Relationship Between Soil Organic Matter and Some Physical Properties in Selected Soils of the Oil Palm Belt of Nigeria

Organic matter is related to all plant nutrients and is a major contributor to tropical soils cation exchange capacity. Its relationship with soils physical properties is currently undermined. Its deliberate inclusion in soils can be used to reverse soil compaction due to high bulk density with improved soil structure and texture. Its relationship with some physical properties such as bulk density, total porosity, particle size distribution (texture) and moisture content was studied in selected soils of the oil palm belt of Nigeria to determine the influence of organic matter on these properties. Such information will assist in the proper management of the soils physical properties for improved organic matter content of soils under oil palm. Results indicated that there was a significant difference in the sand and silt contents due to the influence of the soils parent materials with higher values of sand (870.70 g/kg) recorded in soils under coastal plain sand while higher values of silt (52.70 g/kg) were recorded in soils under alluvium. Soils under basement complex rocks had the highest clay (116.40 g/kg) contents and bulk density values (1.85 gcm -3 ). Highest moisture (75.66 %) content was recorded in soils under alluvium while the highest total porosity values were recorded in soils under shale mixed with sandstone and clay. The study concludes that sand and silt contents had positively significant correlations with organic matter of the soils. It is recommended that organic materials and residues from the oil palm biomass be ploughed back constantly to these soils for improved


INTRODUCTION
The oil palm belt is the Southern part of Nigeria where it occupies about 2.53 million hectares (1). The soils covering these hectares are developed on different parent materials some of which are coastal plain sands, basement complex/sedimentary rocks, alluvium and shale mixed with sandstones and clay and includes soils such as Ultisols, Inceptisols, Oxisols, Alfisols and Entisols (2). The soils generally have low organic matter content due to high temperature and rainfall distribution (3). And contain low activity clay (LAC) that contributes very little to the cation exchange capacities of the soils. Major contributor to the soils CEC is the organic matter content of the soils. Soil organic matter is the highest contributor to the cation exchange capacity of tropical soils (4). Organic matter imparts on many soil properties such as colour and nutrient holding capacity of soils. It also influences water relations through its influence on soil texture and porosity (5). It therefore follows that the physical properties of a soil are as important as its chemical properties. Soils physical properties to a large extent determine how chemical constituents are released for plants' utilization. It has been shown that organic matter influences all soil properties (6). It regulates soil bulk density and helps to improve the aggregate stability of soils (4). It also enhances water infiltration and nutrient movement through soil. It follows that proper management of the nutrient content of soils must take into cognizance the relationship between organic matter content and soil physical properties such as bulk density, texture and moisture content of the soils. In view of this, a study was undertaken to determine the relationship between soil organic matter and some soils physical properties. Its objectives were to determine the: 1.
relationship between soil organic matter content and physical properties such as bulk density, total porosity and moisture content of soils of oil palm under different parent materials 2.
influence of the soils parent materials on physical properties of the soils

MATERIALS AMD METHODS Description of the various study locations:
The study was undertaken in three states of South Southern Nigeria known for oil palm cultivation. 2 the oil palm belt of Nigeria.

Laboratory analysis:
The core samples were carefully placed in the oven after weighing and oven dried at 105 0 C for 24 hours while the profile samples were air dried at room temperature, crushed and passed through a 2 mm sieve and analyzed for soil pH in 1:1 soil to water suspension using a pH meter (7). Soil organic carbon was by the Walkley and Black method (8). Particle size distribution was by the hydrometer method of Bouyoucos (9) while bulk density was by the core method of (10). Total porosity was determined from the relation, 1-Bd/Pd X 100 (10) while soil moisture was determined using the gravimetric method (5).

RESULTS
Sand and silt had the same trend of decreasing with increased soil depths in soils under Alluvium, Coastal Plain Sands and Shale Mixed with Sandstone and Clay. Clay increased with increased soil depths in soils under Alluvium and Coastal Plain Sands parent materials but decreased with increased soil depths in soils under basement complex rocks and shale mixed with sandstone and clay (Table 1). Soil pH and organic matter content of the soils decreased with increased soil depths in all the locations (Table 1)

Influence of parent materials on physical properties of the soils
Sand, silt, bulk density, moisture content and total porosity of the soils were significantly (P < 0.05) different due to the influence of the different parent materials (Table 2). Soils developed on Coastal Plain Sand Parent materials and Alluvium had the highest sand content while soils under basement complex rocks had the highest silt, clay content and bulk density values (Table 2). Soils under alluvium had the highest moisture content of 75.66% while soils under shale mixed with sandstone and clay had the highest total porosity of 49.21 % (Table 2) Relationships between soil organic matter, pH and some physical properties of the soils Soil organic matter was positively and significantly (r = 0.928, P < 0.01); (r = 0.818); (r = 1.000, P < 0.01); and r = 1.000, P < 0.01) correlated with sand, silt, total porosity and moisture content respectively but was negatively and significantly( r = -0.919, P < 0.05) and r = -1.000, P < 0.01) correlated with clay and bulk density respectively in soils under alluvium (Agbarho) (Table 3). Similarly, soil organic matter was positively and significantly (r = 0.986, P < 0.01); (r = 0.916, P < 0.01) and ( r = 1.000, P < 0.01) correlated with sand, silt and Journal of Natural Sciences Research www.iiste.org ISSN 2224-3186 (Paper) ISSN 2225-0921 (Online) Vol. 12, No.7, 2021 total porosity respectively but was negatively and significantly (r = -0.992, P < 0.01) r = -1.000, P < 0.01 and r = -1.000, P < 0.01) correlated with clay, moisture content and bulk density respectively in soils under Coastal plain sands (NIFOR) (Table 4). Similarly, there were positively significant correlations between soil pH and total porosity; soil pH and moisture content and significantly negative correlations between soil pH and bulk density; soil pH and clay; clay and organic matter; clay and sand; clay and silt in soils under alluvium, coastal plain sand, basement complex rocks and shale mixed with sandstone and clay (Tables 3 -6). There was a negatively significant correlation between bulk density and moisture content in soils under alluvium (Table 3) but a positively significant correlation between bulk density and moisture in soils under coastal plain sand, basement complex and shale mixed with sandstone and clay (Tables 4 -6).

DISCUSSIONS:
The result of the study seemed to suggest that much of the silt content had weathered into clay. This could be buttressed by the lower amount of silt that progressively decreased with increased soil depths in contrast with clay which progressively increased with increased soil depths irrespective of parent materials. The bulk density values of soils of the locations were high at the sub soils. The high bulk density values recorded at the sub soils of these locations could be attributed to the very low organic matter contents in the sub soils which were below the critical level. The organic matter contents of the sub soils were below 20 -30 g/kg reported for soils of Southern Nigeria (6). According to (11), organic matter had the tendency of reducing bulk density in soils. Increases in total porosity are often correlated with decreased bulk density (4). This indicated that total porosity and bulk density have an inverse relationship in soils. That inverse relationship was observed in this study such that total porosity tended to decrease with increased soil depths in contrast with bulk density which increased with increased soil depths in all the locations irrespective of parent materials. Total porosity values were higher at the top soils when compared with the sub soil values. This indicated that there were more pore spaces for nutrient and water movement at the top soils when compared to the sub soils irrespective of parent materials. Soil pH was in the acidic ranges as required by oil palm. With respect to the Oil palm, soil pH range of between 5.0 -5.5 is considered highly suitable (12). (13) studied the biophysical properties of selected soils of Delta state and puts the suitability class of Oil palm in Abbi and Sapele soils with moderate acidity (Soil pH of 5.82 -6.21) as S3 (marginally suitable) whereas Agbor, Asaba, Bomadi and Ughelli with slightly acidic soil pH (5.1 -5.5) were classified as S1 (highly suitable) for the cultivation of Oil Palm. With respect to soils of Oil palm, soil pH has been reported to decrease with increased age of the palms (14). The positively significant correlations between soil pH and sand; soil pH and silt; organic matter and sand; organic matter and silt irrespective of parent materials indicates the influence soil pH and organic matter exert on the particle size distribution of soils, especially fractions between 0.2 -0.02 mm (sand and silt). The positively significant correlations between moisture and bulk density in soils formed under coastal plain sand, basement complex rocks and shale mixed with sandstone and clay underscores the influence of water on bulk density of soils. This could be buttressed by the negatively significant correlation obtained between bulk density and moisture content of soils under alluvium in Agbarho. As expected, there was a negatively significant correlation between total porosity and bulk density irrespective of the soils parent materials.

CONCLUSIONS
The following conclusions can be drawn for soils of the oil palm belt of Nigeria-1.
Effects of sand and silt contents on organic matter was found to be higher than clay contents of the soils 2.