Monitoring nitrate in soils with coated wire ion specific electrodes

Abstract

The high sensitivity and low per‐unit cost of nitrate (NO₃‐) ‐ sensitive coated wire electrodes (CWEs) makes them attractive alternatives to chemical extraction for NO₃‐ assessment in soils and solutions. We used CWEs in the field to quantify soil NO₃‐ in fresh soil extracts, in a laboratory incubation comparing CWEs with anion exchange membranes and soil extracts, and in the field in fertilized and unfertilized soils. Freshly calibrated electrodes performed well in the field when used to measure NO₃‐ in soil extracts and when installed directly in soils. The output of electrodes placed in situ in soils responded linearly, decreasing as the NO₃‐ contents of soils were increased. Soil‐installed electrodes performed better in the laboratory than in the field. When the electrodes were left in soils for long periods (days‐to‐weeks), we had problems with both their durability and the stability of their response to NO₃‐. Nitrate‐sensitive CWEs will be used to the greatest advantage in controlled settings where their output can be calibrated frequently and their contact with solution ensured.     

Anion‐exchange membrane,water, and sodium bicarbonate extractions as soil tests for phosphorus

Abstract

Three techniques were evaluated as soil P tests for western Canadian soils: anion‐exchange membrane (AEM), water, and bicarbonate extraction. The AEM, water, and bicarbonate‐extractable total P represented novel approaches to compare to the widely used bicarbonate‐extractable inorganic P (traditional Olsen) soil test. In a range of Saskatchewan soils, similar trends in predicted relative P availability were observed for AEM, water extraction, bicarbonate‐extractable total P, and bicarbonate‐extractable organic P. Correlations between soil test values revealed AEM and water‐extractable P to be most closely correlated, consistent with the similar manner of P removal in the two tests.

Phosphorus availability, as predicted by the tests, was compared to actual P uptake by canola and wheat grown on 14 soils in a growth chamber experiment. P uptake by canola was highly correlated with AEM (r² = 0.86–0.90), water (0.87 ‐0.94), and bicarbonate‐extractable total (0.91) and inorganic (0.92) P. Uptake of P by wheat was not quite as highly correlated with test‐predicted values: AEM (r² = ‐0.73–0.78), water (0.72–0.77), bicarbonate total (0.82), bicarbonate‐inorganic P (0.75).

The similarity in coefficients of determination among test methods indicated nearly identical abilities of the tests to predict soil P availability in the range of soils examined. The AEM and water extractions, unlike bicarbonate, are largely independent of soil type and may prove superior when a wider range of soils is being tested. Bicarbonate‐extractable total P and water‐extractable P suffer limitations in analytical simplicity and cost. In testing for P alone, AEM was considered superior to the other methods due to low cost, simplicity, independence of soil type, and high correlation with plant uptake.     

离子交换树脂膜原位提取土壤有效性养分的探讨

连续3年在实验室内和利用盆栽及田间进行了利用离子交换树脂膜对土壤的原位测定,探讨影响树脂膜对土壤中养分离子吸收的因素;制定出一套包括树脂膜埋置方法,时间,洗脱土粒和解吸被树脂膜吸附的养分等的标准方法,称为植物根模拟法(Plant root simulator)。 通过400多个土壤样品测定结果表明,用树脂膜提取的土壤有效氮、磷、钾和硫与用常规化学法提出的量有显著相关。盆栽条件下,树脂膜原位埋置法对预测供试作物油菜吸取土壤有效养分的能力与常规化学法相当,而硫的效果更好。树脂膜在田间直接埋置能合理地表征土壤中硝态氮的分布,并具有同时提取土壤各种有效养分的能力。由于离子树脂交换膜不受地域影响,具有广泛的通用性,因而在土壤养分有效性的研究中应用前景广阔。     

Influence of soil pH-sorption interactions on imazethapyr carry-over

Soil pH affects imazethapyr sorption-desorption, which in turn can affect persistence and bioavailability. Long-term imazethapyr carry-over has been observed in soil that is below pH 6.5, resulting in significant sugarbeet damage. Imazethapyr concentration decreased rapidly in field soil, regardless of pH. Despite similar amounts of imazethapyr remaining in aged soils at different pH levels, there were differences in bioavailability, which can be explained by sorption-desorption. At low pH more imazethapyr was sorbed than at high pH, but it readily desorbed. At high pH less imazethapyr was sorbed initially, but it did not readily desorb. Thus, after 3 months, the remaining imazethapyr in low-pH soil was desorbable and bioavailable, resulting in injury to canola and sugarbeet. Liming aged, low-pH soil released bound imazethapyr residues, which would then be degraded and result in less carry-over.     

Addition of phosphorus to soils with low to medium phosphorus retention capacities. II. effect on soil phosphorus extractability

Abstract

Knowledge of the change in soil extractable phosphorus (P) as a consequence of soil P fertilization could be useful in discriminating soils with a potential for soil P release to runoff or movement of P along the soil profile. In this research, soils with low to medium P retention capacity were equilibrated for 90 days with soluble P (KH₂PO₄) at rate of 100 mg P kg^‐1 soil. After this period, soil samples both with and without the P addition were analyzed using six conventional methods: 1) Olsen, 2) Bray 1,3) Mehlich3,4) Egner, 5) Houba, dilute CaCl₂ solution, and 6) distilled water, and three “innovative”; P‐sink methodologies: 1) Fe oxide‐coated paper strip, 2) anion exchange resin membrane, and 3) cation‐anion exchange resin membrane. The soils without P addition had low levels of extracted P as determined by all nine procedures. Net increases in the amount of P extracted from the soils with added P ranged from 4.2 mg kg^‐1 (CaCl₂ extraction) to 57.6 mg kg^‐1 (cation‐anion resin membrane extraction). Relationships between change in extracted P and i) physical and chemical characteristics, and ii) soil P sorption properties are also presented and discussed.     

A simple manual method for the determination of phosphate‐extractable (ion‐exchange membrane) sulphate in soils

Abstract

A simple manual method for measuring phosphate‐extractable sulphate in soils using basic laboratory equipment is described. Sulphate is extracted by a phosphated anion‐exchange membrane which, after elution, is measured turbidimetrically as barium sulphate. The results obtained by the proposed manual method compare well with a continuous flow analyser method using a range of Pacific Island soils.     

Effect of swine manure and urea on soil phosphorus supply to canola

Limited information exists as to the effect of liquid swine manure on soil phosphorus (P) availability in Western Canadian soil. Swine manure is most often applied to meet additional requirements for nitrogen (N) and research to date has emphasized N effects. The effect of swine manure and urea on P supply to canola was investigated under controlled environment condition. Canola (Brassica napus) was grown in pots with manure or urea added to two Saskatchewan soils (sandy loam and clay loam) at 0 and 100 mg N kg^‐1. Plants were grown to maturity, and yield and nutrient content were determined. Phosphorus supply rates in soils were measured in the pots using anion exchange resin membrane probes. Additions of swine manure and urea enhanced canola P accumulation and led to a higher proportion of P in seeds. This response was more evident in the manure treatment than with urea. Soil amended with manure significantly increased N and P supply rates in soils as the manure contains N and P. On the contrary, application of urea significantly increased N supply rate, but led to a slight decrease in the measured soil supply rate of available P. Despite the apparent decrease in soil supply of available P in urea treatment, canola maintained its N:P ratio by increasing P absorption, possibly due to a greater root mass.     

Leaching of imidazolinones in soils under a clearfield system

Herbicides with high mobility can leach to deeper layers of the soil and to contaminate underground aquifers. The potential of herbicide leaching in soil can be monitored by chromatography or bioassay methods. This study evaluated the leaching of imazethapyr, imazapic and its commercial mixture (imazethapyr + imazapic) in three tropical soils via a bioassay method. The herbicides were applied in a polyvinyl chloride column and an 80 mm rainfall was simulated. The bioindicator species sorghum (Sorghum vulgare) BRS 655 was sown. Imazethapyr, imazapic and the commercial herbicide mixture showed high leaching in the soils. The presence of imazethapyr and the commercial mixture was detected up to 25 cm deep in Haplic Plinthosol and Oxisol. Imazapic showed less leaching to tree soil, being found at depth 10–15 cm. The pH, texture and iron oxide amount levels affected herbicide leaching. The mobility of the herbicides imazethapyr and imazapic in the Haplic Plinthosol and the Oxisol at depth 25 cm indicates a high risk of groundwater contamination.     

Determination of glucosinolates in canola seeds using anion exchange membrane extraction combined with the high-pressure liquid chromatography detection

A rapid, simple, and reliable method for the determination of individual glucosinolates in canola seeds was developed using a semiquantitative extraction of glucosinolates with anion exchange membranes and HPLC detection. In this one-step extraction procedure, a membrane (7 cm(2)) is placed in the seed suspension prepared by grinding and boiling 0.8 g of seeds in 20 mL of water. After 10 min of shaking on the mechanical shaker, the membrane is removed from the suspension, washed, and transferred to a vial containing 5 mL of 1 N tetramethylammonium chloride. The glucosinolates are eluted from the membrane by shaking the membrane for 10 min with the eluting solvent. The glucosinolate content in membrane eluates is determined by HPLC using sinigrin standards. A coefficient of variation ranging from 1.9 to 7.6% for aliphatic glucosinolates indicated very good reproducibility of the method. Because of the instability of 4-hydroxyglucobrassicin, the coefficient of variation for the determination of this indolyl glucosinolate was 13.9%. To verify the results of the membrane extraction/HPLC detection, this new method was compared with the existing colorimetric and GC procedures. Very good correlation (R(2) = 0.98) was obtained between the total glucosinolates determined by the membrane extraction/HPLC method and the palladate colorimetric procedure for 17 canola varieties. Concentrations of individual glucosinolates in five canola varieties were compared with the GC data. Very good agreement between these two methods was obtained for aliphatic glucosinolates. However, the membrane extraction/HPLC method yielded slightly higher values for 4-hydroxyglucobrassicin than the GC method, possibly indicating that the decomposition of this glucosinolate was reduced during the sample extraction with the membranes. The simplicity and low cost of the membrane extraction/HPLC method make it an attractive alternative to the existing procedures for glucosinolate analysis in canola seeds.     

Evaluation of soil sulfate extractants and methods of analysis for plant available sulfur

Abstract

A steady decline in sulfur additions to Atlantic Canadian soils has prompted the need for an accurate method of determining their plant available sulfur status. Three soils were extracted with five soil extractants ‐ 0.01M Ca(H2PO₄)₂‐H₂O in 2M HOAc, 0.1M CaCl₂, Bray‐1 and de‐ionized water. The soil extracts were analyzed for sulfur or sulfate using inductively coupled argon plasma emission spectrometry (ICAP), AutoAnalyzer (AAN), anion exchange‐high performance liquid chromatography (HPLC‐CD) or atomic absorption spectroscopy (AAS). Results were compared with plant response of sulfur treatments to red clover, ryegrass, canola and wheat in a growth room. Instrument reproducibility and crop response indicated the ideal method of determining plant available soil sulfur was HPLC‐CD using the extractant Ca(H2PO₄)₂‐H₂O.     

Early Supplies of Available Nitrogen to Seed‐Row of a Canola Crop as Affected by Fertilizer Placement

Abstract

A field experiment was conducted at Star City (legal location SW6‐45‐16‐W2); Saskatchewan, Canada from May 2000 to June 2000, to measure nitrogen (N) and phosphorus (P) supply rates from fertilizer bands to the seed‐row of canola crop. Ion exchange resin membrane probes (PRS^TM) were used to measure N and P supply rates in four treatments [80 kg N ha^?1 of urea as side‐row band, 80 kg N ha^?1 of urea as mid‐row band, check/no N (side‐row)/P side‐row, check/no N (mid‐row)/seed placed P]. The treatments were arranged in a randomized complete block design with four replications. Two anion and cation exchange resin probes (PRS^TM) were placed in each plot in the seed‐row immediately after seeding and fertilizing. The probes were allowed to remain in the field for 2 days and replaced with another set of probes every 4 days for a total of 14 days until canola emerged. Ammonium‐N, nitrate‐N and P supply rates were calculated based on the ion accumulated on the probes. Urea side‐row band treatments (fertilizer N 2.5 cm to side of every seed‐row) had significantly higher cumulative available N supply rates than mid‐row band placement in which fertilizer N was placed 10 cm from the seed‐row in between every second seed‐row. No significant differences were observed in P supply rates. The higher N rates (120 kg N ha^?1) resulted in lower grain yield in side‐row banding than mid‐row banding possibly due to seedling damage. However, the earlier fluxes of N into the seed‐row observed with side‐row banding may be an advantage at lower N rates in N deficient soils.     

Effect of the Extraction Period on the Routine Assessment of Soil P Status by Ion‐Exchange Resin Membranes

The cation–anion exchange resin membrane (CAERM) system is one of the most promising alternative methodologies for the assessment of soil phosphorus (P) availability. Nevertheless, the CAERM method has a recommended period of extraction (16 h) that limits its applicability. With the aim to improve the use of CAERM in routine laboratory work, seven extraction times were studied and compared for repeatability and biological accuracy, using 150 samples of soils and covering a wide range of properties. The results showed dissimilar quantities of extractable P, according to the nature of soils and the period of extraction time. On average of all soils, values of extractable P ranged from a 2.4 mg kg^?1 and 26.7 mg kg^?1 in 0.25‐h and 16‐h extraction times, respectively. Relative to the repeatability aspects, the results reveal a tendency of increasing results with the decrease of the period of extraction, showing coefficient variation (CV) values ranging from 5% (16 h) to 14% (0.25 h). All the extraction periods tested for the CAERM presented highly significant values of r (0.824–0.943) between extractable P and each of the three biological parameters: relative yield of ryegrass, P content, and P uptake. Considering the three biological parameters simultaneously, the best results were obtained for extraction periods of 8 h, 2 h, and 4 h. Based on the results, and considering both practical and accuracy aspects, it is conclude that 2 h of extraction is a valid alternative to the 16‐h standard extraction period for the CAERM methodology.     

Use of Ion exchange membranes in routine soil testing

Abstract

We developed and assessed a method for simultaneous extraction of plant available nitrogen, phosphorus, sulfur and potassium using anion and cation exchange membranes (ACEM). The technique was found to be highly suitable for routine soil testing due to its simplicity, rapidness and accuracy. The study compared the amount of nutrients extracted by ACEM with conventional chemical‐based extractants for P and K (0.5M NaHCO₃) and N and S (0.001M CaCl₂) for 135 soil samples representing a wide range of soil types in Western Canada. The nutrient availability predicted by ACEM was significantly correlated with the conventional methods. The correlation was not affected by the two different shaking times tested (one hour and 15 minutes), suggesting that extraction times as short as 15 minutes could be used in ACEM extraction. To evaluate the relative ability of ACEM and the conventional tests to predict actual nutrient availability to plants, canola plants were grown on soils in the growth chamber and actual plant uptake was compared to test‐predicted nutrient availability. Phosphorus and potassium uptake by canola plants was more closely correlated with ACEM extractable P and K (r²= 0.84*** and 0.54***) than with 0.5M NaHCO₃ P and K (r²= 0.70*** and 0.37***). Also, nitrogen and sulfur uptake by canola plants was significantly correlated with ACEM extractable‐NO₃ and ‐SO₄ (r² = 0.60*** and 0.70***) and with CaCl₂ extractable‐NC₃ and ‐SO₄ (r² = 0.57*** and 0.61***). Availability of all four macronutrients can be assessed in a single ACEM extraction. The higher correlation with plant uptake suggests that ACEM is a better index of macronutrient availability than conventional methods. The ACEM soil test could be readily adopted in routine soil analysis because of low cost and simplicity as well as its consistency over a wide range of soil types.     

Anion and cation exchange resin membranes to assess the phosphorus status of some Portuguese soils

Abstract

Ion exchange resin methods were applied to 78 different soils to assess their phosphorus (P) status for predicting their response to P fertilization. The techniques used were anion exchange resin membranes eluted with hydrochloric acid (HCl) (AEM) and cation‐anion exchange resin membranes eluted with HCl (CAEM‐HC1), sodium chloride (NaCl) (CAEM‐NaCl) or water with directly color development (CAEM‐H₂O). Greenhouse studies were conducted with the same soils in order to validate laboratory data. Ryegrass was grown with two levels of P: nil and 150 mg P kg^‐1 of soil. Results indicate that soil P levels are significantly correlated (p<0.001) if extracted with AEM or CAEM, both eluted with HCl, although the CAEM technique had extracted larger amounts of P. Concerning the type of elution, results did not show significant differences (p<0.05) between CAEM‐HC1 and CAEM‐NaCl, but both were significantly correlated (p<0.001) with the results obtained with CAEM‐H₂O. All the techniques used to measure extractable P correlated significantly with relative yield and P uptake by ryegrass, showing their ability to predict soil P availability. Nevertheless, CAEM extraction had higher values of r². Among the three techniques for elution, the levels of correlation with the biological parameters were equivalent. From these results, it was concluded that: (i) exchange resins, specially CAEM, is an accurate method to assess the P fertility status of soils, and (ii) the traditional step of elution can be avoided, allowing the process to be less time consuming, thus more suitable for routine use.     

用离子交换树脂膜方法预测有效氮和硫的供应及氮硫肥对油菜和小麦生长的影响

Field experiments were conducted with five rates （0, 75, 150, 225, and 450 kg P205 ha^-1） of seedbed P fertilizer application to investigate the yield of tomato in response to fertilizer P rate on calcareous soils with widely different levels of Olsen P （13-142 mg kg^-1） at 15 sites in some suburban counties of Beijing in 1999. Under the condition of no P fertilizer application, tomato yield generally increased with an increase in soil test P levels, and the agronomic level for soil testing P measured with Olsen method was 50 or 82 mg kg^-1 soil to achieve 85% or 95% of maximum tomato yield, respectively. With regard to marketable yield, in the fields where Olsen-P levels were 〈 50 mg kg^-1, noticeable responses to applied P were observed. On the basis of a linear plateau regression, the optimum seedbed P application rate in the P-insufflcient fields was 125 kg P205 ha^-1 or about 1.5-2 times the P removal from harvested tomato plants. In contrast, in fields with moderate （50 〈 Olsen P 〈 90 mg kg^-1） or high （Olsen P 〉 90 mg kg^-1） available P, there was no marked effect on tomato fruit yield. Field survey data indicated that in most fields with conventional P management, a P surplus typically occurred. Thus, once the soil test P level reached the optimum for crop yield, it was recommended that P fertilizer application be restricted or eliminated to minimize negative environmental effects.     

Differential tolerance of canola to arsenic when grown hydroponically or in soil 1

Separate studies found canola tolerant to arsenic (As) when grown in hydroponic solutions and generally sensitive to As when grown in soil. Fourteen‐day‐old canola was transferred to pots containing either soil or nutrient solution and then grown for an additional 14 days in a growth chamber at different times for the two different media. Plants were grown in 0.25‐strength Hoagland's solution containing either 0, 0.27, 6.67, or 13.3μM As or in three soils with As added at rates of 0,5, and 10 mg.kg^‐1. Soil‐solution As concentrations were determined via column displacement and were the same or less (0.147 to 4.27μM) than the hydroponic As concentrations. Soil‐solution phosphorus (P) concentrations were determined in the same manner and averaged 9.28μM P compared to 500μM P from the hydroponic solutions. Chlorosis, wilting, and stunted growth—symptoms of As toxicity—occurred in canola at the highest As rate in two of the three soils used. Dry‐matter yields from the third soil were low from all treatments and a lack of response to As additions was probably due to injury from other soil‐related factors. Shoot As concentrations were generally similar from both experiments. Canola did not suffer a yield loss or exhibit As toxicity symptoms when grown in the hydroponic solutions. Leaf P was 8,000 mg.kg^‐1 in the hydroponically‐grown canola and 100 mg.kg^‐1 for the soil‐grown canola. These values are greater than (hydroponic) and lower than (soil media) sufficiency levels for plants similar to canola. High solution P concentrations in the hydroponic solution may have detoxified As by competing with As for uptake in the solution and during metabolism. Phosphate should be added to hydroponic solutions in As studies at levels close to normal soil P levels and added daily to replenish plant uptake.     

中国青海湖地区草地蝗虫与土壤的关系

A growth chamber study was conducted to determine the relationships between the supply of soil available nitrogen （N） and sulfur （S） and canola （Brassica napus） and wheat （Triticum aestivum L. ‘Biggar＇） N and S uptake and yield in three Western Canadian soils. The suitability of one-hour burial with an anion exchange membrane （AEM） was assessed for its utility as a quick test of the available N：S balance in the soil. Canola and wheat were grown on a Luvisolic soil low in available S and on Brown and Black Chernozemic soils low in both available N and S, with different rates and combinations of N and S fertilizers applied. AEM burial was used to assess soil available nitrate and sulfate supply rates after fertilization. Dry matter yield and N and S concentrations in plant tissues were determined after 6 weeks of growth. The soil available N：S ratio determined by AEM burial closely reflected the relative supplies of available N and S as revealed in the N：S ratios of plant tissue dry matter. The highest yields were achieved where the available N：S ratio in soil and plant tissue ranged from 5 to 13. Thus, a one-hour burial of an AEM probe in the field may be a useful tool to quickly test if a balanced N and S supply is present in the soil for optimum crop yield.     

Evaluation of an anion‐exchange membrane for extracting plant available phosphorus in soils

Abstract

Anion‐exchange resins (AER) have been used to determine plant available phosphorus (P) since the fifties and their results have shown strong relationships with plant growth and P uptake irrespective of soil properties. However, this procedure is still not widely used by laboratories because of difficulties in handling resin beads under routine conditions. New kinds and different shapes of resins are being produced each with specific characteristics that must be evaluated before use in laboratory procedures. Thus the objective of this work was to evaluate an AER manufactured in membranes reinforced with a Modacrylic fabric. These anion‐exchange membrane (AEM) sheets are commercially available, making them suitable for soil testing. The membranes were cut in pieces (1.0×7.5 cm) identified as AEM‐strips. The AEM‐strips were soaked in 0.5M HCl for a few days and transferred, after being rinsed with deionized water (DI), to 0.5M NaHCO₃ to convert them to HCO₃ ^‐ form. The AEM‐strips and resin beads in nylon bags recovered 98.4 and 98.0% of the P content in an aqueous P solution, respectively. Three eluent solutions were evaluated with different shaking times. The 0.1M H₂SO₄ and 1.0M NaCl in 0.1M HCl were equally suitable for the molybdenum blue color development without any pH adjustment, while the pH of the 0.5M HCl was too low. The elution of P from the AEM‐strips was independent of time with a 15‐min shaking being adequate for removal of all P from the strips. A comparison of soil sample preparation demonstrated that it was not necessary to vigorously grind or sieve the soil to improve the repeatability of the results. The AEM‐strips were compared with other methods (Pi impregnated filter paper, Mehlich I and Bray 1) using 32 soils from Guatemala with widely varying physico‐chemical and mineralogical properties. Phosphorus extracted by the AEM and Pi procedures (similar principle) were highly correlated and gave similar results irrespective of soil type. The acid extraction (Mehlich I and Bray 1 methods) attacked soil components (apatites) resulting in higher and inconsistent amounts of P extracted which may not be available to plants; the correlation between these methods within soils of similar properties was good, but when all soils were considered together the relationship was not significant. This demonstrated that the acid extraction method for P is not suitable for soils containing apatites, while those based on a sink for P (AEM and Pi) can be applied irrespective of the type of soil.     

Comparison of soil nitrate extracted by potassium chloride and adsorbed on an anion exchange membrane in situ

Abstract

The 2M potassium chloride (KCl) extraction method used to measure soil nitrate (NO₃ ^‐‐N) concentrations in soils may introduce some artifacts caused by soil sampling, processing, and handling. Furthermore, this method provides soil NO₃ ^‐‐N concentrations for soil sampled at a particular time, whereas the dynamics of this anion in situ need to be better understood. In order to develop a reliable in situ method as an alternative, an anion exchange membrane (AEM) was tested for its ability to adsorb NO₃ ^‐‐N from a soil cropped to corn (Zea mays L.) and amended with manure or inorganic nitrogen (N). In a field study, we compared the amount of NO₃ ^‐‐N adsorbed on an AEM and extracted with the 2M KCl method. The AEM was calibrated in the laboratory and placed at 15‐cm soil depth for 2‐wk periods during the corn growing season. Nitrate adsorption on the AEM and KCl‐extractable NO₃ ^‐‐N were larger in the inorganic N treatment than in the manure or the control treatments throughout the growing season. The NO₃ ^‐‐N concentrations measured by the AEM method were correlated with NO₃ ^‐‐N extracted with 2M KCl (r² = 0.78***), suggesting that the AEM method could be used to measure NO₃ ^‐‐N concentrations in agricultural soils.     

Chemical Characterization of Inorganic Phosphorus Desorbed by Ion Exchange Membranes in Short‐ and Long‐Term Extraction Periods

Abstract

The chemical characterization of soil phosphorus (P) desorbed by anion and cation exchange membranes is of major importance to better understand which P forms are available to plants in short‐ and long‐term time periods. Two distinct soils, one acidic and one calcareous, were analyzed for P using two extraction procedures with mixed anion and cation exchange membranes. The short‐term (ST) experiment evaluated the effect of increasing the extraction periods up to 24 h, whereas the long‐term (LT) experiment consisted of a sequential extraction procedure using up to seven successive 24‐h extractions. In both experiments, the Chang and Jackson inorganic P fractionation methodology was carried out after each extraction treatment, and each treatment consisted of three replicates. Data were statistically analyzed by ANOVA and nonlinear regressions. In the ST experiment, increasing the extraction time increased the extracted P according to an asymptotic relationship (y=c?ab ^x ). Extracted P proceeded from the most labile fractions in the acidic soil. In calcareous soils, calcium phosphates may also contribute for extractable P. The LT experiment revealed that a single extraction, regardless of that extraction method, cannot predict the long‐term capacity of soils to supply P to the plants. An exponential relationship (P=a×n ^b ) was found between extracted P and the extraction number. Desorbed P proceeded from the most labile fractions in the acidic soil. However, in calcareous soils, some precaution is needed when considering the biological meaning of the results, because the occluded Fe phosphates also revealed significant decreases, probably due to the redox conditions in which these long extractions are performed.