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.     

Critical Limit of Extractable Phosphorous in a Gleysol for Rice Production in the Senegal River Valley of West Africa

Abstract

Soil‐test correlation and calibration, a useful tool for fertilizer recommendations, has been little used in West Africa. Soils from a long‐term fertility experiment have been used to study the relationship between rice yields and soil extractable phosphorus (P) with Bray 1 and Olsen methods. The Cate and Nelson graphical method was used for critical limits of soil P determination. The critical limits of soil extractable P at 95% relative grain yield were 9 mg P for the Bray 1P and 17 mg P kg^?1 for Olsen P. The Olsen P was more correlated (r=0.63) with rice grain yields than Bray P (r=0.50), but a strong correlation (r=0.92) was also observed between the values of the two methods. Results indicate that at levels less than these critical levels of extractable P, P fertilizers should be applied to increase rice yields.     

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.     

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.     

Mehlich 3 or modified olsen for soil testing in Malawi

Abstract

Soil nutrient extraction methods, which are currently being used in Malawi, are time consuming and require too many resources. The use of a universal soil extractant would greatly reduce resource requirements. The objectives of the study were to (i) compare the universal soil extractants, Mehlich 3 (M3) and Modified Olsen (MO) with ammonium acetate (AA), Bray P1 (BPl), and diethylene triamine penta acetic acid (DTPA) in the amount of nutrients extracted, (ii) determine the relationship among the extractants for the nutrients they extract, and (iii) determine the critical soil‐test levels of phosphorus (P), potassium (K), and zinc (Zn) for a maize crop. Missing nutrient trials involving P, K, and Zn were conducted on thirty sites across Malawi using maize (Zea mays L.). Phosphorus application rates ranged from 40 to 207 kg P₂O₅ ha^‐1. Potassium and Zn were applied at 75 kg K₂O and 10 kg Zn ha^‐1, respectively. Procedures of Cate and Nelson were used to identify soil nutrient critical levels. Results showed that the correlations between M3 and BP1, and MO and BPl were highly significant (r=0.93, 0.94, respectively). Mehlich 3 extractable K and AA extractable K (r=0.90), MO and AA extractable K (r=0.94) were highly significant (P<0.01) and the correlations between M3 and AA and MO and AA extractable calcium (Ca) (r=0.92, 0.90, and 0.94, respectively) were also highly significant (P<0.01). The correlations between M3, MO, and AA extractable magnesium (Mg) (r=0.99) were highly significant (P<0.01). Zinc, copper (Cu), and manganese (Mn) extracted with M3 and DTPA were significantly correlated (r=0.89, 0.87, and 0.95, respectively). Correlations between MO and DTPA extractable Zn, Cu, and Mn were also highly correlated (r=0.89,0.85, and 0.95, respectively). Maize grain yields ranged from 730 to 9,400 kg ha^‐1. Mehlich 3‐P and MO‐P critical levels were 31.5 and 28.0 μg g^‐1, respectively. Mehlich 3 and MO gave a similar critical level of 0.2 cmol kg^‐1 for K while Zn critical levels were 2.5, and 0.8 μg g^‐1 for M3 and MO, respectively. Mehlich 3 and MO were equally effective in separating responsive to none responsive soils for maize in Malawi.     

Correlation of Mehlich 3, bray 1, and ammonium acetate extractable P,K, Ca,and Mg for Alaska agricultural soils

Abstract

Soil test recommendations currently used in Alaska are based on a limited amount of in‐state data along with consideration of data from other states. Recently, Mehlich 3 extractable P has been found to be highly correlated to yield on representative agricultural soils in Alaska. To fully use its multi‐element capability, a study was conducted to correlate Mehlich 3 extractable P and cations (K, Ca, and Mg) with the P and cations extracted by the Bray 1 and ammonium acetate methods respectively.

When Mehlich 3 extractable K and Mg were regressed with ammonium acetate extractable K and Mg respectively, the relationship was essentially one‐to‐one and the relationship held across all soils tested. Significant variation was observed among soils in the extraction of Mehlich 3‐P and Ca relative to Bray 1‐P and ammonium acetate‐Ca. Individual soil character appeared to affect the regressions for extractable P and Ca, even though the R² values were generally high. The regression slopes for Mehlich 3‐P versus Bray 1‐P ranged from 1.01 to 1.88 with Mehlich 3 extracting an average of 66% more P than Bray 1 in the volcanic ash soils, and 12% more in the loess soils. The regression slopes for Mehlich 3‐Ca versus ammonium acetate‐Ca ranged from 0.95 to 1.33, and the former extracted an average of 17% more Ca than the latter. It is suggested that the regression data of P and Ca can be extrapolated to other soils based on soil classification; to extend the soil test data over a geographic base.     

Assessment of nickel's sufficiency critical levels in cultivated soils,employing commonly used calibration techniques

Although Ni has been officially recognized as an essential micronutrient for all higher plants since 2004, research on assessing its sufficiency critical levels with different soil tests is missing in the literature. The objective of the study was to determine Ni critical levels in unpolluted cultivated soils utilizing four methods, employing three commonly used calibration techniques. Ten soils with different physical–chemical properties and low Ni content were treated with Ni at rates of 1, 2, 4, and 8 mg kg^?1. After equilibration for one month, the soils were analyzed for extractable Ni by four methods, namely DTPA, AB‐DTPA, AAAc‐EDTA, and Mehlich‐3. Response to soil‐applied Ni was assessed by a greenhouse pot experiment, with the untreated and Ni‐treated soils in three replications, using ryegrass (Lolium perenne L.). The aboveground biomass of ryegrass was harvested two months after sowing, dry weight of biomass was measured and relative biomass yield was calculated. Nickel's critical levels were determined employing the: (a) graphical technique of Brown and co‐workers, (b) Mitscherlich–Bray equation, and (c) Cate and Nelson graphical technique. According to the first technique, Ni critical levels were ≈ 2 mg kg^?1 for the DTPA and AB‐DTPA methods, and 6.0 and 5.3 mg kg^?1 for the AAAc‐EDTA and Mehlich‐3 methods, respectively. Similar levels were obtained by the Mitscherlich–Bray equation. However, the critical levels assessed by the Cate and Nelson technique were lower and ranged from 0.5 to 1.3 mg kg^?1 for all four methods. Conclusively, Ni sufficiency critical levels for all four methods are expected to range at levels of a few mg Ni kg^?1 of soil. As far as the three calibration techniques are concerned, a distinct boundary between Ni response and non‐response was accomplished by none. However, the fact that 60–74% of the soils were correctly separated into responsive and non‐responsive to added Ni by the graphical technique of Brown and co‐workers suggests that this is the most suitable technique.     

Evaluation of soil test methods for estimation of available phosphorus in some Portuguese soils: A greenhouse study

Abstract

The Egnér‐Riehm method for estimating plant‐available soil phosphorus (P) has been used as the standard soil testing method in Portugal for making fertilizer recommendations. However, this method does not accurately reflect the available P status for wheat in some representative soils from the South Region of the country. Therefore, a pot experiment was established with four Luvisols (LVx, LVv, and two different LVh soils) from the South Region of Portugal in order to evaluate the Egnér‐Riehm, Bray I, Bray II, Olsen, and Anion Exchange Resin (AER) methods for their ability to estimate available P in those soils. Wheat (Triticum aestivum L., cv. Panda) was used as test the crop. The experiment was arranged into a randomized complete block design with three replications and five rates (0, 50, 100,150, and 200 mg kg^‐1) of P added to each soil. Critical soil P levels for LVx were established in case for Bray I (27.9 mg kg^‐1), Bray II (33.5 mg kg^‐1), Egnér‐Riehm (25.4 mg kg^‐1), and AER (14.7 mg kg^‐1) soil test procedures. Regarding the other soils, the critical soil P levels could not be estimated. The obtained results confirm that the development of an universal soil test P exti action is of great importance, and that further research should be conducted in order to evaluate routine soil fertility tests in different pedoclimatic environments.     

Predicting Maize Yield,Nutrient Concentration,and Uptake in Phosphorus- and Potassium-Fertilized Soils: Pressurized Hot Water and Other Alternatives to Mehlich I Extraction in Guatemala Soils

Abstract

Poor accessibility and cost of soil testing reduce effectiveness of fertilizer use on small‐scale subsistence farms, and inadequate funding promotes adoption of soil tests in developing countries with minimal validation. For example, Mehlich I extraction of phosphorus (P) currently used extensively in Guatemala may not be suitable for Guatemala's broad range of soils. At least four alternatives are available but relatively untested [Bray 1, Mehlich III, Olsen, and pressurized hot water (PHW)]. Pressurized hot water is relatively simple and inexpensive but is not yet tested against other extraction methods under variable P or potassium (K) fertilization levels. To determine whether PHW‐extracted nutrients could be used to predict maize yield and nutrient concentration and uptake, soil, plant tissue and grain samples were obtained from a multiple‐site field study, and calibration studies were conducted using five rates of P and three rates of K on soils incubated without plants or cropped with maize in greenhouse and field conditions. In the multiple‐site field study, maize yield related significantly to PHW‐extractable P (r²=0.36) and to leaf P concentration (r²=0.23), but Mehlich I–extractable P did not. In the two soils used in the greenhouse study, maize yield, vegetative P concentration, and total P uptake by maize were predicted by PHW‐extractable P (R²=0.72, 0.75, and 0.90, respectively). In the field experiment, grain yield was not improved by P or K application, but P concentration of maize leaf tissue did relate significantly with PHW‐extracted P (R²=0.40). Mehlich I did not. There were no yield responses to K application in any experiment, but relationships defined between extractable K for all five K‐extraction procedures and soil‐applied K were similarly significant. In comparison, PHW was as good as or better than Olsen whereas Bray 1 and Mehlich III were less consistent. Mehlich I was overall the poorest P extractant. Mehlich I extraction of P should be replaced by one of the four alternatives tested. PHW is the least expensive and, therefore, most viable for use in Guatemala soils.     

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.     

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.     

Soil test calibration studies for formulation of phosphorus fertilizer recommendations for maize in Morogoro district,Tanzania. II. estimation of optimum fertilizer rates

Abstract

Studies were conducted on some soils of Morogoro District to estimate economically optimum phosphorus (P) fertilizer recommendations for maize from soil tests. The studies involved the estimation of maize yield response to added P in pot and field experiments. Maize responded to added P at three sites. At all three sites, the residual effects of added P lasted up to three years suggesting that added P was not strongly fixed in these soils. Yield responses were related to extractable P contents by the Olsen and AB‐DTPA methods through a modification of the Mitscherlich‐Bray equation. Estimated model parameters were incorporated in equations for estimating optimum P fertilizer rates for different cost‐price ratios (p) and marginal rates of return (R). At the p and R values that prevailed in the study area during 1997, recommended P fertilizer rates (P_R) could be determined by the equation: P_R=68.497–1.191T (Olsen P). Calculated P fertilizer rates for P deficient sites in the district ranged from 15 to 60 kg P ha^‐1. This wide range underscored the importance of site specific fertilizer recommendations.     

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

Abstract

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.     

Critical soil phosphorus of a low‐P loess‐derived soil as affected by storage temperature

Abstract

Overwintering soil temperature may influence crop response to phosphorus (P) and indices of P availability in the humid, temperate, transitional climate of Tennessee. The effects of P fertilization and soil incubation temperature on sorghumsudangrass (Sorghum bicolor x S. Sudanese) grown on a Typic Hapludalf was investigated in a greenhouse study. In order to determine the effect of temperature on P availability, soils were incubated prior to cropping, at a constant temperature of 6°C or an average diurnal temperature of 24 and 36°C. Reagent grade Ca(H₂PO₄)₂.H₂O was used as the fertilizer source and applied at rates of 0, 10, 20, and 30 mg P kg^‐1 for the first test and 0, 20, 40, 60, and 80 mg P kg_‐ ¹ for the second test. Critical P concentration in the shoots for optimum yield was found to be 1.3 mg g‐¹, corresponding to soil solution and labile P concentrations of 5.5 μmol L^‐1 and 167 μg g^‐1, respectively. Optimum yield occurred for applications of >65 mg P kg^‐1 and was unaffected by soil incubation temperature. Applied P rates affected extractable P by five chemical extractants (Bray I, Bray II, Mehlich I, Mehlich III, and Mississippi), but soil incubation temperature had no affect. The extractants, however, were poorly correlated to plant P uptake and no one extractant appeared preferable to the others as an indicator of P availability.     

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.     

Evaluation of certain Ni soil tests for an initial estimation of Ni sufficiency critical levels

Although Ni is officially recognized as an essential micronutrient for all higher plants, the majority of the published research on soil availability of Ni focuses on its hazardous role as a heavy metal. The objective of the study was to evaluate certain Ni soil tests in uncontaminated soils for an initial estimation of its sufficiency critical levels. Nickel was extracted from 30 cultivated soils employing the following extraction methods: DTPA, AB‐DTPA, AAAc‐EDTA, Mehlich‐3, 0.1 M HCl, and 0.1 M HNO₃. Ryegrass (Lolium perenne L.) was grown in pots containing the soils, harvested five times, certain plant parameters were determined, and the Cate–Nelson procedures were used for Ni critical levels determination. Among the six methods, HCl was the least reliable extractant for the evaluation of soil available Ni, whereas the most significant (p ≤ 5%) relationships between Ni concentration or Ni uptake by ryegrass and Ni soil tests were consistently obtained for AAAc‐EDTA or Mehlich‐3 extractable Ni. In many cases, > 80% of the variability of Ni concentration or uptake by ryegrass was explained by these two soil tests without the inclusion of other soil properties that affect Ni bioavailability. Sufficiency critical levels of Ni in soil were ≈ 2 mg kg^–1 for both methods. Consequently, as an initial approach, concentrations of AAAc‐EDTA or Mehlich‐3 extractable Ni < 2 mg kg^–1 are probably a good guide to indicate soils that will respond to Ni fertilization.     

Lowland Rice Response to Thermophosphate Fertilization

Abstract

Use of adequate rates of phosphorus (P) in crop production on high‐P‐fixing acid soils is essential because of high crop response to P fertilization and the high cost of P fertilizers. Information on lowland rice response to thermophosphate fertilization grown on Inceptisols is limited, and data are also lacking for soil‐test‐based P fertilization recommendations for this crop. The objective of this study was to evaluate response of lowland rice to added thermophosphate and to calibrate P soil testing for making P fertilizer recommendations. A field experiment was conducted for two consecutive years in central Brazil on a Haplaquept Inceptisol. The broadcast P rates used were 0, 131, 262, 393, 524, and 655 kg P ha^?1, applied as thermophosphate Yoorin. Rice yield and yield components were significantly increased with the application of P fertilizer. Average maximum grain yield was obtained with the application of 509 kg P ha^?1. Uptake of macro‐ and micronutrients had significant quadratic responses with increasing P rates. Application of thermophosphate significantly decreased soil acidity and created favorable macro‐ and micronutrient environment for lowland rice growth. Across 2 years, soil‐test levels of Mehlich 1–extractable P were categorized, based on relative grain yield, as very low (0–17 mg P kg^?1 soil), low (17–32 mg P kg^?1 soil), medium (32–45 mg P kg^?1 soil), or high (>45 mg P kg^?1 soil). Similarly, soil‐test levels of Bray 1–extractable P across 2 years were very low (0–17 mg P kg^?1 soil), low (17–28 mg P kg^?1 soil), medium (28–35 mg P kg^?1 soil), or high (>35 mg P kg^?1 soil). Soil P availability indices for Mehlich 1 extractant were slightly higher at higher P rates. However, both the extracting solutions had highly significant association with grain yield.     

Wheat p requirements on calcareous Morrocan soils: III evaluation by isotherms compared to Olsen extractable P

Abstract

Phosphorus fertilizer recommendations were compared by interpretations from P isotherms, Olsen extractable P and the Mitscherlich‐Bray model based on the Olsen method for 15 soils from the Chaouia (dryland) region of Morocco. The P isotherms were fit to straight line and second degree polynomial equations. The P buffer indexes (PBI) derived from the isotherms were not significantly correlated to P buffer capacities as measured by a single P buffer capacity index, but negatively correlated to Olsen P (r = ‐0.63), relative yield (r = ‐0.76) and P uptake (r = ‐0.66). Phosphorus in solution was a quadratic function of P added in 0.01 M CaCl₂equilibrium solution. The P fertilizer recommendations to maintain soil solution P concentrations at 0.01, 0.12 and 0.20 mg P L^‐1were higher than recommended by direct interpretation of plant response to Olsen extractable P and the quantity based on the Mitscherlich‐Bray model as calculated from Olsen available P values. The P fertilizer recommended to maintain soil solution P of 0.10 mg P L^‐1was significantly correlated with Olsen P (r = 0.71) as was that recommended Mitscherlich‐Bray log transformation model (r = 0.81), and nonlinear least square estimation (r = 0.78). Field research will be needed to evaluate if the P fertilizer recommended to maintain this solution P concentration is adequate for maximum economic wheat grain yield under field conditions     

Divergent Relationships of Phosphorus Soil Tests in Temperate Grassland Soils

Abstract

Various soil tests are used to estimate phosphorus (P) availability for both crop uptake and potential loss to water. Conversion equations may provide a basis for comparison between different tests and regions, although the extent to which information can be interchanged is uncertain. The objective was to determine and quantify relationships between specific soil test extractants for samples taken annually in October and February over 4 years from four sites in each of eight soil series under grassland. The extractants comprised Mehlich‐3, Morgan, Olsen, Bray‐1, lactate–acetate, CaCl₂ (1∶2 and 1∶10 soil–solution ratios), and resin. The results showed distinct relationships for each soil series, for which individual lines regression models (different intercepts and slopes) were superior to a single conversion equation across all soils. The ensuing difference between soils was large and ranged from 1.9 to 8.0 and 9.2 to 15.6 mg kg^?1 P for Morgan and Olsen, respectively, at 20 mg kg^?1 Mehlich‐3 P. Generally, the environmentally oriented tests CaCl₂ and resin correlated best with Morgan. Some soil‐specific limitations were also observed. CaCl₂ was less efficient than Morgan, and Morgan less efficient than Mehlich‐3 on a high Fe–P soil derived from Ordovician‐shale diamicton, compared with the general trend for other soils. This finding suggests that further disparity may arise where evaluation of critical, or other, limits across regions involves even a limited sequence of tests.