Phosphorus Speciation in Soils of Contrasting Lithologies in Imo State, Southeastern Nigeria

London Journal of Research in Science: Natural and Formal
Volume | Issue | Compilation
Authored by Bethel.U Uzoho , E.T. Jaja
Classification: FOR Code: 050399
Keywords: phosphorus, speciation, lithologies, sequential extraction, southeastern and nigeria.
Language: English

Phosphorus speciation provides useful information about the dominant P fractions in soils and their availability for plant nutrition and environmental sustainability. Chemical P fractions (H2O, 0.5 M NaHCO3, 0.1 M NaOH, 1.0 M HCl and H2SO4-P) in soils of contrasting lithologies in Imo State, Southeastern, Nigeria were determined using sequential extraction technique. Also, P fractions were correlated with selected soil properties using correlation analysis. Mean water, NaHC03, NaOH, HCl and H2S04-P in soils of various lithologies varied as 0.11, 0.15, 0.36, 0.07 and 0.14 mg kg-1 respectively in decreasing order of  NaOH > NaHCO3 = H2SO4 > H2O > HCl for the top soil. Also, mean concentrations were 0.12, 0.14, 0.36, 08 and 0.17 mg kg-1 for water, NaHCO3, NaOH, HCl and H2SO4-P respectively P in a decreasing order of NaOH > H2SO4 > NaHCO3 > H2O > HCl for the subsoil. Averaged over lithologies, NaOH-P was higher while averaged over chemical fractions, concentrations in Coastal Plain Sands were better than other lithologies at both soil depths. Chemical P fractions correlated with selected soil properties especially sand, silt, clay, pH, ECEC, clay, OM and exchangeable Al, Ca and Mg. In general, amorphous and crystalline Fe and Al oxides/hydroxides, in addition to clay minerals and organic P compounds associated with NaOH-P dominated P chemistry of the soils.

 

               

Phosphorus Speciation in Soils of Contrasting Lithologies in Imo State, Southeastern Nigeria 

B.U. Uzohoα & E.T. Jajaσ

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  1. ABSTRACT

Phosphorus speciation provides useful information about the dominant P fractions in soils and their availability for plant nutrition and environmental sustainability. Chemical P fractions (H2O, 0.5 M NaHCO3, 0.1 M NaOH, 1.0 M HCl and H2SO4-P) in soils of contrasting lithologies in Imo State, Southeastern, Nigeria were determined using sequential extraction technique. Also, P fractions were correlated with selected soil properties using correlation analysis. Mean water, NaHC03, NaOH, HCl and H2S04-P in soils of various lithologies varied as 0.11, 0.15, 0.36, 0.07 and 0.14 mg kg-1 respectively in decreasing order of  NaOH > NaHCO3 = H2SO4 > H2O > HCl for the top soil. Also, mean concentrations were 0.12, 0.14, 0.36, 08 and 0.17 mg kg-1 for water, NaHCO3, NaOH, HCl and H2SO4-P respectively P in a decreasing order of NaOH > H2SO4 > NaHCO3 > H2O > HCl for the subsoil. Averaged over lithologies, NaOH-P was higher while averaged over chemical fractions, concentrations in Coastal Plain Sands were better than other lithologies at both soil depths. Chemical P fractions correlated with selected soil properties especially sand, silt, clay, pH, ECEC, clay, OM and exchangeable Al, Ca and Mg. In general, amorphous and crystalline Fe and Al oxides/hydroxides, in addition to clay minerals and organic P compounds associated with NaOH-P dominated P chemistry of the soils.

Keywords: phosphorus, speciation, lithologies, sequential extraction, southeastern and nigeria.

Author α: Dept of Soil Science & Technology, Federal University of Technology, Owerri.

σ: Dept of Plant Science & Biotechnology, Rivers State University of Science and Technology, Portharcourt.

  1.  INTRODUCTION

Phosphorus is essential for the nutrition of most biological organisms including animals, microbes and plants and associated with the eutrophication of surface and grouwater systems (Rydin et al., 2011; McMahon and Read, 2013; Wang et al., 2013; Dapeng and Yong, 2014). Its reactivity, solubility and availability vary depending on the dominant chemical fractions present in soils and sediments (He et al., 2010; Camelo et al., 2015).

Chemical P fractions have been operationally defined as water, 0.5M NaHC03, 0.1 M NaOH, 1 M HCl and H2S04 soluble fractions. Water soluble or Resin extractable P fraction refers to the inorganic P in soil solution or the weakly adsorbed on oxy-hydroxide or carbonates that is very mobile or readily available and thus easily lost through plant and microbial uptake or leaching and runoff. The 0.5 M NaHCO3 fraction constitutes the weakly adsorbed inorganic Pi or easily hydrolysable organic Po compounds such as ribonucleic acid and glycerophosphates, 0.1 M NaOH extractable fraction includes inorganic Pi that is associated with amorphous and crystalline Al and Fe oxyhydroxides and clay minerals or organic Po associated with organic compounds like humic and fulvic acids while the 1 M HCl fraction consists of the fraction that is associated with carbonates (Solomon et al., 2002). Availability of these P fractions has been indicated to vary from potential to none availability (Mehmood et al, 2015).                                 

Concentrations of various P fractions in soils and sediments could be estimated using sequential extraction procedures, which involves the extraction of the readily available or weakly adsorbed P fractions first using mild extractants and then the non-readily or unavailable fractions subsequently with more stringent extractants. For instance, the order for P fractions using the fractionation procedure proposed by Hedley et al. (1982) and modified by Tiessen and Moir (1993) has been reported to be an increasing sequence of water or resin, 0.5 M NaHCO3, 0.5 M NaOH, 1 M hot concentrated HCl and residual P fractions (Buehler et al., 2002).                        

Several factors especially physicochemical properties of soils and sediments influence concentration of chemical P forms (Dharumarajan and Sigh, 2016). According to Jalali and Tabar (2011), the degree of P association with different chemical forms strongly depends on soil physicochemical properties. It has been noted that in Western Plain Rajasthan soils, India, all P fractions except HCl (Ca-P) and H2S04 (red P) P fractions were affected by soil OC, EC, ECEC and silt + clay fractions (Devra et al., 2014). Also, increased HCl (Ca-P) and decreased NaOH (Al-P and Fe-P) P fractions have been reported with increased pH in subtropical soils of India (Amaresh, 2010) and subtropical soils of Iran (Adhami et al., 2013). Besides HCl-P that was related with exchangeable Na and Mg and H2S04-P with exchangeable Ca and CaC03, there was no significant correlation between P forms and soil properties in Western Iranian Calcareous soils under different land uses (Jalali and Tabar, 2011).                                          

Distribution of P chemical forms varies with soil lithologies (Camelo et al., 2015; Mehmood et al., 2015; Prietzel et al., 2016). It has been indicated that in calcareous vertisols and aridisols of Northern Jordan, HCl-P fraction dominated due to the calcareous nature of the parent materials (Rawajfih et al., 2010). Also, high HCl-P (Ca-P) has been reported in alkaline soils derived from basic and ultrabasic rocks while high NaOH–P fraction was noted in acidic soils over sandstone and shale parent materials (Amaresh, 2010). Furthermore, in a study of P speciation of western soils of the temperate zone using wet-chemical fractionation and Xane spectroscopy, NaOH-P (Al-P and Fe-P) was reported dominant in soils derived from silicate and mixed silicate-carbonate parent materials (Prietzel et al., 2016).        

Soils of southeastern Nigeria are underlain by different parent materials that affect their properties. Presently, interests on P studies in the soils have been concerned with the P sorption characteristics, organic and inorganic P status and P fertility management (Uzoho and Oti, 2005; Uzoho et al., 2014). There appears to be a dearth of information on the status of P chemical forms in soils of the various lithologies. Such information could be useful in understanding P dynamics of the soils and the adoption of appropriate P management technologies for efficient crop production and environmental sustainability. The objectives of this study were to determine P speciation of soils of contrasting lithologies in Southeastern Nigeria and their correlation with selected soil physicochemical properties.    

  1. MATERIALS AND METHODS

2.1  Study Location

The study locations were Amuruo, Bende, Ihiagwa, Oguta and Okigwe representing five different lithologies (Imo clay shale, Bende- Ameke shale, Coastal Plain Sands, Alluvium and false bedded sandstone) in southeastern, Nigeria.  Amuruo lies between Latitudes 50 481 and 50 531 N and Longitudes 70 201 and 70 251E and underlain by Imo Clay Shale, Bende between Latitudes 50 251 and 50521 N and Longitudes 70 281 and 70 451E and over Bende/Ameke Shale, Ihiagwa (Latitudes 50 211 and 50 271 N and Longitudes 70 021 and 70 151E) and underlain by Coastal Plain Sands, Oguta (Latitudes 50 421  and 50 461 N and Long. 60 471 and 60 491E) over Alluvium and Okigwe (Latitudes 50 451 and 6 0 001 N and Longitudes 70 151 and 70 451E) and over False bedded Sandstone. Soil types of the locations consisted of Eutric Tropofluvent (Oguta), Arenic Kandiudults (Ihiagwa and Okigwe) and Typic Dystrudepts (Amuruo and Bende), with the climax vegetations varying as rice (Oritza Sativa) and Oil palm (Elaeis guinensis) in Amuruo, Cocoa (Theobroma cacao) and Cassava (Manihot utilis) in Bende and Cassava (Manihot Spp) in Ihiagwa, Oguta and Okigwe. Farming and trading in addition to fishing and quarrying in Oguta and Okigwe respectively constituted the dominant economic activities in the locations. Climatic conditions of the locations include a mean annual rainfall range of between 1900-2600 mm, mean daily temperature of 26-330C and relative humidity of between 75-85%, characteristic of the humid tropical zone of southeastern, Nigeria (IPEDC, 2006).  

2.2 Sample Collection, Preparations and Laboratory Analyzes

Soil samples were collected from generic horizons of profile pits dug on each of the five study locations.  A total of forty five (45) soil samples were collected and air dried, sieved using a 2 mm diameter mesh and the fine earth soil fractions (< 2 mm) stored ready for laboratory analyzes. Routine analyses using standard methods were conducted on subsamples of the fine earth soil fractions. Particle size was determined after dispersion with calgon (Gee and Or, 2002), OM using wet oxidation method (Nelson and Sommers, 1996), Exchangeable cations, ECEC and pH (Thomas, 1996), available P (Olsens, 1982) and total N (Bremner, 1996).  

2.3  Phosphorus fractionation

Phosphorus fractionation was conducted on the top and subsoil samples of the soil profiles. The topsoil was taken as soils from the topmost depth while the subsoil was that from the remaining horizons of the profile. Sequential extraction using the procedure outlined by Hedley et al. (1982) was employed for the P fractionation process. The procedure included as follows:

Water Soluble-P fraction (F1)                 

Subsample (1 g) of fine earth soil fraction was shaken with 10 ml de-ionized water in a centrifuge tube for 16 hrs and then centrifuged for 15 mins at 2600 rpm. The contents were decanted into a 10 ml polyethylene bottle.

0.5 NaHC03 –P fraction (F2)                

Residue from F1 above was extracted with 10 ml 0.5 M NaHC03 for 16 hrs, centrifuged at 2600 rpm for 15 mins and the supernatant decanted into a 10 ml polyethylene bottle.  

 0.1M NaOH – P fraction (F3)        

Residue from F2 above was extracted with 10 ml 0.1M Na0H for 16 hrs, centrifuged for 15mins at 2600 rpm and then decanted into a 10 ml polyethylene bottle.

1.0 M HCl- P fraction (F4)                

Residue from F3 was extracted with 10 ml 1.0 M HCl for 16 hrs and centrifuged at 2600 rpm for 15 mins. The supernatant was decanted into a10 ml polyethylene bottle.

H2S04- P fraction                        

Finally, residue from F4 was digested with 10 ml H2S04 in a digestion block after which it was allowed to cool before centrifugation for 15 mins at 2600 rpm. The supernatant was then decanted into a 10 ml polyethylene bottle.

2.4  Statistical Analysis

Data generated from the different P fractions were subjected to Analysis of Variance (ANOVA) and means separated using least significant difference (LSD) at 5% probability. Also correlation between P fractions and selected soil properties was determined using correlation analysis. All statistical analyses were conducted using Genstat statistical package (Buysse, 2004).

  1. RESULTS AND DISCUSSION

3.1  Soil Characterization

Soils were dominantly sandy with the degree depressed with profile depth exception being Amuruo and Bende. Differences in soil textures were related to the lithologies, with soils over Coastal Plain Sands, Alluvial and sandstone sandier than the shale. Soils of Coastal Plain Sands and sandstone parent materials have been reported to be skeletal and coarse textured (Uzoho et al., 2014). Exchangeable cations, ECEC, N and P were low and decreased with soil depths as the OM. High concentration of soil OM and plant nutrients in the surface soil depth could be ascribed to the increased accumulation of plant litter. Linear relationship between soil OM and plant nutrients confirms early observations that fertility of tropical soils depends on their organic matter contents (Uzoho et al., 2007). In general, low nutrient concentration of the soils could be ascribed to the high OM oxidation and base leaching due to the high intense tropical rainfall. Soils were acidic and ascribable to the intense leaching of basic cations (Uzoho et al., 2007; 2014).

3.2  Phosphorus Fractions

Phosphorus fractions varied and differed with soil lithologies. Water soluble P ranged between 0.02- 0.15 (mean = 0.11 mg kg-1) equivalent to 2.13-17.74 (mean = 13.46%) of total P in the topsoil (Table 2) and 0.01-0.18 (mean = 0.12mg kg-1) equivalent to 1.90-17.97 (mean = 12.35%) total P in the subsoil depths (Table 3) of the various lithologies. Variation in concentration amongst lithologies was low with a coefficient of variation (CV) of 9.5% and 7.70% in the top and subsoil depths respectively. In the topsoil, shale (Amuruo) was significantly (LSD 0.05) higher than others while sandstone (Okigwe) was the least (Table 2) whereas Coastal Plain Sands (Ihiagwa) was significantly better and with shale (Amuruo) being the least in the subsoil (Table 3). Low concentration in the topsoil for the sandstone parent material could be ascribable to its skeletal nature and associated leaching, runoff and uptake by plant as well as low retention capacity whereas the large value in the topsoil of shale (Amuruo) parent lithology could be due to the high retention capacity. Similarly, high and low accumulations in the subsoil of Coastal Plain sands (Ihiagwa) and shale (Amuruo) respectively could be due to the differences in leaching losses of the topsoil that was heavier in the former than the later lithologies. It has been reported that water soluble P constitutes the fraction that is readily available in soil solution and thus easily lost through leaching (Mehmood et al., 2015). In the soils studied, water soluble P was significantly                         (P < 0.05) correlated with soil ECEC (r = -0.50), exchangeable Mg (r = -0.39), OM (r = -0.38) and silt (r = 0.61) but not with clay (-0.33), exchangeable Al (r = 0.17), Ca (r = 0.22) and pH            (r = - 30) (Table 4).                

Sodium bicarbonate P fraction (NaHC03-P) ranged between 0.11-0.21 (mean = 0.15 mg kg-1) equivalent to 11.70-23.53% (mean = 19.27%) of total P in the topsoil (Table 2) and 0.13- 0.15 (mean = 0.14mg kg-1) equivalent to 10.49-27.48 (mean = 17.71%) of total P in the subsoil (Table 3) of various lithologies. Variation amongst lithologies was low in the topsoil (CV = 29.80%) but uniform in the subsoil. Shale (Bende and Amuruo) was significantly (LSD 0.05) better than the other lithologies in both top and subsoils exception being alluvium (Oguta) that was similar to shale (Bende) in the subsoil. Sodium bicarbonate P fraction constitutes the weakly adsorbed inorganic Pi or easily hydrolysable organic Po compounds like ribonucleic acid and glycerophosphates (Solomon et al., 2002). Thus shale (Bende) was better in both the top and subsoil depths with much of the P exchangeably held. There was significant (P < 0.05) correlation between NaHC03-P and soil OM (r = 0.52), pH              (r = 0.45), exchangeable Mg (r = 0.41), exchangeable Ca (r = 0.65) and ECEC (r = 0.41) but none with exchangeable Al (r = -0.04), clay (r = 0.26), sand (r = -0.24) and silt (r = -0.26) (Table 4).

Table 1: Physicochemical Properties of Soils of the various Lithologies

Location/PM

Depth

Sand

Silt

Clay

OM

N

Avail P

pH (H20)

Ca

Mg

K

Na

ECEC

 

Cm

g kg-1

 

mg kg-1

 

cmol kg-1

Bende (Shale)

0-8

809.60

20.00

170.40

12.40

0.10

4.55

5.91

3.41

0.40

0.45

0.23

7.49

8-15

553.60

36.00

410.40

9.00

0.20

4.13

5.06

2.17

0.20

0.15

0.13

4.75

15-27

845.60

44.00

110.40

3.80

0.20

3.36

4.97

1.43

0.10

0.15

0.10

2.38

Amuruo(Shale)

0-7

869.60

20.00

110.40

21.70

0.10

4.90

6.48

3.35

0.10

0.37

0.20

7.00

7-14

611.60

18.00

370.40

19.70

0.20

3.50

6.26

2.41

0.80

0.33

0.60

7.26

14-28

689.60

20.00

290.40

17.20

0.30

2.73

5.98

2.43

0.80

0.37

0.08

5.28

Okigwe (Sandstone)

0-8

850.40

24.00

125.60

16.60

0.30

6.30

6.05

0.25

0.14

0.26

0.03

6.68

8-16

809.60

40.00

150.40

10.30

0.20

5.60

6.02

0.25

0.24

0.22

0.01

12.72

16-30

840.60

29.00

130.40

3.40

0.20

5.67

5.39

0.27

0.34

0.28

0.01

21.90

Ihiagwa (Coastal plain Sands)

0-15

893.60

30.00

70.40

15.90

0.20

4.69

5.90

0.51

0.10

0.24

0.04

4.49

15-30

865.60

24.00

110.40

18.30

0.20

5.95

5.86

0.33

0.26

0.26

0.03

8.35

30-45

879.60

10.00

110.40

11.40

0.20

3.43

5.67

0.53

0.12

0.26

0.04

6.95

Oguta (Alluvium)

0-15

919.60

50.00

30.40

8.30

0.30

6.86

5.82

0.59

0.40

0.35

0.01

2.94

15-30

865.60

84.00

50.40

7.20

0.20

4.34

5.73

0.49

0.42

0.30

0.03

2.14

 

30-45

855.20

100.00

44.80

6.60

0.20

3.50

5.43

0.67

0.20

0.33

0.03

3.63

Table 2: Absolute and Relative P Fractions in Top soils of Varying Lithologies

A. Absolute P Fractions (mg kg-1)

Location/PM

H20

0.5 M NaHC03

0.5 M NaOH

0.1 M HCl

H2S04

Total

Amuruo (Shale)

0.15

0.20

0.27

0.09

0.14

0.85

Bende (Shale)

0.11

0.21

0.13

0.06

0.11

0.62

Ihiagwa (Coastal plain Sands)

0.13

0.14

0.53

0.06

0.13

0.99

Oguta (Alluvium)

0.14

0.11

0.37

0.07

0.15

0.84

Okigwe (Sandstone)

0.02

0.11

0.52

0.09

0.20

0.94

LSD 0.05

0.02

0.09

0.02

0.02

0.08

0.02

% CV

9.50

29.80

3.00

12.10

29.70

1.30

B. Relative P Fractions (%)

Amuruo(Shale)

17.65

23.53

31.76

10.59

16.47

Bende (Shale)

17.74

33.87

20.97

9.68

17.74

Ihiagwa (Coastal plain Sands)

13.13

14.14

53.53

6.06

13.13

Oguta (Alluvium)

16.67

13.10

44.05

8.33

17.86

Okigwe (Sandstone)

2.13

11.70

55.32

9.57

21.28

 

   PM = Parent material

Table 3: Absolute and Relative P Fractions in Subsoil of Varying Lithologies

A. Absolute P Fractions (mg kg-1)

Location/PM

H20

0.5 M NaHC03

0.5 M NaOH

0.1 M HCl

H2S04

Total

Amuruo (Shale)

0.01

0.14

0.15

0.08

0.13

0.51

Bende (Shale)

0.16

0.15

0.38

0.08

0.12

0.89

Ihiagwa (Coastal plain Sands)

0.18

0.13

0.60

0.08

0.25

1.24

Oguta (Alluvium)

0.09

0.15

0.34

0.08

0.13

0.79

Okigwe (Sandstone)

0.14

0.13

0.32

0.08

0.21

0.88

LSD 0.05

0.02

0.00

0.09

0.00

0.02

0.13

% CV

7.70

0.00

13.70

0.00

12.80

1.30

B. Relative P Fractions (%)

Amuruo (Shale)

1.96

27.48

29.40

15.67

25.52

Bende (Shale)

17.97

16.85

42.70

6.06

13.47

Ihiagwa (Coastal plain Sands)

14.51

10.49

48.30

6.45

20.17

Oguta (Alluvium)

11.40

18.98

43.00

10.12

16.45

Okigwe (Sandstone)

15.92

14.76

36.40

9.10

23.86

 

PM = Parent material

Table 4: Simple Correlation between P fractions and selected Soil Properties

Soil Properties

P Fractions

 

H20

0.5 M NaHC03

0.5M NaOH

0.1 M HCl

H2S04

Clay

-0.33

0.26

-0.23

-0.57

-0.54

ECEC

-0.50

-0.41

0.44

0.51

0.20

Exchangeable Al

0.17

-0.04

0.28

-0.39

-0.29

Exchangeable Ca

0.22

0.65

-0.32

-0.06

-0.49

Exchangeable Mg

-0.39

0.41

-0.78

-0.34

-0.46

pH

-0.30

0.45

-0.82

0.34

-0.51

Sand

0.28

-0.24

0.20

0.61

0.50

Silt

0.61

-0.26

0.34

-0.25

0.54

OM

-0.38

0.52

-0.77

0.11

-0.72

Ranges of NaOH-P associated with Fe/Al oxy-hydroxide were 0.13-0.53 (mean = 0.36 mg kg-1) equivalent to 20.97-53.53 (41.13%) and 0.15-0.60 (mean = 0.36 mg kg-1) equivalent to 29.40-48.30 (39.96%) total P in the top (Table 2) and subsoils (Table 3) of the various lithologies respectively. Concentrations for the topsoil were very low compared to values obtained for calcareous soils of Jordan (Rawajfih et al., 2010).  Variations amongst various lithologies were low, with CV’s equivalent to 3 and 13.70% in the top and subsoils respectively, but with the later higher than the former. In both top (0.53 mg kg-1) and sub soils (0.60 mg kg-1), concentrations were significantly (LSD 0.05) higher in Coastal Plain Sands (Ihiagwa) than the other lithologies probably due to its high acidity. It has been reported that Al and Fe-P extractble with NaOH increased under high acidity due to high P precipitation and retention by Al and Fe (Amaresh, 2010). Soil properties especially ECEC (r = 0.44), exchangeable Mg (r = -0.78), pH             (r = -0.82) and OM (r = -0.77) correlated significantly (P < 0.05) with NaOH-P while its relationship with clay contents (r =-0.23), silt                (r = -0.34), sand (r = 0.20), exchangeable Al            (r = 0.28) and exchangeable Ca (r = 0.34) was not significant. It has been reported that in sub-tropical soils of Iran, NaOH-P was not significantly correlated with OM, clay, sand and silt but with pH (Adhami et al., 2013) and thus corroborated the observations for the soils studied.  

Concentrations of HCl-P ranged between 0.06-0.09 (mean = 0.07mg kg-1) equivalent to 6.06-10.59 (mean = 8.85%) in the topsoil and extremely low when compared with ranges of 522-1089 mg kg-1 for calcareous Vertisols and Aridisols of Jordan (Rawajfih et al., 2010). Amongst lithologies, variation was very low in the topsoil with a CV equivalent to 12.10%. Also, in the topsoil, shale (Amuruo) and sandstone (Okigwe) were better than the other parent materials. In the subsoil, concentrations were similar amongst parent materials and with mean values equivalent to 0.08 mg kg-1 indicating that Ca-P extractble by NaOH was similar in the subsoil of these acidic soils. High HCl-P (Ca-P) has been reported in high alkaline calcareous soils (Amaresh 2010; Rawajfih et al., 2010). There was significant correlation (P < 0.05) between HCl-P and Clay (r = -0.57), ECEC (r = 0.51), exchangeable Al (r = -0.39) and sand (r = 0.61) but none with exchangeable Ca (r = -0.06), Mg            (r = -0.34), pH   (r =-0.34), silt (r = -0.25) and OM (r = 0.11). It has been noted that in sub-tropical soils of Iran, HCl-P correlated significantly with pH and CCE (Adhami et al. 2013) contrary to observation obtained for the soils studied.  

Residual or H2S04-P ranged between 0.11-0.20 (mean = 0.15 mg kg-1) equivalent to 13.13-21.28 (mean = 17.30%) total P in the topsoil (Table 2) and 0.12-0.25 (mean = 0.17 mg kg-1) equivalent to 13.47-25.52 (mean = 19.89%) total P in the subsoil (Table 3). Mean subsoil (0.17 mg kg-1) concentration was better than topsoil (0.15 mg kg-1) probably due to high leaching losses. Variability amongst lithologies was low, with CV’s equivalent to 29.70 and 12.80% in the top and subsoils respectively and with the former better than the later. Concentrations were significantly (LSD 0.05) better in sandstone (0.20 mg kg-1) and Coastal Plain Sands (0.25 mg kg-1) than other lithologies in the top and subsoils respectively. Concentrations were seriously correlated with soil OM (r = -0.72), sand (r = 0.50), silt (r = 0.54), pH (r = -0.51), exchangeable Mg (r = -0.46), exchangeable Ca (r = -0. 49) and clay (r = -0.54) but not with ECEC (r = 0.20) and exchangeable Al (r = -0.29).

Total P obtained as the summation of all P fractions ranged between 0.62-0.99 (0.85mg kg-1) and 0.51-1.24 (mean = 0.86 mg kg-1) in the top and subsoils respectively, with variability amongst lithologies low (CV = 1.30%) at both depths. Coastal Plain Sands was significantly better in the top (0.99mg kg-1) and subsoils (0.86 mg kg-1) while shale was the least in both top (0.62 mg kg-1) and subsoils (0.51 mg kg-1). High content of Coastal Plain Sands could be attributed to a high inorganic fertilizer application or accumulation of organic materials. Mean values obtained at both soil depths were low relative to a range of 181.1-439.15 mg kg-1 (mean = 310.13 mg kg-1) for deep tropical soils of varying parent materials and altitudes, ascribable to high degree of weathering and leaching Ameresh, 2010).

In general, mean absolute P fractions averaged over lithologies decreased in the order total (0.85) > NaOH (0.36) > NaHCO3 = H2SO4 (0.15) > H2O (0.11) > HCl (0.07) in the topsoil and NaOH (0.36) > H2SO4 (0.17) > NaHCO3 (0.14) > H2O (0.12) > HCl (0.08) in the subsoil. This showed that NaOH-P was best while HCl-P the least at both soil depths of the various lithologies. Similar observation has been reported for sub-tropical Iranian soils (Adhami et al., 2013) and some Southwestern Ethiopian soils (Melese et al., 2015) ascribable to the dominance of Al /Fe-oxides, old age,  high acidity and advanced weathering stage (Ameresh, 2010; Melese et al., 2015). Kolahchi and Jalali (2012) obtained high HCl-P or Ca-P in calcareous soils, high in Ca, Mg and CaCO3. Concentration of total P was higher in the top than subsoils as have been reported for some Southwestern Paraná State, Brazil soils under varying tillage practices (Tales et al.,2012) and Southwestern basin of Dian lake, China soils (Zhang and Li, 2016).

V.  CONCLUSIONS

Phosphorus fractions in soils of the various lithologies varied and decreased in the order NaOH > NaHCO3 = H2SO4 > H2O > HCl and NaOH > H2SO4 > NaHCO3 > H2O > HCl in both top and subsoils respectively. The NaOH-P was higher averaged over lithologies while P concentrations in Coastal Plain Sands were better averaged over P fractions at both soil depths.  Soil properties especially sand, silt, clay, OM, exchangeable Al, Ca, Mg, ECEC and pH affected concentrations of the various P fractions.  

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