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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.












2.1.      Phosphorus (P) dynamics in soil

2.1.1. Soil P transformation

2.1.2. Chemical fertilizer P in soil

2.1.3. Manure P in soil

2.2.      Phosphorus dynamics in the rhizosphere

2.3.      Mechanisms of phosphate solubilization

2.4.      Solubilization of calcium phosphate (Ca-P)

2.5.      Solubilization of iron phosphate / aluminium phosphate (Fe- P / Al- P)

2.6. Soil P fixation

2.7. Organic matter and P interaction in the soil

2.8. Phosphorus mobilization

2.8.1. Phosphorus mobilization by soil micro organisms

2.9. Phosphorus and OM incubation

2.10.    Phosphate Solubilizing Microorganisms (PSMs)

2.10.1. Soil microbial biomass phosphorus and contribution to plant nutrition

2.11.    Organic acids in soils

2.11.1. Composition and concentration

2.11.2. Sorption and persistence of organic acids

2.11.3. Competitive sorption of Decomposable Organic Carbon (DOC) and P

2.11.4. Competition between P and Low Molecular Weight Organic Acids (LMWOAs)

2.11.5. Competition between P and High Molecular Weight Organic Acids (HMWOAs)

2.12.    Mineralization of organic phosphate




3.1       Study location

3.1.1    Climate

3.1.2    Geology

3.1.3    Vegetation

3.2       Soil sampling and preparations

3.3       Laboratory studies I: Phosphorus sorption studies

3.4       Laboratory studies II: Incubation experiments of organic acids

3.5       Field studies

3.5.1    Experimental design

3.5.2    Treatments

3.5.3    Field management

3.6       Post soil sampling

3.7       Laboratory analysis

3.7.1    Soil characterization Particle size pH Organic carbon Available P Total N Cation Exchange Capacity (CEC) Exchangeable Bases Exchangeable Acidity (EA)

3.7.2    Fractionation of soil organic phosphorus NH4Cl – P Aluminium Phosphate (Al – P)   Iron Phosphate (Fe – P) Occluded Phosphate (Occl – P) Calcium Phosphate (Ca – P) Total Phosphorus (Total – P)

3.7.3    Free oxides of iron and aluminium Crystalline form of iron oxide   Crystalline form of aluminium oxide Amorphous forms iron and aluminium

3.8       Statistical analysis




4.1       Physico-chemical properties of the experimental sites

4.2       Phosphorus sorption characteristics of the soil

4.3       Inorganic P fractions

4.3.1    Olsen P

4.3.2    NH4Cl – P

4.3.3    Occluded P (Occl – P)

4.3.4    Aluminium – P (Al – P)

4.3.5    Iron – P (Fe - P)

4.3.6    Calcium – P (Ca – P)

4.3.7    Total –P

4.4       Plant height of maize (4WAP) at Minna in 2011 and 2012 seasons

4.5       Plant height of maize (4WAP) at Mokwa in 2011 and 2012 seasons

4.6       Plant height of maize (7WAP) at Minna in 2011 and 2012 seasons

4.7       Plant height of maize (7WAP) at Mokwa in 2011 and 2012 seasons

4.8       Maize grain yield (t ha-1) at Minna in 2011 and 2012 seasons

4.9       Maize grain yield (t ha-1) at Mokwa in 2011 and 2012 seasons

4.10     Soil Olsen – P (mg kg-1) at Minna in 2011 and 2012 seasons

4.11     Soil Olsen – P (mg kg-1) at Mokwa in 2011 and 2012 seasons

4.12     Soil NH4Cl – P (mg kg-1) at Minna in 2011 and 2012 seasons

4.13     Soil NH4Cl – P (mg kg-1) at Mokwa in 2011 and 2012 seasons

4.14     Soil Occl – P (mg kg-1) at Minna in 2011 and 2012 seasons

4.15     Soil Occl – P (mg kg-1) at Mokwa in 2011 and 2012 seasons

4.16     Soil Al – P (mg kg-1) at Minna in 2011 and 2012 seasons

4.17     Soil Al – P (mg kg-1) at Mokwa in 2011 and 2012 seasons

4.18     Soil Fe – P (mg kg-1) at Minna in 2011 and 2012 seasons

4.19     Soil Fe – P (mg kg-1) at Mokwa in 2011 and 2012 seasons

4.20     Soil Ca – P (mg kg-1) at Minna in 2011 and 2012 seasons

4.21     Soil Ca – P (mg kg-1) at Mokwa in 2011 and 2012 seasons

4.22     Comparison of soil Olsen and NH4Cl – P at Minna and Mokwa locations

4.23     Comparison of maize grain yield at Minna and Mokwa locations

4.24     Distribution of diothinite and oxalate extractable forms of Fe and Al oxides in Minna

4.25     Correlation coefficient between soil P and some physico – chemical properties




5.1       Physico – chemical properties of the soils

5.2       Sorption characteristics of the soils studied

5.3       Inorganic phosphate mobilization

5.4       Effects of organic acids on maize plant height and grain yield

5.5       Post harvest soil inorganic phosphate mobilization

5.6       Distribution of Fe and Al oxides

5.7       Correlation analysis




6.1       Conclusion

6.2       Recommendation for future research








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

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