Equilibrium P Concentration Research Articles

Exploring alternate methods for predicting sorption-desorption parameters for environmental phosphorus loss assessment in poultry litter impacted soils.

Repeated applications of poultry litter (PL) in weathered soils cause soil phosphorus (P) accumulation and is a major environmental concern. Much remains unknown about the distribution of inorganic P (P i ) and organic P (P o ) forms along with their retention and release characteristics in soils that have had a history of PL application. The objectives of this study were to 1) determine P loss risk by investigating the distribution of P i and P o forms, P sorption-desorption parameters, and soil P saturation ratio (PSR; molar ratio of P/[Al + Fe]); and 2) predict the P sorption-desorption parameters using selected soil variables or soil test indicator, PSR. Forty soil samples from 10 random locations within seven pasture fields located in Piedmont soil region were selected and separated into four depths: 0–5, 5–15, 15–30, and 30–45 cm. These fields received annual PL applications ranging from 26 to 92 kg P ha -1 for the past 10 years. The P forms were determined using sequential P fractionation. Soil P retention characteristics were determined using Langmuir isotherm. Soil PSR was determined using oxalate extract (PSR Ox ). Surface 0–5 cm soils had significantly higher concentrations of labile P i , intermediately available P i , organic P pools, higher equilibrium P concentration (EPC), and PSR Ox compared to the lower depths, which indicated saturation of the P sorption sites from repeated PL application. Regression analyses used to predict P sorption-desorption parameters from soil test variables suggest inclusion of multiple soil variables, which may be time-consuming and not always feasible. The strong relationship between P sorption-desorption parameters and PSR Ox indicates that PSR Ox can be effectively used to predict P sorption-desorption parameters for the Piedmont soils without performing time-consuming batch isotherm experiments. Further studies would be needed to validate the relationship for other soil types under different environmental conditions. • P fractionation in poultry litter impacted soils identified intermediately available P i as the dominant pool in 0–5 cm soils. • Surface soils had lower P sorption capacity and higher desorption potential compared to the soils in lower depths. • P saturation ratio (PSR) was strongly related to P sorption-desorption parameters. • PSR can be effectively used to predict P sorption-desorption parameters for Piedmont soil region.

Ranking connectivity risk for phosphorus loss along agricultural drainage ditches

Agricultural drainage systems comprising both in-field pipe drains and surface ditches are typically installed to remove excess water from agricultural land. These drainage networks can provide connectivity between phosphorus (P) sources and surface waters thereby increasing the risk of P loss to rivers and streams. The objective of this study was to derive a farm-scale drainage ranking that categorises drainage ditches in terms of P loss risk based on connectivity and physic-chemical characteristics. Ten pilot farms were selected to characterise drainage networks through ground survey and, sediment and water sampling. Five drainage ditch categories were derived based on landscape setting and connectivity. Each category recorded soluble and reactive P concentrations above environmental water quality standards. To assess the risk of surface ditches as a connectivity vector between agricultural P and surface waters ditches were ranked in order of P loss risk by integrating landscape position and sediment P chemistry. Elevated sediment P with high equilibrium P concentration (EPCo) were associated with ditches connected to farm yards, and in sediment sampled at ditch outlets, suggesting P deposition over time indicative of a legacy P source. The greatest risk of P loss was attributed to ditches connecting farm yards to streams, and ditches that connected the drainage network to surface waters, or Outlets. These results rank connectivity risk for P loss along agricultural drainage ditches for farm level risk assessment to target P loss mitigation measures to the appropriate locations.

Algal Demand Drives Sediment Phosphorus Release in a Shallow Eutrophic Cove

Abstract. Algae play an important role in the internal nutrient cycling of shallow lakes and coves. Algae of shallow water bodies have been shown to thrive and even lead to eutrophic conditions despite a lack of measurable quantities of biologically available phosphorus (P) in the water during summer months. To study how sediment P release and algal growth are connected, water and sediment samples were collected in a shallow eutrophic cove on Beaver Lake in northwest Arkansas. Water quality profiles depicting temperature, dissolved oxygen, nutrients, metals, and photic zone chlorophyll-a were collected weekly from 21 May to 10 July 2018 at three points in the cove: one shallow, one at the cove midpoint, and one at the deepest part of the cove. Cove sediment samples were collected at similar points as the water quality samples for equilibrium P concentration (EPCo) analysis, sediment core incubation, and sediment composition. EPCo for the sediments ranged from 0.024 to 0.027 mg L-1. Sediment cores exposed to aerobic conditions typical of shallow areas had P release rates ranging from 1.37 to 2.02 mg m-2 d-1. Concentrations of soluble reactive P (SRP) in the water column from the weekly water quality sampling averaged 0.002 ±0.003 mg L-1, and photic zone SRP concentrations averaged 0.002 ±0.004 mg L-1 for all sampling sites. The chlorophyll-a concentration increased from 10 to 40 µg L-1 during the period from 21 May to 25 June. When SRP << EPCo, conditions favor the release of SRP from sediments to the overlying water. This was confirmed with the aerobic sediment core incubation in which algal demand was controlled using dark conditions and the release rates were >1.37 mg L-1 d-1. Core aerobic release rates and EPCo conditions both confirmed the release of P under aerobic conditions; however, it appears that algal demand sustained low SRP conditions. This created a nutrient cycle in which algae imposed a nutrient gradient favoring P release by keeping SRP conditions below the EPCo. This study indicates that algal growth potential in shallow water bodies is not limited by SRP concentrations measured within the water column. Studies of shallow water bodies with low SRP concentrations and high productivity should look to the sediments as a source of P to fuel algal growth. Finally, these findings suggest that coves play an integral part in algal production and should not be overlooked when determining the overall P budget for a lake or reservoir. Keywords: Aerobic phosphorus release, Equilibrium phosphorus concentration, Sediment core incubation.

Evaluation of Legacy Phosphorus Storage and Release from Wetland Soils.

To better manage legacy phosphorus (P) in watersheds, reliable techniques to predict P storage and release from uplands, ditches, streams, and wetlands must be developed. Techniques such as the P saturation ratio (PSR) and the soil P storage capacity (SPSC), originally developed for upland soils, are hypothesized to be applicable to wetland soils as well. Surface soils were collected from eight beef ranches within the Lake Okeechobee Watershed, FL, to obtain a threshold PSR value and to evaluate the use of PSR and SPSC for identifying legacy P storage and release from wetland soils. Water-soluble P (WSP) was determined for all soils; the equilibrium P concentration (EPC) was determined for selected soils through the generation of Langmuir isotherms. The threshold PSR for wetland soils, calculated from P, Fe, and Al in a Mehlich 1 solution, was determined to be 0.1; SPSC, calculated using the threshold PSR, was found to be related to WSP. When SPSC was positive, WSP and EPC were minimal. However, both WSP and EPC increased once SPSC became negative. Organic matter (OM) varied from 0.4 to 90 g kg for both positive and negative SPSC, suggesting that OM in wetland soils does not have any effect on P retention and release below the threshold PSR. Moreover, when a wetland or drainage ditch is heavily P impacted, it could be a P source; wetland vegetation may no longer be able to assimilate additional P, resulting in P loss from the soil. This study suggests that the PSR-SPSC concept could be a valuable tool for evaluating legacy P release from wetlands.

The effects of freeze‐and‐thaw cycles on phosphorus availability in highland soils in Turkey

AbstractOngoing global warming may result in colder soil and thawing cycles and will increase the frequency of soil freezing‐and‐thawing‐treated cycles (FTCs) during winter in the cool‐temperate and high‐latitude regions. The purpose of this study was to determine the effects of repeated freeze–thaw cycles on the solubility and adsorption of P in lab and field experiments on Pellustert, Argiustoll, Haplustept, Fluvaquent, and Calciorthid soils, the major soil groups in E Turkey. The results demonstrated that, depending on the soil type, the freeze–thaw cycle could increase the adsorption and desorption of P within a certain temperature range. Repeated freezing and thawing decreased equilibrium P concentration (EPC) and increased P adsorption. EPC and P adsorption were strongly correlated with the number of FTCs. The highest P adsorption and the lowest P desorption was found in Pellustert followed by Argiustoll, Calciorthid, Haplustept, Fluvaquent when refrozen at –10°C for 15 d, then thawed at +2.5°C for 18 h, and 9 times FTC. However, in the field study, the adsorption value was lower than the value obtained from the laboratory condition. It appears that increasing the frequency of freeze–thaw processes depending on increase in temperature that leads to decreased plant‐available soil P pools, thus requires more P fertilizer in soil solution to supply adequate P during the plant‐growth period.

Sorption Indices to Estimate Risk of Soil Phosphorus Loss in the Rathbun Lake Watershed, Iowa

To rank and better understand the risk of P loss from potentially erodible soil materials in the Mollisol-dominated watershed of Rathbun Lake in southern Iowa, we sampled seven representative soil materials at four floodplain sites. We compared the samples by using a variety of characteristics and indices, including particle size distribution; total P, C, and N; P sorption indices; equilibrium P concentration; and degree of P saturation, as assessed by dithionite, oxalate, and Mehlich 3 extraction (M3) methods. None of the Mehlich 3 P values of samples in the present study were high enough to suggest a high risk of water impairment caused by P. Equilibrium P concentration (EPC) values ranged from 0.01 to 0.23 mg L 1 . We found that EPC values were significantly correlated with Fe extractable by oxalate or the M3, as well as with total C and total N. The oxalate and M3 provided generally consistent degree of phosphorus saturation indices, leading us to propose three general risk categories for these soil materials: low, intermediate, and high. We conclude that poorly crystalline Fe oxides and organic matter are likely to exert considerable control over the release of P to stream water from materials eroded from these sites. Moreover, risk rankings based on degree of phosphorus saturation values may not be consistent with interpretations of P mobility that are derived from EPC and sorption indices.

Phosphorus Adsorption and Fractionation in a Two‐phase Olive Mill Waste Amended Soil

Two‐phase olive mill waste (TPOMW) is a by‐product of olive oil extraction, contains up to 90% organic matter, and may be used as soil amendment. To investigate the impact of TPOMW amendments to soil on the fractionation, adsorption, and buffering capacity of P, experiments were conducted on soils collected from a typical olive grove amended for 5 yr with 0, 30, and 60 Mg ha−1 of TPOMW. A modified Hedley fractionation scheme evaluated the change in soil P forms while surface models were used to fit the data. The P fractionation results suggested and increase in HCl‐P fraction. Other P pools, such as residual‐P, NaHCO3–Pi, NaOH‐Pi, NaOH‐Po, soluble‐P, and NaHCO3–Po increased to lesser degrees. A two‐surface Langmuir model fit the adsorption data better than a uniform layer model and underestimation of the P adsorption was observed when the traditional Langmuir equation was used. The TPOMW amendments significantly decreased the P adsorption maxima (α = 0.05) and most of the P was adsorbed on low energy sites, independent of the amendment applications received. The TPOMW soil application decreased indices of P‐binding intensity at both high and low affinity sites, decreased the P equilibrium buffering capacity, and increased the equilibrium P concentration (EPC). Coupled with the low energy of P binding, results implied an increase in the readily soluble P, and thus runoff P losses could potentially increase in TPOMW amended soil.

Comparison of the Phosphorus Sorption Characteristics of a Conservation Reserve Buffer and an Adjacent Crop Field

The U.S. Department of Agriculture's Conservation Reserve Program (CRP) was established by the Food Security Act of 1985. Since its inception, thousands of acres of cropland in stream riparian zones have been converted to conservation buffers through the planting of trees and native grasses. The objectives of this study were to determine the phosphorus (P)–sorption characteristics of the surface soil in a 13‐year‐old CRP buffer and an adjacent continuously cropped production area from which the buffer was created and to assess differences in P‐sorption maxima and P‐buffering capacity between the sites. Phosphorus sorption was modeled with both the simple Langmuir and the two‐surface Langmuir equations. There were significant differences in all P‐sorption parameters between the cropped area and the buffer over most of the depth increments studied. The cropped area soil had higher sorption max (Smax), binding energy (k), and P equilibrium buffering capacity (PEBC) than the buffer soil. However, the buffer had higher equilibrium P concentration (EPC). These findings imply that it may not be appropriate to assume that a buffer will act as a P sink simply because it is not receiving P fertilization. Any assessment of buffer or filter strip effectiveness for P retention should include an examination of the P‐sorption properties of the soils present.

Phosphorus dynamics and loading in the turbid Minnesota River (USA): controls and recycling potential

Phosphorus (P) dynamics in the agriculturally-dominated Minnesota River (USA) were examined in the lower 40 mile reach in relation to hydrology, loading sources, suspended sediment, and chlorophyll to identify potential biotic and abiotic controls over concentrations of soluble P and the recycling potential of particulate P during transport to the Upper Mississippi River. Within this reach, wastewater treatment plant (WWTP) contributions as soluble reactive P (SRP) were greatest during very low discharge and declined with increasing discharge and nonpoint source P loading. Concentrations of SRP declined during low discharge in conjunction with increases in chlorophyll, suggesting biotic transformation to particulate P via phytoplankton uptake. During higher discharge periods, SRP was constant at ~0.115 mg l−1 and coincided with an independently measured equilibrium P concentration (EPC) for suspended sediment in the river, suggesting abiotic control over SRP via phosphate buffering. Particulate P (PP) accounted for 66% of the annual total P load. Redox-sensitive PP, estimated using extraction procedures, represented 43% of the PP. Recycling potential of this load via diffusive sediment P flux under anoxic conditions was conservatively estimated as ~17 mg m−2 d−1 using published regression equations. The reactive nature and high P recycling potential of suspended sediment loads in the Minnesota River has important consequences for eutrophication of the Upper Mississippi River.

Phosphorus fractions and adsorption characteristics in grassland soils of varied soil phosphorus status

Characterization of phosphorus (P) in soils is important both agronomically and environmentally, although the outcome may depend on the technique applied. Consequently, we evaluated fractionation and adsorption, individually and jointly, and relevant ancillary soil attributes, to determine the dominant functional characteristics of soil P in 32 fertilized temperate grassland Inceptisols classified by eight soil series, and by two soil-P index and parent material groups. Residual P was low (30.7%) and organic P (Po) prominent, 42.0% vs. 17.5% for equivalent soils in unfertilized natural ecosystems. Labile fractions comprised 6.8% inorganic P (Pi) and 9.1% Po. The proportional increase in high vs. low index soils (Morgan P > 6.0 mg l−1 vs. ≤ 6.0 mg l−1) was higher for Pi, and highest for labile and moderately labile fractions. Only moderately labile Pi and Po differed significantly between soils of limestone and non-limestone origin. Oxalate extractable Fe (Feox) and buffering (EBC) were higher in the latter. The equilibrium P concentration (EPC) was substantially higher in the high index group, and EBC and binding energy (k) substantially lower, with no significant difference in sorption maximum (Pmax). EBC equated with weak to strong buffering in different soil series, and conformed better than k to ancillary attributes. Pmax correlated in order Alox > clay > OC > Feox, and more broadly reflected sorption attributes than oxalate-based sorption capacity (PSC). Principal component (PC) analysis showed consistent differentiation of P fractions, mostly labile and moderately labile, in PC 1 vs. adsorption and ancillary attributes in PC 2. However, scatterplots of PC scores showed that adsorption characteristics provided better functional differentiation than P fractions for distinguishing individual soil series, which may have implications in selection and interpretation of extractants not only for environmental but also for agronomic soil-tests.

Phosphate and arsenate retention in sediments of the Anllóns river (northwest Spain)

We have studied the distribution and sorption behavior of phosphate and arsenate in bed-sediments of the Anllóns river (NW Spain). As a consequence of the intense gold-mining activity in the past, substantial amounts of arsenic were found in the river sediments. For phosphorus, higher concentrations were found near two sources of P pollution. Sorption isotherms were described by the Freundlich, Langmuir and Tempkin equations. In general, the sediments sorbed more P than As. The equilibrium P concentration (EPC) reveals that sediments act as a scavenger for soluble P; by contrast, equilibrium As concentration (EAC) values were high for the As-rich sediments and correlates well with total arsenic content. Amorphous Fe oxides content, organic matter and fraction of clay plus silt were the main properties of the sediments related with the sorption of arsenate and phosphate. The results obtained provide a first estimate of the sorption behavior and availability of the phosphate and arsenate anions in the sediments of the Anllóns river.

Phosphorus extractability in surface soil samples as affected by mixing with subsoil

Samples taken from the plow layer (Ap horizon)and subsoil (B horizon)of six cultivated soil profiles were analyzed as original samples and as mixtures containing 25% or 50% material from the B horizon. Acid ammonium acetate extractable phosphorus, degree of phosphorus saturation (DPS), and a phosphorus Q/I-plot were determined for each sample and mixture to evaluate the effect of bulking of dissimilar materials on results and to assess the possibilities of reducing P solubility in P-enriched surface soils. The results obtained for the mixtures were compared with mass-weighed average results of the original samples. Measured values of DPS corresponded well and those of acetate-extractable P reasonably well to the estimated values, and the results were linearly correlated with the mass fraction of horizon B material in the mixed samples (r2>0.85).Water-extractable P behaved dissimilarly; the equilibrium P concentration (EPC) estimated from the Q/I-plots decreased dramatically when the fraction of highly sorptive horizon B material increased in the mixture. The marked effect of subsoil material on EPC values may provide a technique to reduce potential losses of soluble P by deep tillage.;

Addition of phosphorus to soils with low to medium phosphorus retention capacities. I. effect on soil phosphorus sorption properties

Twelve soils with low‐medium phosphorus (P) retention capacities were equilibrated for 3 months with soluble phosphate at a rate of 100 mg P kg‐1 soil. The P sorption properties of these soils both with and without added P were studied, including equilibrium P concentration (EPCo), standard P requirement (SPR), soil P sorption capacity (b), maximum buffer capacity (MBC), and P sorption index (P‐SI). In general, the soils with no added P showed low values of all the above parameters. Oxalate extractable aluminum appeared to be the major responsible element for the control of P sorption in these soils. The addition of P to these soils had a considerable effect on their P sorption properties. The changes in EPCo were well correlated with P sorption index (r=0.80; p≤0.01 ). The EPCo values of the soils with and without added P were closely correlated to bicarbonate extractable P (P0lsen) and calcium chloride extractable P (PCaC12), with r=0.80, and r=0.99 (p≤0.001), respectively. Ninety percent of the variability in EPCo was explained by the corresponding variability in POlsen when a curvilinear relationship was adopted. The P sorption properties examined appear to be useful parameters to assess the environmental impact of soil P on the quality of surface waters.

Phosphorus Sorption, Desorption, and Buffering Capacity in a Biosolids‐Amended Mollisol

To investigate the impact of biosolids amendments to soil on the sorption, desorption, and buffering capacity of P, laboratory experiments were conducted on soil samples collected from a field study on a Mollisol amended with three levels of biosolids. The potential for sorption of additional P and the binding intensity of P were evaluated by applying the two‐surface Langmuir model to sorption isotherms. Over the range of equilibrium P concentrations in this study, the ability of the soil to sorb added P decreased due to biosolids amendment. Addition of biosolids to the soil also decreased indices of the P‐binding intensity at both the high‐ and low‐affinity sites. The P equilibrium buffering capacity (PEBC) significantly decreased and the equilibrium P concentration (EPC) significantly increased after biosolids amendment. P desorption from soil samples with and without biosolids amendment was investigated for different equilibration periods and at various liquid/solid ratios. The amount of P that could be desorbed from the soil significantly increased after biosolids amendment. The effects of biosolids amendments on indices of soil P sorption–desorption phenomena (binding energy, PEBC, and EPC) imply a large increase in the P concentration of the soil solution. The increase of soluble forms of P in soil solution of this soil, which was heavily amended with biosolids, could enhance the loss of P in runoff and P movement below the root zone.

Changes in soluble and equilibrium phosphate concentration in selected soils from Italy

Soil phosphate in solution (PsolCpand equilibrium P concentration (EPCo) are important soil P parameters both for agronomical and environmental purposes. Solution P is the main source from which plant roots adsorb P, whereas EPCo give information about the amount and direction of changes between soluble and particulate P that occur during transport of sediment in stream flow. Changes in Psol and EPCo with soil extractable P (Mehlich 3‐P and Olsen P) were determined following equilibration of 13 Italian soils with five rates of fertilizer P (0 to 100 mg kg‐1). The slope of the regression equation (Y=a+bX) for the relationships between Psol and EPCo (Y), and soil extractable P (X) decreased with increasing soil P sorption. Furthermore, at the same level of Mehlich 3 and Olsen P, higher sorbing P soils had lower values of Psol and EPCo. As a result, changes in Psol and EPCo per unit increase of soil extractable P were closely related (R2 of 0.86 to 0.93) to soil P Sorption Index (SI). Equilibrium P Concentration could be estimated from soil solution P for the studied soils.

Influence of cultivation on the distribution of phosphorus in three soils from NE Scotland and their aggregate size fractions

Abstract. The effect of cultivation on the P properties of whole soils and aggregate sizes separated from them was investigated for soil developed on three types of parent material. Cultivation, with associated fertilizer additions, increased both the total soil P content and extractable P of the whole soils. When the soils were separated into aggregate sizes (< 2, 2‐52, 53‐150 and 151 2000 μm), total soil P and resin extractable P increased with decreasing aggregate size. Water extractable inorganic P exhibited the opposite trend, while concentrations of water extractable organic P were similar from each aggregate size. Eroded aggregates can adsorb P or release P to solution, the extent of which will be dependent upon a combination of soil attributes and the prevailing solution conditions. The resulting equilibrium P concentration (EPC) of the eroded material will be markedly different from that of the whole soil. By increasing the P content, particularly of the smaller sized aggregates it is likely that the EPC of the eroded material would favour greater P desorption.

Soil phosphorus sorption and availability as a function of high phosphorus fertilizer additions

Abstract After a 3‐months equilibration of soil with phosphorus (P) (up to four times the respective P sorption capacity), equilibrium P concentration (EPCo), standard P requirement (SPR), P sorption index (SI), and P availability by Bray I, Olsen, water and iron‐oxide paper strip methods were determined on three soils of the Latium region of Italy, widely ranging in their affinity for P. Soil P addition increased EPCo and availability P content and decreased SPR and SI values for all soils with differences between soil types a ffinction of P sorption maximum. The tractional increase of available NaHCO3‐P with added P, i.e. P availability index (F) was 0.486 for the soil with the lowest P sorption maximum, 0.217 for the soil with the highest P sorption maximum, and 0.369 for the third soil presenting an intermediate P sorption (r = 0.997; P<0.01). The results indicate that soil type, in addition to the amount of P added, will determine the potential for a soil to release P to runoff.

Adsorption of Phosphate, Arsenate, Methanearsonate, and Cacodylate by Lake and Stream Sediments: Comparisons with Soils

AbstractThe adsorption of phosphate, arsenate, MSMA (sodium methanearsonate) and cacodylic acid (hydroxydimethylarsine oxide) by 14 sediments from streams and lakes of the Mississippi River alluvial flood plain was measured using a phosphate fixation test and also dilate (1 g/100 mL) sediment‐water slurries at 50 and 500 µg/L P or As. The fixation experiment results were similar to a previous study with soils from the same geographic area in that sorption of the four species were similar and correlated with clay and extractable Fe and Al content. In contrast with the soils results, phosphate was more strongly adsorbed than the arsenicals because the sediments were richer in Fe and Al oxides than soils of the same clay content and the phosphate sorption index was most dependent on sediment Fe and Al content. Conversely, the arsenicals were most correlated with clay or Fe content. Arsenical fixation also exhibited some sediment‐pH dependence indicative of binding with Ca. In the dilute slurry experiments, the high‐clay sediments were extremely adsorptive of all the species and yielded equilibrium P concentration (EPC) values from 0.07 to 0.3 µg/L P. Cacodylate adsorption was dependent on sediment pH. Those sediments with high clay content had a very large unfilled capacity to adsorb phosphate or arsenate, and would act as sinks for P or As inputs into their associated lakes or streams.

Phosphorus Adsorption‐Desorption Characteristics of Two Soils Utilized for Disposal of Animal Wastes

AbstractThe purpose of this study was to determine the effect of animal waste loading rates on P adsorption‐desorption characteristics of two soils. In a laboratory incubation study, Norfolk soil was treated with beef, poultry, or swine wastes; and allowed to decompose under optimum moisture conditions for a period of 30 days. Phosphorus adsorption‐desorption characteristics of the soil were measured at the end of the incubation period. Application of beef, poultry, and swine wastes to a Norfolk soil decreased adsorption capacity of the soil and increased soluble P (in 0.01M CaCl2), acid‐extractable P (0.05N HCl + 0.025N H2SO4), equilibrium P concentration (EPC), and P desorption (after four 1‐hour extractions).In a field study, increased rates of swine lagoon effluent application over a period of 5 years to a Norfolk soil (Site 1) and for 3 years to a Cecil soil (Site 2) also increased soluble P (in 0.01M CaCl2), acid extractable P, P desorbed, and EPC values, and decreased the adsorption capacity. At high loading rates of swine lagoon effluent, soluble P movement occurred to a depth of 75 and 30 cm at Site 1 and 2, respectively. Phosphorus adsorption increased with depth, and EPC values decreased with increasing depths of soil profile. A significant relationship was observed between EPC values, and soluble and acid extractable P.

INTERPRETATION OF PHOSPHORUS UPTAKE BY BARLEY FROM ACID SOILS USING A DIFFUSION MODEL

SummaryPhosphorus uptake by bailey (per cent of dry tissue) grown on four acid soils, each treated with four amounts of lime and P, correlated equally well with EPC (equilibrium P concentration = concentration of P supported in solution at field capacity) and with diffusion supply of P to root cylinder surfaces calculated from the soil indices including EPC, a soil capacity factor maintaining that concentration, and a diffusion coefficient. Calculated rates of P supplied by diffusion to primary root cylinder surfaces were too low to account for P found in plants. If the remainder of the observed P uptake was assigned to root hairs, the ratios of calculated root hair surface area to estimated root cylinder surface areas necessary to account for the difference were in reasonable agreement with ratios for barley reported by other workers.