Crystallization of single and binary iron‐ and aluminum hydroxides affect phosphorus desorption

In acidic soils, phosphorus availability is affected by its strong affinity for mineral surfaces, especially Fe‐ and Al‐hydroxides. Plant roots have developed adaptive strategies to enhance the availability of phosphorus, including producing and exuding low molecular weight organic acids with a high affinity for phosphorus that competes with high molecular weight organic ligands formed during humification and mineralization. The aim of this study was to characterize the kinetics and mechanism of phosphorus desorption from Fe‐ and Al‐hydroxides of variable crystallinity, as well as binary Fe:Al‐hydroxide mixtures. Long‐term desorption experiments (56 days) were conducted with CaCl₂, CaSO₄, citric acid, and humic acid as competitive sorptives. CaCl₂ and CaSO₄ were selected as general inorganic sorptives and citric and humic acids were selected as organic ligands produced by organisms in the rhizosphere or following humification. The cumulative phosphorus desorption increased following the order CaCl₂ < CaSO₄ < humic acid < citric acid. Amorphous ferrihydrite and Fe‐rich Fe:Al‐hydroxides exhibited much less desorption when exposed to inorganic solutions than the crystalline and Al‐rich Fe:Al‐hydroxide mixtures. Models of the desorption data suggest phosphorus desorption with citric acid is diffusion‐controlled for ferrihydrite and Fe‐rich amorphous Fe:Al‐hydroxides. When humic acid was the sorptive, metal‐organic complexes accumulated in the solution. The results suggest organic compounds, especially citric acid, are more important for liberating phosphorus from Fe‐ and Al‐minerals than inorganic ions present in the soil solution.     

低分子量有机酸对土壤磷活化影响的研究

研究两种低分子量有机酸（柠檬酸和苹果酸）对土壤磷活化影响,并用修正的Hedley法测定土壤磷活化前后磷组分的变化。结果表明,低分子量有机酸能持续活化土壤磷,活化强度随低分子量有机酸浓度的增大而增强,并且柠檬酸活化土壤磷的能力强于苹果酸。低分子量有机酸能促进作物有效态无机磷组分（H2O-P和NaHCO3-Pi）的释放;同时还促进有机磷组分（NaHCO3-Po和NaOH-Po）的矿化。在低分子量有机酸浓度达到0.5 mmol/L以上时,其对土壤磷组分的活化量的顺序为:NaOH-Pi HCl-P NaHCO3-Pi H2O-P,即铁铝结合态磷 钙结合态磷 作物有效态磷。低分子量有机酸活化土壤磷的过程中伴有大量铁、铝释放,且铁或铝的释放量与磷活化量之间显著正相关（P0.05）。说明铁、铝结合态磷是低分子量有机酸活化土壤磷的主要磷源,并且其活化机制可能与铁、铝结合态磷的螯合溶解有关。     

Heavy Metal Release from Some Industrial Wastes: Influence of Organic and Inorganic Acids, Clay Minerals, and Nanoparticles

Knowledge about heavy metal release from industrial solid wastes(ISWs) is crucial for better management of their environmental risks. This study was conducted to investigate the effect of organic and inorganic acids, clay minerals, and nanoparticles(NPs) on the release of heavy metals from sugar factory waste, ceramic factory waste, leather factory waste, and stone cutting waste. The influence of the extractants on heavy metal release from these ISWs was in the following descending order: citric acid oxalic acid nitric acid≥ sulfuric acid Ca Cl2. Addition of clay minerals and NPs as adsorbents decreased heavy metal release, which was significantly lower in NP-treated wastes than in the clay mineral-treated wastes. On the other hand, the presence of organic and inorganic acids increased heavy metal adsorption by NPs and clay minerals. These results suggest that NPs can be applied successfully in waste remediation,and organic and inorganic acids play an important role in the removal of heavy metals from the studied adsorbents.     

有机酸对五种人工合成磷酸盐活化作用及活化途径的研究

采用实验室合成和化学浸提方法,研究了高分子量腐殖酸和低分子量柠檬酸在不同浓度条件下对五种人工合成磷酸盐的释磷效应及活化途径。结果表明:两种有机酸均能促进五种人工合成磷酸盐中磷素的释放,且释磷量随有机酸浓度的增加而增大;其中柠檬酸的活化能力明显强于腐殖酸;腐殖酸对三种钙磷的活化效果较好,而Fe-P和Al-P基本不受其影响或影响很小。柠檬酸通过质子和有机阴离子的共同作用来活化人工合成Ca2-P、Ca8-P和Al-P中的磷,Ca10-P中磷的活化几乎完全依靠质子作用,而低浓度有机酸作用下Fe-P的活化则来源于有机阴离子的作用。     

柠檬酸对不同施肥处理灰漠土遗留磷的活化特征

明确土壤磷的活化潜力及释放特征是土壤遗留磷资源化利用的前提,以长期定位施肥的灰漠土为试验材料,选取不施肥（CK）、施氮磷钾化肥（NPK）、氮磷钾化肥与有机肥配施（NPKM）3种处理,以高浓度（10 mmol·L^-1）和低浓度（2 mmol·L^-1）柠檬酸对土壤遗留磷进行多次浸提,探究柠檬酸对长期不同施肥处理灰漠土遗留磷的活化潜力及特征。研究发现：不同施肥处理土壤遗留磷的活化总量为NPK > NPKM > CK,与土壤总磷含量变化一致;高、低浓度柠檬酸对供试灰漠土遗留磷的活化潜力基本相当（>80%）。供试土壤遗留磷的急剧释放伴随土壤pH的大幅下降,说明酸溶解是主要活化机制。高浓度柠檬酸活化整个阶段,NPK处理土壤磷的释放量均高于NPKM处理;而在低浓度柠檬酸处理前期,NPKM释放的磷量大于NPK,相关元素中仅与镁元素的释放一致,可见低浓度柠檬酸活化前期NPKM处理土壤磷的活化很可能来自镁磷。高浓度柠檬酸对灰漠土遗留磷的活化前期以无机Ca₈-P活化为主,后期以无机Ca₁₀-P活化为主。综上,柠檬酸可显著促进不同施肥处理灰漠土遗留磷的活化,鉴于灰漠土遗留磷的高活化潜力,可通过适当减量施肥及土壤磷高效利用管理等措施促进作物再利用。     

Organic Acids and Diffusive Flux of Organic and Inorganic Phosphorus in Sandy-Loam and Clayey Latosols

This study evaluated the effects of organic acids on the diffusive flux of phosphorus (DFP) along time. Treatments were a factorial 2?×?2?×?2?×?2?×?5 design: two soils (clayey Red Latosol and a sandy-loam Red-Yellow Latosol), two organic acids (citric acid, CA, and humic acid, HA), and two phosphorus (P) sources [monopotassium phosphate (KH₂PO₄) and inositol hexaphosphate (IHP)] either with or without heating (100 °C) of the soil. The soil plus treatments, in all combinations, were placed in contact with an anion exchange resin and the P in the resin was measured after 2, 4, 6, 10, and 15 days. The DFP was greater when the soils were treated with CA than when treated with HA. Citric acid was more effective in increasing the DFP from KH₂PO₄, whereas HA was the organic ligand that promoted a larger DFP when the source was IHP.     

Citric acid enhances the mobilization of organic phosphorus in subtropical and tropical forest soils

Low-molecular-weight organic acids are considered to be effective in the release of inorganic phosphorus (P) but their effectiveness to mobilize organic P is not well understood. The aim of this study was to examine the role of three common organic acids (maleic, oxalic, and citric acids) in mobilizing organic P in forest soils. Soil samples tested in this study were collected from either native or plantation forests in subtropical and tropical Australia with 16–87% of soil total P being in organic form. At a concentration of 10 mM organic acid kg^?1 soil, all three organic acids did not enhance the release of inorganic P as compared with water, whereas the three organic acids displayed different capacities in mobilizing organic P. Citric acid significantly enhanced the solubilization of organic P by 34.7% as compared with water; whereas no significant differences were observed in the mobilization of organic P among maleic acid, oxalic acids, and water. The amount of organic P solubilized by citric acid was not correlated with soil pH but increased with increasing soil organic P as the values were below 200 mg kg.^?1 The possible mechanisms of the effective mobilization of organic P by citric acid were discussed. Our results implied that organic P might play an important role in P nutrition of plants in subtropical and tropical forests due to its substantial proportion in soil P and the effective mobilization by organic acids.     

Release of Inorganic Phosphorus from Soils by Low‐Molecular‐Weight Organic Acids

Abstract

Low‐molecular‐weight (LMW) organic acids are found in soils. They originate from the activities of various microorganisms in soils or may be exuded from the roots of living plants. Several of those organic acids are capable of forming stable organo‐metal complexes with various metal ions found in soil solutions. As a result, these processes may lead to the release of inorganic phosphorus (P) associated with aluminum (Al), iron (Fe), and calcium (Ca) in soil minerals. The release of P from soils by LMW organic acids may be important to the P nutrition of plants. Studies on the release of P from soils by a variety of LMW organic acids showed that, in general, the di‐ and tricarboxylic acids were the most effective in releasing P from two Iowa soils, whereas the monocarboxylic, phenolic, and mineral acids released similar amounts of P. Oxalic, malonic, citric, and, in some cases, malic and tartaric acids were the most effective in releasing inorganic P from the two surface soils studied. There was an inverse relationship between the amounts of P released from soils and the pK_a values of the organic acids. The amounts of P released from soils were significantly correlated with the published stability constants for the formation of organic complexes of Al, Fe, or Ca (log K_Al, log K_Fe, or log K_Ca values). In general, the aliphatic acids containing α‐caboxyl and β‐hydroxyl functional groups or phenolic acids containing ortho‐hydroxyl groups were more effective in causing the release of P from soils than similar organic acids having other functional group combinations.     

有机酸培养时间和种类对煤矸石碎屑组成及速效养分的影响

为研究有机酸添加后煤矸石碎屑组成及其速效养分变化,以淮南潘集矿区为研究区域,选取矿区内多处煤矸石山上大块煤矸石人工破碎后作为供试材料,向其中添加10 mmol/L低分子量有机酸和腐殖酸水溶液,恒温培养120 d,在不同培养阶段取出部分样品测定煤矸石碎屑颗粒组成、EC、pH和速效养分等指标。结果表明,有机酸的添加总体上促进了煤矸石碎屑颗粒的细化。其中,黏粒平均增幅达81.90%,极细砂增幅在42%以上,粉粒和极粗砂粒含量平均下降幅度分别为21.05%、62.34%。有机酸添加有助于煤矸石碎屑颗粒基质结构向好的方向发展,并且以柠檬酸和腐殖酸效果最好。培养后的各处理pH值、电导率、速效养分对有机酸种类和培养时间的响应存在显著差异。有机酸添加后,酒石酸、草酸和腐殖酸处理pH均上升较大,平均pH值分别为7.69、7.75和7.71,显著大于对照处理(P<0.05),达到微碱性水平。除苹果酸处理外,其他处理的pH随培养时间的变化表现“N”型特征。电导率的变化与pH值变化正好相反,以苹果酸处理上升最大,比培养前增幅达146.78%;有机酸添加极大促进了煤矸石碎屑碱解氮(AN)和速效磷(AP)的释放,前者以苹果酸效果较好(释放量最大为CK的1.56倍和培养前的13.03倍),后者以柠檬酸效果较好(释放量最大为培养前的2.56倍和CK的3.39倍)。随着培养时间延长,碱解氮和速效磷释放量均逐渐增加;水分浸润和有机酸溶液添加均在前期表现一定抑制煤矸石碎屑颗粒速效钾(AK)释放的作用,后期逐渐表现一定促进释放的作用,但各有机酸处理之间差异不显著。有机酸对煤矸石山生态修复肯有一定积极作用。     

不同磷源对设施菜田土壤速效磷及其淋溶阈值的影响

土壤中磷的移动性不仅取决于磷的数量且与磷肥形态有关。了解不同磷源（有机肥和化肥）对设施菜田土壤磷素的影响对于指导科学施肥和面源污染防治至关重要。本文选取河北省饶阳县3种不同磷含量的农田土壤（未种植过蔬菜的土壤、种植蔬菜30年的塑料大棚土壤和种植蔬菜4年的日光温室土壤）为研究对象,采用室内培养试验和数学模型模拟方法研究有机无机磷源对设施菜田土壤磷素的影响,确定无机肥和有机肥源土壤磷素淋溶的环境阈值。结果表明添加有机肥和无机磷肥都会显著增加3种不同种植年限设施菜田土壤速效磷（Olsen-P）和氯化钙磷（CaCl₂-P）含量,但增加速度不同。对于未种植过蔬菜的低磷对照土壤,磷投入量高于50 mg·kg^-1（干土）后,无机肥比有机肥显著提高了土壤Olsen-P含量。对于已种植蔬菜30年的塑料大棚土壤,高磷投入时[300 mg·kg^-1（干土）和600 mg·kg^-1（干土）],无机肥比有机肥显著提高了土壤Olsen-P含量,低于此磷投入量时有机肥和无机肥处理之间没有显著差异。3种不同农田土壤CaCl₂-P的含量所有处理均表现出无机肥显著高于有机肥处理,尤其是在高磷量[>300 mg·kg^-1（干土）]投入时表现更加明显。两段式线性模拟结果表明,设施菜田土壤有机肥源磷素和无机肥源磷素淋溶阈值分别为87.8 mg·kg^-1和198.7 mg·kg^-1。随着土壤Olsen-P的增加,添加无机肥源磷对设施菜田土壤CaCl₂-P含量的增加速率是有机肥源磷的两倍。因此,建议在河北省高磷设施菜田应减少无机磷肥的投入,特别是土壤速效磷高于198.7 mg·kg^-1的设施菜田应禁止使用化学磷肥和有机肥,在土壤速效磷低于198.7 mg·kg^-1的设施菜田应加大有机肥适度替代无机肥技术的推广。     

Soil fungal isolates produce different organic acid patterns involved in phosphate salts solubilization

Phosphorus availability is a major limiting factor for yield of most crop species. The objective of this study was to compare the solubilization of three sources of phosphorus (P) by different fungal isolates and to determine the possible mechanisms involved in the process. Talaromyces flavus (S73), T. flavus var flavus (TM), Talaromyces helicus (L7b) and T. helicus (N24), Penicillium janthinellum (PJ), and Penicillium purpurogenum (POP), fungal strains isolated from the rhizosphere of crops, are known to be biocontrol agents against pathogenic fungi. The P solubilization efficiency of these fungal strains in liquid media supplemented either with tricalcium phosphate (Ca₃(PO₄)₂; PC), aluminum phosphate (AlPO₄; AP), or phosphorite (PP) depended on the source of P and the fungal species. The type and concentration of organic acids produced by each species varied according to the source of available P. In the medium supplemented with PC, the highest proportion was that of gluconic acid, whereas in the media supplemented with the other P sources, the highest proportion was that of citric and valeric acids. This suggests that the release of these organic compounds in the rhizosphere by these microorganisms may be important in the solubilization of various inorganic P compounds. Results also support the hypothesis that the simultaneous production of different organic acids by fungi may enhance their potential for solubilizing insoluble phosphate.     

Role of Phenolics and Organic Acids in Phosphorus Mobilization in Calcareous and Acidic Soils

Abstract

Phenolic acids (caffeic, CAF; protocatechuic, PCA; p-coumaric, COU; and vanillic, VAN), catechol (CAT), poly-galacturonic acid (PGA), and citric acid (CIT) were compared for their effectiveness in phosphorus (P) mobilization in three soils differing in chemical properties. The addition of organic ligands at 100 μmol g^{? 1} soil increased the concentrations of resin P (P_r), water-extractable P (P_w), and bicarbonate-extractable inorganic P (P_bi), thus improving the phosphorus availability. The magnitude of P mobilization in the calcareous soil can be expressed in the following order: CAF > CAT > PCA = CIT > VAN > COU > PGA, which was consistent with the number of phenolic hydroxyl groups they contained and the position of carboxyl on the benzoic ring. In the two acid soils tested, the order of P mobilization was CIT > CAT > PCA > CAF after 24 h incubation, and CIT > PCA > CAF > CAT after a 14 d incubation. The mobilized P originated partly from the organic P fractions, which could be extracted by 0.5 M NaHCO₃. In addition, P_r decreased and P_w increased during incubation. The exceptions were that the CAF treatment increased P_r and the CIT treatment did not affect P_w. Calcium extraction from the soils after a 1 d or 14 d incubation could not fully account for the P mobilization. The results suggest that the inorganic P dissolution by the organic ligands was not the only mechanism of P mobilization in the calcareous soil, while in acid soils the chelation of metal cations by organic ligands is likely an important factor in P mobilization.     

腐植酸复混肥对葡萄养分吸收利用的影响

为明确不同腐植酸复混肥施用量,对葡萄生长及土壤养分的影响,采取以二年生葡萄幼树进行盆栽试验。结果表明,与等养分无机化肥相比,腐植酸复合肥明显提高了葡萄植株对N、P、K肥的利用率,使土壤中的N素释放减缓,P的移动性和K的含量有较明显提高,并使土壤中有机质含量有增加的趋势。     

The involvement of iron and aluminum in the bonding of phosphorus to soil humic acid

Abstract

With a peat soil similar amounts of phosphorus (P) were coprecipitated with humic acid from alkali extracts over a limited range of strongly acidic pH, whereas with a mineral soil the amount was pH dependent. The difference between the two soils relates to the much greater total amounts of inorganic P and aluminum (Al) present in the extract of the mineral soil. In this acid mineral soil, Al rather than iron (Fe) may be involved in the formation of metal bridges in humic acid‐metal‐inorganic P complexes. Neither Al or Fe were implicated in binding of organic P to humic acid. The P species observed in humic acids was dependent on the pH at which they were precipitated from the alkali extracts. In the peat soil the inorganic P was an order of magnitude lower than the organic P.     

Catalytic synthesis of humic acids from phenolic compounds by Mn(IV) oxide (Birnessite)

Abstract

Extract

Humic acids are natural organic polymers and are important constituents of soils (Kononova 1966). Although the mechanisms of the formation of humic acids are very complicated, the oxidative polymerization of polyphenols is considered to be one of the most important mechanisms (Kononova 1966; Stevenson 1982). The oxidative polymerization can be accelerated by enzymes (e.g., Haider et al. 1975; Flaig et al. 1975) as well as by inorganic components (e.g., Kyuma and Kawaguchi 1964; Shindo and Huang 1982; Kumada 1987). Shindo and Huang (1985) and Shindo and Higashi (1986) compared the relative effectiveness of several oxides, primary minerals, and clay minerals in promoting the synthesis of humic polymers from hydroquinone, and found that the promoting effect of Mn(IV) oxide was most striking among the inorganic components studied. The catalytic power of inorganic components in the synthesis of humic polymers from phenolic compounds may be influenced by the chemical structure of the compounds. However, little information is available on this aspect.     

Potassium fixation of some calcareous soils after short term extraction with different solutions

This investigation was done to determine the release of potassium (K) from five calcareous soils of southern Iran using 0.025 M CaCl₂, HCl and citric acid during six successive extractions and to study the K fixation capacity of the soils after K release experiment. Mineralogical study indicated that Vertisols and Mollisols were dominated with smectites; while other soils had illite, chlorite, palygorskite and smectite. Results indicated that citric acid extracted more K than CaCl₂ and HCl (137 vs. 111 and 113 mg kg^?1, respectively). The analysis of calcium (Ca), magnesium (Mg) and K concentrations in the solutions suggests that the exchange of K with soluble Ca and Mg (originated from dissolution of carbonates by acidic solutions) is the main mechanism of K release, but citrate is able to dissolve K-bearing minerals and release K in slightly calcareous soils. Soils with more illite released more K. Potassium fixation capacity of soils increased after extractions of soils with different extractants from 324 to 471 mg kg^?1, with no significant difference. It is suggested to apply more K fertilizers in K-depleted calcareous soils and use of different solutions for extracting K from soil minerals may be a temporary and short term solution.     

Effect of ph on precipitation of humic acid from peat and mineral soils on the distribution of phosphorus forms in humic and fulvic acid fractions

Abstract

Humic and fulvic acid fractions were isolated from a mineral soil and a peat by adjusting the pH of the alkali extracts to a range of values from 0.2 to 2.5. Total inorganic and organic forms of phosphorus (P) in the acids were measured by chemical analysis and by ³¹P NMR spectroscopy. As the pH of precipitation of the mineral soil humic acid increased, there was an increase in the total P of the humic acid which related to the inorganic P component. In contrast with the peat, the increases observed in the pH range 0.2 to 1.5 were the result of changes in organic P. Using ³¹P nuclear magnetic resonance spectroscopy, the ratio of inorganic to organic P as mono‐ and di‐esters in the peat humic acid was found to increase from 1:4.8 at pH 2 to 1:19 at pH 2.5. In contrast with mineral soil humic acid, the ratio decreased from 1:6.1 at pH 0.2 to 1:1.3 at pH 2.5. The mono‐ester to di‐ester ratio was about 3 in the peat and 10 in the mineral soil and varied little with pH of precipitation. Phosphonates were detected only in the peat humic acid precipitated in the pH range 1.0 to 2.0     

Carbon addition reduces labile soil phosphorus by increasing microbial biomass phosphorus in intensive agricultural systems

Accumulation of inorganic and labile organic phosphorus (P) in intensive agricultural systems leads to P loss from soil which can cause serious environmental problems. Soil microbes are important in mobilizing soil non-available P, however, little is known about the role of soil microbes in immobilizing P to reduce P loss. Here, we test whether stimulating microbial biomass to immobilize P could reduce the amount of labile P available for leaching. The distribution characteristics of Olsen P, organic P and microbial biomass P were determined in three intensive agricultural systems. In addition, we conducted a pot experiment with three P and four carbon (C) levels. CaCl₂ extractable P was measured and used to indicate the risk of P leaching. We found that there was a positive relationship between soil organic C and microbial biomass P. Carbon addition drove the process of P immobilization and reduced CaCl₂ extractable P. Microbial biomass P increased by 64% (p < .05) with the addition of C, and Olsen P and CaCl₂ extractable P decreased by 28% and 17%, respectively. Our results show that C addition increased microbial immobilization of P and reduced forms of labile P susceptible to leaching. Stimulating microbes to immobilize P by adding C to soils may have the potential to reduce P loss from intensive agricultural systems, reducing their environmental impact.     

Underestimation of Available Phosphorus by Resin–Bicarbonate and Olsen Tests in Calcareous Soils treated with Gypsum

In the present study, Olsen [0.5 M sodium bicarbonate (NaHCO₃), pH 8.5] and resin–bicarbonate (HCO₃) tests underestimated available phosphorus (P) in calcareous soils treated with gypsum (CaSO₄). The reaction of CaSO₄ and HCO₃ ^? ion or resin–HCO₃ to form calcium carbonate (CaCO₃) precipitate reduced the strength of the Olsen NaHCO₃ extractant and resin–HCO₃ strip for P extraction. The iron (Fe) oxide–impregnated filter paper (Pi strip) was independent of CaSO₄ influence and thus correctly estimated soil‐available P with respect to plant response to soil‐available P. Two greenhouse experiments were conducted with maize and wheat grown on calcareous soils treated with different rates of CaSO₄. The results confirmed that Olsen and resin–HCO₃ tests should not be used to measure available P or labile P in the P fractionation scheme in the calcareous soils containing significant amounts of gypsum.     

Phosphorus retention and release of anions and organic carbon by two Andisols

Andisols can absorb large amounts of phosphorus rapidly, and then release it slowly, yet the mechanisms by which they retain P and release it for plant growth are poorly understood. Ligand exchange of organic compounds from Al–humic complexes by P and/or Si release – due to breakdown of allophanic microstructure to provide sorption sites – might account for the retention of P, but its extent is not known. We applied a soil column flow-through technique to quantify the release of anions and organic carbon (C) associated with P sorption by two andic soils, and we related the anion release to possible mechanisms for the retention of P. Phosphate (H₂PO₄^–, HPO₄^2–) sorption and concurrent anion desorption were obtained by passing a 1-g P 1^–1 (32 mmol KH₂PO₄ in 1 mm CaCl₂) solution through the soil columns (25 cm³). Total dissolved P, Fe, Al, S, Ca, Mg, K, Mn, organic C and pH were determined in the eluent. Changes in eluent pH and the patterns of the retention of P and corresponding concentrations of Al, Si and organic C in the eluent were similar for the two Andisols. The general pattern and changes in pH of the eluent coincided with changes in the patterns of release of organic C and Si and the rate of P retention. Release of silica accounted for < 6% of the P sorbed and had only a minor role in P retention in these two Andisols. Release of organic C, however, accounted on a molar basis for 40% and 83%, respectively, of the P sorbed. Direct measurements of the pH of the eluent and release of anions and organic C concurrent to P retention contribute to rapid assessment of the controlling mechanisms of P retention. The results indirectly confirm the hypothesis of ligand exchange of solution P with organic complexes held on allophanic surfaces. The organic C release, however, is not specifically related to either the fast or the slow P retention phase. The shift in the controlling P retention reaction associated with a change from the fast to the slow P retention phase is clearly indicated by an abrupt change of the pH of the eluent. This shift, in previous studies identified graphically by a change in slope of the P sorption isotherm, can be identified directly by measuring the pH of the matrix.