No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
The ability of the DGT soil phosphorus test to predict pasture response in Australian pasture soils - a preliminary assessment
Abstract
Diffusive gradients in thin-films (DGT) technology provides an alternative assessment of available phosphorus (P) for a range of crops, suggesting a preliminary examination of the performance of the new DGT-P test, compared to existing bicarbonate extractable Olsen and Colwell P tests, for pastures is justified. This study utilized historic data from the Australian National Reactive Phosphate Rock (NRPR) study (1992-1994) that included 25 experimental sites representing a wide range of soil types and climates used for pasture production. Stored (~19 yr) soil samples were analysed for DGT-P, Olsen P and a single point P buffering index (PBI) and re-analysed for Colwell P. Results showed the traditional bicarbonate extractable Colwell (r2 = 0.45, P < 0.001) and Olsen P (r2 = 0.27, P < 0.001) methods predicted relative pasture P response more accurately, compared to the novel DGT-P test (r2 = 0.09, P = 0.03) when all 3 yr of data were examined. We hypothesize that the harsher bicarbonate extraction used for the Olsen and Colwell methods more accurately reflects the ability of perennial pasture roots to access less labile forms of P, in contrast to the DGT-P test, which does not change the soil pH or dilute the soil and appears unable to fully account for a plants ability to solubilize P. Further studies are needed to compare the capacity of DGT-P to measure P availability in perennial pasture systems and to better understand the soil chemical differences between pasture and cropping systems.
... PTEs' total concentrations were wide-applied to assess their pollution and risk status in real environments, but cannot show their dynamic patterns and possible interactions with other chemicals in soils. Consequently, to have a comprehensive view on the PTEs' mobility and bioavailability in soil environments, their labile concentrations (C DGT ) were collected by deployments of DGT devices in all soil samples [45][46][47]. ...
Seven potentially toxic elements (PTEs) including Cr, Ni, Cd, Zn, Cu, Pb and As of soil samples from Qinghai-Tibet Plateau (QTP) were measured to assess the degree of contaminations. To have a comprehensive understanding of PTEs’ risks in QTP, the single-step nitric acid extraction and diffusive gradients in thin-films (DGT) technique were employed in this study for the determination of PTEs’ total amount as well as bioavailable content, respectively. Haibei state of Qinghai province was the most contaminated sites by PTEs. Behaviors of PTEs should be highly concerned as they can produce continuous bioavailable effects to ecological systems. High DGT concentrations of Zn and Cr showed their remarkable available risks in soils. In the environment with insufficient organic matter and slight pH variations, phosphorus level was the main contributor to affect the mobility of labile PTEs, especially for Pb and Ni. Concentration ratio between DGT measurement and soil solution and correlation analysis confirmed the high bioavailability of Pb, Zn and Cr, leading to short-term mixture effects on ecological systems, while the long-term release dynamics were found for As and Cu. Results from road-side and river-side soils of QTP evidenced the short-term bioavailable risks were possibly attributed to intensive anthropogenic activities including traveling industries and vehicle driving, whereas long-term bioavailable risks was from the natural transportation and deposition.
... Most calibration studies have been conducted on highly weathered soils from tropical and subtropical regions (Australia, Africa, and Southeast Asia), which are known for their high P-fixation potential (Dubus and Becquer, 2001). Based on field and pot experiments, it was found that DGT measurement was generally superior to various equilibriumtype quantity (Colwell,Olsen, and intensity (0.01 mol L −1 CaCl 2 ) tests in explaining the yields of wheat (Mason et al., 2010;Speirs et al., 2013), maize (Six et al., 2012(Six et al., , 2013(Six et al., , 2014, and tomato (Menzies et al., 2005), but not rice (Six et al., 2013) or pastures (Burkitt et al., 2016). ...
Diffusive gradients in thin films (DGT) measurements have been shown to outperform other phosphorus (P) tests in soils with strong P sorption, but this has not been confirmed for moderately weathered European soils. We compared the performance of DGT in predicting wheat grain yield in Swedish long-term fertility experiments with those of standard intensity (water-extractable P (P-H2O)) and quantity (ammonium lactate-extractable P (P-AL)) tests. A Mitscherlich-type model was used to fit wheat yield response to P application rates (0, 15, 30 or 35, and 45 kg P ha⁻¹ year⁻¹) in each individual trial replicate to estimate the maximum yield. For trials with clear plateau-type yield responses and the goodness of fit (R²) > 0.75, relative yields (RYs) were calculated for each P treatment and plotted against the soil P test results (n = 143). The goodness of the Mitscherlich-type fits decreased in the following order: DGT-measured P (P-DGT) (R² = 0.35) > P-H2O (R² = 0.18) > P-AL (R² = 0.13). When excluding soils with P-AL:P-DGT ≥ 0.1 L g⁻¹, R² was considerably improved to 0.55 for P-AL, 0.46 for P-H2O, and 0.65 for P-DGT (n = 61). At 95% of maximum yield, the upper limit of P deficiency for P-DGT was 44.8 (the soils with P-AL:P-DGT < 0.1 L g⁻¹) and 61.9 μg L⁻¹ (all soils), falling within the range reported for other European and Australian soils (6.0–142 μg L⁻¹). We show that in the investigated Swedish soils, DGT performed better than the quantity and intensity tests, which is attributed to its ability to capture P diffusion and resupply from the soil solid phase, similar to plant roots in the rhizosphere.
... Although DGT has been successfully used to assess available P for various crops, it failed to predict the P response of perennial pasture in soils (Burkitt et al., 2016). In a field trial, DGT also failed to predict Cd uptake by onion (Allium cepa), potato (Solanum tuberosum) or spinach (Spinacia olearacea) (Yi et al., 2020). ...
Soil plays an important role in controlling the biogeochemical cycling of nutrients and contaminants in the environment. Reliable analytical techniques are critical to better understand their transformation in soils. Diffusive Gradients in Thin-films technique (DGT) is one of such techniques, which has become increasingly popular in soil research since 1998. Undeniably, DGT attracts increasing attentions from multidisciplinary researchers worldwide. This is mainly due to its inherent merits, including ease of use, in situ measurement, and biomimetic applications. Further, four important aspects of DGT applications in soil research were summarized. They include: 1) chemical speciation, 2) bioavailability measurement, 3) desorption kinetic modeling, and 4) interfacial process mapping of nutrients and contaminants, with main focus on the progresses achieved during the past five years. In short, DGT is effective in studying the biochemical behaviors of nutrients (e.g., phosphate and nitrate), trace elements (e.g., Cd, Ni, Zn, Cu, Pb, and As), and polar organic compounds (e.g., antibiotics, pesticides, and bisphenols) in soils. Toward the end, we proposed several aspects of applying DGT as a versatile tool in soil research.
... Contudo, devido ao efeito de diversos fatores e processos que podem afetar a precipitação de fosfatos, é difícil prever o equilíbrio de precipitação e dissolução de fosfatos (Marschner e Rengel, 2012). O método da resina tem sido reportado como um avanço nos métodos de extração de P no solo Raij, 1998;Raij et al., 2009), assim como o uso de gradientes de difusão em finas membranas (diffusive gradiente in thin films -DGT) (Degryse et al., 2009;Burkitt et al., 2016;Menezes-Blackburn et al., 2016b . A estimativa da disponibilidade de P no solo, levando em consideração a cinética de dessorção do P, e sem o uso de extratores em relações solo:solução não realísticas, têm sido responsáveis por altas correlações com a produção de várias culturas Six et al., 2013;. ...
O cultivo de braquiária ruziziensis (Urochloa ruziziensis) em sistemas de rotação de culturas tem sido sugerido como uma estratégia para aumentar a ciclagem e disponibilidade de fósforo no solo. Foi avaliado um experimento de longa duração em Botucatu/SP, Brasil, em que a braquiária tem sido cultivada na entressafra de soja (Glycine max) desde 2006, com a aplicação de 0, 30 e 60 kg ha-1 de P2O5. Inicialmente, foi testada a hipótese de que o cultivo de braquiária durante a entressafra pode aumentar a disponibilidade de fósforo no solo. A concentração de fósforo no solo extraído com resina foi maior no solo após a braquiária do que no solo com pousio. Surpreendentemente, o teor foliar de fósforo e a produtividade da soja foram menores após a braquiária do que após pousio. Em função dos resultados observados no primeiro capítulo deste estudo, foi conduzido um experimento com o objetivo de mensurar as frações de fósforo disponíveis para plantas de milho (Zea mays) cultivadas após a braquiária, e avaliar a adsorção e dessorção de fósforo no solo. Amostras de solo do experimento de campo foram coletadas para condução de um experimento em vaso, especialmente para possibilitar uma adequada amostragem de raízes de milho. Novamente, a concentração de fósforo extraído com resina não teve correlação com a resposta das plantas. Por meio da calorimetria de titulação isotérmica foi possível verificar uma menor dessorção e maior adsorção de fósforo no solo cultivado com braquiária do que no solo mantido em pousio, possivelmente devido ao acúmulo de fósforo orgânico e maior interação da matéria orgânica no solo com os óxidos de ferro e alumínio. Uma vez que o acúmulo de fósforo orgânico pode resultar em menor disponibilidade de fósforo no solo, um terceiro estudo foi conduzido com o objetivo de avaliar o fósforo orgânico no solo cultivado com braquiária, principalmente o mio-inositol hexafosfato, considerado uma das formas menos disponíveis de fósforo no solo. O mio-inositol hexafosfato foi separado com cromatografia líquida de alta performance após oxidação dos extratos de solo com hipobromito. A braquiária resultou em menor concentração de mio-inositol hexafosfato no solo do que o pousio; apesar disso, o menor acúmulo deste composto não é suficiente para evitar a menor disponibilidade de fósforo no solo. A falta de correlação do fósforo extraído com resina e a resposta das plantas em rotação com braquiária estimulou a condução de uma pesquisa, apresentada no Capítulo 4, com o objetivo de avaliar a cinética de dessorção do fósforo. Por meio do método de gradiente de difusão de fósforo em finas membranas, foi possível observar que o cultivo de braquiária resulta em menor disponibilidade de fósforo devido à redução da mobilidade e menor ressuprimento do fósforo da fase sólida para a solução do solo, o que por sua vez ajuda a explicar como é difícil estimar a disponibilidade de fósforo com os métodos usuais de extração de fósforo, que não levam em consideração estes fatores determinantes da disponibilidade de fósforo. A persistência destes resultados é um indicativo de que não é possível obter aumento da disponibilidade de fósforo no solo com o cultivo de braquiária ruziziensis.
... Of these two, only the Olsen method was able to determine changes to the immediately available P pool at low soil-P concentrations. Further, the Olsen method may be more reliable compared to the DGT method when the aim is to predict plant availability of P or when changes to the immediately plant-available P fraction are to be determined, as described by Burkitt et al. [22]. ...
Extractive tests for determining the plant-availability of soil phosphorus (P) give varying results due to the inherently different characteristics of the extraction solution. Generally, classical soil P tests such as the Olsen or calcium acetate/lactate (CAL) method do not give an indication on the total amount of plant available P, but merely give an indication of the equilibrium between soil and extraction solution. It is also not entirely clear which fractions of P are directly determined through the various methods of extraction, i.e., determined P must not be immediately plant available, as is the case for rock phosphate. It is therefore possible that extraction methods either over or under estimate the amount of P available for plant consumption. In this research, we compared three methods of soil P determination (CAL, Olsen and diffusive gradients in thin films (DGT)) with regards to their ability to determine P species (Ca(H2PO4)2, CaHPO4, Ca3(PO4)2 and Inositol-6-hexakisphosphate) added to soils of high sorption capacity, immediately after as well as two weeks after application. For each of the methods, it could be shown that sorption processes in the soil immediately (0 days incubation) fix P to a point where it is not extractable through any of the described methods. These sorption processes continue over time, leading to a further decrease of determined P. The acidic CAL extraction method gives higher results of extractable P compared to the Olsen method. Due to the extraction of Ca3(PO4)2, the CAL method may overestimate immediately plant-available P. The most suitable methods for the determination of immediately plant available P may therefore be the Olsen and DGT methods. Organic IP6 is not determined by any of the extraction methods. At low concentrations of soil P, the DGT method may fail to give results.
Soil P testing has been widely used to predict crop yields, P uptake, and fertilizer demands in agriculture. Diffusive gradients in thin films (DGT) provides a zero-sink soil P test which mimics diffusion-controlled plant uptake and has previously been found to predict P availability to crops better than conventional quantity-based P tests in highly weathered Australian, though not in European soils. Here we tested the performance of DGT and the Austrian and German standard P quantity test calcium acetate lactate (CAL) to explain the variation of crop yield and P uptake response of winter wheat ( Triticum aestivum L.) and spring barley ( Hordeum vulgare L.) in long-term P fertilization experiments at four different sites in eastern Austria. Phosphorus extracted with DGT (P-DGT) and CAL (P-CAL) correlated well in similar soils but not across sites with large variation in soil and site properties such as carbonate equivalent and water availability. The predictive power of DGT for barley (R ² = 0.42) and wheat grain yield (R ² = 0.32), and P uptake in wheat grains (R ² = 0.36) was clearly superior to that of the CAL, and less dependent on soil properties. The better performance of DGT compared to the quantity test is consistent with diffusion-limited P uptake in the water-limited cultivated soils of eastern Austria. The critical values of P deficiency derived from the Mitscherlich-type fits for barley and wheat at 80% relative yield are 64.9 and 26.2 µg L ⁻¹ , respectively, consistent with differential P demands of the crops.
Field-based cutting trials, which formed part of the National Reactive Phosphate Rock Project, were established across Australia in a range of environments to evaluate the agronomic effectiveness of 5 phosphate rocks, and 1 partially acidulated phosphate rock, relative to either single superphosphate or triple superphosphate. The phosphate rocks differed in reactivity. Sechura (Bayovar) and North Carolina phosphate rocks were highly reactive (>70% solubility in 2% formic acid), whilst Khouribja (Moroccan) and Hamrawein (Egypt) phosphate rock were moderately reactive. Duchess phosphate rock from Queensland was relatively unreactive (<45% solubility in 2% formic acid). Phosphate rock effectiveness was assessed by measuring pasture production over a range of phosphorus levels, and by monitoring bicarbonate-soluble phosphorus extracted from soil samples collected before the start of each growing season. Other treatments included single large applications of triple superphosphate, partially acidulated phosphate rock and North Carolina phosphate rock applied at 2 rates, and the application of monocalcium phosphate and North Carolina phosphate rock sources without sulfur to evaluate the importance of sulfur in the potential use of phosphate rock fertilisers at each site.
A broad range of environments were represented over the 30 sites which were based on pastures using annual and/or perennial legumes and perennial grasses. Rainfall across the network of sites ranged from 560 to 4320 mm, soil pH (CaCl2) from 4.0 to 5.1, and Colwell-extractable phosphorus ranged from 3 to 47 µg/g before fertiliser application. Two core experiments were established at each site. The first measured the effects of phosphate rock reactivity on agronomic effectiveness, while the second measured the effects of the degree of water solubility of the phosphorus source on agronomic effectiveness.
The National Reactive Phosphate Rock Project trials gave the opportunity to confirm the suitability of accepted procedures to model fertiliser response and to develop new approaches for comparing different fertiliser responses. The Project also provided the framework for subsidiary studies such as the effect of fertiliser source on soil phosphorus extractability, cadmium and fluorine concentrations in herbage, evaluation of soil phosphorus tests, and the influence of particle size on phosphate rock effectiveness. The National Reactive Phosphate Rock Project presents a valuable model for a large, Australia-wide, collaborative team approach to an important agricultural issue. The use of standard and consistent experimental methodologies at every site ensured that maximum benefit was obtained from data generated. The aims, rationale and methods used for the experiments across the network are presented and discussed.
Biologically available phosphorus (P) is divided operationally into two sources, dissolved reactive P (DRP) and bioavailable particulate P (BPP). Dilute CaCl2-extractable soil P (CaCl2-P) is considered to be the benchmark method for estimating DRP in soils, whereas P desorbed to iron-oxide impregnated filter paper (FeO-P) is the benchmark method for BPP in soils and sediments. Neither of these methods is in routine use in Australia. Selected soil P analyses were carried out on 31 diverse surface soils to develop relationships between the environmental benchmark methods and the routine soil P tests of Colwell-P, Olsen-P, and the single-point P buffer index (PBI). The index (Colwell-P/PBI) was highly correlated with CaCl2-P (r = 0.925, P < 0.001), and both Olsen-P and Colwell-P were highly correlated with FeO-P (r = 0.955 and 0.828, respectively; P < 0.001). It is suggested that these measures can be used as environmental risk indicators for soil P status. The critical values of these measures for optimum productivity were compared to the values of these measures corresponding to threshold values of currently used environmental risk indicators.
There is a need to be able to identify soils with the potential to generate high concentrations of phosphorus (P) in runoff, and a need to predict these concentrations for modelling and risk-assessment purposes. Attempts to use agronomic soil tests such as Colwell P for such purposes have met with limited success. In this research, we examined the relationships between a novel soil P test (diffuse gradients in thin films, DGT), Colwell P, P buffering index (PBI), and runoff P concentrations. Soils were collected from six sites with a diverse range of soil P buffering properties, incubated for 9 months with a wide range of P additions, and then subjected to rainfall simulation in repacked trays growing pasture. For all soil and P treatment combinations, the relationship between DGT (0–10 mm) and runoff P was highly significant (P < 0.001, r² = 0.84). Although there were significant curvilinear relationships between Colwell P and runoff P for individual soils, there were large differences in these relationships between soils. However, the inclusion of a P buffering measure (PBI) as an explanatory variable resulted in a highly significant model (P < 0.001, R² = 0.82) that explained between-soil variability. We conclude that either DGT, or Colwell P and PBI, can be used to provide a relative measure of runoff P concentration.
A selection of commonly used soil phosphorus (P) tests, which included anion and cation exchange resin membranes, were compared in a glasshouse experiment using subterranean clover, and evaluated in the field at 19 sites from the National Reactive Phosphate Rock Project in 1993 and at 6 sites in 1995. The ability of the soil P tests to predict plant response was used to evaluate the tests. In the glasshouse experiment the resin test was less effective than the Bray 1 and Colwell tests in its ability to assess the level of plant-available P from the different fertiliser treatments. Seventy-one percent of the variation in total P content of the subterranean clover shoots was explained by resin-extractable P values, whereas the Colwell procedure accounted for 81% and the Bray 1 procedure accounted for 78%. Water and CaCl2 extracts were poor predictors of P content. In the field experiments all tests evaluated performed poorly in describing the relationship between soil test P and the level of P applied and relative yield and soil test P over a wide range of soil types and environments. The Bray 1 procedure performed best but the relationship was poor.
Maps are constructed using Geographic Information Systems (GIS) computer technology to identify privately held land in the high rainfall zones of temperate and tropical Australia where highly reactive phosphate rocks (RPRs) are likely to be effective phosphorus (P) fertilisers for permanent pasture. Australia-wide RPR suitability maps were based on annual rainfall, soil pH and the P sorption capacity of the soil. The digitised soil map from the Atlas of Australian Soils and the soil profile acidity map derived from the Atlas, were used to identify land areas with suitable soil properties. The coarse scale of the Atlas, which has only the 1 dominant soil for each 100 ha minimum landscape unit, limits the precision in identifying specific land types. Reactive phosphate rock suitability maps for pasture land in Victoria were also developed using smaller land units and state-wide digitised soil maps for surface pH and surface texture. The GIS maps indicated that there are about 26.5 × 10⁶ ha of land in the high rainfall pastoral zone of Australia that have sufficient annual rainfall and appropriate soil properties for RPR to become effective by the 4th year after changing from annual water-soluble P fertiliser to RPR fertiliser applications. Additional land with a lower rainfall might also be suitable if the soil surface is not excessively sandy. The area of high rainfall pasture land where RPR is likely to be as effective as water-soluble P fertiliser in the first year of application is around 3 × 10⁶ ha. The major portion of this land is in North Queensland, with smaller areas in southern Victoria, in far north-west Tasmania and in the far south-west of Western Australia. More detailed GIS maps for Victoria indicate that RPRs would become as effective as water-soluble P fertiliser by the 4th year on more than 70% of private land where annual rainfall exceeds 700 mm.
Data from more than 580 field experiments conducted in South Australia over the past 30 years have been re-examined to estimate extractable soil phosphorus (P) levels related to 90% maximum yield (C90) for 7 crop species (wheat, barley, oilseed rape, sunflower, field peas, faba beans, potato) and 3 types of legume-based pasture (subterranean clover, strawberry clover, annual medics). Data from both single-year and longer term experiments were evaluated. The C90 value for each species was derived from the relationship between proportional yield responsiveness to applied P fertiliser rates (determined as grain yield in crops and herbage yield in ungrazed pastures) and extractable P concentrations in surface soils sampled before sowing. Most data assessments involved the Colwell soil P test and soils sampled in autumn to 10 cm depth. When all data for a species were considered together, the relationship between proportional yield response to applied P and soil P status was typically variable, particularly where Colwell soil P concentration was around C90. When data could be grouped according to common soil types, soil surface texture, or P sorption indices (selected sites), better relationships were discerned. From such segregated data sets, different C90 estimates were derived for either different soil types or soil properties. We recommend that site descriptors associated with the supply of soil P to plant roots be determined as a matter of course in future P fertiliser experiments in South Australia. Given the above, we also contend that the Colwell soil P test is reasonably robust for estimating P fertiliser requirements for the diverse range of soils in the agricultural regions of the State. In medium- and longer term experiments, changes in Colwell soil P concentration were measured in the absence or presence of newly applied P fertiliser. The rate of change (mg soil P/kg per kg applied P/ha) appeared to vary with soil type (or soil properties) and, perhaps, cropping frequency. Relatively minor changes in soil P status were observed due to different tillage practices. In developing P fertiliser budgets, we conclude that a major knowledge gap exists for estimating the residual effectiveness of P fertiliser applied to diverse soil types under a wide range of South Australian farming systems.
The usefulness of five contrasting methods of soil analysis for estimating the phosphorus fertilizer requirements of wheat in southern New South Wales has been investigated, using yield data provided by 27 field experiments. Because the level of yield of wheat is strongly affected by seasonal environmental conditions poor correlations are obtained between soil analysis and absolute or relative yield of wheat, Much better and often significant correlations are obtained between soil analysis and the absolute increase in yield from fertilizer application. The best correlations were obtained with an 0.5M NaHCO3 extraction of soil phosphorus. A regression response surface calculated from these relationships provides a method for making direct estimates of fertilizer requirements for maximum economic return to farmers under average climatic conditions. The precision of these estimates is limited more by the flatness of the response surface and uncontrolled variation in the field data, than by inadequacies in the representation of available phosphorus by the NaHCO3 analysis.
This study was performed to better understand the chemical behaviour of P in a variety of alkaline soils from southern Australia. To do so, surface soil samples of 47 alkaline cropping soils from Upper Eyre Peninsula in South Australia and from western Victoria were collected. The 22 soils collected from Eyre Peninsula were Calcarosols, and those from western Victoria were Vertosols, Alkaline Duplex soils, Sodosols, and Red Brown Calcareous soils. Parameters included total and amorphous Al and Fe, organic C, organic P, CaCO3 content, P sorption characteristics, phosphorus buffer capacity, calcium lactate (Ca-Lac) extractable P, bicarbonate-extractable (Colwell) P, water-extractable P, anion exchange membrane extractable P (AEM-P), and isotopically exchangeable P (labile P). Concentrations of micronutrients in the Calcarosols were relatively low, considered to be a function of low clay contents. Given very low background Cd concentrations in the soils, it was estimated from Cd measurements that the majority of total P in the soils was derived from previous fertiliser applications. Phosphorus buffer capacities (PBCs) were relatively high in the Calcarosols and moderately high in the other alkaline soils. P sorption behaviour in the Calcarosols was a direct function of CaCO3 content, although in the other alkaline soils, amorphous Al and Fe oxides were the principal determinants of the P sorption behaviour. Both Colwell and Ca-Lac extractants dissolved non-labile P in the highly calcareous soils, whereas AEM appeared to only remove surface-adsorbed P. In addition, Colwell P values were positively related to PBC and to the slope term in the Freundlich model (Kf) when Kf > 10. It is suggested that AEM-P may be a better predictor of P availability in highly calcareous soils compared with the other extractants.
Soil is commonly stored in an air-dried state for extended periods before chemical analysis. The effect of storage on P solubility was assessed by determining NaHCO3-extractable P concentrations in a range of pasture soils from England and Wales (total C = 28.9-80.4 g kg(-1), clay = 219-681 g kg(-1), pH = 4.4-6.8) immediately following air drying and after 3 yr of storage at ambient laboratory temperature. Following storage, NaHCO3-extractable inorganic P concentrations decreased by between 2 and 60% (mean decrease = 21%), while NaHCO3-extractable organic P concentrations increased by between 48 and 156% (mean increase = 95%). The greatest changes occurred in soils of pH < 5.3. The changes appear to result from the disruption of organic matter coatings on mineral surfaces, continuous solid-phase diffusion of phosphate into soil particles, and decomposition of microbial cells. The results have important implications for the determination of NaHCO3-extractable P in stored soils and highlight the importance of working with fresh samples to derive information with relevance to field conditions.
Phosphorus availability is a major factor limiting crop production in highly calcareous soils. Recent field trials on calcareous soils in southern Australia have shown that fluid fertilizers may provide a useful alternative to granular fertilizer products. Fluid sources of P enhance P uptake and yield when compared with granular fertilizers applied at the same rate. This work aimed to compare the behavior of one fluid (technical grade monoammonium phosphate, TG-MAP) and one granular (monoammonium phosphate, MAP) form of P fertilizer in a highly calcareous soil. Changes in soil pH, P diffusion, solubility, and lability (using isotopic dilution techniques) were determined at different distances from the point of application over 5 wk. Furthermore, reaction products in MAP granules were investigated using spectroscopic techniques. The results indicated that P from fluid TG-MAP diffused more and was more available than P supplied as granular MAP. Also, X-ray diffraction (XRD) and energy dispersive X-ray microanalyses (EDXMA) of the MAP granules indicated that a significant percentage (12%) of the initial P remained in the granules even after 5 wk of incubation in the soil. The enhanced P availability of fluid fertilizers observed in field trials compared with granular forms is discussed in relation to differences in the dissolution, diffusion, and reaction processes in soils.
An automatic rising plate meter called the Ellinbank Pasture Meter (EPM) was constructed and evaluated at the Dairy Research Institute (Ellinbank) for measuring the dry matter present on a pasture dominated by green perennial rye grass. The meter readings of pasture height were found to correlate linearly with pasture yield, and the coefficient of variation of calibrations on any one date averaged about 13%. When the data from a large number of calibrations from separate dates were pooled, the coefficient of variation increased to about 18% (RSD 370 kg ha-1). Although there was a high level of repeatability of readings within operators, a substantial degree of variation existed between operators. The diurnal variation in dry matter percentage of pasture had only small effects on the meter readings. The EPM has an advantage over other pasture measuring methods in that its automatic function enables up to one hundred pasture measurements to be made in five minutes. In direct comparison with a manual rising plate meter and a two probe electronic capacitance meter, it was found that there was no significant difference in accuracy between the three meters but the advantages of the EPM are discussed. On perennial rye grass/white clover pastures in an actively growing vegetative state, the EPM has been found to be accurate enough for research purposes yet simple enough for use by farmers and their advisers as an aid to pasture management.
Soil phosphorus (P) sorption is an important and relatively stable soil property which dictates the equilibrium between sorbed and solution P. Soil P sorption measures are commonly adjusted for the effect of current P fertility on the amount of P a soil sorbs. In the case of highly fertilised agricultural soils, however, this adjustment is likely to be inappropriate as it may mask changes in a soil’s capacity to sorb P, which could affect future P fertiliser applications. A study was undertaken to compare adjusted or unadjusted methods of measuring P sorption using 9 pasture soils sampled from southern Victoria which had previously received P fertiliser and lime. The P sorption assessment methods included: P sorption isotherms, P-buffering capacity (PBC) measures (slope between equilibrium P concentration of 0.25 and 0.35 mg P/L), and single-point P-buffering indices (PBI), with methods either adjusted or unadjusted for current P fertility. A single application of 280 kg P/ha, 6 months before sampling, resulted in a general negative displacement of unadjusted P sorption isotherm curves, indicating reduced P sorption on 8 of the 9 soils. Adding the Colwell extractable P concentration to the amount of P sorbed before calculating the slope (PBC+ColP), tended to negate this fertiliser effect and, in 2 of the 9 soils, resulted in a significant increase in PBC+ColP values. Increasing rates of P fertiliser application (up to 280 kg P/ha) resulted in a consistent trend to decreasing PBI values (unadjusted for Colwell P), which was significant at 4 of the 9 sites after 6 months. However, only minimal changes in PBI values were determined when PBI was adjusted for current P fertility (PBI+ColP). Phosphorus sorption properties appeared reasonably stable over time, although 2 soils, both Ferrosols, indicated significant linear increases in PBI values when these sites remained unfertilised for 30 months. Lime significantly increased both PBI and PBI+ColP values at all sites 6 months after application, but the effect generally diminished after 30 months, suggesting PBI measurements should not be taken immediately after liming.
These results demonstrate that unadjusted measures of P sorption are more likely to accurately reflect changes in soil P sorption capacity following P fertiliser applications and suggest that the unadjusted PBI be used in commercial soil testing rather that the currently adjusted PBI+ColP.
Liquid forms of phosphorus (P) have been shown to be more effective than granular P for promoting cereal growth in alkaline soils with high levels of free calcium carbonate on Eyre Peninsula, South Australia. However, the advantage of liquid over granular P forms of fertiliser has not been fully investigated across the wide range of soils used for grain production in Australia.
A glasshouse pot experiment tested if liquid P fertilisers were more effective for growing spring wheat (Triticum aestivum L.) than granular P (monoammonium phosphate) in 28 soils from all over Australia with soil pH (H2O) ranging from 5.2 to 8.9. Application of liquid P resulted in greater shoot biomass, as measured after 4 weeks’ growth (mid to late tillering, Feeks growth stage 2–3), than granular P in 3 of the acidic to neutral soils and in 3 alkaline soils. Shoot dry matter responses of spring wheat to applied liquid or granular P were related to soil properties to determine if any of the properties predicted superior yield responses to liquid P. The calcium carbonate content of soil was the only soil property that significantly contributed to predicting when liquid P was more effective than granular P.
Five soil P test procedures (Bray, Colwell, resin, isotopically exchangeable P, and diffusive gradients in thin films (DGT)) were assessed to determine their ability to measure soil test P on subsamples of soil collected before the experiment started. These soil test values were then related to the dry matter shoot yields to assess their ability to predict wheat yield responses to P applied as liquid or granular P. All 5 soil test procedures provided a reasonable prediction of dry matter responses to applied P as either liquid or granular P, with the resin P test having a slightly greater predictive capacity on the range of soils tested.
The findings of this investigation suggest that liquid P fertilisers do have some potential applications in non-calcareous soils and confirm current recommendations for use of liquid P fertiliser to grow cereal crops in highly calcareous soils. Soil P testing procedures require local calibration for response to the P source that is going to be used to amend P deficiency.
A single solution reagent is described for the determination of phosphorus in sea water. It consists of an acidified solution of ammonium molybdate containing ascorbic acid and a small amount of antimony. This reagent reacts rapidly with phosphate ion yielding a blue-purple compound which contains antimony and phosphorus in a 1:1 atomic ratio. The complex is very stable and obeys Beer's law up to a phosphate concentration of at least 2 μg/ml.The sensitivity of the procedure is comparable with that of the stannous chloride method. The salt error is less than 1 %.
Background and aims
Recent research has demonstrated the high accuracy of a new method for assessment of plant available P in soil called diffusive gradients in thin-films (DGT). The process of P released by additions of bicarbonate to soil samples simulating common soil P tests is yet to be assessed by the new method (DGT). The aim of this study was to identify the pools of soil P extracted by soil test methods (DGT, Colwell and resin) by comparing, in 32P–labelled soils, the specific activity (SA) of phosphorus extracted by common soil test extracts with the SA of wheat plants grown in a range of agricultural soils from southern Australia.
Methods
Wheat (cv. Frame) was grown for 4 weeks in 14 soils that were labelled uniformly with carrier-free 32P. The specific activity (SA) of P (MBq 32P kg 31P−1) in each soil test extract was compared to the SA of P in the wheat plants.
Results
The SA of P in plants were similar to P extracted by the Colwell extractant in only 4 of the 14 soils; while SA in plants and extractants corresponded in 10 of the soils for the resin method and in 12 of the soils for the DGT method. Phosphorus in the Colwell and resin extract solutions had significantly lower SAs compared to P in the plants for 10 and 4 of the soils, respectively, indicating greater extraction of non-labile P sources (unlabelled 31P). Phosphorus in the DGT extractant had significantly lower SA than the plants for 1 soil and in 1 soil the SA was higher. Overall, across all soils, 25 % of P extracted by the Colwell method was non labile compared to 9 % and 2 % for the resin and DGT methods, respectively.
Conclusion
The new DGT method for extraction of soil P has the potential to accurately predict occurrences of P deficiency because it generally extracts the same pool of labile soil P accessed by wheat plants, while methods using bicarbonate solution (e.g. Colwell, Olsen) or water (resin) at wide soil:solution ratios are more likely to measure more non-labile forms of P in soil.
Background and aims
A soil test that samples nutrients only from fractions that are accessible to plants will predict availability and uptake more robustly than empirical tests. This can be tested by comparison of the isotope ratios (specific activity, SA) of the nutrient between plant and the soil extract. This study was set up to assess this requirement for the diffusive gradients in thin films technique (DGT), recently proposed as a soil P test, in comparison with conventional soil P tests viz. Olsen, Colwell, Bray-1, Mehlich-3, ammonium oxalate, anion exchange membranes (AEM) and 0.01 M CaCl2 solution.
Methods
Maize (Zea mays L.) was grown in two P-deficient soils from western Kenya with contrasting P sorption characteristics, amended with a low and a high P rate and labelled with 33P.
Results
The SA in the plant shoot corresponded with that of the extracts of the different soil tests, except for CaCl2 and ammonium oxalate extracts, at the low P rate in the soil with low P sorption capacity, Teso soil. For the high P rate on this soil, differences in SA between maize shoot and soil test were small for all established soil tests, but significant for the Colwell, Bray-1, Mehlich-3 and AEM tests. The SA in the soil extracts was significantly smaller than that in the maize shoot for Sega the strongly P-sorbing soil at both P rates for all conventional tests, including AEM. This indicates that these tests extracted P from a pool that is not accessible to the plant. For the DGT test, however, there was no difference in SA between the maize shoot and the soil test, for any of the treatments.
Conclusions
Most conventional soil tests can extract a fraction of P which is not available to maize. The DGT technique, however, only samples P from the plant-accessible pool.
Aims and background
The DGT test is an emerging soil test to measure available phosphorus (P) in soils. Data from previous pot and field trials suggested a superior predictive power of this test relative to established soil P tests. Modeling also predicted that the critical DGT value (to obtain 80 % of the maximum yield) is plant specific. This study was set-up to test the DGT relative to established soil P tests in predicting the growth response to P addition across different tropical soils marked by P deficiency, for two plants with contrasting P demand.
Methods
Maize (Zea mays L.) and upland rice (Oryza sativa L.) were grown in a greenhouse on 9 contrasting soils that were amended with P at increasing doses. Soil tests were measured on incubated soils for all P treatments.
Results
Shoot dry weight increased with increasing P application by factors 2 to 90. The P doses required to reach 80 % of maximum dry weight or yield ranged from 20 to 580 mg P kg−1 and were always larger for the faster growing maize than for rice. The DGT method and CaCl2 extractions explained relative yield of maize among soils better (R² = 0.84 and 0.69 respectively) than P determined by Olsen, Colwell, Bray-1, Mehlich-3, ammonium oxalate and resin extractions (R² < 0.53). In strong contrast, relative yields of rice were best predicted by Mehlich-3, Bray-1, Olsen and resin P (R² ~ 0.7) compared to DGT (R² = 0.59) and CaCl2 (R² = 0.12). The DGT P concentration in a soil to obtain 80 % of maximum growth, i.e. the critical DGT values, were 73 μg P L−1 (maize) and 7 μg P L−1 (rice). The critical DGT value measured for maize is in correspondence with literature.
Conclusions
For tropical P deficient soils, intensity based indices of soil P availability such as DGT and CaCl2, are superior to quantity based indices (i.e. the established soil P tests based on extraction) for maize with high P demand. However, the reverse is true for rice suggesting that diffusion of P in the soil as measured by DGT is not the main factor explaining P uptake for rice.
Results from long-term field experiments in south-western Australia are presented in the form of relationships between yield, expressed as a percentage of the maximum yield, and soil test for phosphorus (P) values. Maximum yields were not always indicated by well defined yield plateaux. Different methods have been used to estimate the maximum yield value which is used to calculate yield as a percentage of the maximum yield so as to remove interseasonal variation. For all of these methods and for the same site, the same P fertilizer (superphosphate), and the same plant species, the relationship between yield and soil test P differed for different years. Consequently fertilizer recommendations based on the assumption that this relationship is constant are likely to be incorrect. We therefore question the validity of the common practice in soil testing programmes of using percentage yield values to remove interseasonal variation.
The technique of diffusive gradients in thin films (DGT) provides an in situ means of quantitatively measuring labile species in aqueous systems. By ensuring that transport of metal ions to an exchange resin is solely by free diffusion through a membrane, of known thickness, Delta g, the concentration in the bulk solution, C-b, can be calculated from the measured mass in the resin, M, after time, t, by C-b = M Delta g/DAt, where D is the molecular diffusion coefficient and A is the exposure surface area of the membrane. If a sufficiently thick (similar to 1 mm) diffusion layer is selected, the flux of metal to the resin is independent of the hydrodynamics in solution above a threshold level of convection. Deployment for 1 day results in a concentration factor of similar to 300, allowing metals to be measured at extremely low levels (4 pmol L(-1)). Only labile metal species are measured, the effective time window of typically 2 min being determined by the thickness of the diffusion layer. Because metals are quantified by their kinetics of uptake rather than the attainment of equilibrium, any deployment time can be selected from 1 h to typically 3 months when the resin becomes saturated. The measurement is independent of ionic strength (10 nM-1 M). For Chelex-100 as the resin, the measurement is independent of pH in the range of 5-8.3, but a subtheoretical response is obtained at pH <5 where binding to Chelex is diminished. The effect of temperature can be predicted from the known temperature dependence of the diffusion coefficient and viscosity. The application of DGT to the in situ measurement of Cd, Fe, Bin, and Cu in coastal and open seawater is demonstrated, and its more general applicability as a pollution monitoring tool and for measuring an in situ flux, as a surrogate for bioavailability, is discussed.
Phosphate (P) was added to soil in solution. The soil was air‐dried or freeze‐dried and then incubated at a range of temperatures for periods of up to 110 d. The rate of the continuing reaction between the P and soil was measured using the null‐point method, and by measuring the amount of desorption induced by filter paper impregnated with iron oxide (Pi test). The reaction between soil and P continued in both air‐dried and freeze‐dried soil, albeit more slowly than in moist soil. Freezing the soil, whether moist or dry, virtually stopped the reaction.
These results are consistent with the hypothesis that the continuing reaction between P and soil involves a solid‐state diffusive penetration of the soil particles by the sorbed P ions. They also indicate that the common practice of storing soil air‐dry, even for short periods at low temperature, will not preserve the P status of the soil as at sampling. It was estimated that for a sample of soil which remained moist at 25°C for 100d after the addition of 335 μg P g ⁻¹ soil, before being sampled and stored air‐dry at 4°C for 16 years, the measured P i test value would be about 15 μg P g ⁻¹ . This compares with 46 μg P g ⁻¹ which is the estimated P ⁱ test value measured on the same day as sampling.
When samples cannot be analysed for P status immediately following sampling, they should be stored at the lowest convenient temperature, preferably below 0°C.
The ability of the Diffusive Gradients in Thin Films (DGT) technique and two other established testing methods (Colwell, resin)
to predict wheat responsiveness to applied P from 35 field trials across southern Australia was investigated. Regression analysis
of relative early dry matter production and grain yield responses demonstrated that the DGT method predicted plant responsiveness
to applied P more accurately than Colwell P and resin P at sites where maximum yields were reached with P rates used (20 out
of 35). The measured concentration in soils at the DGT surface, CDGT, explained 74% of the variation in response for both early dry matter and grain, compared to 7% for early dry matter and
35% for grain using the resin P method. No significant relationships could be obtained for Colwell P although modifying the
Colwell test data using Phosphorus Buffering Index resulted in a correct response prediction for 11 of the 20 field sites
compared to 18 for DGT and 14 for resin P. These observations suggest that the DGT technique can assess plant available P
in soils with significantly greater accuracy than traditional soil P testing methods. The critical P threshold, expressed
as CDGT, was 255μgL−1 for early dry matter and 66μgL−1 for grain.
KeywordsNutrient availability-Phosphorus deficiency-Soil testing-Soil fertility
Phosphorus (P) deficiency increased the secretion of phytases from roots of various plant species. The secretory phytases were collected with a dialysis membrane tube for 24 hours from roots of sixteen plant species grown with low or adequate supply of P in nutrient solutions. The activity of not only secretory phytase, but also acid phosphatase, increased with the low P treatment in all of the plant species examined. Secretion of phytase by the roots under P-deficient conditions was highest in Brachiaria decumbens CIAT 606, Stylosanthes guianensis CIAT 184 and tomato, moderate in Brachiaria brizantha CIAT6780, Stylosanthes guianensis CIAT 2950, alfalfa, white clover and orchard grass, and lowest in Andropgon gayanus CIAT 621, Stylosanthes capitata CIAT 10280, upland rice, timothy, redtop, alsike clover, red clover and white lupin plants. An immunoreactive protein band that reacted with a polyclonal antibody raised against wheat bran phytase, corresponding to molecular weight 35–40 kD, could be detected in seven of the species tested. These results indicate that the secretory phytase may provide an efficient mechanism for certain plants to utilize inositol hexaphosphate in soil.
Testing for soil phosphate (P) using the Colwell procedure is widely used in south-western Australia to estimate fertilizer applications required for crops and pastures. The relationship between plant yield, expressed as a percentage of the maximum yield, and soil test values is assumed to be constant in different years for the same soil type and plant species. Data from 11 long-term field experiments in south-western Australia show that regardless of whether percentage of maximum or absolute yield is used, the relationship between yield and soil test values is different (1)in different years, for the same site and where the same P fertilizer type has been used. This occurred irrespective of whether the same or different plant species were grown in different years; (2)where different types of P fertilizer had been used, for the same site, same year and same plant species; (3)for different plant species, for the same site, same year, and same type of P fertilizer. We conclude that considerable errors in the recommendation of fertilizer rates may result from the assumption that there is a constant relationship between soil test and yield.
A single solution reagent is described for the determination of phosphorus in sea water. It consists of an acidified solution of ammonium molybdate containing ascorbic acid and a small amount of antimony. This reagent reacts rapidly with phosphate ion yielding a blue-purple compound which contains antimony and phosphorus in a 1:1 atomic ratio. The complex is very stable and obeys Beer's law up to a phosphate concentration of at least 2 μg/ml.The sensitivity of the procedure is comparable with that of the stannous chloride method. The salt error is less than 1 %.RésuméUne méthode spectrophotométrique est décrite pour le dosage du phosphate dans l'eau de mer, an moyen de molybdate d'ammonium, en présence d'acide ascorbique et d'antimoinc. Il se forme rapidement un composé violet bleu, renfermant antimoine et phosphore dans un rapport atomique de 1:1.ZusammenfassungBeschreibung einer Methode zur Bestimmung von Phosphat in Mecrwasser mit Hilfe von Ammoniummolybdat in Gegenwart von Ascorbinsäure und Antimon. Der gebildete blau-violette Komplex wird spektrophotometrisch gemessen.
The effect of potential chemical constraints on the performance of two relatively new soil P testing methods, anion exchange membrane (AEM) and diffusive gradients in thin films (DGT), were evaluated. Exposures to ranges of anion (Cl(-), NO(3)(-), SO(4)(2-) and HCO(3)(-)) concentrations relevant to agricultural soils had minimal effect on P recoveries using DGT. It has also been shown previously that DGT P recoveries are unaffected by varying pH (3-9). In contrast, increasing NO(3)(-) and SO(4)(2-) concentrations in solution reduced the recovery of P using the resin method (anion exchange membrane, AEM) by 24% at 50mgL(-1) NO(3)(-) and by 47% at 12mgL(-1) SO(4)(2-) when the P concentration of the test solution was 2mgL(-1). Phosphorus sorption by the resin decreased with increasing Cl(-) concentrations until there was a 100% decrease at 300mgL(-1) Cl(-) when the P concentration of the test solution was 2mgL(-1) and a 92% reduction at 700mgL(-1) Cl(-) when the P concentration of the test solution was 0.2mgL(-1). There was also a small but significant effect of carbonate species on P sorption to the resin at carbonate concentrations that can occur in agricultural soils. Changing the pH of the solution had minimal effects on the resin P measurements in solutions above pH 4, but below pH 4, resin P measurements decreased dramatically. A poor coefficient of determination for the regression fit between DGT and resin measurements on eight agricultural soils suggested that these two methods are measuring different amounts of P for different soils. Resin P measurements increased significantly, but non-uniformly across soils, when the soil:water ratio was decreased but this did not result in an improved relationship with DGT P. There was a minimal effect on measured P using either Cl(-) or HCO(3)(-) as counter ions on the resin.
Mobility and lability of phosphorus from granular and fluid monoammonium phosphate differs in a calcareous soil
- Lombi
Methods of soil analysis
- S Kuo
Kuo, S. 1996. Phosphorus. In: Methods of soil analysis. Part 3.
Chemical methods (eds D.L. Sparks, A.L. Page, P.A. Helmke &
R.H. Loeppert), pp. 869-919. Soil Science Society of America and
American Society of Agronomy, Madison, WI.
Making better fertiliser decisions for grazed pastures in Australia. Victorian Government, Department of Primary Industries
- C Gourley
- A Melland
- R Waller
- I Awty
- A Smith
- K Peverill
- M Hannah
Gourley, C., Melland, A., Waller, R., Awty, I., Smith, A., Peverill,
K. & Hannah, M. 2007. Making better fertiliser decisions for
grazed pastures in Australia. Victorian Government, Department
of Primary Industries, Melbourne.