Soil Test Phosphorus Research Articles

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.

Long‐term phosphate fertilization strategies evaluation in a Brazilian Oxisol

AbstractThere are concerns related to the application of phosphate fertilizers to weathered soils that present low soil test phosphorus (STP) due to P adsorption in iron oxyhydroxides. Furthermore, long‐term trials are needed to evaluate crop response to corrective P fertilization and its interaction with different maintenance P fertilization strategies in these soils. An experiment involving the combination of three initial corrective P fertilization schemes (control without P correction or with the application of 105 kg P ha−1 as triple superphosphate [TSP] or reactive rock phosphate [RRP]), four P maintenance strategies (a control without the application of maintenance P, or 35 kg P ha−1 yr−1 as TSP, RRP, or a mix of both) and two application methods was cultivated during 16 yr at the Embrapa Cerrados experimental station in Planaltina, DF, Brazil. Corrective P fertilization promoted an early crop yield response. In contrast, high crop yields were only obtained in control treatments with no corrective P fertilization after soil P stocks were increased to a minimum level. With increasing P stocks, broadcast application resulted in slightly better yields. The required residual P stocks in soil to obtain high yields were estimated as equivalent to 113.6 and 205.2 kg P ha−1 for TSP and RRP, respectively. These values allow for STP contents to increase to critical levels, whose value for TSP of 4.1 mg kg−1 Mehlich‐1 P is below that recommended for the region, possibly due to the contribution of organic P forms in the long‐term no‐tillage system.

Are there environmental or agricultural benefits in using forest residue biochar in boreal agricultural clay soil?

Short-term agronomic and environmental benefits are fundamental factors in encouraging farmers to use biochar on a broad scale. The short-term impacts of forest residue biochar (BC) on the productivity and carbon (C) storage of arable boreal clay soil were studied in a field experiment. In addition, rain simulations and aggregate stability tests were carried out to investigate the potential of BC to reduce nutrient export to surface waters. A BC addition of 30 t ha−1 increased soil test phosphorus and decreased bulk density in the surface soil but did not significantly change pH or water retention properties, and most importantly, did not increase the yield. There were no changes in the bacterial or fungal communities, or biomasses. Soil basal respiration was higher in BC-amended plots in the spring, but no differences in respiration rates were detected in the fall two years after the application. Rain simulation experiments did not support the use of BC in reducing erosion or the export of nutrients from the field. Of the C added, on average 80% was discovered in the 0–45 cm soil layer one year after the application. Amendment of boreal clay soil with a high rate of BC characterized by a moderately alkaline pH, low surface functionalities, and a recalcitrant nature, did not induce such positive impacts that would unambiguously motivate farmers to invest in BC. BC use seems unviable from the farmer's perspective but could play a role in climate change mitigation, as it will likely serve as long-term C storage.

Soil phosphorus testing for intensive vegetable cropping

AbstractEnvironmental concerns and rapidly decreasing phosphorus (P) resources caused a renewed interest in improving soil P tests for a more efficient P fertilization. This led to the development of better P fertilizer recommendation systems for major arable crops and grass. Nevertheless, these P fertilizer recommendation systems seem to fail for intensive vegetable crops, with often a very short growing season and limited rooting system. This leads to low P use efficiencies in the horticultural sector. In order to address this problem we set up a study to answer following questions: (1) which soil P test predicts the plant available P content for intensive vegetable crops the best and (2) can new insights, such as combining different soil P tests, improve P fertilizer recommendations for intensive vegetable crops? To this end, bulk samples of 41 soils with very different P status (based on ammonium lactate extractable P) were collected. The plant available P content of these soils was determined using six commonly used soil P tests (P‐CaCl2, P‐water, P‐Olsen, P‐acetate, P‐lactate, and P‐oxalate) and a P fertilizer pot experiment with endive (a very P sensitive vegetable crop) was conducted. Six pots of each soil were planted with endive. Three of these pots received no P fertilization (0P) and three pots received ammonium polyphosphate equivalent to 24 kg P ha−1 (24P). All other factors were kept constant. Relative crop yield of the 0P fertilized plants compared to the 24P fertilized plants was determined. Plotting these relative yields against the P status of the soil per soil P test allowed to fit a Mitscherlich curve through the data. Also the combination of two different soil P tests to predict the relative yield with a Mitscherlich equation was evaluated. The coefficients of variation of the soil P tests, the R2 values and the relative standard errors of the parameter estimates revealed that P‐acetate and P‐water predicted the relative yield of the 0P plants the best and that combining two different soil P tests gave no extra predictive power. This finding may form the basis for the development of a new P fertilizer recommendation system for intensive vegetable crops, leading to an improved P use efficiency in horticulture. In order to develop this new system more data relating soil P test values with RY of intensive vegetable crops should be collected.

Legacy effects of liquid dairy manure in grain production systems

Transferring liquid manure from dairy farms to nearby cash-grain farms has the potential to support high grain yields at reduced cost, while making use of animal wastes that might otherwise cause environmental hazards if not utilized appropriately. Yet, empirical evidence of the legacy of long-term liquid manure application, and its interaction with crop rotation management on grain yield and soil properties is scarce. Here we evaluate the benefits and legacy of repeated liquid dairy manure (115 Mg ha−1 yr−1) over 12 years (2003–2014) on corn yield, soil organic matter (SOM) content, and soil nutrient availability in three annual cash grain cropping systems under different crop rotation management in southern Wisconsin. The three systems include: continuous corn (CC); minimum-till corn-soybean (CS); and an organically managed corn-soybean-wheat/oat-clover rotation (CSW). Results showed that, liquid manure application increased corn yield, soil test phosphorus (STP), and soil test potassium (STK) content across the three systems, and soil pH and nitrate (0–90 cm) in CC over the 12 years. The lack of significant impact of manure on SOM content suggested that the liquid manure application was insufficient to build SOM on these fertile Mollisols. The legacy benefit of liquid manure application on corn yield was significant in 2015, but no longer detectable in 2017, three years after the last application, while STP and STK content remained higher in manure treated plots than no manure plots. In 2017, cation exchange capacity (CEC), soil pH, and soil nitrate content were also greater in manured plots, but the effect was crop rotation specific. We show that manure is an equivalent or superior substitute for synthetic fertilizers providing a potential win-win scenario for both grain and dairy farms in the North Central US, where synthetic fertilizer is costly (cash-grain farms) and land for manure spreading is often limited (dairy farms). Our study suggest that liquid dairy manure application is beneficial for agroecosystem productivity but continual applications are needed to sustain its benefit in the crop production systems in the region.

Soil test phosphorus and phosphorus balance trends: A county‐level analysis in Ohio

AbstractOff‐site transport of phosphorus (P) is a concern with Lake Erie and other Ohio surface waters. Baseline knowledge of and long‐term trends in soil test P (STP) and P balance are important as farmers are under increased scrutiny due to ongoing water quality concerns. The objectives of this work were to examine Ohio county‐level trends in STP, P inputs, P outputs, and P balance. Soil test P data from all 88 Ohio counties were obtained from 1993 to 2015 from the three major soil test laboratories that service Ohio farmers. County‐level P inputs (fertilizer and manure), outputs (crop removal), and balance (inputs minus outputs) were obtained from the International Plant Nutrition Institute's Nutrient Use Geographic Information System database. Regression analysis techniques were used to identify long‐term trends. Across multiple percentile levels, 44–63% of Ohio counties had decreasing long‐term STP trends, while 11–21% had increasing trends. Eighty‐four percent of Ohio counties had a negative P balance in 2014, meaning P outputs exceeded P inputs. Decreasing long‐term trends in STP and P balance, suggest that farmers are taking steps to manage P inputs and STP, which should result in reduced P runoff concentrations.

Predicting soil test phosphorus decrease in non‐P‐fertilized conditions

AbstractMonitoring the availability of phosphorus (P) in soil under continuous cropping facilitates finding deficiency in crops and contributes to improving crop growth and nutrient management models. Soil P availability for crops is usually estimated by soil test P (STP), such as Bray‐1. This is widely used in the Americas. The relationship between the decrease of STP Bray‐1 and cumulative removal of P was evaluated in non‐P‐fertilized areas in long‐term studies. This removal was the sum of annual P removal over the study period as P exported in grains/crop outside the soil. The objectives were to: (a) quantify changes in STP as a function of cumulative P removal, (b) assess the relationship between relative decrease rate of STP and soil variables as well as annual removal of P by crops, and (c) develop a model to predict decrease of STP Bray‐1. Exponential decay functions were used to describe annual cumulative removal of P and STP from soil over time for 12 long‐term studies where no addition of P fertilizer was carried out. Changes in the relative rate of decrease of STP, relative to the initial STP Bray‐1 value at the onset of the experiment, were predicted by the ratio of soil organic matter to clay and silt and the average annual P removal by exponential decay (R2adj = 0.64; RMSE = 3.2 mg kg−1). We propose this predictive model as suitable to provide estimates of the relative decrease rate of STP by Bray‐1 and thereby improve management of P for optimizing crop yield.Highlights STP Bray‐1 decrease and cumulative P removal were related by exponential decay functions. Relative decrease rate of STP Bray‐1 was related to SOM/(clay+silt) ratio and annual P removal. A predictive model of the relative decrease rate of STP Bray‐1 was fitted and validate. Our model is a useful tool to help predict soil P availability and nutrient management.

Impact factors and mechanisms of dissolved reactive phosphorus (DRP) losses from agricultural fields: A review and synthesis study in the Lake Erie basin

Dissolved Reactive Phosphorus (DRP) losses from agricultural fields promote algae growth in water bodies, and may increase the risk of Harmful Algal Blooms (HABs). Using existing data from the Lake Erie Basin, we applied multiple regression analysis to better understand the impacts of both site-specific conditions (e.g., soil types/properties) and management practices (e.g., Agricultural Conservation Practices [ACP]) on annual DRP losses in subsurface and surface runoff. Results showed that soil properties significantly impact DRP losses. Greater DRP losses were associated with increased soil pH and Soil Test Phosphorus (STP). By contrast, soil organic matter (SOM) was inversely correlated with DRP losses. Soil clay content was also inversely correlated with DRP subsurface losses, but had no impact on DRP surface losses. The ACPs evaluated had varied effectiveness on DRP loss reduction. Cropping systems involving soybean could reduce DRP subsurface losses, whereas winter cover crops could cause unintended DRP subsurface losses. Cropping systems involving soybean and cover crops, however, had no impact on DRP surface losses. In addition, no-till and conservation tillage also enhanced DRP losses compared to conventional tillage, particularly for soils with high SOM and/or high clay content.Precipitation amount and fertilizer application rate significantly increased DRP surface losses as expected. Fertilizer application rate, however, had no impact on DRP subsurface losses. The impact of precipitation amount on DRP subsurface losses depends on STP levels. Precipitation amount significantly increases DRP subsurface losses when STP is higher (>41 mg kg−1 in this analysis). The optimal STP level for crop growth is 30 to 50 mg kg−1.Results from this study help us to better understand DRP losses and the effectiveness of ACPs for controlling them. We suggest taking soil surveys and soil tests into consideration when designing and/or implementing ACPs to manage DRP losses. Furthermore, the method we used for this study could be applied to other agricultural regions to investigate impacts of site-specific conditions and management practices on water quality.

Relationships between the Haney H3A and Conventional Soil Tests for Phosphorus and Potassium in Kansas Soils

The Haney H3A soil test procedure has gained popularity in recent years for soil health evaluation and has been used in some circles to adjust fertilizer management practices. However, data relating this test to current soil tests, relative crop yield, or total nutrient uptake are nonexistent in Kansas soils. The objective of this study is to evaluate the correlation between H3A soil test phosphorus (P) and potassium (K) with soil tests currently used in Kansas (e.g. Mehlich-3). Soils from a nitrogen response study were extracted using both Mehlich-3 and H3A (version 4) soil test procedures. Mehlich-3 and Haney extractable P and K were positively correlated (r = 0.90 and 0.91, respectively) in data combined from all sites. Linear regression models fit to the combined data indicate that Mehlich-3 extracts approximately 25% more P and 250% more K. The Root Mean Square Error (RMSE) of these models (15.4 ppm P and 83.4 ppm K) indicate that existing calibration based on Mehlich-3 values are likely not suitable for use with H3A-4.

High Soil Test Phosphorus Effect on Corn Yield

Low corn prices have farmers scrutinizing their nutrient dollar return on investment (ROI). A recent University of Nebraska–Lincoln study examines phosphorus (P) application level ROI in continuous corn over six years. The most cost‐effective method of fertilizer P use proved to be a P‐replacement approach with the P rate equal to the P removed from the previous harvest. The research also found that maintaining soil P availability levels above 25 ppm (Bray‐1) wasn't justified. Earn 0.5 CEUs in Nutrient Management by reading this article and taking the quiz at www.certifiedcropadviser.org/education/classroom/classes/706.

Manure Management at Ohio Confined Animal Feeding Facilities in the Maumee River Watershed

In 2015, 48 permitted Confined Animal Feeding Operations (CAFOs) housed approximately 90% of poultry and 20% of swine and cattle within the Ohio portion of the Maumee River watershed. Recently, concerns about the impact CAFOs may have on nutrient loading in the watershed have been raised. In this study, we used manure management plans and inspection reports obtained from the Ohio Department of Agriculture Division of Livestock Environmental Permitting (ODA-DLEP) to assess how these CAFOs managed their manure for the years 2014 and 2015. A majority of liquid manure was applied between April and October, closely matching the amount of liquid manure planned to be applied during this period. Approximately 79% of the acres under control of the CAFOs that received manure had Bray P1 soil test phosphorus values below 50 ppm. The average distance between a swine CAFO’s livestock holding barn to the fields they control that can receive manure was 1.43 miles while for cattle CAFOs this distance was 1.91 miles. Approximately 78% of manure phosphorus generated on CAFOs was planned to be transferred through Distribution and Utilization, a process in which ownership of manure changes hands, including virtually all solid poultry manure phosphorus. While publicly available data show that, in general, CAFOs in the region are adhering to their state-approved permits, a knowledge gap regarding the management of approximately 80% of manure phosphorus exists due to manure transferred through Distribution and Utilization and manure produced from non-permitted livestock operations.

Soil phosphorus testing on alkaline calcareous soils

Soil phosphorus testing made great strides with multiple chemical tests proposed and implemented for crop fertilizer management programs during the previous century. In the latter part of the previous century, the environmental impact of excess nonpoint phosphorus loading from the landscape (e.g., agricultural lands) to waterbodies became an issue of increased concern, and soil phosphorus testing came to the forefront of management and monitoring. This article will provide a general overview of the usage of phosphorus testing for agronomic purposes in the United States Earn 0.5 CEUs in Nutrient Management by reading this article and taking the quiz at www.certifiedcropadviser.org/education/classroom/classes/725.

Corn root and soil health indicator response to no-till production practices

Crop production guidelines in Brazil currently focus on soil fertility or chemical indicators (i.e., soil pH and nutrient availability) but often neglect physical and biological aspects of soil health. No-tillage (NT) practices have been widely adopted in Brazil primarily to mitigate erosion, but chemical-based guidelines associated with those practices may be contributing to nutrient stratification and a concentration of crop roots within the top few centimeters of the soil profile. Our objective was to quantify relationships between soil physical and chemical attributes and root growth in soils with a long-term NT history (>10-yr). Crop root development and several soil health indicators were measured within the 0 to 7-, 7- to 17- and 20- to 30-cm depth increments of two long-term NT studies on Rhodic Hapludox soils in Brazil. Data from both experiments showed strong organic carbon (OC) stratification, significant differences in soil physical and chemical attributes, and root systems that were concentrated within the surface layer. Soil compaction below 7-cm appeared to be the dominant factor limiting root growth. Soil-test phosphorus (P) and OC were also correlated to root growth. Based on these analyses and other on-farm observations, we concluded that it is important to recognize that despite its benefits, NT practices can result in plant root stratification. Therefore, it is important to not only monitor soil fertility/chemical conditions, but also soil structure and biological indicators of soil health when developing NT guidelines for Brazil.

Quantity and quality of water percolating below the root zone of three biofuel feedstock crop systems

Biofuel demand is on the rise, yet there is limited information available about the likely implications of increased planting of cellulosic biofuel crops on water and nutrient fluxes. To help address this gap, this study quantifies subsurface water, nitrate, and dissolved phosphorus fluxes below feedstock crops managed for biofuel production on naturally well drained soils in southern Wisconsin. Automated equilibrium tension lysimeters were used to measure subsurface (below root zone) fluxes of water and nutrients below continuous corn (CC), monoculture switchgrass (SG), and hybrid poplar (HP) cropping systems for three years. Average water fluxes below SG were nearly double those below CC and HP. Most of this difference can be linked to seasonal events, particularly during late winter and early spring when subsurface water fluxes and increased soil moisture were observed below SG plots, likely due to enhanced macropore flow. Nitrate fluxes were approximately 14 times higher below CC than the 37.4 kg NO3-N/ha three year annual average observed for SG; the nitrate fluxes were negligible below HP despite similar nitrogen application rates. The high flux in one of the two CC plots is likely due to excessive manure applications prior to the study period; however, the nitrate loss potential from CC is much higher than the perennial crops. Dissolved reactive phosphorus (DRP) concentrations were very low below all three cropping systems (average of 0.076, 0.083, 0.034 mg/L in SG, CC, and HP respectively). For the cropping systems evaluated, nitrate fluxes were driven by concentration differences in leachate and not influenced by nominal differences in N fertilizer inputs, whereas DRP fluxes were mainly driven by water fluxes and therefore independent of phosphorus inputs and soil test phosphorus. The data in this paper provide a foundation to better understand differences in fluxes below different biofuel cropping systems.

Impact of an Extreme Storm Event on River Corridor Bank Erosion and Phosphorus Mobilization in a Mountainous Watershed in the Northeastern United States

AbstractMovement of sediment, and associated phosphorus, from stream banks to freshwater lakes is predicted to increase with greater frequency of extreme precipitation events. This higher phosphorus load may accelerate harmful algal blooms in affected water bodies, such as Lake Champlain in Vermont, New York, and Québec. In the Mad River, a subwatershed in central Vermont's Lake Champlain Basin, extreme flooding from Tropical Storm Irene in 2011 caused extensive erosion. We measured stream channel change along the main stem between 2008 and 2011 by digitizing available prestorm and poststorm aerial imagery. Soils were sampled post Irene at six active stream erosion sites, using an experimental design to measure differences in soil texture and phosphorus both with depth (90 cm) and distance from the stream. In addition to total phosphorus (TP), we determined bioavailable (soil test) phosphorus (STP) and the degree of phosphorus saturation (DPS). The six sites represented a 0.87‐km length of stream bank that contributed an estimated 17.6 × 103 Mg of sediment and 15.8 Mg of TP, roughly the same as average annual watershed export estimates. At four sites, the STP and DPS were low and suggested little potential for short‐term phosphorus release. At two agricultural sites where the lateral extent of erosion was high, imagery showed a clear loss of well‐established riparian buffer. Present‐day near‐stream soils were elevated in STP and DPS. An increase in these extreme events will clearly increase sediment loads. There will also be increasing concentration of sediment phosphorus if stream banks continue to erode into actively managed agricultural fields.

Phosphorus availability in selected soils of Rwanda : Comparison of four test procedures

Low levels of plant-available phosphorus (P) are a major factor limiting crop production in sub-Saharan Africa. Soil tests for phosphorus in this region have been applied mainly to highly weathered tropical lowland soils, while the number of investigations on the P status of less weathered tropical highland soils is limited. In this study, we evaluated four soil P tests with regard to their suitability for typical tropical highland soils and Technosols on mine spoils from pegmatite dump material in the Gatumba Mining District (GMD) of Rwanda. The tests comprised Bray 1, Mehlich 3, Olsen P and Pi test. Twelve soils (eleven from Rwanda and a reference soil from Germany) were analyzed for available P. The results reflected extremely low levels of available P in the Rwandan soils compared to the soil from Germany. A ranking from highest to lowest measured P followed the order Mehlich 3 > Bray 1 > Olsen > Pi. High correlations were found between Bray 1, Mehlich 3 and Pi. Extremely low levels of P extracted with Pi compared to Bray 1 and Mehlich 3, however, raised the question of its suitability for the investigated soils. Values of Olsen P generally showed low reproducibility and low correlation coefficients. Results from regression analyses with pH, clay content and soil organic matter also confirmed limited suitability of Olsen. A regional calibration for the detection of plant-available P in the GMD was successful for Bray 1 and Mehlich 3 demonstrating that these were suitable for all soils investigated.

Mitigating Soil Phosphorus Release Using Liquid Water Treatment Residuals

Water treatment residuals (WTRs) have been land‐applied at 20% solids to reduce phosphorus (P) release, but liquid WTRs (2% solids) may be more effective. Our objectives were to show the variability of phosphorus release within a field treated with solid WTRs, determine an optimal application rate for liquid WTRs, and evaluate the long‐term effectiveness of that rate. Six liquid WTR treatment rates (220–4,400 kg/ha) were applied in triplicate to soil cores at varying soil test phosphorus (STP) levels and then flooded. Soluble reactive phosphorus was measured in overlying water, release rates were calculated, and the soil was analyzed for Mehlich‐III and water extractable phosphorus. Release rates were highly variable (2–14 mg/m2/h) within a WTR‐treated field. A small application rate (440 kg/ha) was sufficient to control release at an STP level <100, but greater or repeated application may be necessary for long‐term control.

The Challenges of Managing Legacy Phosphorus Losses from Manure-Impacted Agricultural Soils

Historical application of manure to agricultural land in areas of intensive animal production, like the Delmarva Peninsula, has led to soil test phosphorus (STP) concentrations that far exceed agronomic optimum. While natural soils would typically serve as a sink for newly applied P sources, soils with P accumulation from long-term manure applications often serve as a source of P via a gradual release of dissolved P in runoff or leaching events. These losses of “legacy P” from manure-impacted soils are difficult to control and are linked to water-quality degradation in sensitive water bodies, like the Chesapeake Bay. In this review, we examine how long-term application of manure impacts soil P dynamics under P-buildup and -drawdown scenarios to better understand the behavior of legacy P in soils. We also examine the reasons that traditional best management practices (BMPs) fail to control legacy P losses from soils and discuss potential promising management strategies using the Delmarva Peninsula as a case study.

Four soil phosphorus (P) tests evaluated by plant P uptake and P balancing in the Ultuna long-term field experiment

Soil phosphorus (P) availability was assessed with four different soil P tests on seven soils of the Ultuna long-term field experiment (Sweden). These four soil P tests were (1) P-H<sub>2</sub>O (water extractable P); (2) P-H<sub>2</sub>O<sub>C10</sub> (water extractable P upon 10 consecutive extractions); (3) P-AL (ammonium lactate extractable P) and (4) P-C<sub>DGT</sub> (P desorbable using diffusive gradients in thin films). The suitability of these soil P tests to predict P availability was assessed by correlation with plant P uptake (mean of preceding 11 years) and soil P balancing (input vs. output on plot level for a period of 54 years). The ability to predict these parameters was in the order P-H<sub>2</sub>O<sub>C10</sub> > P-C<sub>DGT</sub> > P-H<sub>2</sub>O > P-AL. Thus, methods considering the P-resupply from the soil solid phase to soil solution performed clearly better than equilibrium-based extractions. Our findings suggest that the P-AL test, commonly used for P-fertilizer recommendations in Sweden, could not predict plant P uptake and the soil P balance in a satisfying way in the analysed soils.

Soil Test Phosphorus and Phosphorus Availability of Swine Manures with Long‐Term Application

Core Ideas Net P additions of 9.8 to 16.1 kg ha−1 were needed to increase soil test P by 1 mg kg−1. Manure P source availability coefficients were 0.99 for liquid manure, 1.08 for solid manure, and 0.97 for composted manure. The long‐term P availability was identical, regardless of the form of swine manure. The long‐term availability of P from swine manure was similar to that of chemical fertilizer. Understanding the form‐specific long‐term effects of manure (liquid, solid, and composted) application on soil P is essential for nutrient management planning. This 8‐yr study aimed to quantify changes in soil test P (STP, Olsen‐P) with application of three forms of swine manure compared with inorganic fertilizer and to calculate the manure P source availability coefficient (PSAC) as the ratio of the specific manure form to inorganic P in providing crop P availability. The STP content increased linearly with both total and net P addition at the surface (0–15 cm) and subsurface (15–30 cm) soil depths, regardless of P source. The slope of STP vs. total or net P addition in the surface layer was steeper for liquid than for solid manure, whereas in the subsurface layer, composted manure showed a greater slope than either liquid or solid manure. The amount of applied P required to increase STP by one unit in the surface layer was 19.9, 15.7, 31.6, and 20.9 kg P ha−1 for inorganic fertilizer and liquid, solid, and composted manure, respectively. When increases in subsoil STP and grain P removal were considered, the net P additions of inorganic fertilizer and liquid, solid, and composted manure needed to increase STP by one unit were 12.2, 9.8, 16.1 and 10.7 kg ha−1, respectively. The PSAC values were 0.99, 1.08, and 0.97 for liquid, solid, and composted manure, respectively. The long‐term availability of swine manure P was largely identical among forms and was similar to that of inorganic fertilizer.