ABSTRACT
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.
TABLE OF CONTENTS
LIST OF FIGURES
LIST OF TABLES
ABSTRACT
CHAPTER ONE
INTRODUCTION
CHAPTER TWO
LITERATURE REVIEW
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
CHAPTER THREE
MATERIALS AND METHODS
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
3.7.1.1 Particle size
3.7.1.2 pH
3.7.1.3 Organic carbon
3.7.1.4 Available P
3.7.1.5 Total N
3.7.1.6 Cation Exchange Capacity (CEC)
3.7.1.7 Exchangeable Bases
3.7.1.8 Exchangeable Acidity (EA)
3.7.2 Fractionation of soil organic phosphorus
3.7.2.1 NH4Cl – P
3.7.2.2 Aluminium Phosphate (Al – P)
3.7.2.3 Iron Phosphate (Fe – P)
3.7.2.4 Occluded Phosphate (Occl – P)
3.7.2.5 Calcium Phosphate (Ca – P)
3.7.2.6 Total Phosphorus (Total – P)
3.7.3 Free oxides of iron and aluminium
3.7.3.1 Crystalline form of iron oxide
3.7.3.2 Crystalline form of aluminium oxide
3.7.3.3 Amorphous forms iron and aluminium
3.8 Statistical analysis
CHAPTER FOUR
RESULTS
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
CHAPTER FIVE
DISCUSSION
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
CHAPTER SIX
CONCLUSION AND
RECOMMENDATIONS
6.1 Conclusion
6.2 Recommendation for future research
REFERNCES
CHAPTER ONE
INTRODUCTION
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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