Manganese retention by selected calcareous soils as related to soil properties

Abstract

Calcareous soils often need supplemental manganese (Mn) to support optimum plant growth, but some reports show that the apparent recovery of applied Mn is very low in such soils, i.e., nearly all of the applied Mn is retained in the soil. This experiment was conducted to find the relationship between the retained Mn and selected properties of calcareous soils. Eleven surface (0–20 cm) soil samples with pH ranging from 7.7 to 8.1 and calcium carbonate equivalent (CCE) ranging from 20 to 50% were used in the Mn adsorption study. Two‐gram subsamples of each soil were equilibrated with 20 mL of 0.01M CaCl₂ solutions initially containing 10 to 200 mg Mn L^‐1. The Mn that disappeared from solution (after 6 h shaking at 25°C) was considered as adsorbed (retained) Mn. The adsorption data showed a highly significant fit to Freundlich and also to the two‐surface Langmuir adsorption isotherms. The coefficients of both isotherms showed significant positive correlations with cation exchange capacity (CEC), organic matter (OM), and CCE of the soils indicating that OM and calcium carbonate are the sites of Mn retention in calcareous soils. Comparison of the adsorption data of this experiment with those of plant Mn uptake of the same soils (published earlier) shows that as the Langmuir second surface adsorption maxima (maximum retention capacity) of the soils increase the plant Mn concentration and uptake decrease.     

Zinc adsorption characteristics of selected calcareous soils of Iran and their relationship with soil properties

Abstract

The apparent recovery of applied zinc (Zn) by plants is very low in calcareous soils of Iran because most of it is retained by the soil solids. Subsamples of 24 surface soil (clay 130–530 g kg^‐1; pH 7.7–8.4; electrical conductivity 0.63–3.10 dS m^‐1; organic matter 6.0–22.0 g kg^‐1; cation exchange capacity 8–20 cmol kg^‐1; calcium carbonate (CaCO₃) equivalent 180–460 g kg^‐1) representing 13 soil series in three taxonomic orders were equilibrated with zinc sulphate (ZnSO₄) solutions and the amount of Zn disappeared from solution after a 24‐h shaking period was taken as that adsorbed (retained) by the soil solids. The adsorption data were fitted to Freundlich (X=AC^B) and Langmuir [X=(K‐bC)/(1+K#lbC)] adsorption isotherms. Backward stepwiseprocedure was used to obtain regression equations with isotherms coefficients as dependent and soil properties as independent variables. Freundlich A and Langmuir K were found to be highly significantly related to pH and clay and increasing as these soil properties increased. But Langmuir b was related only to clay and Freundlich B showed no significant relationship with any of the properties studied. The distribution coefficient (also called maximum buffering capacity), calculated as the product of Langmuir K and b, was also found to be highly significantly related to pH and clay. It is concluded that pH and clay content of calcareous soils are the most influential soil properties in retention of Zn.     

Geochemical Fractionation and Adsorption Characteristics of Zinc in Thai Major Calcareous Soils

Zinc (Zn) is a vital plant nutrient that is widely deficient in Thai cultivated calcareous soils. The chemical fractionation and adsorption of Zn are among the most important solid- and liquid-phase interactions that determine the retention of Zn in the soils. This study aimed to investigate the fractionation and adsorption isotherms of Zn in cultivated Thai calcareous soils. The results of sequential extractions showed that Zn is mainly distributed in residual fractions followed by organic-bound, iron and manganese oxides-bound, carbonate-bound, and exchangeable Zn, respectively. Zinc adsorption was well fitted by the Langmuir and Freundlich isotherms. Thai calcareous soils had high Zn adsorption capacity. Soil pH, organic carbon, calcium carbonate, cation exchange capacity, and extractable calcium were the major soil properties that affected the Zn adsorption isotherms in these soils. Zinc hydroxide was the solid precipitate and the Zn hydroxide ion (ZnOH⁺) was the dominant Zn ion in alkaline equilibrium solution.     

Zinc desorption kinetics from some calcareous soils of orange (Citrus sinensis L.) orchards,southern Iran

Zinc (Zn) desorption is an important process to determine Zn bioavailability in calcareous soils. An experiment was performed to assess the pattern of Zn release from 10 calcareous soils of orange orchards, southern Iran and the soil properties influencing it. For Zn desorption studies, soil samples were extracted with diethylene triamine penta-acetic acid solution at pH 7.3 for periods of 0.083–48 h. Suitability of seven kinetic models was also investigated to describe Zn release from soils. Generally, Zn desorption pattern was characterized by a rapid initial desorption up to 2 h of equilibration, followed by a slower release rate. The simple Elovich and two-constant rate kinetic models described Zn release the best, so it seems that Zn desorption is probably controlled by diffusion phenomena. The values of the rate constants for the superior models were significantly correlated with some soil properties such as soil organic matter (SOM) content, cation exchange capacity (CEC), and soil pH, whereas carbonate calcium equivalent and clay content had no significant influence on Zn desorption from soils. SOM had a positive effect on the magnitude of Zn release from soils, while soil pH showed a negative effect on Zn desorption. Furthermore, the initial release rate of soil Zn is probably controlled by CEC in the studied soils. Finally, it could be concluded that SOM, CEC, and soil pH are the most important factors controlling Zn desorption from calcareous soils of orange orchards, southern Iran.

Abbreviations: Soil organic matter (SOM); Cation exchange capacity (CEC); Calcium carbonate equivalent (CCE); Zinc (Zn).     

Extractability of previously‐applied zinc as influenced by properties of calcareous soils

Abstract

The recovery of applied zinc (Zn) by plants is relatively small. Coupled with lack of leaching, this leads to accumulation of Zn in topsoil which may result in unfavorable growth conditions for the subsequent plants. Different extractants may be used for assessing the Zn status of soils previously treated with Zn sources. The extractability of retained Zn is influenced by soil properties. This experiment was conducted to study the influence of selected properties of calcareous soils on extractability of Zn by three popular Zn soil tests. Twenty samples from surface horizons (0–20 cm) of highly calcareous soils of southern Iran (pH 7.9 to 8.5; calcium carbonate equivalent 16 to 58%) previously treated with three levels of Zn (0, 10, and 20 mg Zn kg^‐1 soil as ZnSO₄#lb7H₂O) in triplicate and under one crop of corn (Zea mays L.) were extracted with DTPA, EDTA‐(NH₄)₂CO₃ and Na₂‐EDTA. Extractability (EXT) in a particular extractant was defined as the slope of the regression line, relating extractable Zn of each soil to the rate of applied Zn, multiplied by 100. The EXT values of soils ranged from 24.9 to 73.0% for DTPA, 47.2 to 84.4% for EDTA‐(NH₄)₂CO₃, and 28.2 to 56.7% for Na₂‐EDTA. Stepwise regression equations showed that cation exchange capacity (CEC) and calcium carbonate equivalent (CCE) followed by clay content were the most influential soil properties in EXT of retained Zn of highly calcareous soils. The EXT values decreased with increase in CEC, and CCE but increased with increase in clay.     

Prediction of residual effects of zinc sulfate on growth and zinc uptake of corn plants using three zinc soil tests

Abstract

A significant portion of chemical zinc (Zn) fertilizers applied to calcareous soils is not absorbed by the first crop and may, therefore, affect the growth and chemical composition of the subsequent crops. This is called the residual effect of Zn. Soil tests may be used to predict such effects. The present experiment was conducted to study the residual effects of zinc sulfate (ZnSO₄) on the second crop of corn (Zea mays L.) grown on selected highly calcareous soils of Iran and to compare the suitability of three soil tests for prediction of the effects. Twenty highly calcareous soils of southern Iran (16–58% calcium carbonate equivalent; pH 7.9–8.5), previously treated with three levels of Zn (0, 10, and 20 mg Zn/kg as ZnSO₄) and under one crop of corn, was used in greenhouse to grow a second crop of corn without additional Zn fertilizer but with uniform application of nitrogen (N), phosphorus (P), and iron (Fe). Soils were sampled before the second crop and extracted with three Zn extradants, DTPA, EDTA‐(NH₄)₂CO₃, and EDTA. Dry weight of plant tops and Zn concentration and uptake after eight weeks under the greenhouse conditions were used as the plant responses to residual Zn. Statistical analyses including F‐test and multiple regression equations showed that the overall effect of previously‐applied Zn on dry matter was nonsignificant, but Zn concentration and uptake were significantly increased. The three soil tests predicted the Zn concentration and uptake equally well. Moreover, DTPA and EDTA soil tests could predict the dry matter of plants at the highest level of previuosly‐applied Zn (20 mg Zn/kg), especially when selected chemical properties of soil, namely, calcium carbonate equivalent or organic matter content, were considered in the regression equations.     

Growth and manganese uptake by soybean in highly calcareous soils as affected by native and applied manganese and predicted by nine different extractants

Abstract

Manganese (Mn) becomes a limiting factor of plant growth under some soil conditions. High pH and abundance of free calcium carbonate in calcareous soils are conducive to Mn‐deficient plants. The Mn status of highly calcareous soils in Iran has not been studied in detail. This experiment was conducted to obtain such information. Twenty‐three surface (0–20 cm) soil samples with pHs from 7.7 to 8.2 and calcium carbonate equivalent (CCE) from 19 to 46% were used in a 7‐week greenhouse study with soybean [Glycine max (L.) Merr.]. The experiment was a 23 × 3 factorial with three replicates, i.e., 23 soils and 3 Mn levels (0, 15, and 30 mg/kg soil as manganese sulfate). Extractable Mn of the untreated soils were determined before planting by nine different procedures. Dry matter yield, Mn concentration of soybean tops, and Mn uptake were used as the measure of plant response. Multiple regression equations showed that the most influencial soil properties in extractability of Mn in highly calcareous soils are cation exchange capacity (CEC) and calcium carbonate equivalent (CCE). Application of Mn increased dry matter, Mn concentration and Mn uptake of soybean plants in most soils. The Mn concentration of plants on some of the soils, however, decreased following the application of Mn. This was attributed to dilution effect caused by enhanced growth. The recovery of applied Mn in all soils was low, persumably due to conversion of the applied Mn to unavailable forms. Regression equations were developed to predict dry matter, Mn concentration, and Mn uptake by plants from soil Mn extracted by water, hydrochloric acid, disodium‐EDTA, and EDTA‐ammonium acetate.     

不同性质铁铝土对砷酸根吸附特性的比较研究

采用批平衡法研究了8种不同性质铁铝土对砷酸根的吸附特性,分别运用Langmuir单表面方程和Langmuir双表面方程对等温吸附数据进行拟合,以较优拟合方程求出土壤对砷的最大吸附量,并采用简单线性相关分析法探讨土壤性质对砷吸附能力的影响。结果显示,铁铝土对砷吸附强烈,吸附等温线均为非线性。双表面方程对吸附等温线的拟合效果优于单表面方程。采用双表面方程预测的吸附量和实测值的决定系数(0.935～0.978)大于采用单表面方程预测获得的决定系数(0.989～0.998)。土壤的砷吸附能力可采用高能表面和低能表面两种吸附位点进行解释。采集于云南昆明的砂页岩母质发育的红壤具有最强的砷吸附能力,根据Langmuir双表面方程计算的最大吸附量为3 498 mg kg-1。土壤的砷吸附能力受到土壤中游离氧化铁、全铝、全铁、黏粒和全磷含量的显著影响,最大吸附量与这几种土壤组成与性质因子呈显著正相关关系。     

Phosphorus requirement of corn and sunflower grown on calcareous soils of Pakistan

Abstract

Determining phosphorus (P) fertilizer requirement through sorption isotherms is considered more accurate than conventional soil testing. Phosphorus sorption isotherms were constructed by using the Missa (Typic Ustochrept), Gujranwala (Udic Haplustalf), Abbotabad (Aquic Entrochrept), and Rasulpur (Ustochreptic Camborthid) soil series which varied in their calcium carbonate (CaCO₃) and clay contents. Quantities of fertilizer‐P (mg P/kg soil) required to adjust the standard soil solution P concentration (0.2 mg P/L) were: Missa, 83; Gujranwala, 51; Abbotabad, 50; and Rasulpur, 8. Langmuir model conformed better with the adsorption data than Freundlich model and indicated that the order of maximum adsorption capacity (b) was: Missa > Abbotabad = Gujranwala > Rasulpur. The P sorption curves were used to develop the soil solution P concentration in the Gujranwala and Missa soils varying from native to 0.5 mg P/L. Soil solution P requirement of corn and sunflower for near‐maximum (95%) biomass production was approximately the same, 0.32 mg P/L. Internal P requirement in whole shoots (≤30 cm tall) were 0. 29% for corn and 0.39% for sunflower.     

Zinc Sorption Behavior in Soils with Divergent Characteristics

Adsorption studies are important to determine the retention and release of applied plant nutrients and the efficiency of fertilization. The present study was conducted to delineate the zinc (Zn) adsorption behavior of soils with varying characteristics. In this study, adsorption behavior of Zn was studied at varying Zn concentrations and temperatures in three different soils collected from the Research Farm of Indian Agricultural Research Institute (IARI), New Delhi (Inceptisols, Typic Haplustepts), soil (Inceptisols, Typic Ustocrept) from a farmer’s field in Shamgarh, Karnal, Harayana, and the Hayathnagar Research Farm of Central Research Institute For Dryland Agriculture (CRIDA), Hyderabad, (Haplustalf). The results of this study revealed that the adsorption of Zn as well as differential buffering capacity (DBC) was greater in Typic Haplustepts of IARI and Typic Ustocrept of Karnal than Haplustalf of CRIDA, Hyderabad. However, the supply power as well as percentage saturation (% sat.) was greater in Haplustalf of CRIDA, Hyderabad, than Typic Haplustepts of IARI and Typic Ustocrept of Karnal The results also revealed that irrespective of the soil types, Zn adsorption increased with an increase in Zn concentration and increase in temperature from 20 °C to 35 °C. When the adsorption data were fitted to isotherms, Langmuir and Freundlich isotherm fits were excellent, as evidenced by high r² (0.71 to 0.99) values. Thermodynamic parameters such as standard free energy (ΔG) were negative in both Typic Haplustepts and Typic Ustocrept soils, indicating spontaneous Zn adsorption reaction, but nonspontaneous reaction was observed in the case of Haplustalf soils.. Interestingly, both standard enthalpy (ΔH) and standard entropy (ΔS) were negative in all the three types of soils studies. The analogy and results of the present study would be useful in deciding the Zn fertilizer needs for a given soil type.     

Kinetics of Copper Desorption from Highly Calcareous Soils

Abstract

Desorption of copper (Cu) is an important factor in determining Cu availability in calcareous soils. Kinetics of native and added Cu desorption by DTPA (diethylene‐triaminepentaacetic‐acid) from 15 highly calcareous soils of southern Iran were studied in a laboratory experiment. Our results showed that two constant‐rate, Elovich, simple Elovich, and parabolic‐diffusion equations were the best‐fitted equations among eight kinetic models used. The copper desorption pattern based on the parabolic‐diffusion equation revealed that the rate of native Cu desorption was higher in the first 2 h followed by a slower release rate, which suggests that two different mechanisms are involved. The trend may describe why the DTPA soil test has been considerably successful in predicting Cu availability in calcareous soils. Stepwise multiple regression equations indicated that CCE (calcium carbonate equivalent), CEC (cation exchange capacity), and clay content are the most important soil characteristics that predict the rate constants of the kinetic models. Mean extractant recovery percentage (ERP) of the soils was only 20%, which indicated that after 20 days, DTPA extracted only one‐fifth of added Cu. Regression equations indicated that as soil OM (organic matter) content increased, the value of ERP decreased. From results reported herein it seems that CCE, CEC, and clay are the most important factors controlling Cu release from highly calcareous soils of southern Iran. However, the initial soil Cu desorption rate is probably controlled by CEC.     

Zinc Adsorption by Mineral Soils of Finland

Abstract

Zinc adsorption by 10 (pH 4.0–6.5) cultivated mineral soils from Finland was studied in batch experiments. Additions of Zn ranged up to 600 mg kg^?1 of soil and the corresponding equilibrium concentrations were 0.1–13 mg 1^?1. In each soil, Zn adsorption conformed to the Freundlich isotherm. Despite a relatively low initial Zn adsorption by the acidic soils, each of the soils proved to have a high potential to adsorb Zn, but the capacity was highly pH dependent. In addition to the conventional Freundlich adsorption isotherms, calculated separately for each soil, extended Freundlich-type isotherms that also incorporate soil pH and other soil characteristics were used to describe Zn adsorption of several soils simultaneously in one equation. The pH-dependent Freundlich adsorption isotherm proved to serve as a practical tool to assess Zn adsorption by soils varying in pH and other characteristics.     

Adsorption isotherms of some heavy metals under conditions of their competitive adsorption onto highly calcareous soils of southern Iran

Recently, application of sewage sludge or effluents resulted in raising the concentrations of some heavy metals in some agricultural soils of Iran. Experiments were conducted to evaluate the competitive adsorption of lead (Pb), copper (Cu), zinc (Zn), and cadmium (Cd) on six calcareous soils. Adsorption characteristics were evaluated by equilibration of 1 g of each soil sample with 20 ml of 0, 10, 20, 30, 40, 50, 100, or 200 mg L^?1 of their nitrate solutions and 0.01 M NaNO₃ as background electrolyte. Furthermore, solid/liquid distribution coefficients (K_d) of studied metals, as an index of soil capacity to resist a change of the soil solution concentration, were calculated. Results indicated that amounts of adsorbed Pb, Cu, Zn, and Cd increased with increase in their concentrations in the contact solutions, but this trend was more pronounced for Pb and Cu than the others. For all studied soils and metals, Langmuir equation described the adsorption behavior fairly well. Furthermore, Langmuir and Freundlich equation parameters were positively correlated to cation exchange capacity (CEC) and smectite contents; whereas, they were negatively correlated to sand content. Considering K_d values, the selectivity sequence of the metal adsorption was Pb > Cu > Zn > Cd. Therefore, the risk of leaching and also plant uptake of Zn and Cd will be higher as compared to those of the other elements.     

Zinc Sorption in Selected Soils

Abstract

The adsorption of nutrient elements is one of the most important solid‐ and liquid‐phase interactions determining the retention and release of applied plant nutrients and the efficiency of fertilization. The study showed that the soils with high cation exchange capacity (CEC), CaCO₃, organic matter contents, and heavy texture adsorbed more zinc (Zn). The alkaline soils from Pakistan adsorbed more Zn than English acidic soils. Langmuir and Freundlich isotherm fit was excellent, and r² values for the Langmuir isotherm were highly significant (r²=0.84 to 0.99). The Langmuir b values, representing the adsorptive capacity of a soil, increased as the texture fineness increased in the soil, with increases in the concentration of adsorptive material (such as organic matter and CaCO₃) and with increases in CEC and pH. The alkaline soils from Pakistan had higher bonding energy constant and higher log Kf values than the acidic English soils. Sequential extraction of Zn in these soils showed that most of the Zn was held in CaCO₃ pool in the alkaline soils, whereas in acidic soils adsorbed Zn was in exchangeable form.     

Zinc adsorption by andepts from the central plateau of Costa rica

Abstract

The adsorption of Zn by soil and subsoil samples from the slopes of the Irazu and Poas volcanoes in Costa Rica was correlated with soil properties and represented by Langmuir isotherms.

Soil pH, texture and free aluminum and sesquioxide content correlated significantly with Zn adsorption. No correlation with organic matter content was found probably due to the variable nature and properties of its compounds. The adsorption maxima ("b”; values) calculated by the Langmuir equation are higher for the soils developed on the more weathered Poas than for the more recent Irazu ash. The bonding energy constants ("K") followed a similar trend.     

Distribution of zinc forms in highly calcareous soils as influenced by soil physical and chemical properties and application of zinc sulfate

Abstract

Water‐soluble zinc (Zn) fertilizers are rapidly converted to insoluble forms in calcareous soils resulting in lower efficiency of such fertilizers. A knowledge of distribution of native and applied Zn in such soils is necessary for understanding the fate of applied Zn fertilizers and finding ways to increase their efficiency. This experiment was conducted to obtain such information in selected highly calcareous soils of Iran. A sequential extraction method was used to fractionate the Zn forms of surface horizons (0–20 cm) of 20 highly calcareous soils [16 to 58% calcium carbonate (CaCO₃) equivalent; pH 7.9 to 8.5] which had previously received 0, 10, or 20 mg Zn/kg as zinc sulfate (ZnSO₄) and had been under one corn (Zea mays L.) crop in the greenhouse. The forms determined were exchangeable (EXZN), sorbed (SRZN), organic (ORZN), carbonate (CRZN), residual (RSZN), and sum of forms (SMZN). The native SMZN ranged from 32.4 to 66.7 mg/kg with a mean of 49.9 mg/kg. Application of 10 and 20 mg Zn/kg as ZnSO₄ increased the mean to 57.7 and 62.7 mg/kg, respectively. Concentration of different forms of Zn in the soils was determined to be in the following order: RSZN >>> CRZN > SRZN > EXZN > ORZN. The concentration of native EXZN+SRZN+ORZN forms constituted less than 5% of SMZN, while concentration of CRZN alone ranged from 4.37 to 16.05% with a mean of 8.36%. Application of ZnSO₄, while significantly increased the concentration of all forms of Zn, had a pronounced effect on CRZN. Averaged over all soils, 58 and 60% of the applied ZnSO₄ was converted to CRZN for the 10 and 20 mg Zn/kg, respectively. Regression equations relating different Zn forms to soil physical and chemical properties indicated that the Zn forms are significantly influenced by soil properties.

It was concluded that conversion of applied ZnSO₄ to CRZN was mainly responsible for retention of this fertilizer in highly calcareous soils, making it temporarily unavailable to plants, and therefore decreasing its apparent recovery by the first crop.     

Selective sequential dissolution techniques for trace metals in arid‐zone soils: The carbonate dissolution step

Abstract

Dissolution capacity and kinetics of carbonates by sodium acetate (NaOAc)‐acetic acid (HOAc) at various pHs were studied. A comparative study of the selectivity, specificity, and effectivity of NaOAc‐HOAc solution on carbonate bound fraction during the sequential selective dissolution procedure was conducted by comparing the dissolution of major and trace elements from arid zone soils by this buffer solution at various pHs. The effect of the pH of NaOAc‐HOAc solution on the following fractions in the sequential selective dissolution procedure was also studied. NaOAc‐HOAc solution at pH 5.5 at a soil to solution ratio of 1:25, can dissolve all the carbonate from calcareous soils with 10–20% of carbonate; at pH 5.0 it can dissolve all the carbonate in soils with about 30–50% calcium carbonate (CaCO₃). A second extraction with fresh buffer solution at pH 5.0 is required for soils with more than 50% of carbonate. Six hours of extraction time is generally sufficient for complete carbonate dissolution. For most of agricultural soils in arid and semi‐arid zones, the attack of the buffer solution at pH 5.0 on other solid‐phases seems to be limited. But the buffer solution at pH 5.5 would be better for some forest soils with low carbonate content and high organic matter content. The part of carbonate fraction not be dissolved in this step is released in the following steps: easily reducible oxides fraction (ERO), organic matter fraction (OM), and reducible oxides fraction (RO), leading to gross misinterpretation of the elemental partitioning in arid zone soils.     

Role of Different Soil Fractions in Copper Sorption by Soils

Abstract

To evaluate contributions of organic matter, oxides, and clay fraction to copper (Cu) adsorption in six characterized soils, adsorption isotherms and distribution coefficients were obtained by a batch experimental method. Copper adsorption isotherms from untreated soil, organic matter removed from samples, and organic‐matter‐ and oxide‐removed samples were compared with curve patterns and correlated to Langmuir and Freundlich models. Copper sorption data on untreated soils described L or H‐curves, whereas in soils deprived of any component, their curves were S‐type. Distribution coefficients allowed knowing Cu adsorption capacity of untreated soil and of organic matter, oxides, and clay fraction. Soil organic matter is the main component that affects Cu adsorption as long as soil pH is near neutrality. At acid pH, oxides are the main component that affects Cu adsorption, although to a much smaller extent than organic matter near neutral conditions. Soil pH is the main soil factor that determines Cu adsorption.     

锌在石灰性土壤中的吸附

本文用平衡法研究石灰性土壤对锌的吸附.供试土壤为徐州丰县的黄潮土表层(0-15厘米).结果表明:吸附要用两个Laagmuir方程来描述,可以假想成两个L型表面,采用平移坐标法求出最大吸附量与实验结果基本相符.pH对锌的吸附影响很大,高pH(7.5)时,CaCO₃组分对锌的吸附的贡献约占70%,低pH时,对吸附的贡献较小.     

Phosphorus Management of Lucerne Grown on Calcareous Soil in Turkey

ABSTRACT

Lucerne or alfalfa (Medicago sativa L.) is grown as a forage crop on many livestock farms. In calcareous soils in eastern Turkey, lucerne production requires phosphorus (P) additions as the soils are naturally P deficient. Phosphorus sorption isotherms were used to estimate P fertilizer needs for lucerne grown for two years in a 3-cut system on a calcareous P deficient Aridisol in eastern Anatolia, Erzurum province, Turkey. Annual P applications ranged from 0–1200 kg P ha^?1. The Langmuir two-surface adsorption equation was used to derive the maximum P sorption capacity of unamended soil and to determine soil solution P, maximum buffer capacity (MBC), equilibrium buffer capacity (EBC), and P saturation at the optimum economic P rate (OEPR) for dry matter (DM) production. Soils were tested for Olson P at the onset of the study and after two years of P applications. In both years, tissue was analyzed for P content at flowering prior to first cutting. The OEPR (2-year average) was 754 kg P ha^?1 yr^?1 corresponding with a soil solution P concentration of 0.30 mg L^?1, a DM yield of 8725 kg DM ha^?1, and $528 ha^?1 annual profit. The P content of leaves at flowering increased linearly with P application beyond 100 kg P ha^?1 and was 3.2 g kg^?1 P at the OEPR. The unfertilized soil had an EBC, MBC, P saturation, and X_max of 3304 mL g^?1, 3401 mL g^?1, 6%, and 1086 mL g^?1, respectively, whereas two years of fertilization to the OEPR decreased EBC and MBC to 358 mL g^?1 and 540 mL g^?1, and increased P saturation and Olsen P to 56% and 32 mg kg^?1, respectively. These results suggest a P saturation >50% or Olsen P >30 mg kg^?1 are needed to maintain an optimum soil solution concentration of 0.30 mg L^?1 in this calcareous Aridisol. Similar studies with different soils and initial soil test P levels are needed to conclude if these critical soil test values can be applied across the region.