PHOSPHATE SORPTION BEHAVIOR IN BAJOA AND GOPALPUR SOIL SERIES

COURSE TITLE: PROJECT THESIS COURSE NO: SS-4106

SOIL SCIENCE DISCIPLINE KHULNA UNIVERSITY

KHULNA

DECEMBER, 2014

PHOSPHATE SORPTION BEHAVIOR IN BAJOA AND GOPALPUR SOIL SERIES

COURSE TITLE: PROJECT THESIS COURSE NO: SS-4106

This project thesis paper has been prepared and submitted to Soil Science Discipline, Khulna University, for the partial fulfillment of the four years professional B. Sc. (Hons) degree in Soil Science.

Submitted By

Md. Ali Reza

Student ID: 101309 Session: 2012-2013

PHOSPHATE SORPTION BEHAVIOR IN BAJOA AND GOPALPUR SOIL SERIES

APPROVED AS TO STYLE AND CONTENT BY

.

KHANDOKER QUDRATA KIBRIA

Associate Professor Chairman of Examination Committee

Soil Science Discipline Khulna University, Khulna

Bangladesh

Soil Science Discipline Khulna University

Khulna December, 2014

DECLARATION

This project thesis paper has been prepared and submitted to Soil Science Discipline, Khulna University, for the partial fulfillment of the four years

professional B. Sc. (Hons) degree in Soil Science.

Supervisor

..

Md. Sadiqul Amin czn_bd@yahoo.com Associate Professor

Soil Science Discipline Khulna University, Khulna

Candidate

Md. Ali Reza

Student ID: 101309

Soil Science Discipline

Khulna University, Khulna

Dedicated To

My Beloved Parents

Whose owes can never be fulfilled

Acknowledgement First of all, I would like to express gratefulness to Almighty God who has enabled

me to accomplish this thesis work and to complete this project thesis paper

successfully in due time.

I would be privileged and proud to express unfettered gratification, sincere

appreciation and profound respect to my honorable teacher and supervisor Md

Sadiqul Amin, Associate Professor, Soil Science Discipline, Khulna University,

for his sincere supervision, valuable instruction, enthusiastic guidance,

constructive criticism and constant encouragement during the thesis work and the

preparation and compilation of this thesis paper.

I express my heartiest gratitude and sincerest appreciation to Md. Sanaul Islam

Associate Professor, Soil Science Discipline, Khulna University, for his advice,

encouragement and for all possible help during this thesis work.

I am really grateful to Afroza Begum, Head, Soil Science Discipline, Khulna

University, for his advice, encouragement andenthusiastic guidance to doing this

thesis work.

I am also thankful to Mr. Abdullah and Mr. Mahfuz, laboratory attendants for their

cooperation in the laboratory.

Special appreciation goes to my friends Anindita Das, Arpita Jotder and Farhana

Naznin Ema for their excellent co-operation.

Finally, I would like to express my gratitude to my parents for their moral,

inspiration and encouragement throughout the work.

December, 2014 Author

Md. Ali Reza

Table of Contents Title Page

No.

Table of content I

List of Table IV

List of Figures V

Chapter One: Introduction

1. Introduction 1

1.2. Objective of the research 3

Chapter Two: Literature Review

2. Literature Review

2.1. Phosphorus status of soils in Bangladesh 4

2.2. Forms of phosphorus in soil 4

2.3. General characteristics of soil phosphorus 4

2.4. Role of phosphorus in soil 6

2.5. Soil phosphorus status and its availability 7

2.6. Phosphorus dynamics in soil 8

2.7. Requirement of phosphorus in plant 10

2.8. Soil factors affecting P uptake 10

2.8.1. Phosphate Buffering Capacity 11

2.8.2. Phosphate Diffusion in Soil 11

2.9. Phosphorus sorption and desorption 12

2.10. Adsorption isotherm 14

2.11.Types of adsorption isotherm 15

2.12. Factors affecting P sorption from soils 18

Chapter Three: Materials and Methods

3. Materials and Method

3.1. Sample Collection 22

3.1.1. Bajoa Series 22

3.1.2. Gopalpur Series 22

3.2. Processing of soil sample 23

3.3. Physical analysis of soil 23

3.4. Chemical analysis of soil 23

3.4.1. Soil pH 24

3.4.2. Electrical Conductivity (EC) 24

3.4.3. Determination of organic carbon 24

3.4.4. Cation Exchange Capacity (CEC) 24

3.4.5. Available Phosphorus (P) 24

3.5. Phosphate sorption experiment 24

3.6. Fitting of phosphorus adsorption data in three equations 25

Chapter Four: Results and Discussion 27

4. Results and discussion 27

4.1. Physical Characteristics 27

4.1.1. Particle size analysis 27

4.1.2. Texture 27

4.2. Chemical Characteristics 27

4.2.1. Soil reaction or (pH) 27

4.2.2. Electrical conductivity (EC) 27

4.2.3. Organic Carbon 28

4.2.4. Available P 28

4.3. Phosphate sorption behavior 28

4.4. Freundlich adsorption isotherm 29

4.5. Langmuir adsorption isotherm 30

4.6. Temkin adsorption isotherm 31

4.7. Multi-point adsorption equations 32

4.8. Correlation between soil chemical parameters with phosphateadsorption 33

5. Summary and Conclusion 36

6. Reference 37

Appendice 48

List of Tables Table No. Title Page No. Table 3.1 Table 4.2

Location and environmental conditions of the soils Phosphate adsorption parameters calculated from the isotherm

23 33

List of Figures Figure No. Title Page No. Fig 4.1 Fig 4.2 Fig 4.3 Fig 4.4 Fig 4.5 Fig 4.6 Fig 4.7

Phosphate sorption capacity of soils with different rates of phosphate application Freundlich adsorption isotherm for phosphorus in Gopalpur soil series Freundlich adsorption isotherm for phosphorus in Bajoa soil series Langmuir adsorption isotherm for phosphorus in Gopalpur soil series Langmuir adsorption isotherm for phosphorus in Bajoa soil series Temkin adsorption isotherm for phosphorus in Gopalpur soil series Temkin adsorption isotherm for phosphorus in Bajoa soil series

29 29 30 30 31 31 32

Chapter I Introduction

1. Introduction

Phosphorus (P) is an important naturally occurring element in the environment that can

be found in all living organisms as well as in water and soils. It is an essential component

for many physiological processes related to proper energy utilization in both plants and

animals. It is a component of key molecules such as nucleic acids, phospholipids, and

adenosine triphosphate (ATP). Phosphorus is also a critical element in natural and

agricultural ecosystems throughout the world (Onweremdu, 2007), as its limited

availability is often the main constraint for plant growth in highly weathered soils of the

tropics (Bunemannet al., 2004). Consequently, plants and animals cannot grow without a

steady supply of this nutrient (Theodorou and Plaxton, 1993). The efficiency of applied P

fertilizers to soils is low (He et al., 1994), because fast adsorption and/or precipitation

reactions occur due to the existence of clay minerals and to the presence of soil

components such as Ca, Al and Fe (Reddy et al., 1980). However, long-term continuous

application of P in the form of inorganic fertilizer and organic manures results in build-up

of soil P which ultimately increases the likelihood of elevated P in the soil solution and

surface runoff. Excess surface runoff containing P from soil eventually gives rise to

freshwater eutrophication. Therefore, frequent small applications of P have been

proposed as more economical in the long term (Linquistet al., 1996). Soils vary greatly in

the amount of P required to provide an adequate supply of available phosphorous to

plants. Plants also vary in their P requirements for optimal growth (Vander Zaaget al.,

1979).

Most of the phosphorus used by plant is inorganic phosphorus (Pionke and Kunishi,

1992). Different forms of phosphorus account for different portions in different soils, and

can be transformed under certain conditions (Fixen and Grove, 1990). The phosphorus

absorbed by plant directly comes from the soil solution, and there is a dynamic

equilibrium between phosphorus in the soil solution and on the surface of clay particles

(Barrow, 1983). Such equilibrium is governed by P sorption and release from the solid

phase and plant P uptake (Sharma et al., 1995).

Better management of phosphate fertilization can be achieved by studying the P sorption-

desorption behavior of the soil that reflects the partitioning of P between soil solid phase

and soil solution. Understanding sorption-desorption of P on soil gives insight into the

mechanisms of soil P retention and release. From an agricultural viewpoint, maintaining

soil P concentrations in an optimum range for plant growth is essential to sustain soil

fertility and ensure the production of plants. Environmental concerns associated with P

focus on its stimulation of biological productivity in aquatic ecosystems, being

transported from agricultural fields to surface waters.

Sorption or fixation is the process by which phosphorus binds to the soil, thereby

becoming unavailable for leaching or run off. The adsorption of phosphate is the process

in which phosphate ions in solution react with atoms on the surface of soil particles

(Abedin and Salaque, 1998). This is an important property affecting both the fate of

phosphate fertilizer and the availability of phosphate to plants. When the soil P

equilibrium is disturbed by adding fertilizer, reaction between fertilizer and soil takes

place in two steps: a rapid step leads to adsorption of P and a slow reaction converts P to

a more firmly held form

An effective soil test can help to predict the fertilizer requirement of crops. However,

conventional soil tests provide information only about plant available P

(Barrow, 1978).

(Fixen and

Grove, 1990) and do not estimate the amount of P fertilizers needed unless calibrated for

the particular soil under test (Fox and Kamprath, 1970). Furthermore these tests do not

correctly predict the fertilizer-P requirement for a particular soil - crop system (Rashid

and Hussain, 1998). Consequently, P sorption isotherms, which relate concentration of P

in soil solution with P sorbed by the soil, have been used to correctly predict the P

fertilizer requirement of crops (Shah et al., 2003; Okunolaet al., 2010).

The soil P buffering capacity may be the limiting factor in P uptake (Holford, 1976; Nair

and Mengel, 1984). Phosphorus buffering capacities are derived from adsorption

isotherm plots (Holford 1976, Parfitt, 1978). Such relationships for P in the soil system

can be obtained by fitting data to suitable isotherm equations, such as the Langmuir,

Freundlich equation and the Temkin equation. The adsorption capacities of soils have

been important criteria in soil classification (Rajan, 1973; Breeuwsmaet al., 1986). The

adsorption curves provide an adequate basis for estimation of P requirements across a

diversity of soils and environment (Van Der Zee et al; 1979). Adsorption isotherms have

an advantage over conventional methods of soil testing, because the isotherms consider

both intensity and capacity factors (Rajan, 1973; Tiarks, 1982). The relationships were

found tobe highly correlated. Use of P sorption isotherms for determining P requirements

can increase theefficiency of P fertilization in crop production.

1.2. Objectives of the research

The objectives of this study are to:

To study P sorption of selected soils.

To obtain sorption isotherms to evaluate the phosphate requirement of the selected soils.

To determine the best isotherm for the data.

Chapter II Review of Literature

2. Literature Review

2.1. Phosphorus status of soils of Bangladesh

Phosphorus status of Bangladesh soils covering the major soil types was assessed in

terms of parent materials and physiography. Total P concentration ranged from 172 to

604 mg kg-1 in the topsoil and from 126 to 688 mg kg-1 in the subsoil, and varied with

the physiography to which the soils belonged. In most soils, the available P concentration

was much higher for the topsoil than for the subsoil. The inorganic P concentration was

higher than the organic P concentration, except for one soil series from the Old

Himalayan Piedmont Plain, and was significantly and positively correlated with the total

P concentration. The high content of total phosphorus in these soils may be due to the

presence of phosphorus containing minerals in the soil materials. Islam and Mandal

reported that the contents of total phosphorus in soils of Bangladesh varied with the

variation of organic matter, pH and clay contents. In general, the P status was critically

low in paddy soils of the terrace area. Normal growth in this area is expected to be

difficult without application of P fertilizer. The available phosphorus contents in the soils

varried widely 2 to 15 mg kg-1

Phosphorus can be considered as one of the key elements in soil development because of

its great ecological significance and because practically all the P in natural systems must

be derived from the soil parent material. Soil phosphate can be broadly divided in three

categories: P in soil solution, P in equilibrium with the soil solution and fixed P

.Only 0.5 percent of the total P in the soil occur as

available P.As the soils are alkaline in reaction and contain calcareous materials, the

contents of available phosphorus are expected to be low because of its fixation.

However, the available P ratio in the soils indicated a poor releasing capacity of available

P in soils.Thus application of phosphatic fertilizers is recommended for maintaining the

nutrient balance for sustainable productivity.

2.2. Forms of phosphorus in soil

. Solution

P is immediately available for plant uptake and its concentration depends on the root

uptake and on the sorption and/or desorption reactions occurring between soil solution

and the solid phase. Data on an adequate optimum concentration of phosphorus in the soil

solution are scarce and inconsistent; in fertile arable soils they range from 0.03 to 0.3

ppm.An adequate concentration of phosphorus in soil solution and its refilling from the

solid phase of soil are necessary for the required production of agricultural crops.The

equilibrium P concentration in the soil solution is also a good parameter for improving P-

soil testing. It agrees with the opinion that only an unconditionally necessary level of

soluble forms of phosphorus should be maintained in agriculturally productive soils to

ensure the necessary yield of agricultural crops. Recently adsorbed P at soil surfaces, can

be readi1y available due to P desorption into the soil solution. With increasing time, P

becomes more strongly retained by the soil matrix and therefore less available to plants

(Barrow and Shaw, 1975).The P fixation in soils depends upon many factors, namely the

pH of the soil, organic matter content, type of clay and sesquioxides. Morel et al.;

(1989) evaluated the phosphate fixing capacity of soils by the isotopic

exchange techniques in north-east France and reported that there was a significant

correlation between amount of phosphorus fixed, pH, exchangeable cations, clay content

and soluble phosphate. Soils around Beemanique near Rose-Belle, were found to have a

maximum P-fixation of 95.7%. Clay content is important when considering P sorption

(Reddy et al., 1980) due to substitution reactions between phosphate anions with OH-

anions or H2O molecules which are fixed to Al and/or Fe hydrous oxides present on the

surface of clay particles, resulting in the formation of a single bond (chemisorption)

which makes P less readily- available. Precipitation reactions between P with Al and/or

Fe have also been proposed as an alternative mechanisms that reduce soil P availability to

plants (Reddy et al., 1980; Iramuremyeet al., 1996), in most cases, chemisorption and

precipitation reactions are not easy to differentiate (Sample et al., 1980). In calcareous,

neutral and slightly acidic soils where CaCO3 is present, it reacts with available P.

Dissolution of Ca takes place forming Ca2+. P precipitates that undergo subsequent

reactions producing less Ca-P soluble forms .

2.3. General Characteristics of Soil Phosphorus

Phosphorus (P) is one of the most abundant elements and is essential for plant growth as

well as an important component in the developmental processes of agricultural crops

(Zhuo et al., 2009a) . Approximately two-thirds of inorganic P and one third of organic P

are not available in soil, especially in soils of variable charges. The rate of P use during

crop growth is very low. Phosphates fixed by Fe, Al, and Ca in soils is a major cause of

low phyto availability (McBeath et al., 2005), because at least 70-90% of P that enters the

soil is fixed, making it difficult for plants to absorb and use. Therefore, research by

pedologists and plant nutritionists has primarily focused on the issue of increasing the P

use-rate of plants, both at local and international scales. Resolving this issue is

fundamental towards the continued development of agriculture. Organic supplements

have been reported to increase P availability in P-fixing soils (Iyamuremye and Dick,

1996; Agbenin and Igbokwe, 2006) and humic substances enhance the bioavailability of

P fertilizers in acidic soils (Hua et al., 2008). Decomposition products from manure such

as humic acids and citrate were reported to have greater affinity for Al oxides than for

PO4 (Violante and Huang, 1989). Various researchers have investigated the activity of P

in soil from the perspective of low-molecular-weight (LMW) organic acids, the

incorporation of different organic fertilizers, and soil types. The addition of LMW

organic acids activates Al-P and Fe-P in neutral and acidic soils and Ca-P in alkaline soil

(Zhuo et al., 2009b), causing an increase in the levels of readily-available P for use by

plants. The authors found that the P adsorption capacities of soils were dependent on the

type of organic fertilizer applied and the available soil type. Other studies have shown

that when poultry, cattle, and goat manure are applied to highly weathered tropical soil,

the P-sorption efficiency of the soil and P-buffering capacity decreased with an

increasing incubation period (Azeez and Averbeke, 2011). Variations in organic products

from supplements have also been reported to influence P sorption (Hue, 1991).

2.4. Role of Phosphorus in plants

Phosphorus is one of the seventeen essential nutrients required for plant growth

(Ragothama, 1999). Plant dry weight may contain up to 0.5% phosphorus and this

nutrient is involved in an array of process in plants such as in photosynthesis, respiration,

in energy generation, in nucleic acid biosynthesis and as an integral component of several

plant structures such as phospholipids (Vance et al., 2003). Phosphorus plays a critical

role in energy reactions in the plant. Deficits can influence essentially all energy

requiring processes in plant metabolism. Phosphorus stress early in the growing season

can restrict crop growth, which can carry through to reduce final crop yield. Deficiencies

during early growth generally have a greater negative influence on crop productivity than

P restrictions imposed later in growth. Plants respond to P deficiencies by adaptations

that increase the likelihood of producing some viable seed. The adaptations increase the

ability of the plant to access and accumulate P and include modification of rhizosphere

pH, diversion of resources to root production, increased root proliferation in high-P

regions, and formation of associations with vesicular arbuscularmycorrhizae. Plants differ

in strategies adopted and in efficiency of P absorption. The low availability of

phosphorus is due to the fact that it readily forms insoluble complexes with cation such as

aluminum and iron under acidic soil condition and with calcium and magnesium under

alkaline soil conditions whereas the poor P fertilizer recovery is due to the fact that the P

applied in the form of fertilizers is mainly adsorbed by the soil, and is not available for

plants lacking specific adaptations. Moreover, global P reserves are being depleted at a

higher rate and according to some estimates there will be no soil P reserve by the year

2050 (Vance et al.,2003; Cordell et al., 2011).

2.5. Soil phosphorus status and its availability

Despite its importance for normal plant growth and metabolism, P is one of the least

accessible nutrients. Many soils are inherently poor in available phosphorus content

(Barber, 1995) although the total amount of P in soil may still be high (Vance et

al., 2003). This is evident from the extremely low soil solution P concentration (

nutrients (Clarkson, 1981); consequently, plants do not deplete the total volume of the

rooted soil layer but only that part of the soil which is in the immediate vicinity of the

roots (Fohse and Jungk, 1983).

Phosphorus is commonly bound to iron and aluminium oxides and hydroxides through

chemical precipitation or physical adsorption (Kochian et al., 2004). As a result of

adsorption, precipitation and conversion to organic forms, only 10-30% of the applied

phosphate mineral fertilizer can be recovered by the crop grown after the fertilization

(Syers et al., 2008). The rest stays in the soil and may be used by crops in the following

years. Because of low P solubility and desorption, only a small proportion of phosphate

ions exist in the soil solution for plant uptake even under optimum P fertilization making

P fertilizer recovery to be lower compared to other nutrient containing fertilizers. This

suggests that chemical fertilizer application alone is not a cost effective way of increasing

crop production in many P-limiting soils (Tilman et al., 2002). Therefore, the use of

genotypes/cultivars with improved root traits able to unlock and absorb P from bound P

resources and/or effectively utilizing the absorbed P is of paramount importance for

enhancing the efficiency of P fertilization.

2.6. Phosphorus dynamics in soil

Soil P exists in various chemical forms including inorganic P (Pi) and organic P (Po).

These P forms differ in their behavior and fate in soils (Turner et al., 2007). Pi usually

accounts for 35% to 70% oftotal P in soil (calculation from Harrison, 1987). PrimaryP

minerals including apatites, strengite, and variscite arevery stable, and the release of

available P from theseminerals by weathering is generally too slow to meet thecrop

demand though direct application of phosphaterocks (i.e. apatites) has proved relatively

efficient for cropgrowth in acidic soils. In contrast, secondary P mineralsincluding

calcium (Ca), iron (Fe), and aluminum (Al)phosphates vary in their dissolution rates,

depending onsize of mineral particles and soil pH (Pierzynskiet al.,2005; Oelkers and

Valsami-Jones, 2008). With increasingsoil pH, solubility of Fe and Al phosphates

increases but solubility of Ca phosphate decreases, except forpH values above 8

(Hinsinger, 2001). The P adsorbedon various clays and Al/Fe oxides can be released

bydesorption reactions. All these P forms exist in complexequilibria with each other,

representing from very stable, sparingly available, to plant-available P pools such aslabile

P and solution P .In acidic soils, P can be dominantly adsorbed by Al/Feoxides and

hydroxides, such as gibbsite, hematite, andgoethite (Parfitt, 1989). P can be first adsorbed

on thesurface of clay minerals and Fe/Al oxides by formingvarious complexes. The

nonprotonated and protonatedbidentate surface complexes may coexist at pH 4 to 9,while

protonated bidentateinnersphere complex is pre-dominant under acidic soil conditions

(Arai and Sparks, 2007). Clay minerals and Fe/Aloxides have large specific surface areas,

which providelarge number of adsorption sites. The adsorption of soilP can be enhanced

with increasing ionic strength. Withfurther reactions, P may be occluded in nanopores

thatfrequently occur in Fe/Al oxides, and thereby becomeunavailable to plants (Arai and

Sparks, 2007).In neutral-to-calcareous soils, P retention is domi-nated by precipitation

reactions (Lindsay et al., 1989),although P can also be adsorbed on the surface of

Cacarbonate (Larsen, 1967) and clay minerals (Devauet al., 2010). Phosphate can

precipitate with Ca, gen-eratingdicalcium phosphate (DCP) that is available toplants.

Ultimately, DCP can be transformed into morestable forms such as octocalcium

phosphate and hydroxyapatite (HAP), which are less available to plantsat alkaline pH

(Arai and Sparks, 2007). HAP dissolution increases with decreaseof soil pH (Wang and

Nancollas, 2008), suggesting thatrhizosphere acidification may be an efficient strategy to

mobilize soil P from calcareous soil. Po generally accounts for 30% to 65% of the total P

in soils (Harrison, 1987). Soil Po mainly exists in stabilized forms as inositol phosphates

and phosphonates, and active forms as orthophosphate diesters, labile ortho-phosphate

monoesters, and organic polyphosphates (Condronet al., 2005). The Po can be released

through mineralization processes mediated by soil organisms and plant roots in

association with phosphatase secretion. These processes are highly influenced by soil

moisture, temperature, surface physical-chemical properties, and soil pH and Eh (for

redox potential). Po transformation has a great influence on the overall bioavailability of

P in soil (Turner et al., 2007). Therefore, the availability of soil P is extremely complex

and needs to be systemically evaluated because it is highly associated with P dynamics

and transformation among various P pools .

2.7. Requirement of Phosphorus in plant

P is an important plant macronutrient, making up about 0.2% of a plant's dry weight. It is

a component of key molecules such as nucleic acids, phospholipids, and ATP, and,

consequently, plants cannot grow without a reliable supply of this nutrient. Pi is also

involved in controlling key enzyme reactions and in the regulation of metabolic pathways

(Theodorou and Plaxton, 1993). After N, P is the second most frequently limiting

macronutrient for plant growth. The uptake of P poses a problem for plants, since the

concentration of this mineral in the soil solution is low but plant requirements are high.

The form of P most readily accessed by plants is Pi, the concentration of which rarely

exceeds 10 m in soil solutions (Bieleski, 1973). The form in which Pi exists in solution

changes according to pH. The pKs for the dissociation of H3PO4 into H2PO4 and then

into HPO4 2 are 2.1 and 7.2, respectively. Therefore, below pH 6.0, most Pi will be

present as the monovalent H2PO4 species, whereas H3PO4 and HPO4 2 will be present

only in minor proportions. Most studies on the pH dependence of Pi uptake in higher

plants have found that uptake rates are highest between pH 5.0 and 6.0, where

H2PO4dominates (Furihataet al., 1992), which suggests that Pi is taken up as the

monovalent form.

For efficient use of phosphorus fertilizer requirements should be assessed as accurately as

possible. Recommendations for phosphate fertilization must take into account of the

specific plant requirement,the available level of phosphate in the soil and soil P

adsorption characteristics. Bache and Williams (1971) have reported that phosphate

requirements of a soil can be estimated using quantity: intensity plots. Fox (1981/1982)

advocated that phosphate adsorption curves are the only method for precise fertilizer

recommendations, and especially if the properties of the soil are unknown.

2.8. Soil factors affecting P uptake

According to Kamprath and Watson (1980) the factors affecting the supply of P to plants

are the amount of soil P (quantity), the concentration of soil solution P (intensity), and

movement of P to roots (diffusion). In any assessment of the available P in soil by

chemical tests, one needs to consider the relationship between quantity, intensity and

diffusion and factors influencing availability of these components of P to plants.

2.8.1. Phosphate Buffering Capacity

The P equilibrium between solid phase and solution phase is characterized by phosphate

buffering capacity of a soil. This is the ability of a soil to maintain its P concentration in

solution as P immobilized by soil through various processes or as P is added by

fertilization. Thus the availability of soil phosphate is described by both intensive and

extensive parameters which are determined by the concentration of phosphorus in soil

solution and quantity of phosphate adsorbed on the soil solids respectively. These two

parameters determine the buffering capacity of soil (Barrow, 1967; Holford, 1976). The

buffering capacity of a soil is the slope of the adsorption isotherm at some arbitrary

concentration, which may reflect the actual P concentration found in the solution (Beckett

and White, 1964). It is an indication of the ability of the soil to replace a unit change in

soil solution P and maintain a productive solution concentration. The soil P buffering

capacity may be the limiting factor in P uptake

Soils with high phosphate adsorption maxima have higher phosphate buffering capacity

than those with low phosphate adsorption maxima

(Holford, 1976; Nair and Mengel, 1984).

(Rajan, 1973). According to Kamprath

and Watson (1980) the buffering capacity of acid and neutral soils is a function of the

amounts and crystallinity of hydrated oxides of Fe and Al, whereas in calcareous soils the

amounts of exchangeable Ca and CaCO3

Diffusion of phosphorus through the soil to the roots is the dominant mechanism

governing the P-uptake by roots growing in all soils, except those extremely high in

phosphorus.

determine the P buffering capacity.

2.8.2. Phosphate Diffusion in Soil

Barrow (1989) suggested that phosphate mostly moves to plant roots by

diffusion and it is only the phosphate in the soil solution that is free to move.

The concentration gradient in soil across the root surfaces is an important factor

influencing P diffusion (Kamprath and Watson, 1980).Soil texture is another factor

affecting diffusion of P (Olsen and Watanabe 1963).As the clay content increases, the

diffusion coefficients increase due to a decrease in tortuosity and an increase in buffering

capacity. The diffusion of phosphate persists until the equilibrium is established. Since

the diffusion of phosphorus occurs essentially in the liquid phase and an individual

phosphate ion spends a relatively short time in this phase, the diffusion coefficient of

phosphorus in the soil solution will be different from that in free solution. Diffusion

coefficient of phosphate through soil is in the range of 10-8 to 10-11 cm2s-1. Fitter

(1992) stated that a phosphate ion normally moves less than a millimeter through the soil

in a day. The diffusion coefficient for phosphate ion in water is 0.89 X I0-5cm2s-1.

Phosphorus diffusion through soil is slower than in pure water for three reasons, (i) soil

water occupies only part of the soil so the cross-sectional area for diffusion is less; (ii) the

diffusion path is tortuous because the water is present as films around soil particle; and

(iii) most of the diffusible phosphorus is adsorbed on soil surfaces which equilibrate with

and buffers the small amount of phosphorus in soil solution. All the factors that govern

the rate phosphorus diffusion to the root and the extent of root growth are important in

determining the availability of phosphorus to growing plants in a soil.

2.9. Phosphorus sorption and desorption

Establishment of phosphate (P) retention and release capacity of soils is essential for

effective nutrient management and environmental protection. The main source of plant

available P is generally termed the labile pool. This provides fairly rapid exchange with

soil solution, maintaining the solution concentration. The remaining fraction is the non-

labile pool. This contains a large quantity of insoluble phosphate, which is very slowly

released into the labile pool. Various organic and inorganic phosphates constitute these

labile and non labile pools. In general the labile pool can be considered as orthophosphate

adsorbed onto surfaces of clay minerals, hydrous oxides and carbonates plus iron and

aluminium phosphates. The relationship between the quantity of phosphorus in the labile

pool and the soil solution concentration depends particularly on soil texture and pH

(Archer, 1988).

When soluble P compounds are added to the soil, they react rapidly with various soil

components and are quickly converted to slowly available forms thus creating one of the

main problems relevant to the maintenance and improvement of soil fertility. Phosphorus

fixation is a serious problem in alkaline and calcareous soils (Sharif et al., 2000). The soil

can rapidly and firmly adsorb large amounts of P from solution and once adsorbed, they

are difficult to release (Huang, 1998). In calcareous soils, the dynamics of P is controlled

by many soil properties that strongly retain P and consequently maintain low P

concentration in soil solution (Bertrand et al., 1999).

In recent years plant available phosphate is dependent on the factors - the concentration

of phosphate in the soil solution, amount of exchangeable phosphate and the relative rate

of adsorption from the soil.

The first two factors are static and can be related to each other by adsorption isotherms.

The exchangeable P can be measured by using a suitable technique. Desorption by

Olsen's extraction method (0.5 M NaHC03 solution) has been well correlated with plant

yield in a wide range of soils (Rashid and Rowell, 1988).

The immediate source of phosphate-P to plant roots is the soil solution. Phosphate

deficiency in soil usually occurs from too low concentration of orthophosphate in the soil

solution rather than from an inadequate total P content. The concentration of P in the

solution is governed by a dynamic equilibrium between solid and solution phases where

phosphate is continually released from and re-adsorbed by the solid phase. Any change in

the P concentration of soil solution will initiate physico-chemical processes to re-

establish the equilibrium.

Phosphate adsorption is a process in which phosphate ions in solution react with atoms on

the surface of soil. When soluble phosphate compounds are added to the soil they

undergo a series of complex reactions. These compounds react rapidly with soil minerals,

by precipitation reactions and adsorption onto surfaces, and the availability of this added

P declines. A simple cation exchange may be essentially completed within minutes,

whereas the adsorption of orthophosphate can continue increasing for two days even after

these proceed at a very slow rate for some months (Wild, 1988).

The equilibrium between solid phase and solution phase P is usually expressed by the

buffering capacity of a soil in the shape of adsorption isotherm, which is a line showing

the relationship between quantity of P adsorbed by the soil and the changing

concentration of P in the surrounding solution. According to Holford (1989) the buffering

capacity or sorptivity of a soil is controlled by the two fundamental soil properties: one is

the extent or number of P-reactive sites, and the other is the affinity of these sites for P.

These processes may be on the soil colloidal surfaces (adsorption) or in the solution

(precipitation). Numerical estimates of these properties can be obtained by fitting a

suitable equation such as Langmuir or Freundlich equation to the adsorption isotherm.

Some parameters of adsorption can be obtained from these equations to compare the

behaviour of P in soil.

The phenomenon of phosphorus sorption and desorption in soils is widespread because of

its agricultural importance, and has been the subject of considerable study and

controversy (Barrow, 1978). Several factors determine the flux of P to plant roots. These

were stated as mainly intensity, quantity, capacity and mobility factors. Phosphate

concentration in soil solution represents the intensity factor and it has been shown that

plants vary in this requirement. The quantity factor is the amount of table P in soils. The

capacity factor is described as the quantity of P retained at specific conditions of

intensity. Predicting requirements for P fertilizers using P sorption isotherms takes into

account both intensity and capacity factors, and may be considered a valid concept

relevant in the P supply mechanism and fertilizer management.

Phosphorus sorption capacity is an important soil characteristics that affects the rates and

plant response to Phosphorus fertilizer application (Fox and kamprath, 1970). Several

factors apart from the nature of the soils used may affect adsorption of phosphate. Barrow

(1978) enumerated these as: the period and temperature of contact between soil and

phosphate solution, the method of shaking, the P solution: soil ratio; the identity and

concentration of the supporting electrolyte used; the moisture content of the soil prior to

treatment, and previous addition of phosphate or other specifically adsorbed anions.

2.10. Adsorption isotherm

The process of adsorption is usually studied through graphs know as adsorption

isotherm.An adsorption isotherm is an invaluable curve describing the phenomenon

governing the retention (or release) or mobility of a substance from the aqueous porous

media or aquatic environments to a solid-phase at a constant temperature and Ph.

Adsorption equilibrium is established when an adsorbate containing phase has been

contacted with the adsorbent for sufficient time, with its adsorbate concentration in the

bulk solution is in a dynamic balance with the interface concentration. In order to

optimize the design of an adsorption system for dye removal from solutions, it is

important to find the most appropriate correlation for the equilibrium curve (Crini and

Badot, 2008).

The adsorption capacity of soils have been important criteria in soil classification

(Breeuwsmaet al., 1986). The adsorption curves provide an adequate basis for estimation

of P requirements across a diversity of soils and environment (Van Der Zee et al.,

1979).Muljadiet al., (1966) and Olsen and Khasawneh (1980) revealed that the isotherm

from a plot of phosphate retained against different equilibrium concentrations could be

divided into three regions corresponding to three distinct stages in soil phosphate

interaction: (a) The first region corresponding to low 27 phosphate addition resulting in

practically complete adsorption or a negligible fraction of the added phosphate remaining

in the equilibrium solution. The adsorption isotherm rises steeply and remains close to the

Y-axis; (b) the second region is the strongly curved portion of the isotherm which is

convex to the Y-axis. Bache (1964) showed that adsorption in this region varies

logarithmically with the equilibrium phosphate concentration; and (c) The third portion

of the isotherm approaches linearity and occurs at medium to high phosphate

concentrations. Here the adsorption varies linearly with the amount of P in equilibrium in

solution. At high level of this region, the slope of line is small and the isotherm, for most

soils, tends to become more or less parallel to the X-axis.

2.11. Types of adsorption isotherm

The reaction between phosphate and soils in particular has been described

mathematically by several adsorption isotherm equations i.e. Langmuir equation (Bolster

and Hornberger, 2007; Jiao et al., 2008), Freundlich equation (Zhang and Selim, 2007;

Jiao et al., 2008), Temkin equation (Anghinoniet al., 1996 and Ahmed et al., 2008) and

Elovich equation (Olsen and Watanabe, 1957; Fox and Kamprath, 1970). Among these

equations the Langmuir,Freundlich and Temkin equations are the most frequently used to

describe the relationship between equilibrium P added and P sorbed by the soils.

2.11.1. Langmuir Isotherm

Langmuir equation, proposed by Langmuir in 1916 for adsorption of gases on clean solid

surfaces, was first used by Olsen and Watanabe (1957) to describe phosphate adsorption

in soils. It is based on the assumption that the energy of adsorption is independent of the

surface coverage. In its linear form, the Langmuir equation can be written as:

C/X = 1/Kb + C/b

Where C = equilibrium concentration of phosphate in solution (g P/ml),

X= mass of phosphate adsorbed (g)/ mass of soil (g)

K= adsorption maximum (mg P/g soil), b is related to the binding energy of soil.

A plot of C/X against C should give a straight line, from which the adsorption maximum

K, is the inverse of the slope and the constant b, (b = slope/intercept) related to energy of

adsorption or binding energy can be readily calculated.

The use of Langmuir equation appears satisfactory because its derivation is acceptable on

theoretical grounds and it contains parameters, which have physico-chemical significance

(Olsen and Watanabe, 1957; Holfordet al, 1974) representing the extensive (adsorption

capacity) and intensive (affinity) properties of the adsorbent for the adsorbate(Holford,

1982). However, deviations from the expected linearity have been reported at high

phosphate additions (Olsen and Watanabe, 1957).

Langmuir isotherm can often be used to give a measure of the energy by which

phosphorus is bonded to the solid and an adsorption maximum. Based on this maximum,

calculation of the degree of phosphate saturation can be made, which has been shown to

be related to plant uptake of soil phosphorus (Holford and Mattingly, 1976 b).

2.11.2. Frendluich Isotherm

Freundlich equation was the first model to be used in describing phosphate retention

(Russell and Prescott, 1916). Barrow (1978), advocated that the adsorption data from

dilute solution could be fitted to Freundlich equation in the following form:

X= KC1/n

Where K and n are empirical parameters, a is the sorption energy and n the sorption

constant.

C is the equilibrium concentration of adsorbate in mg/L and

X = mass of adsorbed P (g)/mass of soil (g)

The equation was originally empirical, without any theoretical physico-chemical

foundation, and no significance can be attached to the coefficients (Olsen and Watanabe,

1957). It implies that energy of adsorption decreases exponentially as the fraction of

covered surface increases (amount of adsorption). Freundlich equation can be derived

theoretically by assuming that the decrease in energy of the adsorption with the

increasing surface coverage is due to surface heterogeneity. Freundlich equation is

normally used in its logarithmic form;

log X = 1/n log C + log a.

A plot of log X against log C should give a straight line. Though it is the oldest

adsorption equation in the literature on phosphate adsorption, it has been shown to give

better fits to adsorption data than the Langmuir isotherm (Fitter and Sutton, 1975)

especially in many soils over limited concentration ranges

Freundlich relationship has been used only for the theoretical treatment of phosphate

adsorption. It has not been possible to compare quantitatively adsorption data for soils

obtained from plot of Freundlich equation because the equation was assumed to be

empirical

(Barrow and Shaw, 1975).

(Arshedet al., 2000). However, some workers suggested that the intercept and

slope of a linear Freundlich plot could be used to compare phosphate adsorption in soils

(Kuo and Lotse, 1974; Holford, 1982). Freundlich equation has also the limitation that it

does not predict a maximum adsorption capacity. Despite its limitation the equation was a

better fit to phosphate adsorption isotherms in most of the soils than the most widely used

Langmuir equation (Fitter and Sutton, 1975) and as good fit as the more complex two -

surface Langmuir equation (Barrow, 1978; Sibbesen, 1981).

2.11.3. Temkin Adsorption Equation

X/m= a + B lnC

Where X/m =, mass of adsorbed P (g)/mass of soil (g)

C is the equilibrium P concentration (ug/ml),

and a and B are parameters.

A plot of X/m against ln C gives a straight line if the adsorption process fits the model.

The values of a and B are obtained from the intercept (a) and the slope (B), respectively.

The B value of Temkin equation is considered as the P-buffering capacity (retention

capacity of adsorbed P) of soil (ug/g), (Anghinoniet al, 1996).

2.12. Factors affecting P sorption from soils

The soil characteristics that influence P fixation include the amount and type of clay-

fraction minerals, soil pH, soil organic matter content, time of reaction, exchangeable

Al3+, soil redox condition (Sanchez and Uehara, 1980), and root exudates.

2.12.1. Soil pH

Soil pH has profound effect on the amount and manner in which soluble phosphates

become adsorbed. When soil is acidic, the dominant P ion species present is H2PO4- and

when soil becomes alkaline (Higher pH), the dominant ion becomes PO43-(Gillian and

Sample, 1968). Adsorption of phosphorus by iron and aluminium oxides also declines

with increasing pH (White, 1980). Gibbsite (Al(OH)3 adsorbs greatest amount of

phosphate between pH 4 and 5. Phosphorus adsorption by goethite (an-FeOOH)

decreases steadily between pH 3 and 12

)

(Huang, 1975). Phosphate availability in most

soils is at a maximum in the pH range of 6.0 to 6.5 (Tisdale et al., 1985). At lower pH

values the retention results from the reaction with iron and aluminium and their hydrous

oxides. Above pH 7.0 the ions of calcium, and magnesium and their carbonates cause

precipitation of added phosphorus, which decreases its-availability.

2.12.2. Effect of Soil Carbonate

Lajtha and Bloomer (1988) regarded calcium carbonate as the primary geochemical agent

capable of retaining P in the soils of a desert ecosystem. The presence of calcium or

magnesium ions must accompany high pH values. At pH values above 7.5 the ions of

calcium and magnesium as well as their carbonates cause precipitation of the added P,

and their availability decreases. If the increase of these ions (calcium and magnesium)

continue, there will be a decrease in solubility of soil phosphorus. However, liming acid

soils increase the solubility of phosphorus. In calcareous soils calcium bound phosphate

Ca-P is the most important and dominant P fraction (Kuo and Lotse, 1972). The reaction

of added phosphate with calcareous soils as CaCO3 involves initial adsorption of small

amounts of phosphate followed by precipitation of high levels of Ca-P.

2.12.3. Effect of Clay Mineralogy

Several workers have reported a significant correlation between clay content and P

sorption parameters (Chauderyet al.,2003). The clay content of a soil has great impact on

phosphate adsorption. Soils containing large quantities of clay will adsorb more

phosphate than those with less clay content.Many studies have shown that there are close

relationships between clay content and P sorption by calcareous soils of Mediterranean

regions .

2.12.4. Effect of organic matter

Several authors noted correlation between organic C and the amount of P adsorbed by

soils (Woodruff and Kamprath, 1965). According to Tisdale et al, (1985) the availability

of phosphorus increased from decomposition of organic residues has been due to: (a) the

formation of phosphohumic complexes which are more easily assimilated by plants, (b)

anion replacement of the phosphate by the humate ions, and (c) the coating of

sesquioxide particles by humus to from a protective cover and thus reduces the phosphate

retention capacity of the soil. It was suggested that certain organic anions form stable

complexes with iron and aluminium, thus preventing their reaction with phosphorus by

blocking the adsorption sites (Leaver and Russell, 1957). It was further stated that these

complex ions release phosphate previously retained by the same mechanism. Harter

(1969) suggested that it is OH groups in organic matter which affects phosphate

adsorption through anion exchange, while the results of Appeltet al, (1975) showed that it

is the Al and to lesser extent the adsorbed by the organic colloids which are active in P

adsorption. Organic matter does lower the adsorption of P. it also provides a method of

increasing the P availability without the use of fertilizers. The evolution of carbon

dioxide after the decomposition of organic residues has a favorable effect on phosphate

availability. The gas is dissolved in water to form carbonic acid which is capable of

decomposing certain primary soil minerals. On the basis of available evidence, it is clear

that the addition of organic materials to mineral soils may increase the availability of soil

phosphate.

Soils with a high content of organic matter (colloidal) had very low capacities to adsorb P

(Fox and Kamprath, 1971).Soluble P fertilizer was readily leached from such soils; the

addition of large amounts of exchangeable Al almost completely.

2.12.5. Ionic Strength of Soil Solution

Both organic and inorganic anions compete with phosphate for adsorption sites to varying

extent. In some cases it may result in a decrease in the adsorption of added phosphate or

desorption of retained phosphate. Weakly held inorganic anions such as nitrate and

chloride are of little significance, whereas specifically adsorbed anions like hydroxyl,

sulphate and molybdate are competitive.

The strength of bonding of the anions with the adsorption surface determines the

competitive ability of that anion. For example, sulphate, even though considered to be

specifically adsorbed anion, is unable to desorb much phosphate (Zhang et al., 1987).

Species and concentration of cations in the soil solution also influence the adsorption of P

by soils.

Divalent cations enhance P sorption more than monovalent cations(White, 1981). For

example clays saturated with Ca2+ have the capacity to retain greater amounts of P than

those saturated with Na+ or other monovalent cations. The explanation for this effect of

Ca2+ involves the making of positive charges edge sites of crystalline clay minerals more

accessible to P anions for sorption. On the other hand, both organic and inorganic anions

compete to varying degrees for P sorption sites, resulting in some cases, a decrease in the

sorption of added P (Moshi et al., 1974).

2.12.6. Soil Mineral Type

Numerous studies show that aluminosilicate clay minerals play an important role in P

sorption by soils. The surface charge of clay minerals (and oxides) is partly pH dependent

so that anion exchange capacity increases as pH decreases. Soils with significant contents

of iron and aluminium oxides have large phosphorus fixation capacities because of their

high surface areas. The higher the Al and Fe oxide contents of soil clay and the less

crystalline (more amorphous) the soil minerals, the greater an acid soils P fixation

capacity. This is largely attributed to the greater surface area which these conditions

represent (Quintero et al., 1999). Among the layer silicate clays, 1:1 type clays have a

greater phosphate retention capacity than 2:1 type clays. Soils containing large amounts

of kaolinite group clay minerals will retain larger quantities of added phosphate than

those containing the 2:1 type clay minerals.

2.12.7. Effect of temperature

Temperature affects most physical processes and the speed of chemical reactions

generally increases with a rise in temperature. If temperature at which phosphate reacts

with soil is increased, the rate of reaction is considerably increased (Barrow, 1989). It

also has an important theoretical application. As the temperature increases, the kinetic

energy of the molecules increase, which enables them to jump over the energy barrier

into a new reaction state High temperatures are expected to slightly increase the molar

solubility of compounds such as apatite, hydroxyapatite, octacalcium phosphate, variscite

and strengite. Increase in temperature also stimulates biological activity which enables

phosphate to be released from organic residues. Wild, in 1950 estimated that an increase

in temperature from 298 K to 308 K increased P adsorption in soils. The soils of the

warm regions of the world generally adsorb more phosphates than the soils of temperate

regions. These warmer climates also give rise to soils with higher contents of the hydrous

oxides of iron and aluminum. Many workers agreed that phosphorus retention increases

at higher temperatures (Muljadiet al., 1966; Kuo and Lotse, 1974).

2.12.8. Plant Root Geometry

Phosphate uptake is more dependent on plant root activity than is the case for other major

nutrients. Plant root geometry and morphology are important for maximizing P uptake,

because root systems that have higher ratios of surface area to volume will more

effectively explore a larger volume of soil (Lynch, 1995). For this reason mycorrhizae are

also important for plant P acquisition, since fungal hyphae greatly increase the volume of

soil that plant roots explore.In certain plant species, root clusters (proteoid roots) are

formed in response to P limitations. These specialized roots exude high amounts of

organic acids (up to 23% of net photosynthesis), which acidify the soil and chelate metal

ions around the roots, resulting in the mobilization of P and some micronutrients

(Marschner, 1995).

Chapter III Materials and

Method

3. Materials and Methods

The study was conducted in laboratory with two soil series of Ganges Floodplain in

Bangladesh to evaluate the sorption behavior of Phosphate.

3.1.Sample Collection

Soil samples were collected from the top soil (0-15 cm) by using composite soil sampling

method as suggested by the Soil Survey Staff of the USDA (USDA, 1951) from different

location as follow-

3.1.1. Bajoa Series

Bajoa soils are developed in tidal floodplain basins. They are seasonally shallowly to

moderately deeply flooded, poorly drained soils developed in tidal deposits. It contains a

grey to olive grey silty clay loam subsoil with moderate to strong blocky structure in the

B horizon. They have five phases: Highland; medium high land, non-saline; medium

lowland and medium lowland, flood hazard.

3.1.2. Gopalpur Series

Gopalpur series includes poorly drained, seasonally flooded soils developed in

moderately fine textured Gangetic Alluvium. They are developed on the summits to

upper slopes of gently undulating ridges. Gopalpur soils consist of olive-brown, friable,

calcareous clay loams and silty clay loams usually overlying a moderately fine textured

substratum. They show a weak coarse blocky structure in the sub soil with patchy

coatings on ped faces. They are calcareous and moderately alkaline in reaction

throughout the profile. They have five phases: Highland, smooth relief; highland,

irregular relief; medium highland, irregular relief and medium lowland, flood hazard.

Table 3.1 General information of studied soil

Characteristics

Soil Series

Bajoa Gopalpur

Location

Village: Milemara

Union: Batiaghata

Upazila: Batiaghata

District: Khulna

Village: Dokkhinpara,

Chaderdanga

Union: chaderdanga

Upazila: Batiaghata

District: Khulna

Latitude &

longitude

N: 22043.219

E: 089028.256

N: 22040.727

E: 089027.151

Land type Medium high land High land

Land use Seasame-Boro-Falow Seasame-Boro-Falow

Effervicence Slightly Calcarious Calcarious

3.2. Processing of soil sample

The collected soil samples were dried in air spreading on separate sheet of papers. After

drying in air, the larger aggregates were broken gently by hammering with a wooden

hammer. Then the crushed soils was passed through a 2.0 mm sieve. The sieved soils

were then preserved in plastic container and labeled properly. These were later used for

various physical and chemical analyses.

3.3.Physical analysis of soil

The particle size analysis of the soils was carried out by hydrometer method as described

by Gee and Bauder (1986). Textural classes were determined using Marshalls Triangular

Coordinator system.

3.4. Chemical analysis of soil

3.4.1. Soil pH

Soil pH was determined electrochemically using a 1:2.5 (w/v) soil: distilled water ratio

using a pH meter.

3.4.2. Electrical Conductivity (EC)

The electrical conductivity of the soil was measured at a soil: water ratio of 1:5 by the

help of EC meter.

3.4.3. Determination of organic carbon

Organic carbon of samples was determined by Tyrins method as outlined by Jackson

(1962).

3.4.4. Cation Exchange Capacity (CEC)

The CEC of the soils was determined by extracting the soil with neutral 1 N NH4OAc at

pH 7, followed by the replacing the ammonium in the exchange complex by 1 N KCl at

pH 7 (Black, 1965). The displaced ammonium was determined by colorimetric method at

685 nm wavelength as suggested by Baethgen and Alley (1989).

3.4.5. Available Phosphorus (P)

Available Phosphorus was extracted from the soil with0.5 M NaHCO3 (Olsens Method)

at pH 8.5 and Molybdophosphoric blue colour of analysis was employed for

determination (Jackson, 1967) and P was determined by ascorbic acid blue colour method

(Murphy and Riley, 1962).

3.5. Phosphate sorption experiment

For determining phosphate adsorption characteristics of the soils, phosphorus sorption

was estimated according to the procedure outlined by (Nair et al,.1984).One gram of air-

dried sieved soil was taken into a 50 mL centrifuge tube. Seven initial P concentrations,

namely 0, 1, 2, 5, 10, 25 and 50 g P mL-1 in 0.01 M CaCl2 solution were added

separately to each centrifuge tube using a soil/solution ratio of 1:20 (w/v). The resultant P

contents were 0, 20, 40, 100, 200, 500 and 1000 g P g-1 soil. The centrifuge tubes were

then shaken and equilibrated for 16 h. The mixtures were centrifuged and the

supernatants were analyzed for phosphate following the ascorbic acid blue color method

(Murphy and Riley 1962). Sorbed P was inferred from the difference between the

concentration of soluble P added in the initial solution and the concentration of P in the

solution at equilibrium. Each treatment was replicated three times.

3.6. Fitting of phosphorus adsorption data in three equations

The sorption values of each soil were fitted according to the Langmuir, Freundlich and

Temkin equations.

Linear form of the Langmuir (1918) equation is:

CX-1 = (KL bL)-1 + CbL

-1

where, X is the amount of P sorbed (mg kg-1), C is the equilibrium P concentration (mg

L-1) in solution, bL is the adsorption maximum (mg P kg-1), KL is the bonding energy

constant (L mg-1 P). A plot of CX-1 (y-axis variable) against C (x-axis variable) will yield

a straight line with a slope of 1/bL and a y-intercept of 1/KLbL. The Maximum Buffer

Capacity (MBC) of the soil, which is the increase in sorbed P per unit increase in final

solution P concentration, was estimated from the product of Langmuir constants KL and

bL (Holford, 1979).

Freundlich (1926) equation is:

X = KfC

Logarithmic form of the

N

Freundlich (1926) equation is:

log X = log Kf

+ N log C

where, X is the amount of P sorbed (mg kg-1), C is the equilibrium P concentration (mg

L-1) in solution, Kf is the proportionality constant (mg kg-1), N is the empirical constant.

A plot of log X (y-axis variable) against log C (x-axis variable) will yield a straight line

with slope N and a y-intercept log Kf

Temkin equation (

.

Temkin and Pyzhev, 1940) is:

X = a log C + b

where, X is the amount of P sorbed (mg kg-1), C is the equilibrium P concentration (mg

L-1

) in solution, a and b are constants. A plot of X (y-axis variable) against log C (x-axis

variable) will yield a straight line with slope b and y-intercept.

Chapter IV Results and Discussion

4. Results and discussion

The investigation was conducted to study the phosphate sorption of two soils of Ganges

Floodplain. The results of the study and discussion are presented here.

4.1. Physical Characteristics

4.1.1. Particle size analysis

The sand percentage of Gopalpur and Bajoa series were 8% and 12% respectively. Sand

percentage of Bajoa were greater than Gopalpur. The silt percentage of Gopalpur and

Bajoa were 58% and 57% respectively. The silt percentage of Bajoa were greater than

Gopalpur series. The clay percentage of Gopalpur and Bajoa were 34% and 31%

respectively. The clay percentage of Bajoa were greater than Gopalpur.

4.1.2. Texture

The texture of Gopalpur and Bajoa were both silty clay loam. SRDI staff (1965-86) found

that on the Ganges Tidal Floodplain, soils of river banks and extensive basins are heavy

clay to silty clays.

4.2. Chemical Characteristics

4.2.1. Soil reaction or (pH)

The pH of the Bajoa and Gopalpur soil series were 7.9 and 6.9 respectively. It indicates

that the soils are neutral to moderately alkaline soils. This is a common feature of the

Ganges Tidal Floodplain soils of Bangladesh (Brammer, 1971). This may be due to the

presence of low exchangeable bases in the surface soil. The almost equal range of pH in

all the soil series is probably due to the existence of strong buffering systems

(McConchie, 1990).

4.2.2. Electrical conductivity (EC)

The EC value of the Gopalpur and Bajoa soil series were 1.8 and 5.52 dSm-1. These

results indicate that the soils of the study area were nonsaline to moderately saline.

4.2.3. Organic Carbon

The organic carbon percentage of the Gopalpur and Bajoa soil series were 0.464% and

0.696% indicating the lower amount of organic matter (BARC, 2005).

4.2.4. Available P

Phosphorus content of the Gopalpur and Bajoa soil series were 15 ppm and 9 ppm, which

indicates low level of Phosphorus (BARC, 2005).Bhuiyan (1988) observed that the

available phosphorus of different soil series of Bangladesh ranged from 2.2 to 14 ppm.

Bajoa was below the critical level of available P.

4.3. Phosphate sorption behavior

Soils were equilibrated with 0.01 M calcium chloride solution containing graded

concentrations (0 to 50 g mL-1) of phosphorus. The resulting change in the sorbed

phosphate was then calculated from analysis of the equilibrium solution. Phosphate was

sorbed at all other rates in different amounts and proportions by the soils included for the

present study (except 0 g P mL-1 application) (Fig. 4.1). At 0 g P mL-1, there were

some desorption in all the soil series. Desorption of P at control was also reported by

several other scientists (Vaananenet al., 2008; Afsaret al., 2012). In the present study,

phosphate sorption increased gradually with increasing phosphate application in all the

soil series. Increase in P sorption with increasing phosphate in equilibrium solution was

also reported by other scientists (Afsaret al., 2012). At phosphate application rates, the

Bajoa soil series, occupying non salinity, sorbed the greater amount of phosphate. On the

other hand, the Sara soil series of moderately salinity sorbed the lower amount of

phosphate. P sorption was higher in Bajoa soil series than Gopalpur.

Fig. 4.1. Phosphate sorption capacity of soils with different rates of phosphate application

4.4. Freundlich adsorption isotherm

The R2

Fig. 4.2.Freundlich adsorption isotherm for phosphorus in Gopalpur soil series

value of the Freundlich equation of Gopalpur and Bajoa soil series were 0.8008

and 0.7062 where greater value 0.8008 was observed in Gopalpur soil than Bajoa soil

(Fig. 4.2 and 4.3). The intercept of Gopalpur and Bajoa soil series were 0.8998 and

0.8234. The slopes of the sorption curves show that the amount of P sorbed by the soils

differed between the soil series. The greater slope of 1.11 was observed in Gopalpor soil

than 0.8234 in Bajoa soil series.

0

50

100

150

200

250

300

350

400

0 200 400 600 800 1000 1200

Ads

orbe

d P

(g

P p

er g

soil)

Applied P (g P per gram soil)

gopal SORBED g P per g soil

bajoa SORBED g P per g soil

y = 1.11x + 0.899R = 0.800

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

0 0.5 1 1.5 2 2.5 3

Log

X

Log C

Fig. 4.3.Freundlich adsorption isotherm for phosphorus in Bajoa soil series

4.5. Langmuir adsorption isotherm

The Langmuir adsorption isotherm explains the adsorption maxima and energy of

adsorption. The Langmuir equation was found to be a favorable method to explain the P

adsorption in most soils. The R2 value of the Langmuir equation of Gopalpur and Bajoa

soil series were 0.9834 and 0.9988 respectively (Fig. 4.4 and 4.5). The greater value of R2

was observed in Bajoa Soil and the lowest in Gopalpur soil series. The intercept of

Langmuir equation ofGopalpur and Bajoa soil series were 0.4698 and 0.0034. The greater

value of intercept was observed in Gopalpur Soil than Bajoa soil series. The slope of

Langmuir equation of Gopalpur and Bajoa soil series were 0.0078 and 0.0031. The

greater value of slope was observed in Gopalpur soil and the Bajoa soil series.

Fig. 4.4. Langmuir adsorption isotherm for phosphorus in Gopalpur soil series

y = 0.823x + 1.369R = 0.706

0

0.5

1

1.5

2

2.5

3

3.5

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5

Log

X

Log C

y = 0.007x + 0.469R = 0.983

0.001.002.003.004.005.006.007.00

0.00 100.00 200.00 300.00 400.00 500.00 600.00 700.00 800.00

CX

-1

C

Fig. 4.5. Langmuir adsorption isotherm for phosphorus in Bajoa soil series

4.6. Temkin adsorption isotherm

The R2

Fig. 4.6.Temkin adsorption isotherm for phosphorus in Gopalpur soil series

value of theTemkin equation of Gopalpur and Bajoa soil series were 0.8097 and

0.8077 where greater value 0.8097 was observed in Gopalpur soil than value 0.8077 in

Bajoa soil (Fig. 4.6 and 4.7 and ).The slope of Temkin equation of Gopalpur and Bajoa

soil series were 105.91 and 96.146. The intercept of Temkin equation was greater for

Gopalpur (63.109) thanBajoa soil series (99.484).

y = 0.0031x + 0.0034R = 0.9988

-0.50

0.00

0.50

1.00

1.50

2.00

2.50

-100.00 0.00 100.00 200.00 300.00 400.00 500.00 600.00 700.00 800.00

CX

-1

C

y = 105.9x - 63.10R = 0.809

-100.00-50.00

0.0050.00

100.00150.00200.00250.00300.00350.00

0.00 0.50 1.00 1.50 2.00 2.50 3.00

X

Log C

Fig. 4.7.Temkin adsorption isotherm for phosphorus in Bajoa soil series

4.7. Multi-point adsorption equations

The P sorption data of the two soil series were plotted according to the conventional

Langmuir, Freundlich and Temkin equations (Table 4.2). Among the three adsorption

equations, Langmuir equation was best fitted to the equilibrium P sorption data (R2 =

0.9834,0.9988). Different phosphate sorption parameters were calculated from the three

sorption equations (Table 4.2). The Langmuir equation is used to derive the maximum P

sorption capacity (bL) of the soils. The bL values of the Gopalpur and Bajoa soil series

were 128.20 and 322.58 mg P kg-1. An increasing trend in the bL values was observed

along the catena of the studied soil samples. It should be noted that the bLvalues are more

empirical curve-fitting parameters than true sorption maxima, since input concentration

were not sufficient to saturate the soil (D'Angeloet al., 2003). The energy of adsorption

expresses the binding energy required to adsorb phosphorus. The P binding energy KL of

the Gopalpur and Bajoa soil series were 0.016 and 0.911. Maximum P buffering capacity

(MPBC) is the product of P sorption capacity (or monolayer coverage in mol P kg-1of

soil) and phosphate affinity constant related to the binding strength (Dalal and

Hallsworth, 1976) and regulates the partition of P between solution and solid phase. The

Maximum P Buffer Capacity (MPBC) of the Gopalpur and Bajoa soil series were 2.0512

and 293.87 where the Bajoa soil series had the greater value than Bajoa soil series.

Unlike P buffering capacity, MBC does not vary with solution phosphate concentration

(Poteet al., 1999). Majumdaret al. (2004) suggested that management practices such as

y = 96.14x + 99.48R = 0.807

-100.00-50.00

0.0050.00

100.00150.00200.00250.00300.00350.00400.00450.00

-2.00 -1.00 0.00 1.00 2.00 3.00 4.00

X

Log C

soil conservation measure, application of manure, etc., influence MBC of P. The

phosphate sorption data were also adequately described by the Freundlichand the Temkin

equations. Among the two soil series, Bajoa soil series had the greater Kf

X: Total sorbed P; C: Equilibrium P concentration in solution; K

values (23.39)

than the Gopalpur (7.93) soil series determined from the Freundlich equation (Table 4.2).

The Buffering index (BI) of the Gopalpur and Bajoa soil series were 8.802, 19.17 where

the Bajoa soil series had the greater value than Gopalpur soil series.

Table 4.2. Phosphate adsorption parameters calculated from the isotherm

f and N are empirical constants; BI:

Buffering index; bL: Phosphate sorption maximum; KL

The phosphate sorption capacity increased along the catena of the soil. The higher P

sorption capacity of the Bajoa soil series might be attributed to its high organic matter

and clay content. Significant relationships between P sorption capacity and several soil

properties like organic matter and clay contents have been reported by several authors

(

: P binding strength; MBC: The maximum buffer

capacity of the soil; a and b of Temkin equations are constants.

4.8. Correlation between soil chemical parameters with phosphate adsorption

Adsorbed amount of phosphorus from the soil series were correlated with each other and

with other soil chemical properties.

Daly et al., 2001; Hossainet al., 2012).Soil containing high contents of clay adsorb more

P than those with small amounts (Pena and Torrent, 1990).

Freundlich equation Langmuir equation Temkin

equation

Soil series

log X =

log Kf N + N

logC

K BI f

CX-1 = (KL bL)-1 +

CbL

b-1

KL MBC L X = a logC

+ b

Gopalpur y = 1.11x

+ 0.8998 1.11 7.93 8.802

y = 0.0078x

+ 0.4698 128.20 0.016 2.0512

y = 105.91x

- 63.109

Bajoa

y =

0.8234x +

1.3692

0.82 23.39 19.17 y = 0.0031x

+ 0.0034 322.58 0.911 293.87

y = 96.146x

+ 99.484

There is a significant correlation between maximal value of surface-adsorbed phosphorus

and clay content of soil suggesting that clay content affects the ability of a soil to adsorb

P applied to soil. Therefore, as clay content increases, the P sorption capacity of a soil

increases. The sorbed P will increase the ability of a soil to replenish the soil solution as a

plant withdraws P. The role of clay content on P sorption has been reported by some

researchers (Pena and Torrent, 1990; Afifet al., 1993)

Organic matter is known to influence phosphorus sorption greatly (Quanget al., 1996).

Soils that are highly weathered and the presence of organic matter reduces P- sorption

capacity due to direct result of competition for sorption sites between phosphate and

organic ligands (Hakim, 2002). Khan (1999) reported that less phosphorus adsorption in

soils with high organic matter is due to occupation of adsorption spaces by organic

anions. In the present study, the amount of P sorption was positively correlated to organic

matter but the correlation coefficients were not significant. Organic matter was also

positively correlated with the maximum buffering capacity, the buffering capacity and the

sorption capacity obtained from the Freundlich plot.

A negative and significant relationship exists between P sorption maximum and pH(Fig.

4.5). pH has been considered to be a measure of exchange acidity which measured Al3+

in addition to H+. The negative relationship between P sorption maximum and pH shows

that sorption decreases with increasing soil pH. This can be partly explained by the

decrease in positive charge of hydroxy-surfaces with increasing pH, namely the

disappearance of -H2O

+ ligands that have very strong affinity for phosphate (Shang et al.,

1992). The negative relationship between P sorption and pH is consistent with the

findings of other workers (Ioannouet al., 1994; Zhang et al., 1996).

Among the sorption equations, Langmuir equation was best fitted to the sorption data.

Similar results have been reported for Langmuir equation over Freundlich and Temkin

equations by other scientists as well (Gichangiet al., 2008; Moazedet al., 2010). Based on

R2 values, the Freundlich equation was better in predicting the P sorption capacity of

calcareous soils than the other two equations (Zhou and Li 2001). The good fit of the

Langmuir adsorption equation indicates that the P sorption affinity of soils remained

constant with increasing surface saturation (Mead 1981).

The Bajoa soil series had the highest KL values. Mehadi and Taylor (1988) suggested that

high KL value indicates strong bonding of phosphate by soil particles. As a result, due to

the highest KL values, the Bajoa soil series will retain P better Gopalpur soils and

possibly be the better sink at similar P adding rates.

The Gopalpur soil series had the lowest KL

values. The soils that show lower P buffering

capacities may need more frequent application of p fertilizer than soil with relatively high

buffering capacities.

Chapter V Summary and

Conclusion

5. Summary and Conclusion

In this study, P adsorption isotherms were obtained for two soil samples. The isotherms

were similar indicating similarity in the nature of adsorption reaction, but differed in the

intrinsic characteristics such as the slopes of the isotherms and adsorption capacity.

Maximum P adsorbed by the soil ranged from 19.96 to 345.29 g P g-1 soil. Differences

in the amounts adsorbed may be attributed to differences in soil texture, clay mineralogy,

organic matter contents in the soil.

In the investigation it was noted that Langmuir equation provided the best fit to P

adsorption data in the soils compared to the Freundlich and Temkin equations. The r2

values calculated from the Langmuir plots of soils ranged with an average of 0.991,

followed by the Freundlich plots with an average r2 = 0.7535, and Temkin plot with

average r2 = 0.8087. Langmuir equations were able to explain the P behavior in the soils

with respect to buffering capacity and P supplying capacity. The Maximum P Buffering

Capacity (MPBC) of the soilsare 0.0512 and 293.87. MPBC were higher in the Bajoa soil

series and lower in Gopalpur soil series. Phosphate sorption maximum were higher in the

Bajoa soil series and lower in Gopalpur soil series. Phosphorus binding strength

werehigher in the Bajoa soil series and lower in Gopalpur soil series. In soils with high P

sorption capacities and strong P binding energy like ours, add to soils a less soluble P

source that releases P to soil solution in smaller concentrations and slow down the P

fixation reaction in soil and to maintain fertilizer P in plant available form for a long

period.

P sorption data enabled the prediction of the amount of fertilizer needed for a soil. This

will prevent the arbitrary addition of fertilizer to soil which can cause eutrophication.

From the physical and chemical properties of the soils studied, soils which contain larger

amounts of the soil properties (texture, clay mineralogy, organic matter, aluminum, iron

and calcium contents of the soil) and the sorption parameters derived from the three

equations contain more P.

Chapter VI References

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