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Soil testing is the most widely used tool to predict the need for fertiliser phosphorus (P) application to crops. This study examined factors affecting critical soil P concentrations and confidence intervals for wheat and barley grown in Australian soils by interrogating validated data from 1777 wheat and 150 barley field treatment series now held...
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Context 1
... from Queensland came from studies conducted in the 1960s, and very few since 1979, so the Vertosol dataset was dominated by experiments during earlier eras of cropping. Only 24% of the treatment series for P were gathered since 1990, an era of cropping when the use of varieties with high yield potential (and higher water-use efficiency) (see Fig. 1), herbicides for weed control, early sowing with minimum tillage, and crop residue retention have become common practice. Eighty per cent of the treatment series for wheat and 90% for barley were classified as class A trials (see Materials and methods) compared with the 80% average for the whole database ( Watmuff et al. ...
Context 2
... the Australian grain growing regions, mean wheat yield has been ~1 t/ha in 8 of the last 40 years, during extreme droughts (Fig. 1). Hence, we used 1 t/ha as a filter for screening out treatment series with low maximum yield. For treatment series with yield <1 t/ha, there was either no relationship between Y max and Colwell-P, or the relationship fitted had a lower critical concentration than for the remainder of the treatment series (Appendix 1). The one exception ...
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Citations
... The higher nutrients levels found in (NH 4 ) 2 SO 4 -SD (high S) and NH 4 H 2 PO 4 -SD (high P) digestates had no effect on the plant growth. The critical soil values of S and P in the soil is 2.8 and 23 mg kg − 1 (Anderson et al., 2013;Bell et al., 2013). The field soil used in this study recorded 2.8 mg S kg-1and 49 mg P kg-1. ...
Raw liquid anaerobic digestate was synthesised into nutrient-dense solid digestates via acidification and evaporation. Acidification retained ammonium in the digestate whilst also donating the anion to free ammonium to form an ammonium salt. Digestate was treated with the addition of sulphuric, nitric, and phosphoric acid resulting in the formation of ammonium sulphate, ammonium nitrate and ammonium phosphate, respectively then evaporated into a solid fertiliser product. FTIR, XRD and SEM-EDS collectively confirm that the addition of acids completely converted the free ammonium in the raw digestate into their respective ammonium salt counterparts. Compounds of potassium chloride, silicon dioxide, calcium carbonate, magnesium ammonium phosphate, sodium nitrate, and sodium chloride were identified in all solid digestate samples. Plant growth and grain yield was higher in urea ammonium nitrate, raw liquid digestate and acidified digestate products compared to control and unacidified solid digestate. Urea ammonium nitrate and ammonium nitrate solid digestate had the highest dry shoot, likely due to the high available nitrogen found in both fertilisers. Overall, acidification and evaporation of liquid digestate can efficiently transform it into a valuable solid fertiliser with a high nutrient density. This process not only has the potential to mitigate handling and storage constraints of low nutrient density digestate in anaerobic digestion facilities but also offers a sustainable alternative to conventional fertilisers.
... Defective tubers and those with a size lower than 50 mm were discarded and not accounted into the total yield. In each experiment, the relative yield (RY) was calculated as the ratio between the total tuber yield at a specific P rate and the maximum total tuber yield observed in P fertilized plots, multiplied by 100 (Bell et al. 2013). We evaluated RY since P critical levels are generally obtained from on-farm experiments that relate soil or plant P tests with RY (Zamuner et al. 2016b). ...
The objectives of our study were to (i) evaluate the association between the phosphorus (P) nutrition index (PNI) and total or soluble P in petioles (PpeT and PpeS, respectively) throughout potato’s (Solanum tuberosum L.) growing season and (ii) evaluate the use of PpeT and PpeS as predictors of the P nutritional status and tuber yield. Five potato (cv. innovator) field trials were carried out during two growing seasons, where four different P rates were evaluated. Whole plants were harvested, and the P content was determined and used to calculate the PNI. Also, petioles were harvested to determine PpeT and PpeS. At crop maturity, total and relative yield (RY) were determined. From all evaluated dates, PpeT and PpeS presented a stronger capacity to predict RY at 70 days after planting (r = 0.65 and 0.66, respectively), when PpeT and PpeS presented a 2.96 and 1.48 g kg−1 threshold below which RY was negatively affected, respectively. The PNI was mostly better associated with PpeT than with PpeS. The PNI presented a moderate capacity to predict the potato tuber RY which was stable throughout the growing season and presented an average critical threshold of 0.96. Depending on the sampling moment, all three evaluated indexes (PNI, PpeT, and PpeS) resulted to be suitable in-season plant analysis to assess the P nutritional status of potatoes. However, based on the simplicity of the quantification and the observed predictive capacity, the methodologies based on petiole analysis are advantageous as compared with PNI.
... There are numerous practical ways to deal with such situations depending on the prevalent soil types. For example, in a range of Australian soils, combining soil test P values with soil P buffering capacity (which represents the soil capacity to sorb P and thus influence P availability after fertilizer addition) correlated better with fertilizer responses than the soil P tests alone (Bell et al., 2013). However, crop rotation should also be taken into account (Neuhaus et al., 2015). ...
... However, for agronomic management, it can be adequate. For example, a 10% level of precision gives a range of plus or minus 2 to 5 mg P/kg for Colwell P. While this may seem large, this range is of the scale of the statistical error around critical values for calibrations of Colwell P with grain yield (Bell et al. 2013). Hence, a 10% level of precision is adequate for decision-making with respect to P fertiliser. ...
Purpose
A poor understanding of the nature of variation of soil properties within a field can lead to management decisions that reduce productivity or increase off-site environmental risks.
Methods
The variability in total C%, total N%, plant-available (Colwell) P, and pH in CaCl2 at multiple depths is examined from two sites near Wagga Wagga and Yerong Creek in the mixed farming zone of southern New South Wales, Australia. The minimum number of cores required to estimate the mean for each soil property with a given level of precision (1%, 5%, and 10%) was determined and the distribution of the sample data was described by calculating the skewness and kurtosis.
Results
The number of cores (42 mm diameter) required to estimate the mean was least for soil pH (3 cores) at both sites and greatest for Colwell P (25 and 58 cores) at a 10% precision in the surface 0.1 m of soil at the Wagga Wagga and Yerong Creek sites, respectively. The distributions of soil chemical properties were greatly varied with skewness and kurtosis both influencing the data. The mean value of the pH distributions sometimes exceeded the mode, leading to an underestimation of the extent of soil acidity. The mean value for Colwell P also often exceeded the mode, indicating an overestimate of the P status of the soil from a productivity perspective but concurrently underestimating the risk of off-site losses, associated with high P values at some locations. There was also an overestimation of the organic C fractions at these sites. Noteworthy in our data was a broad distribution across a large range in all soil parameters at the soil surface which, despite a generally normal distribution at that depth, gave a mean value that represents a relatively constrained approximation of the true fertility status of the soil.
Conclusion
This study highlights the importance of understanding the nature of the variability in soil properties for interpreting soil test results appropriately for agronomic and environmental purposes. Due to its highly variable nature, Colwell P could not be reliably measured within the level of precision assumed under existing soil sampling guidelines used in Australia.
... This constrains our ability to relate it to crop and pasture responses in the field. For example, in determining critical soil phosphorus (P) levels for crop production, Bell et al. (2013b) noted that the > 2 mm fraction of soil can comprise up to 60% of the soil mass but is discarded and only the < 2 mm fraction is used for soil analysis. Whilst there is a small amount of information on P adsorption by the > 2 mm fraction of ironstone gravels in south west Western Australia (SWWA) (Weaver et al., 1992), its paucity does not allow for development of empirical models, nor the improvement of agronomic decisions, or justification for inclusion of the > 2 mm fraction in soil testing. ...
... There are already well established soil chemical procedures to measure bicarbonate-extractable P for the < 2 mm fraction (Colwell, 1965) and for measures of P buffering index (PBI) (Burkitt et al., 2002) or P retention index (PRI) (Allen and Jeffery, 1990). For pasture based systems in Australia, both Colwell P and PBI are necessary to inform agronomic decisions around P requirements (Gourley et al., 2019), whilst for cropping systems a dependency of critical Colwell P values on soil PBI is not as clear (Bell et al., 2013b;Moody, 2007). ...
... The measurement of Colwell P in the > 2 mm fraction of up to 41 mg P kg − 1 for the 2-4 mm fraction and 18 mg P kg − 1 for the 8-10 mm fraction suggests that the > 2 mm fraction will contribute P to pastures and crops. The Colwell P values for the > 2 mm fraction is similar to the critical values and ranges reported to achieve 90% relative yield for winter cereal crops in Australia (Bell et al., 2013a;Bell et al., 2013b). The contribution of the > 2 mm fraction to P retention and Colwell P of the entire unsieved soil will depend on the relative quantities, size distribution, and chemical reactivity of the > 2 mm fraction compared to the < 2 mm fraction. ...
The > 2 mm fraction of soils is often excluded from laboratory analysis and glasshouse experiments, but is known to influence whole soil physical, chemical and biological properties. The historical focus on the < 2 mm fraction has led to a knowledge gap in analytical procedures and flow-on effect for agronomic response and advice when the > 2 mm fraction is present, especially when it is porous and chemically reactive.
An ironstone gravel soil (Endopetric Pisoplinthic Plinthosol (Arenic)) from south west Western Australia was separated into < 2, 2–4, 4–6, 6–8 and 8–10 mm fractions. Physical analysis included specific surface area (SSA), optical mineralogy, XRD and SEM. Chemical analysis included phosphorus (P) sorption, P retention index (PRI), P buffering index (PBI) and sodium bicarbonate extractable P (Cowell P) on intact and ground samples, and intact mixtures of the < 2 mm and > 2 mm fractions. Δ NaF pH was used as a surrogate P retention measure on intact mixtures of the < 2 mm and > 2 mm fractions.
P adsorbed onto the > 2 mm fraction can be measured as Colwell P, suggesting this fraction can provide P to plants. Colwell P determined on ground samples was correlated 1:1 with Colwell P on intact counterparts. Grinding of samples resulted in large increases in PRI and PBI, and is not a supported sample preparation, neither is the use of end over end shakers due to surface abrasion of the > 2 mm fraction increasing P sorption, however the use of Δ NaF pH as a surrogate measure of P sorption offers some potential.
Phosphorus adsorption decreased with increasing particle size but adsorption by the > 2 mm fraction is likely significant in the context of an entire unsieved soil and was influenced by SSA. The > 2 mm fraction had higher SSA than their physical size would imply, possibly influenced by the thickness and mineralogy of the exterior coating (rind). Our results show that the > 2 mm fraction contains reactive surfaces that can contribute to the nutrient holding capacity and plant available P of soils.
... Notably, finding lower required P input to achieve near-maximum yield occurred in Decile 1 and in Decile 10 rainfall seasons and in both healthy and marginal P status soil. Soils here were at the low end (2020) or above (2021) the 90% confidence interval range of Colwell P in 0-10cm for maximum yield in Tenosols (16-26 mg/kg, Bell et al 2013). It is most likely a combination of complex soil moisture interactions with other aspects of site conditions (e.g. ...
Key messages • Early sowing is well associated with increased yield crop potential, but when soil moisture was adequate for crop emergence, optimum phosphorus applications at sowing for wheat yield varied with sowing time, from low in April-sown crops to high in June-sown crops. • The effect was seen in early-mid and mid-late maturing wheats, in high and low rainfall years, in healthy and marginal P status soils and is somewhat counter-intuitive to common practice of increasing inputs to support a high yield potential. • The results suggest opportunities for fine tuning on-farm management of P inputs for efficiency and improved yield outcomes and have implications for advisory and decision support systems based on P response data that have not factored sowing time into field trial interpretations.
Aims
An investigation of the application of studies and methodologies from the eastern states to phosphorus supply in wheat crops at different times of sowing in Western Australia.
Results
As a general observation, yields were greatest in April or May sown wheat, with significant yield penalties evident by delaying sowing until mid June. Yield increased in response to increasing P application rate. Increases were greater as the crop was sown later and were observed for both varieties and in both years.
Differences in P responses were primarily due to depression of yield with nil P applied at later sowing times, indicating decrease in plant available soil P pool and/or decrease in capacity of plants to access, take up and utilise the P from the labile soil pool.
Taking the near-maximum yield point from fitted P response curves (90% or 95%) gave distinctly different optimum fertiliser P application requirements for each time of sowing.
The difference in optimum P requirement between the start and end of an 8-to-9 week April-June sowing program is in the order of 10-15 kg P/ha, and the requirement for applied P appears to be lower when the early sowing times correspond with a higher April monthly rainfall.
Notably, finding lower required P input to achieve near-maximum yield occurred in Decile 1 and in Decile 10 rainfall seasons and in both healthy and marginal P status soil.
Conclusion
Opportunity may exist for wheat growers sowing into conditions of good soil moisture in April and early May, to decrease P inputs while still achieving near-maximum growth and yield potential. Coupled with shifting P resources to crops sown later in the season may have a proportionately greater impact on increasing yields and contribute to an overall improvement in yield and returns across a cropping program than maintaining stable P throughout sowing.
The opportunity relies on utilisation of the residual soil P pool and careful management and monitoring will be important due to risk of high P removal rates with early sowing.
... Gourley et al. (2019) notes the importance of soil PBI to the determination of critical Colwell P values for pastures. A similar meta-analysis for crop nutrient response relationships in Australia could find no PBI-dependent critical P values (Bell et al. 2013), which the authors attributed to insufficient data and a range of knowledge gaps relating to current cropping practices, including minimum tillage and soil characterisation. In addition, BFDC lists the minimum requirements for a trial to qualify for inclusion in its national dataset. ...
Phosphatic fertilisers have made grazing in the south-west of Western Australia (WA) viable. However, there is evidence that a large proportion of pasture paddocks exceed soil test critical values at which 95% of maximum yield is achieved as identified in the national Better Fertiliser Decisions for Pasture (BFDP) project. Of 22000 soil samples collected between 2009 and 2020, 56% exceeded the critical value for phosphorus (P), although there were constraints to potassium (K) and sulfur (S) and from soil acidity. Soils with available P exceeding the critical value are expected to lead to excessive losses of P to waterways, resulting in eutrophication. A trial program was established to validate the critical P values from BFDP so that concerns can be addressed about the relevance of these critical P values to WA conditions and to contemporary pasture varieties. Measured relative yields for 19 trials in the first year were mostly within 10% of that predicted from BFDP for soils with a P buffering index (PBI) >10. Soils with PBI <10 had measured relative yields up to 25% greater than predicted by BFDP, suggesting response calibrations for low PBI soils may require adjustment in the BFDP dataset. Some pasture yield gaps occurred when soil pH and P were low. Application of nitrogen (N), K and S almost doubled the yield when P was limiting or sufficient. Agronomic advice and practice should seek to optimise these multiple inputs, thereby optimising P use rather than applying P to levels above the critical value.
... Critical values used to indicate deficiency within the 0-10 cm soil layer (Table 1) are taken from Bell et al. (2013aBell et al. ( , 2013bBell et al. ( , 2013c, Brennan and Bell (2013) and Anderson et al. (2013). ...
Balancing nutrient inputs and exports is essential to maintaining soil fertility in rainfed crop and pasture farming systems. Soil nutrient balances of land used for crop and pasture production in the south-west of Western Australia were assessed through survey data comprising biophysical measurements and farm management records (2010–15) across 184 fields spanning 14 Mha. Key findings were that nitrogen (N) inputs via fertiliser or biological N2 fixation in 60% of fields, and potassium (K) inputs in 90% of fields, were inadequate to balance exports despite increases in fertiliser usage and adjustments to fertiliser inputs based on rotations. Phosphorus (P) and sulfur (S) balances were positive in most fields, with only 5% returning losses >5 kg P or 7 kg S/ha. Within each of the three agroecological zones of the survey, fields that had two legume crops (or pastures) in 5 years (i.e. 40% legumes) maintained a positive N balance. At the mean legume inclusion rate observed of 20% a positive partial N budget was still observed for the Northern Agricultural Region (NAR) of 2.8 kg N/ha.year, whereas balances were negative within the Central Agricultural Region (CAR) by 7.0 kg N/ha.year, and the Southern Agricultural Region (SAR) by 15.5 kg N/ha.year. Hence, N budgets in the CAR and SAR were negative by the amount of N removed in ~0.5 t wheat grain, and continuation of current practices in CAR and SAR fields will lead to declining soil fertility. Maintenance of N in the NAR was achieved by using amounts of fertiliser N similar to other regions while harvesting less grain. The ratio of fertiliser N to legume-fixed N added to the soil in the NAR was twice that of the other regions. Across all regions, the ratio of fertiliser N to legume-fixed N added to the soil averaged ~4.0:1, a major change from earlier estimates in this region of 1:20 under ley farming systems. The low contribution of legume N was due to the decline in legume inclusion rate (now 20%), the low legume content in pastures, particularly in the NAR, and improved harvest index of lupin (Lupinus angustifolius), the most frequently grown grain legume species. Further quantifications of the effects of changing farming systems on nutrient balances are required to assess the balances more accurately, thereby ensuring that soil fertility is maintained, especially because systems have altered towards more intensive cropping with reduced legume production.
... A survey of 109,000 contemporary samples revealed that the 43% of the samples had PBI values of 15-35, and about 22% of the samples had PBI values of 35-70 (Weaver and Wong, 2011). The corresponding Colwell P concentrations required for non-limited wheat yields on these soils are 16 and 22 mg Pkg −1 , respectively (Wong et al., 2012;Bell et al., 2013), and were derived from the national Making Better Fertiliser Decisions for Cropping project and database in Australia ( Dyson and Conyers, 2013, bfdc.com.au). Choosing these Colwell P concentrations required for non-limited wheat yields derived from a national dataset that includes hundreds of trials provides confidence that the current Colwell P level, which exceed the critical value of 16 mg Pkg −1 (Fig. 6), will ensure maximum yield, if all other constraints are met. ...
Managing phosphorus (P) is a global priority for environmental water quality due to P lost from agricultural land through leaching, runoff and subsurface flow. In Western Australia (WA), following decades of P fertiliser application to crops and pastures in low rainfall regions, questions have been raised about this region’s contribution to environmental P loss. This study was conducted on the Fitzgerald River catchment in the south Western Australia (WA) with mixed cropping and grazing land uses and a mediterranean climate with low mean rainfall (~350 mm yr⁻¹). Phosphorus forms were monitored continuously over a three-year period in five separate streams, each draining a defined sub-catchment. The P concentrations in streams consistently exceeded Australian and New Zealand Environment Conservation Council (ANZECC) trigger values throughout the monitoring period. Of the measured total P concentration, ~75% was dissolved P (DRP; <0.45 μm) and 80% of that fraction was in the filterable reactive form (FRP). These water quality measurements and other independent soil investigations at this site, suggest that transport of dissolved P rather than erosion of sediment-bound P was dominant in this environment. Based on extractable soil P (Colwell P) and the P buffering index (PBI), predicted concentrations of dissolved reactive P (DRP) in soil solution in topsoils (0-10 cm) across this catchment, generally exceeded ANZECC’s values of 0.07 mg PL-1. The level of exceedance was spatially variable. Streams draining areas with the lowest predicted DRP concentrations also had the lowest measured FRP concentrations. Elsewhere stream water FRP concentrations depended on both DRP concentration and the PBI of the land being drained. Our findings suggest that deployment of practices that physically filter runoff, for example riparian vegetation, would be ineffective in restricting P transport into stream in this environment. This conclusion is consistent with previous findings of the ineffectiveness of riparian buffers on coarse textured sandy soils in higher rainfall areas of south west WA (Weaver and Summers, 2014). A reduction in DRP losses without yield loss could be achieved by following evidence-based fertiliser advice from soil testing to limit losses of legacy P (Rowe et al., 2016).”
... The large differences in mean critical concentrations among crops (0.09-0.58 mg kg −1 STP H2O-CO2 ; 0-36 mg kg −1 STP AAE10 ) at 95% of maximum predicted yield are representative of the variation in crop response to STP in general. A wide range of crop-specific critical concentrations was found in numerous studies (Johnston, 2005;Colomb et al., 2007;Cadot et al., 2018;Sucunza et al., 2018;Smolders et al., 2020), although similar values were also shown for wheat and maize (Tang et al., 2009;Wu et al., 2018) and wheat and barley (Bell et al., 2013). ...
Phosphorus (P) management in agroecosystems is driven by opposing requirements in agronomy, ecology, and environmental protection. The widely used maintenance P fertilization strategy relies on critical concentrations of soil test P (STP), which should cause the lowest possible impact on the environment while still ensuring optimal yield. While both soil P availability and crop yields are fundamentally related to pedoclimatic conditions, little is known about the extent to which soil and climate variables control critical STP. The official P fertilization guidelines for arable crops in Switzerland are based on empirically derived critical concentrations for two soil test methods (H2O-CO2 and AAE10). To validate those values and evaluate their relation to pedoclimatic conditions, we established nonlinear multivariate multilevel yield response models fitted to long-term data from six sites. The Mitscherlich function proved most suitable out of three functions and model fit was significantly enhanced by taking the multilevel data structure into account. Yield response to STP was strongest for potato, intermediate for barley, and lowest for wheat and maize. Mean critical STP at 95% maximum yield ranged among crops from 0.15–0.58 mg kg−1 (H2O-CO2) and 0–36 mg kg−1 (AAE10). However, pedoclimatic conditions such as annual temperature or soil clay content had a large impact on critical STP, entailing changes of up to 0.9 mg kg−1 (H2O-CO2) and 80 mg kg−1 (AAE10). Critical STP for the AAE10 method was also affected by soil pH. Our findings suggest that the current Swiss fertilization guidelines overestimate actual crop P demand on average and that site conditions account for large parts of the variation in critical STP. We propose that site-specific fertilization recommendations could be improved on the basis of agro-climate classes in addition to soil information, which can help to counteract the accumulation of unutilized soil P by long-term P application.
