P Transformation In The Soil System Biology Essay


Phosphorus is essential macro nutrient being required by plant in relatively larger quantities ranging from 0.2-0.8 % of plant dry matter (Mengal and Kirkby, 1987, Mill and Jones, 1986). It plays a vital role in several physiological processes, photo-synthesis, respiration, energy storage and transfer, and cell enlargement (Mengal and Kirkby, 2000). It is present in seeds and fruit in large quantity and is essential for seed formation, root development, cell division, flowering and maturity of crops (Brady, 1984).

The soil solution P concentration varies from 0.003 to 0.3 mg kg-1 soil. However, the optimum level of P depends on the crop species, growth pattern and total production capacity as well as on the capacity of soil to maintain the required P level in soil solution. If yield potential is low, maximum corn yield can be obtained with 0.01 mg kg-1, but 0.05 mg kg-1 P is needed under conditions of high yielding varieties (Halvin, et al., 1999).

Phosphorus is found in different pools, such as organic and inorganic or mineral forms. Organic P is bound in organic matter consisting of plant or animal residues and living soil microbes of which 20 to 80% is found in phytic acid (insoitol hexaphosphate) (Richardson, 1994). Soil microbes release immobile form of P into the soil solution and also responsible for the immobilization of P. Various chemical process like dissolution, hydrolyses, oxidation, complexation, adsorption and precipitation govern the level of mineral P concentrations in soil solution. As such P in soil is categorized as P in soil solution, P in labile pool, and P in non-labile fraction that can be released only very slowly into the labile pool. The available forms of P to the crop are H2PO4-, HPO42- and PO43- (collectively called orthophosphate) where the relative concentrations of these forms are pH dependant. H2PO4- and HPO42- each representing 50 percent of solution P at a nearly neutral pH (6.0-7.0) but at pH 4.0-6.0, H2PO4 is about 100 percent whereas at pH 8.0 H2PO4 represents 20 % while HPO4- accounts for 80 percent of total P in solution (Black, 1968). Mostly all crop take up H2PO4- more readily than HPO42- ion, however, above pH 7.00, the relative uptake of the divalent ion (HPO42-) is greater than that of monovalent ion (H2PO4), (Ahmed and Rasheed, 2003). It is important to note that usually limited uptake of P occur in bulk soil because of low availability. Plant root morphology and geometry influence the P uptake, the root system having higher ratios of surface area to volume will more effectively explore a large volume of soil (Lynch, 1995).

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Although the total amount of P in the soil may be high but it is often present in unavailable forms or out side of the rhizosphere. The application of P either in the form of organic matter or chemical fertilizers to the soil is necessary to ensure optimum crop productivity in many agricultures systems. However, the recovery of applied P from chemical fertilizer to the crop is quite low in a growing season. Once applied to the soil, more than 80% of the P becomes immobile and unavailable for plant uptake due to factors such as adsorption, precipitation, or conversion to the immobilized organic form (Holford, 1997). Only few unfertilized soils consisting high amounts of total P and having the capacity to release P fast enough to support the high growth rates of crop seldom require additional P application.

The dynamics of P transformation in the soil system and its fixation and release characteristics have been the subject of numerous research investigations. Phosphorus exists in soil in various insoluble forms, like the phosphate of iron, aluminum and calcium. Solubility of these phosphate compounds is quite low, and only a small fraction of total P is available for uptake. The P sorption capacity is an important soil characteristic that affects the P concentration in solution and plant response to phosphate fertilizer application (Holford and Mattingly, 1976) and mobility of P in soil (Mackey et al., 1986). Different factors influencing P retention and availability in the soils are the nature and amount of soil components, P concentrations, pH, temperature, various ionic species, and saturation of sorption complexes (Tisdale et al., 1989).

The reactions taking place in the soil and the properties of various phosphate materials collectively determine the effectiveness of any source of P under any set of soil and cropping conditions. When single super phosphate (SSP) or triple super phosphate (TSP) are applied to the soil, water vapor moves rapidly into each fertilizer granule and the solution formed is extremely acidic (pH 1.8) and is concentrated with Ca and P. The P thus produced is either absorbed by plant root or it is precipitated as dicalcium phosphate on the surface of CaCO3 particles in calcareous soils. In contrast the pH of saturated solution of DAP is around 8.0. However, the net impact of its application in soil is acidic due to the nitrification of NH4 component of DAP that releases H+ into soil solution (Saleem, 1992). Khattak and Khan (1996) have presented an excellent review on DAP chemistry in alkaline soils.

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Pakistani soils are deficient (80-90%) in P (Memon, 1996) and needs supplemental P application to maintain adequate P level to support optimum crop production. High soil pH and calcareous nature of the soils reduce the P availability due to sorption and fixation that need to be addressed while applying P fertilizers. The soils of Peshawar valley slightly to strongly calcareous (lime 5.0 to 26 %) with neutral to strongly alkaline in reaction (pH from 7.2 to 9.1) (Staff, Soil Survey of Pakistan, 1973). Most of these soils contain less than 1% organic matter and have low to medium available P (Ahmad et al., 1990). In these soils high calcium activity coupled with high pH, favor the precipitation of relatively insoluble dicalcium phosphate, hydroxyapatite, carbonate apatite, and octacalcium phosphate.

To improve the P availability, one has to reduce the contact of fertilizer granule with bulk of soil through advance application techniques like banding and side dressing. To avoid the losses due to P fixation, sorption or complexation, the P should be applied in proper amount as per requirement of the crop. It needs that the soil should be evaluated for potential P release in soil solution, critical level of P for the crop and potential input from the supplementing fertilizer. This research work is planned to evaluate the potential of P sorption in various calcareous soil series and the critical levels of P in soil solution in diverse soil series for optimum plant growth.

The soils of Pakistan are calcareous and phosphatic fertilizers efficiency is reducing because of formation of insoluble Ca-phosphate. This study will mainly focus on the establishment of the minimum level of soil solution P concentration for optimum crop growth and yield in calcareous soil series of NWFP.

Specific objectives

The specific objectives of this research are as follows.

To establish relationship between soil solution [P] and plant growth.

To evaluate adsorption-desorption studies on P in diverse soil series in the laboratory using Langmuir and Freudlich models.

To correlate the value of P adsorbed with [P] in soil solution leachate and P uptake by plants.

Correlation of adsorption capacity of these soils with soil properties such as soil pH, lime content and clay content



The research approach will include selection of calcareous soil series with varying lime content, collection of surface soil samples for pot experiments and adsorption-desorption studies. A field experiment will be conducted to investigate the effect of various levels of P fertilizers on crop growth in calcareous soils. In pot experiment, water soluble [P] representing solution P, [P] will be correlated with plant [P] and plant growth to determine the minimum [P] required for optimum yield.

Adsorption-desorption studies on P in diverse soil series will be conducted in the laboratory using Langmuir and Freundlich models. The value of P adsorbed will be correlated with [P] and P uptake by plants.

During the first year of the research project, main activities will include, selection of sites representing important soil series, collection and processing of soil samples for pot, evaluating the P levels required in soil solution for optimum growth and performing adsorption-desorption studies to correlate P adsorption to soil texture, lime content and CEC.

During the second year, chemical analysis of soil and plants will be performed on the pot experiment along with the field experiment.

During third year chemical analysis of soil, plants and statistical analysis of field data will be accomplished. Writing of research paper and dissertations will be completed.

3.1 Selection of soil series and collection of soil samples.

Three soils series with diverse lime content (10, 20, and 25%) will be selected with the help of Soil Survey of Pakistan Peshawar region, using recently developed map (Soil survey of Pakistan, 2008). Surface 0-30 cm soil samples will be collected and processed for use in lab and pot experiments.

3.2 Pot experiment: P uptake by crop in soil plant leachate system

The pot experiment will be conducted with the objective to correlate the [P] in soil solution with the plant growth in different selected soil series. The [P] fertilizer will be applied at the rate of 0, 10, 20, 40 and 60 mg P kg-1 to pots containing 10 kg air-dried sieved soil. The experiment [5 P - 3 soils] will be arranged in randomized complete block design (RCBD) with three replications. Triple super phosphate (TSP) will be used as P source. All pots will be supplied with recommended level of N and K as 60 and 30 mg N: K2O kg-1. All NPK will be applied in water solution form. Soil will be analyzed for water soluble P at different growth periods to correlation soil solution [P] with plant [P] and growth. Pots will be supplied with irrigation water as per requirement and leachate, if any from each pot will be collected and analyzed for leached ions and P. The crop will be harvested after maturity and nutrient concentration in plant and soil will be determined accordingly. Recovered P in leachate, plant uptake and soil P will be correlated with soil lime and fertilizer input to determine level of P fertilizer and soil solution P for optimum crop yield in calcareous soil.

3.3 Field Experiment

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To verify the results of pot experiments and conclusion drawn from adsorption studies, a field experiment will be conducted on one of the soil series using the following treatments 0, 45, 90, 135, 180 kg P2O5 ha-1 (0, 10, 20, 40 and 60 mg P kg-1) which will be supplied in form of triple super phosphate (TSP). Treatments will be arranged in RCB design with 3 replications and plot size of 3x5 m2. All plots will be given equal doses of N and K at 120 and 60 kg N: K2O in the form of urea and SOP. Water soluble P at 0-30 cm deep soil before sowing, at germination, boot stage and at time of harvest will be analyzed and correlated with plant [P] and growth. AB-DTPA extractable P before sowing and after crop harvest will also be determined and correlation with water soluble P and plant growth. Soil and plant samples collected after crop harvest will be analyzed for other soil physico-chemical properties and nutrient concentrations. These soil properties will be used to correlation studies regarding P availability and plant growth.

3.4 Phosphorus adsorption study in selected soil series

Soil P adsorption will be studied by shaking 5 g of soil sample in 50 mL solution containing 0, 2.5, 5, 10, 20, 30, 40, 50 mg P L-1 in 0.01 M CaCl2 in 100 mL Erlenmeyer flask at room temperature (25o C ±2). Shaking time of 24 h will be selected for obtaining equilibrium after conducting an initial study over a range of shaking time (from 0 to 24 hours). Amount of P adsorbed will be calculated from the difference of the amount of initial and final concentration in the soil equilibrium solution. The adsorption data will be fitted to Langmuir and Freundlich equations. The adsorption isotherm is given by a plot of adsorbed phosphate (c/x/m, ug mL-1) against equilibrium solution C. This approach has been used by many workers to measure the phosphate adsorption capacity of soil (Barrow, 1970: Bache and Williams, 1971; Rajan and Fox, 1975; Anderson et al., 1974; and Gebbert and Coleman, 1974).

Both Langmuir and Freundlich equations will be used to describe the adsorption data and to find the best fit for each soil. A common form of Langmuir equation is

x/m = KCB/(1+KC) ------------------------------------------- [Eq.1]

Where C = P concentration of equilibrium soil solution (µg mL-1)

x/m = Amount of P sorbed (µg P g-1 soil)

b = Adsorption maximum (µg g-1 soil)

K = A constant (mL µg-1) related to the bonding energy of the soil with P

The linear form of this equation is:

C/(x/m) = 1/kb + C/b -------------------------------------------------------­­­­------- [Eq. 2]

The P adsorption maxima (b) will be calculated for each soil from Langmuir isotherm.

A common form of Freundlich equation is as follows:

x/m = KC1/n -------------------------------------------------------------------------- [Eq.3]

Where K and n are empirical constants and other terms are the same as defined in Langmuir equation

The linear form of Freundlich equation is as follows:

Log x/m = 1/n log C + log K ---------------------------------------------------------- [Eq.4]

3.5 Statistical Analysis

Using the statistical M. Stat Computer Programme, two way analysis of variance with five levels of P and three soil series [5 P - 3 soils - 3 reps] will be performed on the data collected during these studies. The means will be compared with each other by using least significant difference test (LSD) at 5% probability (p<0.05).

Correlation between equilibrium P concentration (C, ug mL-1) and C/x/m against phosphate remaining in solution C, ug L-1) will be determined, using M Stat programme to find out the slope and intercept and hence adsorption maxima (b) and binding energy constants (k) for different soil series.

Regression analysis will be performed to determine relationship between the amount of P sorbed and the soil properties such as, soil pH, CaCO3 and clay content.

On the basis of these studies, adsorption data will be used to calculate P required for maintaining the [P] in solution. Through soil pot experiment the solution [P] predicted will be tested for validation in a soil-plant-leachate system.

Crop biomass, [P] in tissue and [P] in leachate will be correlated with each other.

3.6 Laboratory Analyses

3.6.1 Soil pH (Mclean, 1982)

A 10g of soil sample will be taken in a conical flask and add 50mL of distilled water will be added to make 1:5 soil-water suspensions. Suspension then will be shaked in a mechanical shaker for 30min. suspension will be filtered through Watt man No. 42 filter paper and pH meter will be used for pH determination.

3.6.2 Soil Texture (Gee and Bauder, 1986)

Sand, silt and clay fractions of the soil (Soil texture) will be determined by Bouyoucos hydrometer method as reported by Gee and Bauder (1986). 50 g soil will added with sufficient amount of water and 10 mL of 1 M sodium metahexaphospate and dispersed mechanically for 10 min. The soil will then be transferred to 1 L cylinder and the vol. will be adjusted with distilled water. Hydrometer readings will be noted after 40 sec and 2 h which will account for silt+clay and clay content in the suspension, respectively. Sand fraction will determined by difference.

3.6.3 Lime (CaCO3) Content of Soils (Richard, 1954)

Lime content of soil will be determined by acid neutralization method. In this method five grams of air-dried, sieved soil sample will be taken in a conical flask and 30 mL of 0.5 N HCl will be added to it. The suspension will be heated on hot plate for 5 min. after the boiling started; it will be removed and allowed to cool down. The suspension will be filtered and filtrate will be titrated against 0.25 N NaOH. The mL of NaOH used for acid neutralization will be used for calculating percent lime content of soil. The following formula will be used for calculating the % lime content.

weight of soil (g)% CaCO3 = [(mL of HCl x N of HCl ) - (mL of NaOH x N of NaOH)] x 0.05 x 100

Where 0.05 g meq-1 the weight of CaCO3.

3.6.4 Soil Organic Matter (Nelson and Sommers, 1996)

One g of air-dried soil will be taken in a conical flask and 10 mL of 0.5 N K2Cr2O7 and 20 mL of conc. H2SO4 will be added to it. The mixture will be allowed to stand for 30 min. to complete the reaction. Then 200 mL of distilled water will be added and suspension will be filtered. Indicator, 2-3 drops of orthophenolphthalein will be added to the filtrate and titrated against 0.5 N FeSO4.7H2O until the color change to dark brown after a series of different colors.

weight of soil % SOM = [(mL of K2Cr2O7 x N) - (mL of FeSO4.7H2O x N)] x 0.69

3.6.5 Water-soluble P (Adams, 1974)

Five g of soil sample will be taken in a conical flask and 50 mL of distilled water will be added to it. The contents of the flask will be shaken continuously for 10-15 min, then the mixture will be centrifuged at 100 rpm for 15 min. The supernatant solution will be filtered through Whatt man No. 42 filter paper and 10 mL of aliquot will be taken into 25 mL volumetric flask. The volume will be raised to 25 mL by adding distilled water and 4 mL of reagent B (ascorbic acid). Standard solution, containing 0.1, 0.3, 0.5, 0.7 and 0.9 ug P mL-1 will be used along with blank. The P concentration will be determined spectrophotometerically using required standard solutions.

3.6.6 AB-DTPA extractable P (Soltanpour and Schwab, 1977)

To a soil sample weighing 10 g (air-dried), 20 mL of the ABDTPA extracting solution (pH 7.6) will be added in 125 mL Erlenmyer flask. The soil mixture will be shaken through mechanical shaker for 15 min, and filtered through Whatman No. 42 filter paper. Phosphorus concentrations will be determined by the ascorbic acid method as discussed for water-soluble P, using Lambda 35 spectrophotometer.

3.6.7 Plant P (Benton et al., 1991)

Plant samples will be collected and washed with distilled water and dried in oven at 60-70C0 for 48 h. After air drying, the samples will be grinded and stored in glass bottles. All parts of plant samples will be than analyzed for P, using wet digestion technique.


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