742-746. [CrossRef]
- Herlihy M, McGrath D. Phosphorus fractions and adsorption characteristics in grassland soils of varied soil phosphorus status. Nutr Cycling Agroecosys. 2007; 77: 15-247. [CrossRef]
- Norrish K, Taylor RM. The isomorphous substitution of iron by aluminium in goethites. J Soil Sci. 1961; 12: 294-306. [CrossRef]
- Li W, Wang LJ, Liu F, Liang XL, Feng XH, Tan WF, et al. Effects of Al3+ doping on the structure and properties of goethite and its adsorption behaviour towards phosphate. J Environ Sci. 2016; 45: 18-27. [CrossRef]
- Hamdi W, Pelster D, Seffen M. Phosphorus sorption kinetics in different types of alkaline soils. Arch Agron Soil Sci. 2014; 60: 577-586. [CrossRef]
- Wong MTF, Swift RS. Role of organic matter in alleviating soil acidity. New York: Marcel Dekker Inc.; 2003. pp. 337-358. [CrossRef]
- Oburger E, Jones DL, Wenzel WW. Phosphorus saturation and pH differentially regulate the efficiency of organic acid anion-mediated P solubilization mechanisms in soil. Plant Soil. 2011; 341: 363-382. [CrossRef]
- Kuzyakov Y, Jones DL. Glucose uptake by maize roots and its transformation in the rhizosphere. Soil Biol Biochem. 2006; 38: 851-860. [CrossRef]
- Yamaguchi N, Okazaki M, Hashitani T. Proton consumption with phosphate adsorption on amorphous Fe(III) hydroxide. Soil Sci Plant Nutr. 1996; 42: 681-690. [CrossRef]
- Ajwa HA. Metal-induced sulfate adsorption by soils. PhD thesis, Iowa State University, accessed 20 Octoer 2019, Dissertions and Theses: A&I (ProQuest) database.
- Sposito G. Chemistry of soils. New York; Oxford University Press: 2008. pp. 344.
- Von Uexkull HR, Mutert E. Global extent, development and economic impact of acid soils. Dordrecht, Netherlands: Kluwer Academic Publication; 1995. pp. 5-19. [CrossRef]
- Sumner ME, Noble AD. Soil acidification: The world story. New York: Marcel Dekker; 2003. pp. 1-28. [CrossRef]
- Schumacher BA, Shines KC, Burton JV, Papp, ML. A comparison of soil sample /homogenization techniques. U.S. EPA, Washinton, D.C. 1990. accessed 19 Oct 2014, http://www.epa.gov/esd/cmb/research/papers/bs120.pdf.
- Mendoza-Cortez JW, Cecílio-Filho AB, Grangeiro LC, de Oliveira FHT. Influence of phosphorus fertilizer on melon (Cucumis melo L.) production. Aust J Crop Sci. 2014; 8: 799-805.