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Determining of Soil Phosphorus Requirement with Application of Freundlich Adsorption Isotherm

April 2010
Asian Journal of Agricultural Research 4(4):226-231

DOI:10.3923/ajar.2010.226.231

Authors:

Abd Ali Naseri

Shahid Chamran University of Ahvaz

Yaser Hoseini

University of Mohaghegh Ardabili

Hadi Moazed

Shahid Chamran University of Ahvaz

Fariborz Abbasi

Agricultural Research, Education and Extension Organization

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Phosphorus (p) adsorption characteristics of 5 soil samples were studied in soils of Southeastern Iran during 2009/2010 cropping season. Some soil properties as well as selected P-adoption characteristics were studied in these soils, Results show for this study Freundlich adsorption isotherm has good match with phosphorus adsorbed data and has good R-square. Freundlich sorption isotherms were used to evaluate the P requirement of 5 soils in a lab study. The soils were medium to heavy textured, acidic in reaction and were highly calcareous. Amount of P sorbed by the soils increased with increasing P in equilibrium solution. Quantities of P retained on sod solid phase were significantly correlated (p<0.05) with clay content of the soils. Maximum P was sorbed by a soil that had the maximum (64%) Clay content, there were negative relationship between P adsorption capacity and SOC and Ca content of soils.

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Unlocking the secrets of wheat production: Evaluating phosphorus levels and seeding rates in alluvial soil conditions

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Aug 2023

This research study was conducted to determine the effect of different phosphorus levels and seeding rates on the growth, phenology, and yield of wheat. The experiment used a randomized complete block design with split plot arrangement and four replications. The main plots consisted of six seeding rates, ranging from 50 kg/ha to 175 kg/ha, while the sub-plots received six different P levels, ranging from 50 kg/ha to 200 kg/ha. In consecutive years, the study revealed significant effects of phosphorus levels, seeding rates, and their interaction on various growth characteristics and yield parameters of wheat. These included days to 50% heading and 50% maturity, leaf area index (LAI), leaf area duration (LAD), crop growth rate (CGR), net assimilation rate (NAR), and grain yield (GY). Phosphorus (P) level of 140 kg/ha resulted in the highest CGR, NAR, and GY in both years of the study. The highest interaction values for CGR, NAR, and GY were achieved with the combination of 140 kg P/ha and a seeding rate of 125 kg/ha. By utilizing a seed rate of 175 kg/ha and a P level of 200 kg/ha, the study revealed the highest values for LAI and LAD.

Unlocking the secrets of wheat production: Evaluating phosphorus levels and seeding rates in alluvial soil conditions

Article

Full-text available

Aug 2023

A research study was conducted to determine the effect of different P levels and seeding rates on the growth, phenology, and yield of wheat. The experiment used a randomized complete block design with split plot arrangement and four replications. The main plots consisted of six seeding rates, ranging from 50 to 175 kg/ha, while the sub-plots received six different P levels, ranging from 50 to 200 kg/ha. In consecutive years, the study revealed significant effects of phosphorus levels, seeding rates, and their interaction on various growth characteristics and yield parameters of wheat. These included days to 50% heading and 50% maturity, leaf area index (LAI), leaf area duration (LAD), crop growth rate (CGR), net assimilation rate (NAR), and grain yield (GY). Phosphorus (P) level of 140 kg/ha resulted in the highest CGR, NAR, and GY in both years of the study. At a seeding density of 125 kg/ha, similar findings were observed, with the maximum CGR, NAR, and GY recorded. The highest interaction values for CGR, NAR, and GY were achieved with the combination of 140 kg P/ha and a seeding rate of 125 kg/ha. By utilizing a seed rate of 175 kg/ha and a P level of 200 kg/ha, the study revealed the highest values for LAI and LAD. Additionally, the maximum protein content was observed in treatments where seeds and P were applied at 50 and 200 kg/ha, respectively. In summary, the study provided conclusive evidence that the application of suitable seeding rates and P levels can yield significant improvements in the growth, phenology, and yield of wheat. The findings could be beneficial for farmers and researchers in improving wheat production.

PHOSPHATE SORPTION CHARACTERISTICS OF GANGES MEANDER FLOODPLAIN SOILS OF BANGLADESH

Article

Full-text available

Sep 2022

Three representative soils belonging to Amjhupi, Ishurdi and Sara series from the Ganges meander floodplain soils in Jessore district were collected to observe phosphate sorption characteristics and to evaluate the best fitted adsorption equation for the soils. The highest phosphate sorption was observed in Amjhupi series and the lowest was in Sara soil series. Differences in P sorption were significantly (p<0.05) influenced by clay content, CEC and pH of the soils. Free CaCO3, EC, OM and available P were found insignificant in affecting P adsorption. The Langmuir equation was best fitted to the equilibrium P sorption data and maximum R-square. Amjhupi soil series would require more P to be supplied to maintain a desired P level in soil solution than others.

Phosphate Sorption Capacities of Some Soils in Madhupur Tract of Bangladesh: Role of Aluminium, Iron and Clay Contents

Article

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Jul 2018

An experiment was carried out to study the phosphate sorption capacities of five soil series, namely Noadda, Salna, Gerua, Belabo and Chandra in the Madhupur Tract of Bangladesh. Nine composite soil samples representing the five soil series were found to exhibit varied sorption capacities. However, most of the soils showed the highest amount of phosphate sorption at 25 µg P/mL of application. The sorption data for most of the soils showed a good fit into Langmuir adsorption isotherm. The values of Langmuir-derived maximum sorption capacity (Q 0) varied widely and was in the following order: Belabo, site 9 (1000 µg/g) > Gerua, site 5 (995 µg/g) > Noadda, site 4 (500 µg/g) > Noadda, site 7 (250 µg/g) = Belabo, site 2 (250 µg/g) > Salna, site 6 (167 µg/g) = Gerua, site 8 (167 µg/g) > Salna, site 3 (143 µg/g) > Chandra, site 1 (56 µg/g). The highest sorption capacity of Belabo (site 9) soil could be attributed to its total Al, total Mn, and amorphous Fe contents. The calculated Langmuir coefficient b values were found to be higher than the threshold value of 0.07 mL/µg for the soils in question meaning the soils are not prone to loss via surface and subsurface flow. A strong relationship (r = 0.799**) was obtained between phosphate sorption at 10 µg P/mL of application and the clay content in the studied soils. The aluminium content was also found to have a significant relationship (r = 0.684*) with the phosphate sorption at 10 µg P/mL of application. However, the total free iron oxides and the amorphous Fe did not correlate well with the phosphate sorption at 10 µg P/mL of application. Therefore, the clay and total aluminium contents were primarily responsible for the phosphate sorption capacity of the acidic soils of the Madhupur tract under investigation.

Phosphate sorption indices as affected by the calcareousness of soils

Article

Full-text available

Jan 2019

An experiment was carried out to study the effects of calcareousness on phosphate sorption indices of soils using three representative calcareous soils, namely Sara (Aquic Eutrochrept), Gopalpur (Aquic Eutrochrept), and Ishurdi (Aeric Haplaquept) series of Bangladesh. Three non-calcareous soils, namely Belabo (Typic dystrudepts), Sonatala (Aeric Endoaquepts) and Ghatail (Aeric Haplaquept) series were also selected for comparison purposes. Phosphate sorption indices of soils were calculated using Langmuir and Freundlich isotherms. Isotherms were constructed taking one gram of air-dried sieved (< 2 mm) soil into a 50 ml centrifuge tube, and subsequently adding seven initial P concentrations, namely 0, 1, 10, 25, 50, 100 and 150 µg/ml to each centrifuge tube employing a soil/solution ratio of 1 : 20 (w/v). According to the Langmuir equation, the amount of phosphate sorbed followed the order: Sonatala > Ghatail > Sara = Gopalpur > Ishurdi > Belabo. The abundance of amorphous iron rather than the calcareousness was putatively responsible for the high phosphate sorption capacity of soils. Maximum phosphorus buffering capacity (MPBC) of the calcareous soils ranged from 33.4-62.8 l/kg. Langmuir and Freundlich equations produced different values for soil P requirements (SPR) at 0.2 and 1.0 mg P/l. Calcareous soils would require 27-44 mg P/kg soil to attain 0.2 mg P/l soil solution, which is deemed sufficient for crop growth. The soils would require 32-58 mg P/kg soil to reach 1.0 mg P/l soil solution, which is regarded to be safe for soils in terms of susceptibility to P losses. The calculated Langmuir constant b values were higher than the threshold value of 0.07 l/mg for two of the calcareous soils. Therefore, even though the non-calcareous soils sorbed more phosphate, higher bonding energy of P sorption for calcareous soils makes them less vulnerable to loss via surface runoff.

International Journal of Advanced Research in Biological Sciences Phosphate sorption potential of some acid soils of Bangladesh as tested by Freundlich and Temkin equation

Article

Full-text available

Jan 2015

To investigate the phosphate sorption potential of soils, an incubation experiment was performed with five representative soil series of Bangladesh namely the Borkal (Typic Dystrudepts), the Pahartali (Aeric Endoaquepts), the Raojan (Aeric Endoaquepts), the Noapara (Typic Endoaquepts) and the Manu (Aeric Dystrudepts). Calcium chloride solution of 0.01 M containing 0, 1, 10, 25, 50 and 100 mg phosphorus dm-3 was used to equilibrate the soils and then the sorbed phosphate was measured. The phosphate sorption curves showed almost similar magnitude of changes. Phosphate sorption by the soils was observed in the order of Borkal>Noapara>Manu>Raojan>Pahartali soil series and reflected with the organic matter content of soils. Between Freundlich and Temkin equation, Freundlich model showed better fit to sorption data at higher P concentrations. The coefficient of determination (R 2) values were used to compare the suitability and applicability of Freundlich and Temkin equation. Equilibrium P concentration (EPC 0) value (P concentration at zero fixation) was estimated by using Temkin adsorption equation and the values were between 0.05 mg dm-3 and 0.45 mg dm-3. The highest EPC 0 was obtained for Pahartali and the lowest for Noapara soil series. Therefore, Freundlich equation may be more suitable for evaluating phosphate sorption capacities of agricultural soils.

Effect of Incubation Period of Phosphorus Fertilizer on some Properties of Sandy Soil with Low Calcareous Content, Southern Algeria

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Mar 2015

Fertilization practices can contribute to modify the properties of the soils, in particular salinity and the pH of soils. The main goal of this study is to demonstrate the impact of the physicochemical properties changes related to applied phosphorus fertilizers on phosphorus availability in light textured soils having low calcareous content. A short-term incubation experiment during 105 days was carried out in field conditions. Three different phosphorus fertilizers (monoammonium phosphate, fosfactyl and single super phosphate) were added in a sandy soil. The principal results indicated that the application of these fertilizers causes an enhancement in phosphorus availability in this soil. Through incubation period, the contribution of phosphorus showed a tendency to increase in the electrical conductivity value of the soil. The SSP fertilizer shows a highest degree of soil salinization with 7.57% compared to the control without phosphorus addition. On the other side the P fertilizers tested exert an acidifying effect, the addition of monoammonium phosphate causes a decrease in pH values of soil in the range of 6.71% compared to untreated samples. Therefore, the pH values of the control reflect the degree of acidification of phosphorus fertilizers used which constitutes an advantage in our Saharan climatic conditions. This study demonstrates that it is essential to choose appropriate fertilizers according to the components of the soils in order to have a suitably reasoned mineral fertilization and respectful of the physicochemical environment of soil and undergrounds waters properties.

Évaluation de la résilience des systèmes de culture légumineuses - céréales face à la sècheresse : Analyse multi-échelle de l’efficience de l’utilisation de l’eau et de l’azote par la validation des modèles biophysiques

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Mar 2022

Kherif Omar

The effects of climate change worse natural agro-climatic constraints and limit the develop-ment of Algerian agriculture. Cereal crop cultivation is often undertaken on poor soils with extensive rain-fed practice. Unfortunately, the Means (supplemental irrigation and nitrogen fertilizers) proposed for increasing cereal yields have a high cost with environmental impacts, which thus penalize their agro-economic efficiency. In this context, with regard to ecological principles and the related ecosystem services, the introduction of a legume in an association system with cereals could improve the sustainability of agro-ecosystems. Our study aims to assess and estimate the performance of associated crops in terms of the use and resources storage (especially C and N elements). An approach combining the analysis of experimental data (via the agro-ecological competitive indices) and quantitative modeling (MOMOS and STICS models) of agro-ecological indicators were applied to assess the C, P and N cycles. First, we carried out the estimation of the exchange rates for C and N between plants, soil, microbial biomass and nodular symbionts. The parameters of the MOMOS model were also improved by the measurements of the state variables (plant, soil and weather) and validated the current calibrated models (MOMOS-C and MOMOS-N) at the plot, farm and territory scales, for the associated species (common bean-maize). In the intercropping system, we found that all the daily exchanges of C and N between plant organs and microorganisms vary greatly between phrenological stages, a considerable part of the fixed N is preferentially stored in the nodules of the associated legume, promoting micro-bial functioning. We have confirmed the robustness of this parameterized version of MOMOS to assess and analyze the mechanisms regulating global C and N exchanges between plant symbionts, soil, microbial biomass and the atmosphere. On winter intercrop "wheat-chickpea" the first evaluation with competitive indices has demonstrated two conditions which make intercropping advantageous; (I) Sufficient supply of nitrogen to maximize symbiotic fixation of N2 and contribute to the nutrition of the cereal crop; (II) with a dominance of the legume plants, interspecific competition between chickpea and durum wheat was directly affected by changes in climatic conditions or N fertilizer, the intercrop wheat was more competitive than legume under conditions of low rainfall and N application. The calibration and validation of the STICS model for chickpea in rain-fed conditions were carried out, based on an optimization method (simplex algorithm). A high precision of the cal-ibration was statistically confirmed for most of the variables of interest. The model was able to capture the temporal dynamics of the variables studied in contrasted situations (cropping sys-tems, growing seasons and N application rate). Finally, this calibration work allows the STICS model to effectively simulate innovative cultivation practices based on crop diversification (grain and cereal legumes) and nitrogen fertilization management.

Effect of Phosphorus Application on Durum Wheat in Alkaline Sandy Soil in Arid Condition of Southern Algeria

Article

Full-text available

Jan 2015

The present study is the part of research program on the rational fertilization of durum wheat based on the long-term trials in El-Menia situated in arid region of south eastern Algeria. This study includes only the results of three years of experimentation. Three levels of phosphorus (30, 60 and 90 kg h a -1 P205) from three sources Mono-Ammonium Phosphate, Single Super Phosphate and Fosfactyl were applied on durum wheat (Triticum durum var. Carioca) under center pivot irrigation system. The experiments were made Two-Factor Hierarchical Designs during growing seasons 2008/2009, 2009/2010 and 2010/2011. Yield components, grain and straw yields; N, P content and their uptakes by grain were measured. For most of t he tests, the fertilizer source had no effect on the traits studied except for phosphorus content where single super phosphate give the best value (0.4±0.004%). Indeed, 30 and 60 kg ha-1 P205 give the best 1000-gram weight and grain yield regardless of the year. For straw yield, 90 and 60 kg h a-1 give the best average values. Nitrogen (N) uptake grains have not been influenced by the doses of phosphorus regardless of year. The P content, grain yield, number of spikes/m2 and N uptake increase with increase P uptake regardless of year. While number of grains/spike, N content and straw yield increased with the phosphorus uptake by year.

Response of Two Groundnut (Arachis hypogaea L.) Varieties to Sowing Date and NPK Fertilizer Rate in a Semi-Arid Environment : Growth and Growth Attributes

Article

Mar 2011

The main objective of the present investigation was to determine the growth attributes of two groundnut varieties as influenced by sowing date and NPK compound fertilizer rate. Treatments were factorial combinations of three sowing dates (mid-June, end-June and mid-July), three fertilizer rates (10 kg N+13 kg P+13 kg K ha -1, 20 kg N+26 kg P+26 kg K ha -1 and 30 kg N+39 kg P+39 kg K ha -1) and two varieties (SAMNUT-22 and SAMNUT-23). Fertilizer rate x sowing date constituted the main plot, while varieties were assigned to the sub-plot as the experiment was laid out in split-plot design with four replications. Canopy spread at 9 Weeks After Sowing (WAS) declined with delay in sowing in 2005. Application of 20 kg N+26 kg P+26 kg K ha -1 increased canopy spread signifieantly. The widest canopy spread in 2005 resulted from the application of 30 kg N+39 kg P+39 kg K ha -1 to mid-June sown crop. Dry matter at 9 WAS in 2004 declined 21.2% when sown in mid-July while at 12 WAS It declined 25.1% and by 23.3% in 2005. Variety SAMNUT-23 accumulated 18.6% greater dry matter than var. SAMNUT-22. Durmg the penod 9-12 WAS crop growth rates were highest for the mid-June sown crop and 20 kg N+26 kg P+26 kg Kha -1 fertilizer rate, with a value that was 59.4 and 50.1% higher than those for the lowest and highest fertilizer rates, respectively. Delayed sowing delayed 50% flowering. Variety SAMNUT-22 flowered 8 days later than var. SAMNUT-23 and when sown early (in mid-June) and treated with 20 kg N+26 kg P+26 kg Kha -1 out-performed SAMNUT-23 in growth attributes.

Phosphate sorption characteristics of major soils in Okinawa, Japan

Article

Full-text available

Feb 2000

Phosphate sorption isotherms were determined for 16 representative major soils developed from different parent materials on Okinawa. Phosphate sorption characteristics were satisfactorily described by the Langmuir equation, which was used to determine phosphorus (P) sorption maxima of the soils. Phosphate sorption maxima ranged from 630 to 2208 mg P kgsoil (mean 1,362 mg P kg). The standard P requirement (i.e., the amount of P required to attain 0.2 mg P L equilibrium solution) followed the same trend as sorption maximum (r =0.94***), with values ranging from 132 to 1,020 mg P kg soil (mean 615 mg P kg). This mean value corresponds to fertilizer addition of 923 kg P ha indicating that the soils have high P fertilizer requirements. Results of simple linear regression analysis indicated that sorption maximum was significantly correlated with clay content, organic matter, oxalate iron (Fe), pyrophosphate Fe, DCB aluminum (Al), oxalate Al, and pyrophosphate Al, but not with DCB Fe, pH, or available P content. The best regression model for predicting sorption maximum was the combination of clay, organic matter, pyrophosphate Fe, and DCB Al which altogether explained 79% of the variance in sorption maximum. The equation obtained could offer a rapid estimation of P sorption in Okinawan soils.

Yield and phosphorus efficiency of some lowland rice varieties at different levels of soil‐available phosphorus

Article

Full-text available

Nov 1998

A field experiment was conducted on an Aerie Haplaquept soil to study the effect of phosphorus (P) deficiency in soil on the P nutrition and yield of five modern varieties of rice, viz., Purbachi, BR1, BR3, BR14, and BR29, popular with the rice farmers of Bangladesh. Soil‐available P in the different plots of the experimental field varied widely, from 2.8 to 16.4 ppm. This plot to plot variation in soil‐available P content resulted from differences in the total amounts (0 to 480 kg ha) of P the plots had received over a period of 8 years in a long‐term P fertilizer trial conducted previously in the same field. Phosphorus deficiency in soil drastically reduced the grain yield of all the rice varieties. In severely P deficient plots, where soil‐available P was around 3 ppm, the yield was less than 1 ton ha while in plots containing an adequate P level, i.e., >6 ppm, the yield was more than 41 ha. Rice yield increased linearly with an increase in soil P content up to 6 ppm, and the highest grain yield for any variety, obtained at 6–7 ppm of soil‐available P leveled off at this point. Soil P deficiency not only decreased rice yield severely but also decreased P content in straw and grain drastically. However, differences among rice varieties were noted in P nutrition, particularly at low soil P levels. The rice varieties differed markedly also in respect of internal P efficiency. The BR29 showed the highest internal P efficiency both at low and high soil P levels. In all the rice varieties, internal P efficiency decreased with an increase in soil P levels.

Phosphorus Dynamics in a Highly Weathered Soil as Revealed by Isotopic Labeling Techniques

Article

Full-text available

Sep 2004
SOIL SCI SOC AM J

Isotopic labeling techniques have the potential to elucidate soil P dynamics and the fate of P sources added to the soil, but they have rarely been applied to highly weathered tropical soils. We collected soils from two crop rotations [continuous maize (COM; Zea mays L.) and maize-crotalaria (MCF; Crotalaria grahamiana Wight & Arn.) fallow rotation] in a field experiment in Kenya and incubated them for 9 wk after addition of a plant residue or inorganic phosphorus (Pi), both labeled with 33P and added at 6 mg P kg-1 soil, or after carrier-free labeling of isotopically exchangeable soil phosphorus (soil IEP). The amount of P and recovery of 33P were determined in resin-extractable Pi (P resin), microbial P (Phex), and in a 0.1 M NaOH extract of samples from which Presin and Phex had been removed. The Presin increased after addition of Pi, while P hex increased after plant residue amendment, involving considerable microbial uptake of soil P. The recovery of 33P in Presin followed the order added Pi > soil IEP > plant residue, and decreased steadily from 7 to 22% after 1 d to 3 to 5% after 9 wk. The recovery of 33P in Phex remained constant throughout the incubation, being greater after plant residue amendment (15%) than in the other two treatments (4-7%). An additional 66 to 76% of 33P was recovered in the NaOH extract, as much as 27% of which was in organic phosphorus (P o) after plant residue amendment and 2 to 8% in the other two treatments. Similar to P dynamics after plant residue amendment, the comparison of the two rotations indicated a shift toward Phex and Po with increasing microbial activity due to previous fallow biomass incorporation.

Sorbed phosphate at standard supernatant concentration as an estimate of the phosphate need of soils

Article

Jan 1965

R. S. Beckwith

Phosphate sorption capacity of soils has meaning only if the equilibrium supernatant solution concentration is specified. Measurements have been made, on a variety of Queensland soils, at an equilibrium concentration of 0.2 p.p.m. P ; reasons for this choice of cencentration are discussed. Phosphate sorption values measured in this way appear to parallel the phosphate needs of legumes growing on a number of the soils examined in the laboratory. The approach is put forward for testing by others on present and future phosphate rate trials. Present phosphate sorption measurements are interpreted as indicating (1) that even where native phosphate is inadequate, or has been depleted by cropping, heavy-textured grey and brown soils of the brigalow lands will only require small field applications of superphosphate. (This statement may not apply to soils containing free carbonate in the surface). (2) that phosphate requirements of krasnozems vary considerably but may exceed 1 ton of superphosphate an acre in some areas. Loss of the surface horizon by erosion, or mixing the subsoil With surface soil, could increase the phosphate requirement of some of these soils. (3) that the phosphate status of soils formed from phyllite in the Gympie district is intermediate between these extremes. Here also the subsoils must be expected to have larger phosphate requirements than the surface soils.

Phosphorus characterization and correlation with properties of selected Benchmark soils of the United States

Article

Jan 2002

Twenty-one benchmark soils of the United States, including surface and subsurface horizons and satellites, from the Water Erosion Prediction Project (WEPP) were analyzed for phosphorus (P), using methods that include total (TP), water-soluble (WP), Bray 1 (BP), Mehlich No. 3 (MP), Olsen (OLP), New Zealand P Retention (NZP), organic (OP), anion exchange resin (AEP), and acid oxalate (P-o). Objectives of this study were to determine relationships among soil P test values and other soil properties. Knowledge and understanding of these relationships are important to researchers when evaluating soil P data sets for use in predictive models for agronomic, soil genesis, or environmental purposes. Important relationships that were developed, using simple or multiple linear regression models, among P methods and other soil properties, e.g., organic carbon (OC), total N (TN), dithionite-citrate extractable iron and aluminum (Fe-d, Al-d), and clay are as follows: TP (mg/kg) = 229.02 + 27.76 Al-d (g/kg) + 27.44 OC (g/kg) + 4.14 Fe-d (g/kg), r(2) = 0.89, p < 0.01, n = 263 (all soils) OP (mg/kg) = 114.07 + 38.07 TN (g/kg) - 14.74 pH + 6.94 OC (g/kg), r(2) = 0.80, p < 0.01, n = 262 (all soils) BP (mg/kg) = -1.82 + 1.11 MP (mg/kg), r(2) = 0.96, p < 0.01, n = 268 (all soils) P-o (mg/kg) = 16.02 - 24.27 Al-o (g/kg) + 25.59 Fe-o (g/kg) + 19.33 OC (g/kg)r(2) = 0.79, p < 0.01, n = 203 (non-calcareous) NZP (%) = 16.92 + 1.37 Al-d (g/kg) + 0.28 clay (%), r(2) = 0.91, p < 0.01, n = 203 (non-calcareous).

Phosphate sorption isotherms for evaluating phosphorus requirement of wetland rice soils

Article

Jun 1985

Phosphate sorption isotherms were developed for five Philippine wetland rice soils using the conventional technique and a modified one. In the conventional method, P requirements of soils varied between 280 and 810 g P/g soil. In the modified method, they varied from 160 to 540 g P/g soil at 0.2 ppm P in solution. Soils with high P-sorption capacities had vermiculite and halloysite as the dominant clay minerals. Soil reduction by flooding decreased P-sorption by 28–70 percent at 0.2 ppm P in solution. The decrease in P-sorption due to soil reduction was greatest in a crystalline soil with vermiculite and halloysite as the dominant clay minerals and least in a soil with dominant X-ray amorphous silicates in the clay fraction.Desorption of freshly adsorbed P under reduction was greater in HCO 3 – solution than in CaCl2 and it increased with level of applied P. Desorption patterns of freshly adsorbed P were similar to adsorption patterns but values of P in solution were lower at desorption. Soils varied with respect to desorption of freshly sorbed P. Desorption studies indicate that soils vary in intensity factor with respect to P and thus influence P availability to plants. Use of P-sorption and P-desorption data obtained under reduced soil condition was proposed for detecting P needs of submerged rice soils.Results of a pot study with IR36 at different levels of solution P (reduced) in one soil indicated a high degree of correlation between adjusted P levels and the measured growth parameters. About 0.12 ppm P in the soil solution or 0.46 ppm P desorbed in HCO 3 – solution (equivalent to 100 mg P/kg soil) was adequate for near-maximum plant height, tiller production, total dry matter yield, plant P content, and total P uptake.