Soil Test P Research Articles

Quantifying phosphorus loads from legacy-phosphorus fields

Agriculture is a major contributor to eutrophication across the world. Such is the case for Lake Erie where P loads support recurrent harmful cyanobacterial blooms. To mitigate these blooms, targets have been set for total and dissolved reactive P (TP/DRP) loads during the annual and March-July periods. To accelerate progress towards meeting these goals, a targeted management approach aimed at the fields with the greatest P loads has been suggested. A public–private partnership recruited legacy-P fields to quantify P loads in runoff by leveraging the positive relationship between soil test P (STP) and discharge P concentrations. Legacy-P fields were previously defined as having one or more zones with STP >100 ppm (two-fold above agronomic needs); all others were termed agronomic-P fields. Edge-of-field monitoring was performed during 2021 on 11 legacy-P and 13 agronomic-P fields (total n = 24). Legacy-P fields had 2-fold greater March-July DRP loads but only 0.5-fold greater annual TP loads, though both were statistically nonsignificant effects (g≈0.5). Effects were smaller (g≈0.2) for March-July TP and annual DRP. Over half of the legacy-P fields lacked meaningful (<1% of total) surface loads with little to no observed runoff. Further analysis revealed that legacy-P fields had a stronger effect on subsurface discharge, with a statistically significant effect (g = 0.81, p = 0.05) on the March-July DRP concentrations. Statistical significance in this study was obscured due to a large degree of variance and limited sample size, however, the differences reported here could have practical importance to managers working to address eutrophication across Lake Erie and other watersheds.

Decrease in soil test phosphorus levels under omitted phosphorus fertilizer application

AbstractMany European cropped soils have high soil test P (STP) values in the top soil because of P accumulation over many years of fertilizer application. This should allow to save P fertilizer applications for some years without STP values decreasing to a level that might negatively impact crop yield. However, the way STP develops under omitted P fertilizer application is not well understood. We examined STP development under omitted P fertilizer application for timeframes between 7 and 46 years on 96 unfertilized treatments (P0 treatments) of 43 European long‐term P field experiments, using five different STP methods. For comparability, values obtained by different STP methods were converted to Olsen‐P concentrations. We fitted exponential decay curves to Olsen‐P data of each P0 treatment defined by initial Olsen‐P values (Olsen‐Pi), rates of decrease (k) and asymptotes (A), reflecting minimum obtainable STP. Subsequently, we analysed whether the variables most commonly recorded in experiments, are sufficient to explain the variation in model parameters, these variables being P export, clay content, Corg and pH as well as average annual temperature and precipitation. We found that out of our predictor variables, soil clay content, precipitation and temperature were showing the most prominent effects on the parameters Olsen‐Pi, A or k. However, the amount of variation explained by the considered variables was too low to potentially facilitate a prediction of STP decrease, and various P0 treatments showed no clear Olsen‐P decrease or unexpectedly high asymptotes. This hints at a strong influence of the P sorption capacity of the soil with often high potential for replenishment from less available P pools. In connection with P introduction from the subsoil or possibly from surrounding plots, the extension of timeframes of omitted P fertilizer application without reaching critical STP values for crop production, might be explainable. Corresponding effects could not be analysed because of lack of data for most P0 treatments, calling for the additional determination of, for example, the maximum P sorption capacity, total P and subsoil P in future experiments.

Soil phosphorus, hydrological risk and water quality carrying capacities in agricultural catchments

To support profitable agricultural production, nutrients, including phosphorus (P) are applied to soils. However, to avoid over-application and mobilisation of excess P, in-soil concentrations must be maintained at the agronomic optimum (crop requirement) through soil test P (STP) data. Areas above optimum STP (e.g., Olsen P) status have been linked to elevated instream soluble reactive P (SRP) concentrations. For example, when this status is combined with hydrologically sensitive areas (HSAs), excess P can be mobilised and transported directly to surface waters. Catchment carrying capacities for high STP are a possible management strategy to reduce these pressures. The aim of this study was to investigate the transferability of catchment carrying capacity approaches using primary and secondary datasets. Field by field STP status and LiDAR derived HSAs (2 m grid resolution) were compared with instream SRP concentrations using combinations of least squares regressions. The high range of STP catchment carrying capacities (15 % − 44 %, depending on the regression used) was influenced by the variation of instream SRP concentration thresholds (48 – 71 µg L-1) that are determined using altitude and alkalinity factors. However, a single SRP threshold of 35 µg L-1 reduced the catchment STP carrying capacity to a smaller range (10 % − 16 %), with a mean of 13 %. The analysis showed that instream particulate P concentrations were also related to above optimum STP but to a lesser degree and that all HSAs were vulnerable to P loss when soils were above optimum STP. Targeted management strategies should follow a “treatment-train” approach starting with reducing the catchment or farm area above agronomic optimum STP to a carrying capacity (proposed here as 13 %), followed by interception measures located at HSA breakthrough and delivery points to reduce both instream SRP concentration and load.

Long‐term changes in soil phosphorus in response to fertilizer application and negative phosphorus balance under grass rotation in mineral soils in Nordic conditions

AbstractConsiderable amounts of residual fertilizer phosphorus (P) have accumulated in the agricultural soils of Finland since the 1960s, and the P fertilizer application recommendations have been lowered. It is unknown how much P intensively managed silage grass can obtain from the accumulated reserves without a loss of yield. In two field experiments on sandy loam conducted in 2003–2020, four consecutive grass (70% timothy, 30% fescue) rotations were performed (4 or 5 years each, including the establishment year). The grass received mineral P fertilizers (PF; 16 kg P ha−1 year−1), cattle slurry (PS; 11 kg P ha−1 year−1) or no P (P0). The organic P (Po) and inorganic P (Pi) pools in 2003 and 2020 samples were determined following the Hedley procedure using H2O, NaHCO3, NaOH and HCl as sequential extractants. Soil test P (STP) was monitored annually using ammonium acetate extraction. The results showed that the cumulative P balance (P0: −344 to −412 kg ha−1; PF and PS: −101 to −198 kg ha−1) was highly negative, resulting in declining STP. Still, after 18 years, the grass showed no consistent yield response to P fertilizer application. The most significant Pi decline occurred in the Pi–NaHCO3 (~30%) and Pi–NaOH (~50%) pools, while the changes in Po were negligible. This study and international comparisons, Mehlich‐3, degree of P saturation and the result of Hedley in other studies, suggest that these soils, initially above the critical STP level, contain plenty of legacy P and can provide perennial grass with sufficient P for a long time.

Balancing phosphorus fertilization for sustainable maize yield and soil test phosphorus management: A long-term study using machine learning

ContextExcessive phosphorus (P) applications to ensure enhanced crop growth and increased profitability are commonly used, leading to declined P use efficiency and high P accumulation in most cultivated soils across north-western India. However, the effects of consistently applying excessive P fertilizers on soil test P (STP) levels, P availability, and maize production outcomes are poorly understood. ObjectivesThe study aims to find the long-term impacts of different fertilizer P application rates on maize yield, agronomic P efficiency, optimal P rates for yield maximization, and predictive models for sustainable crop production and environmental protection across soils with different STP status. MethodsThe current study collected and analysed maize plant and soil data spanning from 2011 to 2021 from an ongoing long-term field experiment conducted on three different STP status soils i.e., medium P (MPS), high P (HPS), and very high P (VHPS) soils. The experiment involves five different fertilizer P application rates No-P, 13, 26, 39, and 52 kg P ha−1. ResultsThe results indicated a significant increase in maize grain yield and P bioavailability with the application of 13 kg ha−1 of fertilizer P in VHPS (>50.0 kg P ha−1) and 26 kg ha−1 in both HPS (22.5–0.0 kg P ha−1) and MPS (12.5–22.5 kg P ha−1). Improved agronomic efficiency reducing P rate to half of the recommended 26 kg P ha−1 was estimated in VHPS. The economically optimal P rate (EOPR) model consistently sustains crop yield with lower P rates. Among the different machine learning algorithms, multiple linear regression (MLR) was found more reliable in predictive accuracy. The decrease in predicted grain yield and increase in predicted soil P accumulation at P rates of 26 kg P ha−1 and above in varying P status soils underscores the need for judicious P fertilization practices to avoid negative impacts on yield. ConclusionsThe P application required to obtain an average maize yield of 6000 kg ha−1 with relatively high P use efficiency, fertilizer P applications of 35.2 kg ha−1 in VHPS, 37.1 kg ha−1 in HPS, and 41.1 kg ha−1 in MPS based on STP status is more economical. Smart agriculture, using machine learning algorithms helps in ensuring effective P utilization over the long term while minimizing negative ecological impacts. ImplicationsThe sustainable growth of maize crops will necessitate a reduction in P application rates, and P optimization should need further attention to explore the microbial dissolution potential of residual P.

Conventional soil test phosphorus failed to accurately predict dissolved phosphorus release in agricultural hydromorphic soils in Brittany, Western France

Researches have proved that agricultural phosphorus (P) loss contributes significantly to surface water eutrophication. Various soil test P (STP) methods have been developed to assess the P loss risk from agricultural soils. In the intensively-cultivated Brittany region of Western France, hydromorphic soils in wetland domains exhibit high risks of leaching and transferring dissolved P - the most bio-available form of P - to surface waters. It remains unclear whether STP conventionally developed for well-drained soils can accurately predict the risk of dissolved P release from these hydromorphic soils. In this study, we measured the dissolved reactive P (DRP) concentrations in soil solutions sampled in situ from 26 hydromorphic soils in the Brittany region and examined their relationship with several STPs available on the corresponding soils, such as the degree of soil P saturation, the equilibrium soil P concentration, or the soil Olsen P, Dyer P, and water extractable P contents. DRP concentrations ranged from 0.01 to 0.310 mg P l−1 (mean = 0.075 mg P l−1), highlighting the potential of hydromorphic soils as hotspots for DRP release in agricultural landscapes. Correlations between DRP concentrations and STPs were relatively weak (0.09 < r2 < 0.64), indicating that conventional STPs are generally unable to accurately predict the DRP release risks in hydromorphic soils. Tentatively, Olsen P showed promises as a useful risk indicator, with a relatively high r2 value of 0.6 and wide inclusion in the current STP database, especially in the Brittany region. Nevertheless, this hypothesis requires further evaluation with additional data. This study confirms the high risk of dissolved P release from hydromorphic soils in agricultural wetland domains and emphasizes the need for developing specific risk assessment tools to these hydromorphic soils.

Comparison of Bray‐1 and Mehlich‐3 extraction of P and K in Wisconsin silt loam soils

AbstractPhosphorous (P) and potassium (K) fertilizer rate recommendations are based on soil nutrient concentrations, which can be measured using a variety of lab methods. In Wisconsin, Bray‐1 extractant is used to measure soil test P (STP) and soil test K (STK), while most neighboring states measure STP and STK using Mehlich‐3 soil test extractant. This research aims to quantify the relationship between Bray‐1 and Mehlich‐3 extraction methods for both STP and STK for silt‐loam soils in southern Wisconsin. Soil samples were collected from three fields in November 2021. STK and soil test phosphorus STP were measured using both Bray‐1 and Mehlich‐3 extraction methods. The relationship between Bray‐1 and Mehlich‐3 extractants was quantified using linear regression. Soil test phosphorus as measured using Bray‐1 extractant was linearly related to STP as measured using Mehlich‐3 extractant (p &lt; 0.001; Adj. R2 = 0.97), and the relationship between STK results from Bray‐1 extraction and STK results from Mehlich‐3 extraction was also linear (p &lt; 0.001; Adj. R2 = 0.91). For the same soil samples, Mehlich‐3 extractant removed slightly more P and K as compared to Bray‐1 extractant. Due to the strong linear relationship between Mehlich‐3 and Bray‐1 extractants, simple regressions can be used to estimate Bray‐1 STP or STK from a soil test report that was generated using Mehlich‐3 lab methods, and vice versa, on WI silt‐loam soils.

Soil phosphorus balance in Minnesota soils and its effects on soil test phosphorus and soil phosphorus fractions

AbstractPhosphorus (P) fertilizer recommendations based on the build and maintain (B&amp;M) or the sufficiency philosophies were compared at six sites across Minnesota. Various levels of soil test P (STP) levels were established over four growing seasons. Applied P and P removed in harvested grain were monitored and used to develop a soil net P balance (Net P). Linear regression of net P with changes in STP was highly significant. Initial STP levels could be maintained at four of the six sites with a negative Net P. At those same sites, a zero Net P would tend to slowly increase STP over time. A sequential soil P fractionation analysis was conducted on soils from the six sites at the initiation and at the end of the 4‐year period. Nine total soil P fractions were extracted that represented both inorganic P (Pi) and organic P (Po) in the labile and non or less labile soil P pools. A positive linear relationship between Net P and changes in Pi fractions was significant. As Net P increased, so did the changes in Pi in the Resin, BiCarb, and NaOH fractions. These three fractions accounted for 66% to &gt;100% of the changes in Net P with Resin and NaOH accounting for the majority of Net P. Changes in Net P that were accounted for in the less labile P pools, Sonic and HCl fractions, variable, and difficult to determine. There was little effect of Net P on Po fractions.

Influence of soil test phosphorus level and leaching volume on phosphorus leaching

Phosphorus (P) is an essential nutrient in agricultural production, yet its losses contribute to eutrophication in freshwater systems. To mitigate this, there is increasing interest in quantifying soluble P leaching losses and including them in P indices. The objectives of this research were to 1) evaluate water soluble P leaching loss from topsoils (0-15 cm) across a range of initial soil test P (STP) levels; 2) determine the effects of soil type, physiochemical soil properties, and leaching volume on P leaching loss; and 3) determine the predictability of P leaching losses from soil tests. Intact soil cores (3.81 cm in diameter, 0–15 cm deep) were collected from six agricultural fields in Minnesota and leached with deionized water offsite. Additional columns were collected to identify the influence of three leaching volumes on P leaching load. Phosphorus leaching loss was impacted by soil type, initial STP level, and the volume of the leaching event. Leachate P concentration increased with increasing STP level and remained consistent among leaching volumes. Concentrations were not diluted with increased leaching volumes. Consequently, leaching volume was the primary driver of total P load. Several linear and machine learning models were employed to predict P leachate concentration. Phosphorus concentration was best predicted using a Ridge Regression model using two soil tests: the degree of phosphorus sorption (DPSOP) and water extractable P (WEP) (R2 = 0.58, RMSE = 0.06). The model prediction of P leaching loss is promising, although further research on subsoil interactions is needed. This article is protected by copyright. All rights reserved

Thresholds of target phosphorus fertility classes in European fertilizer recommendations in relation to critical soil test phosphorus values derived from the analysis of 55 European long-term field experiments

Phosphorus (P) fertilizer recommendations of individual countries may differ in many aspects, but often the main principle is to reach or maintain a target range of plant-available P in soil. Within this target P fertility class, the soil is expected to supply enough P to the crop, while P fertilization replaces what is exported by the harvested crop. However, the threshold values of the target P fertility classes are based on a multitude of different soil test P (STP) methods and vary by a factor of up to three, even for countries using the same STP method. This study aimed to provide a comparison of the thresholds of target P fertility classes of different European countries and critical soil test P values (Pcrit; STP below which the average relative yield falls below 95% due to P insufficiency) derived from the analysis of data from 55 long-term field experiments in eight European countries. To overcome the issue of diverging STP methods, all values were converted to Olsen-P using empirically based conversion equations from the literature. Converted threshold values varied by a factor of up to five. For the experimental data, we fitted multi-level Mitscherlich-type models to determine Pcrit values of unfertilized soils corresponding to 95% of maximum yield. We found an average Olsen-Pcrit value of 15 mg P kg−1 (adj. R² = 0.37; RMSE = 14.1% relative yield; n = 2368; 55 experiments), which lies far below several country-specific thresholds of target P fertility classes. Crop-specific analyses resulted in higher Olsen-Pcrit values for sugar beet (22 mg P kg−1), potato (19 mg P kg−1) and winter rapeseed (18 mg P kg−1). Among the texture classes (loam, sand, silt and clay), sandy soils exhibited the highest average Olsen-Pcrit value (22 mg P kg−1). We consider a reevaluation of extraordinarily high country-specific thresholds as well as an inclusion of crop type and soil texture (where not already implemented) to be a reasonable measure towards more cost-effective and environment-friendly P fertilization.

A management framework for phosphorus use on agricultural soils using sorption criteria and soil test P

The variation in sorption and desorption of phosphorus (P) among soil types is not captured in current agronomic advice for agri-environmentally sustainable use of P. Phosphorus use is typically based on soil test P (STP) and soils are assumed to have the same rate of response to added P, regardless of sorption properties. The development of P sorption categories, coupled with STP information could improve fertiliser decision making, by making it more site specific and soil type specific. A framework for P sorption specific advice is proposed here integrating soil P sorption dynamics with STP for agronomic and environmental management, at farm and catchment scale. Using a national population of agricultural soils, laboratory measurement of Langmuir sorption maximum (Smax50, mg kg−1) and binding energy (k50, l mg−1), were coupled with STP (Morgan P) to derive novel categories for P management advice, specifically addressing the build-up and draw-down phases of P in soils. In addition to measured values, Smax50 and k50 were predicted from MIR spectroscopy and pedotransfer functions and used to allocate soils into these new sorption categories. The allocation of soils into a P management category using predicted values indicated that pedotransfer functions offered greater reliability (90% allocation accuracy using an independent test set), however MIR spectroscopy is faster and less resource intensive (67% allocation accuracy using an independent test set). Phosphorus sorption dynamics should be interpreted alongside soil test P and P Index information so that water quality policy can consider the difference between high and very high STP soils based on sorption information. In the absence of laboratory data on P sorption, soils can be classified into P management classes using predicted values from spectroscopy (rapid and cheap) or pedotransfer functions (greater reliability). Further development of the MIR methodology is recommended along with field validation.

Estimating dissolved phosphorus losses from legacy sources in pastures: The limits of soil tests and small-scale rainfall simulators.

A legacy of using P fertilizers on grazed pastures has been enhanced soil fertility and an associated increased risk of P loss in runoff. Rainfall simulation has been extensively used to develop relationships between soil test P (STP) and dissolved P (DP) in runoff as part of modeling efforts scrutinizing the impact of legacy P. This review examines the applicability of rainfall simulation to draw inferences related to legacy P. Using available literature, we propose a mixing layer model with chemical transfer to describe DP mobilization from pasture soils where readily available P in the mixing layer is rapidly exhausted and contact time controls DP concentrations responsible for subsequent DP mobilization. That conceptual model was shown to be consistent with field monitoring data and then used to assess the likely effect of rainfall simulation protocols on DP mobilization, highlighting the influence of soil preparation, scale and measurement duration, and, most important, hydrology that can facilitate the physical transport of P into and out of surface flow. We conclude that rainfall simulation experimental protocols can have severe limitations for developing relationships between DP in runoff and STP that are subsequently used to estimate legacy P contributions to downstream water resources.

Crop Production and Phosphorus Legacy with Long-Term Phosphorus- and Nitrogen-Based Swine Manure Applications under Corn-Soybean Rotation

The traditional manure management strategy, based on crop N needs, results in accumulation of phosphorus (P) in soil due to the imbalance of N/P ratio between crop requirement and manure supply. This study was conducted from 2004 to 2013 to evaluate the effects of P-based liquid and solid swine manure (LMP and SMP, for P-based liquid and solid swine manure, respectively) application, in comparison with N-based application (LMN and SMN, for N-based liquid and solid swine manure, respectively), on crop yield and soil residual P under corn (Zea mays L.)–soybean (Glycine max L.) rotation in a Brookston clay loam soil of the Lake Erie basin, ON, Canada. Chemical fertilizer P (CFP) and non-P treatments were included as controls (CK). For liquid manure treatments, corn yield for LMN showed a lower annual corn yield (7.82 Mg ha−1) than LMP (9.36 Mg ha−1), and their differences were even statistically significant at p &lt; 0.05 in some cropping years. The annual corn yield of LMP was also higher than those of SMP (7.45 Mg ha−1) and SMN (7.41 Mg ha−1), even the CFP (8.61 Mg ha−1), although the corresponding yield differences were not significant (p &lt; 0.05) in some cropping years. For soybean, the plots with P application produced an average of 0.98 Mg ha−1 greater annual yields than CK. No significant differences were found between CFP and manure treatments. The annual corn yield of SMN was close to that of the CK (7.19 Mg ha−1). The grain P removal (GPR) of SMN (18.6 kg ha−1) for soybean was significantly higher than that of the other treatments. The above-ground-P uptake (AGPU) in SMN, for both corn and soybean, was significantly higher than that of the other five treatments. The soil test P (STP) presented clear stratification, concentrating in the top 30 cm soil depth after 10 years of application. The contents of STP with LMN and SMN increased from 7.1 mg P kg−1 to 12.4 and 45.5 mg P kg−1, respectively. The sum of STP mass (0–30 cm) with LMP (31.6 kg ha−1) was largely identical to that with CFP (30.1 kg ha−1); however, with SMN (173.7 kg ha−1), it was significantly higher than the rest of the treatments. Manure P source availability coefficients were averaged at 1.06 and 1.07 for LMP and SMP, respectively. The addition of phosphorus-based liquid or solid swine manure can overcome the drawback of traditional N-based applications by potentially reducing the adverse impact on water quality while sustaining crop agronomic production.

Long-term effect of poultry litter application on phosphorus balances and runoff losses.

Assessment of annual and cumulative impacts of phosphorus (P) management strategies at field and watershed scales is needed to improve crop use efficiency and minimize environmental impacts. The objectives of this study were (a) to assess relationships among P balance, soil test P (STP) concentration, and runoff dissolved reactive P (DRP) concentration from fields receiving different poultry litter application rates (0.0-13.4 Mg ha-1 ) and (b) to determine the effect of long-term poultry litter application to fields on watershed DRP loss. Nutrient management practices, crop yield, STP, and runoff losses were assessed from nine fields and two watersheds located near Riesel, TX, from 2000 to 2015. Field-scale P balances were largely controlled by P application rate and exhibited a positive relationship with STP and runoff DRP flow-weighted mean concentration. Using a before-after control-impact experimental design that included monitoring at both field and watershed scales showed the influence of field P management on watershed DRP loss varied according to both source (i.e., P application rate, impacted area) and transport (i.e., hydrological connectivity) factors. Increased risk of watershed DRP loss was observed during wet years and years with two poultry litter applications to fields within the watershed. The percentage of the total watershed area receiving high rates of poultry litter also played a critical role in determining the risk of DRP loss. Findings highlight the impact of long-term P management strategies on DRP loss at both field and watershed scales and show the importance of incorporating hydrologic connectivity when assessing conservation effects on water quality.

Understanding the environmental impact of phosphorus in acidic soils receiving repeated poultry litter applications.

With rising global demand of poultry products, a surge in poultry production would warrant safe disposal of waste byproducts such as poultry litter (PL). A dilemma exists over environmental phosphorus (P) loss risk and agronomic utilization of PL in highly weathered soils with high P fixation capacity. The objective of this study was to determine P forms and their distribution in highly weathered Piedmont soils located in high density poultry operation (HDPO) areas and evaluate environmental P loss risk using soil P storage capacity (SPSC) approach. Soil samples from agricultural fields with 10±2years PL application history were collected from surface (0-15cm) and subsurface (15-30cm) depths. Approximately 64±11% of total P was in non-reactive P (NRP) form, 35±19% in moderately reactive P (MRP) forms, and<1% in highly reactive P (HRP) form. Phosphorus sorption index (PSI) was higher in subsurface (316Lkg-1) compared to surface soils (150Lkg-1). The SPSC calculated based on a distinct soil threshold P saturation ratio (PSR; ratio of P/[Al+Fe], all elements expressed in moles) was higher in subsurface (17mgkg-1) than surface (-150mgkg-1) soils. Repeated application of PL resulted in P saturation of surface soils (SPSC<0) and represents a source of P to the environment. The NRP form decreased, and MRP forms increased when a) soil test P (STP) rating transitioned from low to extremely high, and b) SPSC changed from positive to negative. Results indicate that P release in soil solution is predominantly controlled by buffering action of MRP forms since HRP was minimal and NRP is mostly unavailable in highly weathered soils. A holistic approach that includes STP for maintaining agronomic productivity along with SPSC to minimize environmental P loss risk will be desirable for sustainable management of PL in HDPO.

Mehlich 3 as a generic soil test extractant for environmental phosphorus risk assessment across Alabama soil regions

AbstractIndividual states in the United States have adopted different soil test P (STP) methods for agronomic and environmental purposes. Working between soil regions and state borderlines can be complicated by using multiple STP methods. For example, Alabama uses Mehlich 1 (M1) for noncalcareous soils (Appalachian Plateau [AP], Coastal Plain [CP], Limestone Valley [LV], Piedmont Plateau [PP]) and Lancaster (La) for calcareous soils (Blackland Prairie [BP]). Mehlich 3 (M3) has been proposed as an efficient and possibly a generic extractant for a broader range of soils. The objective of this study was to evaluate whether M3 can be used as a common or generic extractant for environmental P loss risk assessment for both calcareous and noncalcareous soils. More than 769 soil samples were collected from 68 agricultural fields representing noncalcareous and calcareous soils, and their extraction efficiencies for M1, M3, and La were compared. Mehlich 1 extracted 13.8, 13.1, 6.3, and 6.2% of total P; whereas M3 extracted 29.8, 35.4, 10.5, and 14.8% of total P from AP, CP, LV, and PP, respectively. There was no significant difference between La and M3 extractable P for both acidic and alkaline soils of BP. The conversion factor of 1.42 (95% confidence interval [CI95%] = 1.36–1.47) was found suitable to convert M1‐P to M3‐P values for noncalcareous soils, and 0.92 (CI95% = 0.87–0.98) was suitable to convert La‐P to M3‐P values for calcareous soils. The linear relationship between water‐soluble P (WSP) and extractable P was stronger for M3 than for M1 and La for all soil regions, as indicated by a greater correlation coefficient. Results suggest that M3 can be adopted as a generic extractant for environmental P loss risk assessment for calcareous and noncalcareous soils.

Implications of choosing different interpolation methods: A case study for soil test phosphorus

AbstractMany farmers have their fields grid soil sampled to plan for variable rate P fertilizer application. Grid soil samples are often interpolated to create fertilizer application maps. However, most farmers and other practitioners do not compare interpolation methods. The objective of this study was to evaluate the performance of different grid soil sampling interpolation methods on P fertilizer prescription maps. Grid soil samples were collected from six fields and interpolated via geographically weighted regression (GWR), random forest (RF), and inverse distance weighting (IDW). At four out of six site–years, the root mean square error of soil test P (STP) was 7 to 16% lower for the GWR method compared with the RF method, and GWR identified the low‐STP areas better than RF. Geographically weighted regression may outperform RF and IDW because of its lower error and reduced sensitivity to individual high‐ and low‐STP values. Evaluating multiple interpolation techniques and comparing maps both visually and using global error rates can improve the decisions made by farmers and other practitioners.

Spatial Variability of Soil Phosphorus Indices under Two Contrasting Grassland Fields in Eastern Canada

Phosphorus (P) is an essential nutrient for grassland production systems. However, continuous applications of P fertilizers result in soil P accumulations, increasing the risk of P losses in runoff and erosion. This study aims to investigate the field-scale variability of soil-test P (STP) in two contrasting grassland fields using descriptive statistics and geostatistics for accurate recommendations on soil sampling strategy and sustainable approaches to P management. A young grassland (YG; 2 years) and an old grassland (OG; 10 years under permanent pasture) were classified as humo-ferric podzol and received organic fertilizers. Soil samples were collected in 16-m by 16-m triangular grids at two depths (0–5 and 5–20 cm). They were analyzed for available P and other soil elements extracted using the Mehlich-3 method (M3). The agri-environmental P saturation index (P/Al)M3 was calculated. Phosphorus accumulation was observed in OG (0–5 cm) as a result of long-term manure applications. Repeated applications of organic fertilizers can impact the long-term buildup of soil P, thus decreasing soil P va-riability and spatial dependence in permanent grasslands. A soil sampling strategy focusing on the 0–5 cm layer should be retained in permanent grasslands for sustainable P recommendations in Eastern Canada.

Stratified Soil Sampling Improves Predictions of P Concentration in Surface Runoff and Tile Discharge

Phosphorus (P) stratification in agricultural soils has been proposed to increase the risk of P loss to surface waters. Stratified soil sampling that assesses soil test P (STP) in a shallow soil horizon may improve predictions of P concentrations in surface and subsurface discharge compared to single depth agronomic soil sampling. However, the utility of stratified sampling efforts for enhancing understanding of environmental P losses remains uncertain. In this study, we examined the potential benefit of integrating stratified sampling into existing agronomic soil testing efforts for predicting P concentrations in discharge from 39 crop fields in NW Ohio, USA. Edge-of-field (EoF) dissolved reactive P (DRP) and total P (TP) flow-weighted mean concentrations in surface runoff and tile drainage were positively related to soil test P (STP) measured in both the agronomic sampling depth (0–20 cm) and shallow sampling depth (0–5 cm). Tile and surface DRP and TP were more closely related to shallow depth STP than agronomic STP, as indicated by regression models with greater coefficients of determination (R2) and lesser root-mean square errors (RMSE). A multiple regression model including the agronomic STP and P stratification ratio (Pstrat) provided the best model fit for DRP in surface runoff and tile drainage and TP in tile drainage. Additionally, STP often varied significantly between soil sampling events at individual sites and these differences were only partially explained by management practices, highlighting the challenge of assessing STP at the field scale. Overall, the linkages between shallow STP and P transport persisted over time across agricultural fields and incorporating stratified soil sampling approaches showed potential for improving predictions of P concentrations in surface runoff and tile drainage.

Yield response to soil test phosphorus in Switzerland: Pedoclimatic drivers of critical concentrations for optimal crop yields using multilevel modelling

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.