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LITERATURE REVIEW
Phosphorus deficiency in soils is a wide spread problem in the world (Harrison 1987). It is believed to be the second most important soil fertility problem through out the world next to nitrogen (Warren 1992) and often the first limiting element in acid tropical soils (Buehler et.al., 2002).
Also in the Sub-Saharan Africa, P is a limiting nutrient in many soils of the semi-arid tropics and in acid, highly weathered soils of the sub-humid and humid tropics (Buresh et al., 1997). Oxisols and andisols are major soils in the sub-humid and humid tropics of Africa (Deckers, 1993) and are characterized by low total and available P content and high P retention capacity (Friesen et al.,1997). In addition, andepts and oxisols have a high P fixation capacity (Sanchez and Uehera, 1980).
In acid soils, P is fixed in to slightly soluble forms of precipitation and sorption reaction with Fe and Al compounds as well as crystalline and amorphous colloids (Sanchez and Uehera, 1980). Phosphorus sorption was highly correlated with the clay and total free Fe-oxide contents extracted by Dithionite-Citrate-Bicarbonate (DCB) in ultisols and alfisols derived from the savanna and rainforest zones of West Africa (Juo and Fox, 1977). Arudino et al., (1993) found that sorption capacity of acidic alfisols from South Africa were highly correlated with the DCB extractable iron oxides and with amorphous Fe and Al oxide content (Oxalate extractable). Based on P sorption isotherms for 200 soils from West, East and South Africa, Warren (1992) concluded that fertilizer requirements tend to follow the order andisols> oxisols> ultisols> alfisols> entisols. With the exception of andisols, there is, in general, a direct relationship between P sorption by soils and the surface area of Fe and Al oxides. Clay content in soils also affects P sorption. For example, millet producing soils of West Africa in the Sudano-Sahellian agro-ecological zone are generally sandy in texture, have a low sorption capacity and only need low to medium inputs of P to maintain an adequate pool of labile P (Manu et al., 1991).
In calcareous alkaline soils, solid-phase CaCO3 is the dominant factor affecting P availability. Data for 19 soils from different agricultural areas of West Asia and North Africa showed that CaCO3, Fe-oxides, amount and reactivity of silicate clays as well as P fertilizer addition rate and time after application affect the availability of P in calcareous soils (Afif et al., 1993). Iron oxides particularly the more reactive forms have a modifying influence on P fractions in calcareous soils, despite the dominant influence of CaCO3 (Ryan et al., 1985). With 20 calcareous soils in the USA, Sharpley et al., (1984) found a negative correlation between labile P and CaCO3 content after six months of incubation.
Sorption and desorption of phosphorus
Phosphorus sorption is the removal of labile P from the soil solution, due to the adsorption on, and absorption into the solid phases of the soil, mainly on to surfaces of more crystalline clay compounds, oxihydroxides, or carbonates (Hollford and Mattingly, 1975). The term “labile P” is commonly used to represent mobile P, which is available (or rapidly becomes available by reactions with fast kinetics) as a nutrient for plant growth, including soluble P and that which has been deposited by the slow reaction (which is not readily available) (McGechan and Lewis, 2002). Although soil P sorption has been studied intensively, relatively less has been done on the P desorption in soils and sediments. Desorption refers to the release of P from the solid phase in to the solution phase. Desorption occurs in soils when plant uptake depletes soluble P concentrations to very low levels, or in an aquatic system when sediment – bound P interacts with natural waters with low P concentrations (Pierzynski et al., 1994). Interest in P desorption studies are rising due to the importance of P on soil fertility and pollution (Sharpley, 1985). Intensive animal husbandry in Europe has led to the production of large amounts of animal manures, and the disposal of manures on the agricultural land have led to increased soil P tests (Gerke, 1992). Many soils have become saturated and contributed to surface water eutrophication (Sharpley, 1985; Mozaffari and Sims, 1994; Penn et al., 1995; Sharpley, 1996; Pote et al., 1998). Similar problems also occur where sewage sludges has been disposed on land (Gerke, 1992; Sharpley and Sisak, 1997).
P sorption and desorption rates
Phosphorus sorption capacity is an important soil characteristic that affects the rate and plant response to P fertilizer application. (Fox and Kamprath, 1970; Hollford and Mattingly, 1975). Phosphorus sorption by soils is usually rapid at first but then slows with time (Dimirkou et al., 1993). The initial fast P sorption rates are presumably due to reaction with surface sites of metal oxides or hydroxide particles that are exposed to the solution phase. Slow P sorption that continues after the initially rapid sorption is ascribed to the slow diffusion in to the soil aggregates (Willet et al., 1988), or due to the slow formation of P containing minerals (Van Riemsdijk et al., 1984; Lookman et al., 1995; McGechan and Lewis, 2002).
The P desorption rate in the soils are of particular interests in respect to the bioavailability and the pollution risk as a result of P translocation to deeper layers and by surface runoffs (Pote et al., 1996; Li et al., 1999; Paulter and Sims, 2000). Desorption kinetics can also be classified in to fast and slow rates (Munns and Fox, 1976). The fast P pool presumably represents primarily P bound to the reactive surfaces that are in direct contact with the aqueous phase (Hingston et al., 1974, Madrid and Posner, 1979). The relatively higher surface coverage of soil with P and thus, easy replacement of the adsorbed phosphate may be attributed to a higher initial P desorption from the soil (McGechan and Lewis 2002). Other possible contribution to the fast desorbing pool may be the less soluble P salts originating from recent fertilizers applications that are not yet in equilibrium with reactive hydrous oxides (Lookman et al., 1995). Complexed P with organic material may also be part of the fast desorbing pool (Gerke, 1992). The slow P release rate from the second pool is either the result of slow dissolution rates or from slow diffusion from interior sites inside oxyhydroxide particle (McDowell and Sharpley, 2003). The extent to which this slow reaction is then reversible (desorption) is fundamental in determining the residual effectiveness of added phosphate.
Phosphorus status of South African soils
Phosphorus deficiency is the most widespread and economically important nutrient deficiency in the higher rainfall areas of South Africa. The problem of satisfying the P requirements of plants is twofold. Firstly the soils are severely deficient in P and secondly, the plant availability of applied fertilizer P tends to be rapidly reduced through reactions with soil components (Bainbridge et al., 1995). The main reasons for the low plant availability of phosphate are presence of ferric Fe (III) – and aluminum (Al) oxyhydroxides (Sposito, 1989; Bainbridge et al., 1995) and low organic material content of South African soils (Applet et al., 1975; Stevenson, 1982; Iyamuremye and Dick, 1996; Baldock and Skjemstad, 1999).
The studies of Reeve and Sumner (1970) revealed a wide variation in the P sorption capacities of some oxisols in Kwa-Zulu-Natal province. Similarly McGee (1972), in evaluating P sorption in soils of Guateng, Mpumalanga, North West and Free State provinces found considerable variation in their sorption capacities. Bainbridge et al., (1995) determined the P-sorption isotherms of 50 soil samples from a number of localities in the Kwa-Zulu-Natal province. They reported that the amount of P sorbed ranged from 5-1174 mg kg -1 and that the highest sorption occurred in the highly weathered red and yellow-brown clay soils with a high organic carbon content in the A horizons (Inanda, Kranskshop and Mgwa forms). This agrees with the findings of Haynes (1984) who had indicated that ferric and aluminum ions complexed with organic matter provide additional sites for P sorption. In an effort to identify soil properties responsible for P sorption, Henry and Smith (2002) constructed phosphorus isotherms for 21 selected soils from the Republic of South Africa and reached to the conclusion that the citrate bicarbonate dithionite- Al to be an important factor in P sorption although other soil constituents such as clay percentage, organic matter, citrate bicarbonate dithionite-Fe and Bray II P content also contributed to P sorption characteristics of the soils. Estimates of the phosphorus requirement of 20 selected soils of the South African tobacco industry were interpolated from phosphorus sorption isotherms and the results showed that the phosphorus required varied widely and is influenced by both the level of Bray II P content and the P fixation capacity of the soil (Henry and Smith, 2003). Although P sorption has been found to increase with increasing soil clay content, a considerable variation in sorption capacities have been obtained in different soils with similar clay contents (Johnston et al., 1991). It has been shown further that, soils with predominantly 1:1 type clay material (i.e. highly weathered red and yellow brown clay soils) sorp much more P than the soils with predominantly 2:1 type clays.
Van Zyl and Du Preez (1997 I) have tried to study the effect of farming practices such as tillage, fertilization and liming on the phosphorus fractions in soils from the summer rainfall area (250-300S; 240-300E) in South Africa by comparing the phosphorus level of selected virgin and cultivated areas. They found that PT(total P) increased in the case of cultivation, which is attributed to use of fertilization as opposed to the virgin land. They also reported the influence of cultivation on the phosphorus fraction of the same soils and found that most of the inorganic fractions increased as the result of cultivation although the effect was not significant for the residual Pi fraction. NaHCO3-PO was found the most depleting organic fraction due to cultivation ascribing its easily minerlizable property as opposed to the other organic fractions (Van Zyl and Du Preez, 1997II). In a long-term experiment (>15 years) on yellowish brown sandy clay loam (Avalon) and a red sandy clay (Clovelly) soil in Ermelo, Mapumalanga province, Du Preez and Claassens (1999), concluded that the NaOH-extractable P (moderately adsorbed P) was mainly responsible for the replenishment of the labile soil P pool.
Relatively little information is available on areas pertaining to the long-term P desorption studies. Recently, studies related to the desorption kinetics of residual and applied phosphate to an acid sandy clay soils of Piet Retief, Mpumalanga were carried out over a 56-day period using hydrous ferric oxide in dialysis tubes (DMT-HFO) as a specific phosphate sink, followed by a sequential phosphate extraction. The total amount of phosphate desorbed during the stated period was reported to be virtually equal to the decrease in the NaOH (moderately labile) extractable inorganic phosphate fraction revealing the active participation of this fraction in the desorption process (De Jager, 2002). In an endeavor to investigate the fate of the applied P in soils, Ochwoh et al., (2005) also carried out the same experiment for sandy clayey soil (Ferric Luvisols) from Rustenberg (high P fixing) and a red sandy loam soil (Ferric Acrisols) from Loskop (low P fixing). The results showed that 30-60 % of the added P was transformed into the less labile P pools with in one day and 80-90% after 60 days. In the same study made by Ochwoh (2002), an attempt was made to determine the P desorption rates by successive DMT-HFO extractions after the transformation of the applied P followed by sequential extraction. They observed the transformation and redistribution of the applied P during incubation periods and proved that all the so- called unlabile soil P pools contributed to the labile P pool by different proportions.
Chemical extractants
Soil phosphate testing is used to predict plant yield from the amount of P already present in soil. This requires knowledge of the relationship between plant yield and soil P test values, where the yield measured later on in a season is related to soil P test values measured on soil samples collected earlier in season (Kumar et al., 1992). Soil testing for P is done using a chemical extractant. A large number of extractants have been suggested by various researchers (Tan, 1996) and the choice of appropriate soil test reagent depends on many factors, among which are the following:
- The soil and extractant type (Kleinman et al., 2001)
- The nature of the crop (Ibrikci et al., 1992) and
- The fertilizer type (Indiati et al., 2002).
The suitability of a specific soil P tests for soils is dependent on the pedogenic properties of the soils. For instance, Bray-1, Melich-1, and to a lesser extent, Melich- 3, are not considered suitable for calcareous soils because soluble P may be precipitated by CaF2, a product of the reaction between NH4F and CaCO3. Generally, acid extractants provide inconsistent measures of soil P in calcareous soils. Some extraction methods, however, such as Olsen, are considered suitable over a wide range of soils, from acidic to calcareous (Kleinman et al., 2001). Dilute acidic extractants such as Melich-1 (M-1) have been used on acidic soils. Investigations involving the M-1 test in Florida’s acidic soils suggested excessive P recommendations for other crops such as watermelon [Citrus lanathus Thunb]. The M-1 dissolves Ca-P compounds in soils containing apatite and predicts high P values (Ibricki et. al., 1992). The Mehlich-3 (M-3) extractant was developed to predict nutrient requirements of plants over a wide range of soil chemical characteristics for macro- and micronutrients. The M-3 contains fluorides, which enhances the extraction of Al- phosphate through complexation reaction. According to Menon et al. (1990), acid extractants used in Bray-1 and 2 procedures, may extract more P from soils than the amount accumulated by plants. Acid extractants are capable to dissolve aluminum phosphate and calcium phosphate (Leal et al., 1994) giving high P values that do not reflect the level of available P. In general, acidic extractants have been found very effective in estimating available P in acidic soils. The same methods may not be appropriate when used in calcareous soils because of neutralization by the soil carbonates. In addition, acidic solutions may overestimate P from soils fertilized with water-insoluble fertilizer P such as phosphate rock (PR), by dissolving more P from PR than the plant could use.
Selection of appropriate soil test reagent also depends on the crop type. Crop species are known in their efficiency for utilization of nutrients from the soil. For instance, peanut [Arachis hypogia L] has been shown not to respond to phosphorus even in the soils testing low in Olsen extractable P where as wheat grown on the same field shown marked responses to residual as well as direct P application. Total P removed by peanut and wheat was comparable. It was, therefore, postulated that peanut perhaps taps some of the reserve P-fractions in the soil that are not readily available to other crops like wheat and mustard as the result of long-term fertilizer P application (Pasricha et al., 2002). A similar report was obtained on some soils of western Quebec (Canada), which were brought in to cultivation in the 1940s for some forage grasses. Grass grown on fine textured soils of the area did not respond to P fertilizer during the first two growing seasons during a 3-year in situ study (Ziadi et al., 2001). These soils initially had low Melich-3 extractable P contents and very high clay contents. Some studies using chemical extractions reported that the Melich-3 soil test might underestimate the P availability in clay soils (Cox, 1994). The lack of response of forage grass to P fertilizers suggests a significant contribution of the P reserves, which was not predicted by the Melich-3 extractant.
List of tables
List of figures
Declaration
Acknowledgements
Abstract
Chapter
General introduction
Chapter Literature review
2.1 Sorption and desorption of phosphorus
2.2 Phosphorus sorption and desorption rates
2.3 Phosphorus status of South African soils
2.4 Chemical extractants
2.5 The sequential extraction of phosphorus
2.6 Methods to investigate and describe phosphorus desorption
2.6.1 Use of P free solutions
2.6.2 Use of materials that bind phosphate
Chapter 3 Kinetics of phosphate desorption from long-term fertilized soils of South Africa and its relationship with maize grain yield
3.1 Introduction
3.1.1 Theory
3.2 Materials and methods
3.2.1 Sampling procedure and experimental site history
3.2.2 Soil characterization
3.2.3 Long term desorption study
3.2.4 Field data
3.2.5 Data analysis
3.3 Results and discussion
3.3.1 DMT-HFO extractable P
3.3.2 Plant growth as related to phosphorus desorption kinetics
3.4 Conclusion
Chapter 4 Effect of long-term phosphorus desorption using dialysis membrane tubes filled with hydrous iron oxide on phosphorus fractions
4.1 Introduction
4.2 Materials and methods
4.2.1 Fertilization history and soil analyses
4.2.2 Long term desorption study
4.2.3 Fractionation procedure
4.2.4 Field data
4.2.5 Data analysis
4.3 Results and discussion
4.3.1 P recovery and distribution
4.3.2 Effect of P level and extraction time on the labile P (DMT-HFO-Pi + HCO3-Pi+Po) fraction
4.3.2.1 DMT-HFO extractable Pi
4.3.2.2 0.5M NaHCO3-extractable Pi
4.3.2.3 0.5M NaHCO3-extractable Po Effect of P level and extraction time on the less labile
P (0.1M NaOH-Pi + 0.1M NaOH-Po +1M HCl-Pi) fraction
4.3.3.1 0.1M NaOH-extractable Pi
4.3.3.2 0.1M NaOH-extractable Po
4.3.3.3 1M HCl-extractable Pi
4.3.4 Effect of P level and extraction time on the stable P [(C/HCl-Pi + C/HCl-Po +(C/H2SO4 +H2O2-P)] fraction
4.3.4.1 C/HCl- extractable Pi
4.3.4.2 C/HCl- extractable Po
4.3.4.3 C/H2SO4 + H2O2 – extractable P
4.3.5 Plant growth as related to phosphorus fractions
4.4 Conclusion
Chapter 5 Effect of shaking time on long term phosphorus desorption using dialysis membrane tubes filled with hydrous iron oxide
5.1 Introduction
5.2 Materials and methods
5.2.1 Long term phosphate desorption experiment
5.2.2 Modification of the shaking time
5.2.3 Field data
5.2.4 Data analysis
5.3 Results and discussion
5.3.1 DMT-HFO-Pi
5.3.2 Plant growth as related to phosphorus desorption kinetics
5.4 Conclusion
Chapter 6 Short cut approach alternative to the step-by-step conventional soil phosphorus
fractionation method
6.1 Introduction
6.2 Materials and methods
6.2.1 Long-term desorption study
6.2.2 Fractionation procedure
6.2.3 Short cut approach to a modified fractionation procedure
6.2.4 Field data
6.2.5 Data analysis
6.3 Results and discussion
6.3.1 Modifications made on the C/HCl step of Tiessen and Moir (1993) method
6.3.2 DMT-HFO-extractable Pi
6.3.3 C/HCl extractable Pi
6.3.4 Plant growth as related to phosphorus extracts by DMT-HFO and C/HCl
6.4 Conclusion
Chapter 7 Long-term phosphorus desorption using dialysis membrane tubes filled with hydrous iron oxide and its effect on phosphorus pools for Avalon soils
7.1 Introduction
7.2 Materials and methods
7.2.1 Fertilization history and soil analysis
7.2.2 Long term desorption study
7.2.3 Fractionation procedure
7.2.4 Green house experiment
7.2.5 Data analysis
7.3 Results and discussion
7.3.1 Percent P distribution
7.3.2 Changes in inorganic P
7.3.2.1 DMT-HFO extractable Pi
7.3.2.2 0.5M NaHCO3-extractable Pi
7.3.2.3 0.1M NaOH-extractable Pi
7.3.2.4 1M HCl-extractable Pi
7.3.2.5 C/HCl extractable Pi
7.3.3 Changes in organic P
7.3.3.1 0.5M NaHCO3-extractable Po
7.3.3.2 0.1M NaOH- extractable Po
7.3.3.3 C/HCl- extractable Po
7.3.4 C/H2SO4 +H2O2- extractable P
7.3.5 Plant growth as related to phosphorus fractions
7.4 Conclusion
Chapter 8 Phosphate desorption kinetics study for Avalon soils and its relationship with plant growth
8.1 Introduction
8.2 Materials and methods
8.2.1 Long term desorption study
8.2.2 Green house experiment
8.2.3 Data analysis
8.3 Results and discussion
8.3.1 Long term desorption study of P
8.3.2 Plant growth as related to phosphorus desorption kinetics
8.4 Conclusion
Chapter 9 General conclusions and recommendations
9.1 Kinetics of phosphorus desorption and its relationship with plant growth
9.2 The dynamics of phosphorus and the relationship between fractional pools and plant growth
9.3 Effect of varying shaking time on phosphorus desorption
9.4 Short cut to the combined method
9.5 General remarks
9.6 Research needs
References
