One mechanism by which plants can mobilize organic and inorganic forms of phosphorus (P) in soils is by exudation of low molecular weight organic acids. Laboratory and field trial were carried out during 2011 and 2012 cropping seasons to study the effects of additions of organic acids ( citric, oxalic and tartaric acids) on the mobilization of phosphate of soils from Minna and Mokwa, both in Southern Guinea Savanna of Nigeria. For initial laboratory studies and prior to field cultivation, soil samples were collected from these locations and incubated at 25± 1oC and 40% moisture content for three weeks with citric, tartaric or oxalic acids at 1.0 mmol kg-1 of soil. Soil Olsen P and inorganic P fractions were analyzed. The experimental design used during the field trial was split plot design with organic material sources (orange waste, amaranthus and tamarind pulp) assigned to the main plot while the rates (0, 2.0, 4.0, 6.0 and 8.0 tons ha-1) of application occupied the sub-plots. Each treatment received three replications in each of the locations. Maize was planted during the two cropping seasons as test crop. Both agronomic, Olsen – P and soil inorganic P data were determined. The results indicated that Olsen – P and NH4Cl – P were significantly increased by treating with the three organic acids. Al phosphate (Al – P), Fe phosphate (Fe –P), occluded phosphate (Occl – P) and Ca phosphate (Ca – P) were also mobilized and released in various degrees in each of the locations irrespective of the cropping season. The relative fractions of inorganic P was in the order Occl – P > Fe – P > Al – P > Ca – P. The effect of organic acid sources on maize plant height at 4 and 7 weeks after planting in the two locations were not significant during 2011 cropping season, but significant during 2012 cropping season. However, the effect of sources of organic acid and their rates of application on maize grain yield was significant in each location and the season. It was also observed that the results of soil inorganic P after the field trial followed similar trend with what was obtained from the initial laboratory studies (Occl – P > Fe – P > Al – P > Ca – P), but the effect was much lower. The order of increased mobilization of phosphate by these organic acids was citric acid > tartaric acid > oxalic acid and orange waste ˃ tamarind pulp ˃ amaranthus leaves respectively for both initial laboratory studies and field trial. Also, it could be concluded that hydroxyl acids i.e tricarboxylic acids such as citrate form stronger complexes than those containing single COOH groups. The pattern of P mobilization by addition of organic acids differed from one location to another. The comparison suggested that the mobilization of P was highly soil dependent, and the soil P status such as amount and distributions of P fractions may be important for solubilization of P after the addition of organic acids. These three organic acids therefore have the potentials to increase the availability of available P. The practical implication of these processes is that organic residues could be used as a strategic tool to reduce the rates of fertilizer P required for optimum crop growth on acidic and P-fixing soils of Nigeria.



Phosphorus (P) is an important plant nutrient and the reactions of phosphate with soil components have been extensively studied from the point of view of soil fertility, soil chemistry and environmental concerns (Parfit, 1978; Sanyal and De Datta, 1991; Hue et al., 1994; Wang et al., 2007). In tropical and subtropical acidic soils, low P availability becomes one of the limiting factors for plant growth; at the other extreme, accumulation of soil available P has negatively affected water quality (Sharpley, 1995). The misapplication of phosphate fertilizers usually causes eutrophication of water bodies, unbalanced plant nutrition and low P utilization efficiency.  When soil phosphate levels are too low, P deficiency in plant represents a major constraint to agricultural production (Palomo et al., 2006). One problem is that P fertilizer  can largely be fixed by the oxides, hydroxides and oxyhydroxides of Iron (Fe)  and Aluminium (Al) and clay minerals in an acidic soils, which makes it less available or effectively unavailable to plants (Fankem  et al., 2006). This is because the availability of both applied and native P is controlled largely by, the sorption and desorption characteristics of the soil.

Variable charge minerals are also the major components of most soils of the tropics that affect P unavailability to plants. Such is the case with soils of Nigeria which is dominated by sesquioxides and low activity clays (Bala, 1992). The most likely areas appear to be those dominated by Oxisols, Ultisols and Alfisols. The low amount of total and available P in these soils make investigation into problems associated with phosphorus availability imperative. Already, the widespread occurrence of P deficiency in most arable land in Nigeria has led to the intensive use of P fertilizer.  It has been reported that land utilization also influences P sorption capacity (Odunze, 2009).

Due to the low solubility and high sorption capacity of P in soil, the supply of phosphate can be a major constrain to plant growth. There is overwhelming evidence, however, to suggest that some plants can directly modify the rhizosphere to gain access to previously unavailable soil P reserves. This can include the deregulation of P membrane transport systems, the manipulation of root hair length or density, the release of phosphates to replace organically bound soil P and the release of organic acid and H+ to solubilize inorganic P (Tinker and Nye, 2000).

Researches into management practices to increase phosphate availability in a weathered soil, and at the same time curtail its leaching to contaminate lakes, streams and ground water remains highly imperative. Efficient use and alternative management of phosphate fertilizers are critical to ensure global food production and security (Cordell et al., 2009).The application of combined organic – inorganic inputs has been one management practices suggested to increase P availability in weathered soils (Agbenin and Igbokwe, 2006).

Soils contain complex, aromatic, relatively high molecular weight (i.e., > 2000) organic acids such as humic and fulvic acids (Hue et al., 1994). However, structurally simpler organic acids also exist in the soil such as low molecular weight (citric, oxalic, succinic, malic, tartaric acids) C-, H-, and O- containing compounds. These organic acids are characterized by the possession of one or more carboxyl groups (Jones, 1998). Soil organic acids are derived from plant and animal residues, microbial metabolism, canopy drips and rhizosphere activities (Hue et al., 1994; Wang et al., 2007).

 In a review of organic acid in the rhizosphere, Jones (1998) indicated that typical concentrations of organic acids in the soil ranges from 0.1 – 100 µmol L-1. Although the existence of organic acids in soils is short lived, organic acids may be produced and formed continuously. Hence, organic acids have a very important chemical significance (Jones, 1998) especially for the mobilization of various phosphates in soil (Marschner, 1995). In addition, Jones (2000) and Palomo et al (2006) reported that secretion of organic acids (such as citric, tartaric, oxalic acids e.t.c.) from plant root was the major mechanism for enhancing P availability in soils and hence improving crop yields. The supply of P to plants is also strongly influenced in the rhzosphere by the presence of organic acids (Hue et al., 1994). This introduces the concept that it may be possible to mimic a plant’s release of organic acids by artificially incorporating acids into the soil which would increase P availability in soils with low P status.

Citric, tartaric, and tannic acids derived from degradation of humic substances have greater affinity for Al and Fe oxides than phosphate (Violante and Huang, 1989). Thus, these organic acids can compete strongly with P for adsorption sites on Al and Fe oxide systems. In soils with appreciable amounts of these oxides, phosphate sorption will be severely curtailed (Bar-Yosef, 1996). Organic acids/substances can be sorbed to both the external and internal surfaces of the mineral colloids. Fulvic, humic citric and  tartaric and  acids were reported to be bound to the structural cations of edges and hydroxyl Al and Fe coatings on mineral colloids (Huang, 2004)

The uptake of P from soil through root exudation is mostly from various inorganic phosphate. Although the mobilization is very complex, some understanding of the mechanism have been gained. Hinsinger (2001) reported that the solubility of Ca increases with a decreasing pH of the environment due to H+ released of organic acids from plant roots. The cheletion of Fe3+, Al3+ and Ca2+ by organic anions lead to the release of inorganic P bound by these cations (Jones et al., 2003), and organic anions that compete with P adsorption on the surface of soil particles further stimulate the desorption of adsorbed anions (He et al., 1998)

Although the competitive adsorption of P and organic ligands by synthetic clay minerals and oxides have been extensively studied (Sibanda and Young, 1986; Kafkafi et al.,1988; Violante and Gianfreda, 1995; Violante et al.,1996), there is a limited information on the fate of P in the presence of organic acids in natural soils (Yuan, 1980; .He et al., 1997). Therefore the exact mechanism among soil inorganic colloids, organic acids and P has not been well- understood.  While some detailed studies have been carried out on some soils of the derived savannas of Nigeria especially in terms of P sorption and desorption characteristics, very little attention has been given to the soils of the Southern Guinea part of the Nigeria Savanna (Tsado, 2008). Thus, the need arises for specific studies aimed at understanding the effect of some selected organic acids on phosphate mobilization in these soils. This will facilitate making specific recommendations for P availability to plants with a view to boosting agricultural productivity in the Southern Guinea agro ecology.

This study investigated the effect of some selected organic acids on the availability of phosphate in some of the soils of the Nigerian Southern Guinea Savanna. Thus, the specific objectives were to:

  1. investigate the effect of different organic compounds on the adsorption of phosphate in some major soil types of Southern Guinea Savanna Zone of Nigeria,
  2. study the influence / regular application of different rates of the organic material applications to the preceding year cropping and 
  3. evaluate phosphate sorption characteristics of these soils with a view to efficient management of P fertilization.



2.1. Phosphorus (P) dynamics in soil

2.1.1. Soil P transformation

Soil P exists in various chemical forms including inorganic P (Pi) and organic P (Po). These forms differ in their behavior and fate in soils (Hansen et al., 2004; Turner et al., 2007). Pi usually account for 35 to 70 5 of total in soils (Harrison, 1987). Primary P minerals including apatites, strengites and varisites are very stable and the release of available P from these minerals through weathering is generally too slow to meet crop demand through applications of phosphate rocks (i.e. apatites) and has only proved relatively efficient for crop growth in acidic soils. In contrast, secondary P minerals including calcium (Ca), aluminium (Al) and iron (Fe) phosphates vary in their dissolution rates depending on their size of mineral particles and soil pH (Pierzynski et al., 2005; Oelkers and Valsami-Jones, 2008). With increasing soil pH, solubility of Fe and Al phosphates increases but the solubility of Ca phosphates decreases, except for pH values above 8 (Hinsinger, 2001). The P adsorbed on various clays and Al/Fe oxides can be released by desorption reactions. All these P forms exist in complex equilibra with each other ranging from very stable, sparingly available to plant-available P pools such as labile and solution P.

In acidic soils, P can be dominantly adsorbed by Al/ Fe oxides and hydroxides such as gibbsite, hematite and goethite (Parfitt, 1989). P can be first adsorbed on the surface of clay minerals and Al/Fe oxides forming various complexes. The nonprotonated and protonated bidentate surface complexes may co-exist at pH 4 – 9, while the protonated bidentate innersphere complex is predominant under soil acidic conditions (Luengo et al., 2006; Arai and Sparks, 2007). Clay minerals and Al/Fe oxides have large specific surface areas, which provide large number of adsorption sites. The adsorption of soil P can be enhanced by increasing ionic strength.  With further reactions, P may be occluded in nanopores that frequently occur in Fe/Al oxides, thereby becoming unavailable to the plants ( Arai and Sparks, 2007).

In neutral- to- calcareous soils, P retention is dominated by precipitation reactions (Lindsay et al., 1989), although P can also be adsorbed on the surface of Ca carbonate (Larsen, 1967) and clay minerals (Condron et al., 2005). Phosphates can precipitate with Ca, generating dicalcium phosphates (DCP) that are available to plants. Ultimately, DCP can be transformed into more stable forms such as octocalcium phosphates and hydroxyapatites (HAP), which are less available to plants at alkaline pH ( Arai and Sparks, 2007). HAP accounts for more than 50% of total Pi in calcareous soils from long – term fertilizer experiments and dissolution increases in soil pH (Wang and Nancollas, 2008) suggesting that rhizosphere acidification may be an efficient strategy to mobilize P in calcareous soils.

 Po generally may account for 30 to 65% of the total P in the soils (Harrison, 1987). Soil Po may exists in stabilized forms as inositol phosphates and phosphonates and active forms as orthophosphate diesters, labile orthophosphate monoesters and organic polyphosphates (Turner et al., 2002; Condron et al., 2005). The Po can be released through mineralization processes mediated by soil organisms and plant roots in association with phosphatase secretions. These processes are highly influenced by soil moisture, temperature, surface physical and chemical properties and soil pH  and Eh (for redox potential). Po transformation has a great influence on the overall bioavailability of P to plant (Turner et al., 2007).

2.1.2. Chemical fertilizer P in soil

The modern terrestrial P cycle is dominated by agriculture and human activities (Oaklers and Valsami-Jones, 2008). The concentration of available soil Pi seldom exceeds 10 µM (Bieleski, 1973), which is much lower than that in plant tissues where the concentration is approximately 5 to 20 Mm Pi (Raghothama, 1999). Because of the low concentration and poor mobility of plant available P in soils, applications of chemical P fertilizers are needed to improve crop growth and yield. The major forms od phosphate fertilizers include Mono Calcium Phosphate (MCP) and Mono Potasium Phosphate (MPP). Application of MCP can significantly affect soil physical properties. After application to soil, MCP undergoes a wetting process, generates large amounts of protons, phosphates and DCP and eventually form a P-saturated patch (Benbi and Gilkes, 1987). This P-saturated patch forms three different reaction zones including direct reaction, precipitation reaction and adsorption reaction zones. The direct reaction zone is very acidic (pH  = 1.0 – 1.6), resulting in an enhanced mobilization of soil metal ions. These metal ions can also react with high concentrations of Pi in the zone thus causing further precipitation of Pi. The amorphous Fe –P and Al – P that thereby form can be partly available to plants. In calcareous soil, new complexes of DCP and MCP can be formed and with time DCP is gradually transformed into stable forms of Ca – P (othocalcium  phosphate or apatite). Because the Pi concentration is relatively low, P adsorption by soil minerals is dominat in the outer zone (Moody, et al, 1995).In contrast, the application of monopotassium phosphorus has little influence on soil physical and chemical properties (Lindsay, et al., 1989). Therefore, the matching P fertilizer types with soil physical and chemical properties may be efficient strategies for rational use of chemical fertilizer P.

2.1.3. Manure P in soil

The application of manure is widely practiced to increase the productivity of soils that contain inadequate levels of organic carbon. The effects of manure on P availability in various soils has been widely studied, and the general conclusion has been that it is a source of P; interact with soil components in a manner that increases P recovery by crops; and enhances the effectiveness of inorganic P fertilizer. P added from manure and other sources, however, tends to become less available to plant with the passing of time.(Sample et al., 1990). Whalen and Chang (2001) also observed that manure application guidelines are frequently based on N requirements of the crops, and P is therefore over supplied and liable to either accumulate or be removed by surface or subsurface transport. As regards the eventual status of fertilizer P in the soil, it is interesting to note that the manure and mineral (KH2PO4) fertilizer appear to contribute to different P pools (Griffin et al., 2003). The later is efficient at increasing CaCl2 extractible P and Mehlich – 3 P while manure has a greater effect on modified Morgan P as well as other types of P. 

Manure can be applied to increase P fertility, the total P content in manure is very variable and nearly 70% of total P in manure is labile. In manure, Pi account for 50 – 90% (Dou et, al., 2000). Manure also contains large amount of Po, such as phospholipids and nucleic acids (Turner and Leytem, 2004), which can be released to increase soil Pi concentrations by mineralization. Furthermore, small molecular organic acids from mineralization of humic substances in manure can dissolve Ca – P, and especially from citrate. It can efficiently weaken the nanoparticle stability of HAP, by controlling the free Ca availability and thereby the nucleation rate (Martins et al., 2008). P adsorption to soil particles can be greatly reduced through applying organic substances. The humic acids contain large numbers of negative charges, carboxyl and hydroxyl groups which strongly compete for adsorption sites for Pi. Manure can also change soil pH and thus alter available P.

2.2. Phosphorus dynamics in the rhizosphere

The rhizosphere is the critical zone of interactions among plants, soils and microorganisms. Plant roots can greatly modify the rhizosphere environment through their various physical activities, particularly the exudation organic compounds mucilage, organic acids, phosphatases and some specific signaling substances, which are key drivers of various rhizosphere processes. The chemical and biological processes in the rhizosphere not only determine the mobilization and acquisitions of soil nutrients as well as microbial dynamics, but also control the nutrient – use efficiency of crops and, thus profoundly influence crop productivity (Hinsinger et al., 2009; Richardson et al., 2009; Wissuwa et al., 2009; Zhang et al., 2010).

Due to its low solubility and mobility in the soil, P can be rapidly depleted in the rhizosphere by plant uptake, resulting in a P gradient concentration in a radial direction away from the root side (Wissuwa et al., 2009; Zhang et al., 2010). In spite of total soil P content usually exceeding plant requirements, the low mobility of soil P can restrict its availability to plants. Soluble P in the rhizosphere soil solution should be replaced 20 to 50 times per day by P delivery from bulk soil to the rhizosphere to meet plant demand (Marschner, 1995). Therefore P dynamics in the rhizosphere are mainly controlled by plant root growth and function and also highly related to physical and chemical properties of the soil (Neumann and Romheld, 2002). Because of unique properties of P in soils low solubility, low mobility and high fixation by soil matrix as well as the availability of P to plant are dominantly controlled by two key processes; (1) Spatial availability and acquisition of P in terms of plant roots architecture as well as mychorizal association, and (2) bioavailabilty and acquisition of P based on rhizosphere chemical and biological processes (Zhang et al., 2010).