Change Point in Phosphorus Release from Variously Managed Soils with Contrasting Properties

Abstract

Investigating the relation between concentration or release of phosphorus (P) into soil solution (CaCl₂‐P, determined by 0.01 M CaCl₂ extraction of soils) and soil test phosphorus (Olsen P, or 0.5 N NaHCO₃‐extractable soil phosphorus) for 10 widely ranging and variously managed soils from central Italy, a change point was evident where the slopes of two linear relationships meet. In other words, it was possible to distinguish two sections of the plots of CaCl₂‐P against Olsen P, for which increases of CaCl₂‐P per unit of soil test P increase were significantly (p<0.05) greater above than below these change points. Values of change point ranged from 14.8 to 253.1 mg kg^?1 Olsen P and were very closely correlated (p<0.001) to phosphorus sorption capacity of soils. Similar change points were also previously observed when Olsen P (and also Mehlich 3 P) of surface soils was related to the P concentration of surface runoff and subsurface drainage. Because insufficient data are available relating P in surface soils and amount of P loss by overland, subsurface, or drainage flow, using the CaCl₂ extraction of soil can be convenient to determine a change point in soil test P, which may be used in support of agricultural and environmental P management.     

Different Fractions of Soil Potassium,Olsen Phosphorus,and Available Sulfur Status of Intensively Cultivated Berpura Soil Series of India and Nutrient Indexing of Rice Crop

Berpura alluvial soil series of the Indo‐Gangetic Plains is situated in the Ambala District of the Haryana State of India. Soils of this series had medium concentrations of both potassium (K) and phosphorus (P) and large concentrations of sulfur (S) before 1970. To study different fractions of K, Olsen P, and 0.15% calcium chloride (CaCl₂)–extractable (available) S of soils of the Berpura series and to create nutrient indexing of rice crops growing on this series, surface soil samples were collected from 100 farmers' fields after the harvest of the wheat crop in 2005. During kharif season of same year, samples of upper two leaves at anthesis growth stage of rice crop were also collected from the same 100 farmers' fields that had earlier been sampled for soil analysis. Analysis of soil samples showed more K depletion in soils of this series, of which 86% of farmers' fields were deficient in ammonium acetate (NH₄OAc) K (available K). Thirty and 62% of leaf samples of the rice crop growing on the 100 fields of this series were extremely and moderately deficient in K, respectively. The mean values of water‐soluble, exchangeable, nonexchangeable, lattice, and total K were 10.6, 30.3, 390.0, 8204, and 8635 mg kg^?1, respectively. In soils of this series, 0.123, 0.351, 4.517, and 95.009% of total K were found in water‐soluble, exchangeable, nonexchangeable, and lattice K forms, respectively. On the other hand, long‐term farmers' practice of more application of P fertilizer in wheat crop has resulted in P buildup in the soils of the Berpura series. Olsen P in soils of farmers' fields of this series ranged from 9.0 to 153.0 mg kg^?1, with the mean value of 41.8 mg kg^?1. Eighty‐two percent of leaf samples of rice crops grown on this series without application of P fertilizer were sufficient in P. The analysis of soil and rice crops for P and K proved the suitability of 0.5 M sodium bicarbonate (NaHCO₃) and 1 N NH₄OAc for extracting available P and K, respectively, in alluvial soils of the Indo‐Gangetic Plains. The 0.15% CaCl₂–extractable S in this soil ranged from 9.6 to 307 mg kg^?1 with a mean value of 34.6 mg kg^?1. Four and 26% of soil samples had low and medium, respectively, in 0.15% CaCl₂–extractable S. S deficiency was recorded in rice crops, as 29% of the leaf samples were extremely deficient in S and 58% were moderately deficient in S. This indicated the unsuitability of the 0.15% CaCl₂ to extract available S from the Udic ustochrept utilized for cultivation of rice crops.     

Integration of Soil pH with Soil‐Test Values of Zinc for Prediction of Yield Response in Rice Grown in Mollisols

Seventeen Mollisols having pH_(1:2) in the range of 6.00 to 8.42 were analyzed with five extractants, and the extractable zinc (Zn) ranges were 0.84 to 2.75 mg Zn kg^?1 soil for diethylenetriaminepentaacetic acid (DTPA) (pH 7.3), 0.91 to 2.72 mg Zn kg^?1 soil for DTPA + ammonium bicarbonate (pH 7.6), 1.82 to 7.18 mg Zn kg^?1 soil for Mehlich 3, 1.22 to 3.83 mg Zn kg^?1 soil for ethylenediaminetetraacetic acid (EDTA) + ammonium carbonate, and 0.88 to 1.18 mg Zn kg^?1 soil for 1 mol L^?1 magnesium chloride (MgCl₂) (pH 6.0). Zinc extracted by DTPA (pH 7.3) and Mehlich 3 showed significant positive correlation with sand content, whereas only Mehlich 3 showed negative correlation with soil pH. All extractants showed significant positive correlation with each other except for 1 mol L^?1 MgCl₂‐extractable Zn, which had significant positive correlation with only Mehlich 3– and EDTA + ammonium carbonate–extractable Zn. A greenhouse experiment showed that Bray's percentage yield of rice was poorly correlated to extractable soil Zn but had a significant and negative linear correlation with soil pH (r = ?0.662, significant at p = 0.01). Total Zn uptake by rice had a significant positive correlation with 1 mol L^?1 MgCl₂– and Mehlich 3–extractable Zn. A proposed parameter (p extractable Zn + p OH^?) involving both soil extractable Zn and pH terms together showed significant and positive correlation with Bray's percentage yield and total Zn uptake of rice. The calculated values of critical limits of soil Zn in terms of the proposed parameter were 14.1699 for DTPA (pH 7.3), 13.9587 for DTPA + ammonium bicarbonate, 13.7016 for Mehlich 3, 13.9402 for EDTA + ammonium carbonate, and 14.1810 for 1 mol L^?1 MgCl₂ (pH 6.0). The critical limits of Zn in rice grain and straw were 17.32 and 22.95 mg Zn kg^?1 plant tissue, respectively.     

Comparison of Iron Oxide–Impregnated Paper Strips with Other Extractants in Determining Available Soil Phosphorus

Abstract

Iron oxide–coated strips (P_i) can serve as a sink to continuously remove phosphorus (P) from solution. In this way, P extraction is analogous to the P absorption by plant roots. The objective of this study was to compare the iron oxide–coated paper strips with other chemical extraction methods to estimate the plant P availability for corn (Zea mays) growing in the greenhouse in some soils of Hamadan province of Iran. Sixteen soil samples with different physicochemical properties were analyzed for available P using Olsen, Colwell, Mehlich‐1, 0.01 M CaCl₂, AB‐DTPA, and 0.1 M HCl methods and p_i. Furthermore, the effects of two P levels (0 and 200 mg P kg^?1) on the plant indices (P uptake, relative yield, and plant responses) were studied in a greenhouse experiment using 10 soil samples. The results showed that the amount of extractable P decreased in the order of 0.01 M CaCl₂<AB‐DTPA<p_i<Olsen<Colwell<Mehlich‐1<0.1 M HCl. The amount of P extracted by the p_i method was significantly correlated with other extractants. The amounts of P extracted by all chemical methods were significantly correlated. The results of a pot experiment showed that the amount of P extracted by the p_i method was significantly correlated with the plant P uptake. However, the other methods were not significantly correlated with P uptake. The results of this experiment showed that p_i method was able to predict the plant availability of soil P.     

Evaluation of universal extractants for determining plant available potassium in intensively cultivated soils

Abstract

Eighteen soils from northwestern Switzerland were used to study the value of seven universal extractants (CaCl₂; DB‐DTPA; Mehlich 1, 2, and 3; Morgan‐Wolf; and NH₄OAc‐EDTA) for predicting plant available potassium (K) as compared to a bioassay (a modified Neubauer test with winter rye). These extractants were evaluated on the basis of K uptake by the bioassay test and the soil K status. In order to create the sufficiency level of exchangeable K for plant growth, soils were treated with 0, 20, 40, 80, and 160 mg K/kg of soil. The range of K uptake by the bioassay tests was between 89.2 and 403.0 mg/kg of soil for the control pots, and 136.6 to 495.8 for the K treatments with optimal conditions for plant growth. The average amounts of K extracted by the seven universal extractants, in ascending order, were: CaCl₂ < Morgan‐Wolf < Mehlich 1 < Mehlich 2 < NH₄OAc‐EDTA < Mehlich 3 < DB‐DTPA. The highest simple correlation with K uptake versus the bioassay test was obtained with the DB‐DTPA (r = 0.89) extractant and the lowest with the Mehlich 1 (r = 0.53) extractant. The DP‐DTPA, NH₄OAc‐EDTA and Mehlich 3‐K procedures showed an advantage over K procedures based on water soluble and exchangeable K pools in the investigated soils in order to predict the amount of plant‐available K. A simple regression and the Cate‐Nelson graphic method offer the possibility of assessing the soil‐K status using K values obtained by these universal extractants and to calibrate them against K forms as follows: exchangeable, water soluble, and non‐exchangeable.     

Comparison of Two Soil Phosphorus Analytical Methods in Croatia

Abstract

The purpose of this article was to compare soil phosphorus (P) extraction by sodium bicarbonate solution (Olsen P) and by ammonium lactate (AL P) and to create a model for prediction of Olsen P using ordinary soil‐fertility control data. The soils data used in this study included Olsen P, pH_KCl, pH_H2O, organic matter, AL P, and AL K. Soil pH_KCl ranged from 3.5 to 8, organic matter up to 5%, AL K up to 400 mg kg^?1, and AL P up to 200 mg kg^?1. Olsen P and AL P were significantly correlated, and the difference between them was influenced by soil pH. Regression models included all soil data grouped by soil pH range, which significantly decreased the difference between predicted and measured Olsen P. The validation of the model was conducted on new data sets from field fertilization trials. The results show that Olsen P can be related to AL P and used for fertilizer recommendations instead of AL P.     

Phosphorus Extraction Methods for Water Treatment Residual–Amended Soils

Abstract

Water treatment residuals (WTR) can adsorb tremendous amounts of phosphorus (P). A soil that had biosolids applied eight times over 16 years at a rate of 6.7 Mg ha^?1 y^?1 contained 28 mg kg^?1 ammonium–bicarbonate diethylenetriaminepentaacetic acid (AB‐DTPA), 57 mg kg^?1 Olsen, 95 mg kg^?1 Bray‐1, and 53 mg kg^?1 Mehlich‐III extractable P. To 10 g of soil, WTRs were added at rates of 0, 0.1, 1, 2, 4, 6, 8, and 10 g, then 20 mL of distilled deionized H₂0 (DI) were added and the mixtures were shaken for 1 week, filtered, and analyzed for soluble (ortho‐P) and total soluble P. The soil–WTR mixtures were dried and P extracted using DI, AB‐DTPA, Olsen, Bray‐1, and Mehlich‐III. Results indicated that all methods except AB‐DTPA showed reduced extractable‐P concentrations with increasing WTR. The AB‐DTPA extractable P increased with increasing WTR rate. The water‐extractable method predicted P reduction best, followed by Bray‐1 and Mehlich‐III, and finally Olsen.     

Comparisons of soil test phosphorus by Olsen,Bray P1, Mehlich I and Mehlich III methods

Abstract

A study was undertaken to evaluate the agreement among different university laboratories performing the Olsen, Bray P1, and Mehlich I tests for P on a diverse group of noncalcareous agricultural soils and to develop relationships among the Olsen, Bray P1, Mehlich I, and Mehlich III soil tests. For each test, the results from the individual laboratories were highly correlated (r² ≥ 0.90) and in almost all instances the slopes of the equations describing the relationships among laboratories approached one, The results indicate that the Olsen, Bray P1 and Mehlich I soil tests may be performed with a high degree of precision when standard soil test procedures are followed.

Of the three most commonly performed tests in the U.S. (Olsen, Bray P1, and Mehlich I), the Olsen and Mehlich I tests were the most highly correlated (r² = 0.87) although the Mehlich I test removed approximately one and one half times more P than did the Olsen test. Bray P1 and Olsen and Mehlich I P were less highly correlated (r² ≤ 0.72) and the relationships between these variables were influenced by the texture of the soils. The quantity of P removed by the Bray P1 test was on the order of two and three times greater than that removed by the Olsen and Mehlich I tests, respectively. The Bray P1 and Mehlich III soil tests were highly correlated (r² = 0.97) and similar quantities of P were extracted from the soil by the two tests.     

Development of a New Soil Extractant for Simultaneous Phosphorus,Ammonium, and Nitrate Analysis

Abstract

A new soil extractant (H³A) with the ability to extract NH₄, NO₃, and P from soil was developed and tested against 32 soils, which varied greatly in clay content, organic carbon (C), and soil pH. The extractant (H³A) eliminates the need for separate phosphorus (P) extractants for acid and calcareous soils and maintains the extract pH, on average, within one unit of the soil pH. The extractant is composed of organic root exudates, lithium citrate, and two synthetic chelators (DTPA, EDTA). The new soil extractant was tested against Mehlich 3, Olsen, and water for extractable P, and 1 M KCl and water‐extractable NH₄ and NO₂/NO₃. The pH of the extractant after adding soil, shaking, and filtration was measured for each soil sample (5 extractants×2 reps×32 soils=320 samples) and was shown to be highly influential on extractable P but has no effect on extractable NH₄ or NO₂/NO₃. H³A was highly correlated with soil‐extractable inorganic N (NH₄, NO₂/NO₃) from both water (r=0.98) and 1 M KCl (r=0.97), as well as being significantly correlated with water (r=0.71), Mehlich 3 (r=0.83), and Olsen (r=0.84) for extractable P.     

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.     

Comparison of Multi-element to Single‐Element Extractants for Macro‐ and Micronutrients in Acid Soils from Spain

Abstract

Different chemical reagents are used to assess plant‐available nutrients from soils with similar properties. The use of different extractants is a serious limitation when comparing results between different soil‐testing laboratories, often leading to large differences in fertilizer recommendations for similar crops.

In this study, 80 samples from acid soils from Galicia (Spain) were used to compare several soil nutrient extractants. Traditional and tested extractants for acid soil such as Bray 2 and ammonium acetate were used to evaluate multielement extractants such as ethylenediaminetetraacetic acid–ammonium acetate (EDTA‐aa), ammonium bicarbonate–diethylenetriaminepentaacetic acid (AB‐DTPA), and Mehlich 3.

Linear regression analyses were performed to relate the amount of each nutrient obtained by traditional soil extractants to the amount obtained by multielement extractants. Strong correlation was found between extractable Bray 2 P and Mehlich 3 P (r²=0.97, slope=0.87, and intercept=?0.48). The slope of the regression line between EDTA‐aa‐extractable calcium (Ca) and that from ammonium acetate (Aa) approached 1∶1 (r²=0.86). Similar results were obtained for magnesium (Mg) (r²=0.99). Soil zinc (Zn) concentrations extracted by Mehlich 3 and EDTA‐aa were similar; slope of the regression line was 0.95 (r²=0.88). With regard to copper (Cu), Mehlich 3 extracted approximately 20% more Cu than EDTA‐aa.

The results showed that Mehlich 3 and EDTA‐aa are suitable for assessment of plant available phosphorus (P), potassium (K), Ca, Mg, Cu, Zn, and iron (Fe) in acid soils.     

Release Kinetics of Nonexchangeable Potassium by Different Extractants from Soils of Varying Mineralogy and Depth

Abstract

Nonexchangeable potassium (K) release kinetics of six major benchmark soil series of India as affected by mineralogy of clay and silt fractions, soil depth and extraction media was investigated. The cumulative release of nonexchangeable K was greater in smectitic soils (353 mg K kg^?1 at 0‐ to 15‐cm depth and 296 mg K kg^?1 at 15‐ to 30‐cm depth, averaged for 2 soils and 3 extractants) than in illitic (151 mg K kg^?1 at 0‐ to 15‐cm depth and 112 mg K kg^?1 at 15‐ to 30‐cm depth) and kaolinitic (194 mg K kg^?1 at 0‐ to 15‐cm depth and 167 mg K kg^?1 at 15‐ to 30‐cm depth) soils. Surface soils exhibited larger cumulative K release in smectitic and illitic soils, whereas subsurface soils had larger K release in kaolinitic soils. Among the extractants, 0.01 M citric acid extracted a larger amount of nonexchangeable K followed by 0.01 M CaCl₂ and 0.01 M HCl. The efficiency of citric acid extractant was greater in illitic soils than in smectitic and kaolinitic soils. Release kinetics of nonexchangeable K conformed fairly well to parabolic and first‐order kinetic models. The curve pattern of parabolic diffusion model suggested diffusion controlled kinetics in all the soils, with a characteristic initial fast rate up to 7 h followed by a slower rate. Greater nonexchangeable K release rates in smectitic soils, calculated from the first‐order equation (b=91.13×10^?4 h^?1), suggested that the layer edge and wedge zones and swelling nature of clay facilitated the easier exchange. In contrast to smectitic soils, higher release rate constants obtained from parabolic diffusion equation (b=39.23×10^?3 h^?1) in illitic soils revealed that the low amount of exchangeable K on clay surface and larger amount of interlayer K allowed greater diffusion gradients, thus justifying the better fit of first‐order kinetic equation in smectitic soils and parabolic diffusion equation in illitic soils.     

Soil Phosphorus Tests for Flooded Rice Grown in Contrasting Soils and Cropping History

Soil phosphorus (P) tests for flooded rice (Oryza sativa L.) generally present uncertainties for estimating P availability. Bray 1, 1% citric acid, Mehlich 3, Olsen extractants (dry samples), and Bray 1 extractant after 3 days (BI3) and 7 days (BI7) of anaerobic incubations were evaluated to estimate P availability for rice in 43 Uruguayan soils. Field trials were conduced at each site (0, 13, 26, and 39 kg P applied ha^?1). Relative yield and absolute and relative yield increases were determined. Extracted P was variable for the different tests. For silty soils, P availability was better estimated by citric acid, Mehlich 3, and Bray 1, with similar soil P critical concentrations (6?8 mg P kg^?1). The BI3 and BI7 tests showed greater soil P critical concentration but poorer correlations with yield indexes. This study contributes to the scientific basis of P fertilization for flooded rice, promoting more effective fertilizer use and minimizing environmental P losses.     

Comparison of four phosphorus extraction methods on three acid southeastern soils

Abstract

Phosphorus extractants have not been tested extensively in the Southeast. An experiment was carried out to compare four P extractant methods using samples from a field P‐K factorial experiment with soybeans (Glycine max (L.) Merr.) at three locations in Georgia over four years. There were five P rates ranging from none to 80 kg ha^‐1. Soils and plant tissue were sampled at mid‐summer and yields were recorded. The four P extractants compared were Olsen, Mehlich 1, Mehlich 2, and Bray 1. Quadratic regressions for soil P versus plant P and P rates were not significant compared to linear regressions. There were no significant yield responses to P. All extractants except Olsen were similar in their response to added fertilizer P as measured by linear r² values. Olsen P gave lower linear r² values both with P rate and with plant P. Mehlich 1 values were highly correlated with Mehlich 2 (0.94**) and Bray 1 (0.96**). Mehlich 2 and Bray 1 gave nearly the same soil P values with linear regressions of slope of 1.0 and low intercepts. Results from these experiments show that Mehlich 1, Mehlich 2, or Bray 1 could be used successfully on these soils, but that Olsen should be avoided.     

Determination of Appropriate Soil Test Extractant for Available Phosphorus in Southwestern Nigerian Soils

Evaluation of five soil phosphorus (P) extractants was done on southwestern Nigerian soils from sedimentary and basement complex parent materials to determine the relationship between the extractants and the most appropriate extractant for the soils. The soils differed in properties. Generally, soils from the basement material had less available P compared with sedimentary material. Olsen extracted the greatest P. Bray 1 measured 67% of Olsen P, Hunter measured 52%, Mehlich measured 42%, and Ambic measured 24%. Positive and significant regression (P < 0.001) existed among Bray 1, Olsen, Mehlich, Hunter, and Ambic extractants. The strongest relationship was found among Olsen, Mehlich, and Ambic P. The relationship between maize P uptake and extracted P was quadratic, whereas the relationship with Mehlich was logarithmic. Bray, Mehlich, and Olsen P were the significant contributors to the maize P uptake and dry-matter yield. Extractants in order of P extraction were Olsen > Bray 1 > Hunter > Mehlich > Ambic.     

Effect of Five Phosphate Rocks on Red Clover (Trifolium pratense L.) Yield in Pot Trial

Abstract

Phosphorus (P) availability in five phosphate rocks with different P solubility was compared with that in single superphosphate and superphosphate+lime in a pot experiment with red clover as test plant on a Lamellic Arenosol with sand soil texture and on a Haplic Luvisol with clay loam soil texture, both strongly acid with low P supply. Phosphorus rates in the pot experiment were 0, 100, 400, and 1600 mg total P₂O₅ kg^?1. On both soils, there was a weak correlation between total added P and red clover P responses. If P solubility of the PRs was also taken into account, the correlation between formic acid–, citric acid–, or neutral ammonium citrate–soluble P amounts added and red clover responses became much stronger. Soil P availability was estimated by water, Olsen, Lakanen‐Erviö, and ammonium lactate tests. Among the P extractants studied, Olsen soil P test gave the best correlation with red clover yields.     

Evaluation of Universal Extractants for Determination of Some Macronutrients from Soil

Evaluation of nutrient status in soil is important for nutritional, environmental, and economical aspects. The objective of this work was to find out the most suitable universal extractant for determination of available phosphorus (P) and nitrate (NO₃-) and exchangeable potassium (K), calcium (Ca), and magnesium (Mg) from soils using 0.01 M calcium chloride (CaCl₂), 0.01 M barium chloride (BaCl₂), 0.1 M BaCl₂, 0.02 M strontium chloride (SrCl₂), Mehlich 3, and ammonium bicarbonate diethylene triamine penta acetic acid (AB-DTPA) extractants. Composite surface soil samples (0–20 cm) were collected from the Eastern Harage Zone (Babile and Haramaya Districts), Wolaita Zone (Damot Sore, Boloso Bombe, Damot Pulasa, and Humbo Districts), and Dire Dawa Administrative Council by purposive sampling. The experiment was carried out in a completely randomized design (CRD) with three replications. Results indicated that the greatest correlations were found between Mehlich 3 and Olsen method and also between 0.02 M SrCl₂ and Olsen method for available P. The amount of NO₃^– extracted by 0.02 M SrCl₂ was significantly correlated to the amount determined by 0.5 M potassium sulfate (K₂SO₄). The amounts of exchangeable K, Ca, and Mg determined by ammonium acetate (NH₄OAc) method were significantly correlated to the amount determined by universal extractants tested. In general, both 0.02 M SrCl₂ and Mehlich 3 can serve as universal extractants for the macronutrients considered in this study with the former being more economical when NO₃^– is included.     

Appraisal of Some Soil Tests for Zinc Availability to Late‐Sown Wheat Grown in Mollisols

Abstract

Five soil extractants, namely, 0.005 M diethylene triamine pentaacetic acid (DTPA) (pH 7.3), 0.005 M DTPA+1 M ammonium bicarbonate (pH 7.6), Mehlich 3, 0.01 M ethylene diamine tetraacetic acid (EDTA)+0.05 M ammonium carbonate (pH 8.6), and 1 M magnesium chloride (MgCl₂) (pH 6.0), were evaluated to predict the response of wheat to zinc (Zn) application in Mollisols. These extractants could be arranged in the following decreasing order of their Zn extracting power: Mehlich 3>0.005 M DTPA+1 M ammonium bicarbonate>0.01 M EDTA+0.05 M ammonium carbonate>0.005 M DTPA>1 M MgCl₂. The critical limits of Zn in soil, below which the yield response to late sown wheat (var. UP‐2338) to Zn application could be expected, were 0.57 mg 0.005 M DTPA (pH 7.3) extractable and 1.72 mg Mehlich 3–extractable Zn kg^?1 soil. The critical limit of Zn in whole shoot at 60 days after emergence was found to be 26.1 mg Zn kg^?1 plant tissue. The DTPA and Mehlich 3–extractable soil Zn also correlated significantly and positively with Zn concentration in whole shoot at 60 days after emergence and total Zn uptake by wheat at harvest.     

Automated Modification of the Molybdenum Blue Colorimetric Method for Phosphorus Determination in Soil Extracts

A flow injection analysis (FIA) method capable of automation for molybdate reactive phosphorus (P) determination in soil extracts is described. Results obtained using this method in three soil extracts [calcium chloride (CaCl₂), Olsen, and Mehlich I] were the same as those provided by the manual molybdate blue colorimetric method. Linear range extending to 2 mg P L^?1, detection limits ranging from 6 to 26 µg L^?1 depending on the soil extract, and accurate recoveries from P‐spiked samples were achieved. The sensitivity of the system was around 0.3 absorbance units per mg P L^?1, and the sampling frequency was 72 samples h^?1, higher than those described for most of the flow injection methods.     

Determination of Soil Feature Effects on Plant-Available Phosphorus Extraction using Response Surface and Cate–Nelson Methodology

In this study, four soil extraction methods (Olsen, Soltanpour, Mehlich 3, and water saturation) were used to identify optimal concentrations of phosphorus (P) required for plant growth. Olsen soil extraction for P was the most appropriate method for soil types of this study as the greatest correlation coefficient for soil-test P and with plant factors was achieved. The optimal amount of soil features (pH, organic carbon, lime, gypsum, and clay) determined by using response surface methodology (a new optimization method) were 7.49, 0.66, 41.82, 4.21, and 31.34, respectively. More soil P was extracted when the soil had optimal amounts of these features, showing each feature had a significant effect on extracted soil P. Furthermore, the graphical method of Cate–Nelson determined the optimal amounts of P using Olsen, Soltanpour, Mehlich 3, and saturation extract methods for wheat as 15, 6.5, 35, and 1.5 mg kg^?1 soil in nongypsic soils and 17, 3.5, 45, and 2.5 mg kg^?1 soil in gypsic soils.