Permanent wilting points in soils have been found to correlate significantly with particle size fractions. This study was conducted to establish functional relationship between soil particle size fractions and permanent wilting point of soils of coastal plain sands in southeastern Nigeria. A total of 102 surface samples were collected from three different dominantly Ultisols toposequences (i.e., 34 samples from each). Permanent wilting point experiment was carried out in pots with the 102 samples in the greenhouse while the particle size analysis was carried out in the laboratory. There was significant correlation among the textural separates, permanent wilting point correlated significantly with clay (
A fundamental concept in modeling is to simplify reality into ideas called models which contain only those elements that are meaningful or that could lead to a particular desired objective. In the most simplified form but typically models are simplified representations of a system, such as a soil system. Soil water retention is defined by capillarity, which is the result of adhesion and cohesion. In addition, capillarity depends on the structure of the soil pores. Since water retention of soils is affected by physical properties, such as structure and texture, it is possible to develop empirical relationships to predict soil water retention. Developments in computer modeling accelerate these studies [
Particle sizes are the major components of a mineral soil. Particle sizes, their derivates, and solute transport in soils have been found to be correlated and dependent [
Soils of southeastern Nigeria formed on unconsolidated coastal plain sands are characterized by the dominance of sandy textured fragments comprising larger quantities of coarse over fine-textured materials, have low fertility due to dominance of low activity clays and inherent low organic matter contents [
In the past, attempts were made to correlate basic soil properties such as size fractions (sand, silt, and clay) and organic carbon with water content held at certain hydraulic potentials (usually at 1/3 atm and 15 atm). This was made in order to estimate water content at field capacity and permanent wilting point and the availability of water to plants [
The objective of this study therefore was to determine the functional relationship between soil particle sizes and permanent wilting point of soils of coastal plain sands in southeastern Nigeria. For this aim, the use of particle size distribution and pot indicator (maize) plants, which are measured easily and inexpensively in most areas, were employed.
The study was carried out in three different toposequences of soils namely, soils from Holy family college (HFC), Technical college (TCHC), and Science college (SCC) all located in Abak Local Government Area in Akwa Ibom State in southeastern Nigeria. Soils of the three locations were derived from coastal plain sands and have been altered by cultivation. In Holy family college,
In the field, surface samples were collected on each of the toposequence/landform (about 100 m × 60 m) plots at 5 m^{2} interval (i.e., grid nodes) with the aid of Dutch auger. In each of the sampling areas (the schools), nine (9) replicate samples were collected perpendicular to the direction of the upper slope, sixteen (16) were also collected from the middle slope, while nine (9) were collected again from the bottom slope. A total of 102 samples [(9 × 3) + (16 × 3) + (9 × 3)] were collected from the three toposequences. Particle sizes may not have varied very differently from the three toposequences, but samples were collected as described to increase replication so as to enlarge the degree of freedom in estimating the error variance in conducting this experiment. Samples were processed, and particle size analysis was carried out [
Data collected were summarized using descriptive statistics, and normality of distribution was tested with skewness and kurtosis. The general linear model univariate procedure was used to model the values of the parameters based on their relationship to categorical and scale predictors. Correlation and multiple regression analysis were carried out to find out the way variables relate with each other. All statistical analysis was carried out with the aid of SAS [
The mean and median were used as primary estimates of central tendency, while standard deviations, skewness, kurtosis, minimum, and maximum were used as estimates of variability (Table
Descriptive Statistics of some physical properties of soils of the study sites.
Variable | SD | SE | Skewness | Kurtosis | Mean | Median | Min. | Max. |
---|---|---|---|---|---|---|---|---|
PWP (%) | 4.33 | 0.43 | 0.02 | −0.39 | 15.04 | 14.80 | 4.44 | 27.40 |
Sand (g kg^{−1}) | 19.49 | 1.93 | −0.73 | −0.14 | 889.85 | 895.65 | 843.20 | 926.20 |
Coarse sand (g kg^{−1}) | 47.81 | 4.73 | −0.30 | −1.47 | 760.78 | 782.45 | 668.00 | 825.40 |
Fine sand (g kg^{−1}) | 36.57 | 3.62 | 0.62 | −0.56 | 128.83 | 119.50 | 79.40 | 239.20 |
Silt (g kg^{−1}) | 13.62 | 1.35 | 1.03 | 1.31 | 42.64 | 40.55 | 20.80 | 89.60 |
Clay (g kg^{−1}) | 12.92 | 1.28 | 0.32 | −0.36 | 67.52 | 64.80 | 40.20 | 99.40 |
SCR | 0.28 | 0.03 | 2.70 | 10.45 | 0.66 | 0.62 | 0.26 | 2.08 |
Clay ratio | 3.04 | 0.30 | 0.75 | 1.03 | 14.37 | 14.44 | 9.05 | 23.88 |
PWP: permanent wilting point, SCR: silt clay ratio, SD: standard deviation, and SE: standard error of mean.
Table
General linear model of some parameters in the topographic positions of the study sites.
Upper | Middle | Lower | |
---|---|---|---|
PWP (%) | 14.40^{b} | 15.85^{a} | 16.44^{a} |
Sand (g kg^{−1}) | 881.34^{b} | 889.28^{b} | 874.00^{a} |
Fine sand (g kg^{−1}) | 124.91^{c} | 105.89^{b} | 161.52^{a} |
Coarse sand (g kg^{−1}) | 765.75^{c} | 793.27^{b} | 712.57^{a} |
Silt (g kg^{−1}) | 47.83^{a} | 38.44^{b} | 45.94^{a} |
Clay (g kg^{−1}) | 60.83^{b} | 62.31^{b} | 80.04^{a} |
Silt clay ratio | 0.84^{b} | 0.62^{a} | 0.58^{a} |
PWP: permanent wilting point.
Means followed by different letters within the same row indicate significant difference at
Table
Pearson correlation coefficients for soil properties of the study sites.
PWP | Sand | Fine sand | Coarse sand | Silt | Clay | SCR | |
---|---|---|---|---|---|---|---|
PWP (%) | |||||||
Sand (g kg^{−1}) | −0.14 | ||||||
Fine sand (g kg^{−1}) | −0.07 | −0.39^{**} | |||||
Coarse sand (g kg^{−1}) | 0.003 | 0.71^{**} | −0.93^{**} | ||||
Silt (g kg^{−1}) | 0.004 | −0.75^{**} | 0.25^{*} | −0.51^{**} | |||
Clay (g kg^{−1}) | 0.21^{*} | −0.72^{**} | 0.33^{**} | −0.54^{**} | 0.08 | ||
SCR | −0.17 | −0.26^{**} | 0.12 | −0.20^{*} | 0.81^{**} | −0.47^{**} | |
Clay ratio | −0.26^{**} | 0.62^{**} | −0.22^{*} | 0.42^{**} | 0.03 | −0.97^{**} | 0.58^{**} |
PWP: permanent wilting point, SCR: silt clay ratio, ^{*}Significant at 5%, ^{**}Significant at 1%.
Multiple regression analysis was carried out in order to establish contributions of predictors (clay and coarse sand) to permanent wilting points. This was because clay contents of soils play a key role and as such a major predictor for modeling permanent wilting point of soils [
PWP = −2.99 + 0.10 (clay) + 0.02 (coarse sand), (
The regression line gave a fair fit to the observed data since it explains 54% of the total variation in permanent wilting point around the mean of clay and coarse sand contents. This therefore goes a long way to say that prediction of permanent wilting point of Ultisols formed on coastal plain sands soils of humid tropical southeastern Nigeria will effectively depend on reliability of determination of clay and coarse sand contents of the soils.
The soil properties were neither skewed nor kurtous in the entire sites and thus outliers did not dominate the central tendency. There was correlation between the permanent wilting point and the soil particle sizes and between the particle sizes themselves. Clay and permanent wilting point correlated significantly positively while coarse sand and clay correlated highly significantly but negatively. The permanent wilting point of soils in the upper slope positions differed significantly with those of the lower positions so as did the clay and coarse sand contents differ accordingly. Higher permanent wilting points were discovered at the lower slope positions. The regression line explains 54% of the total variation in permanent wilting point around the mean of clay and coarse sand contents. Prediction of permanent wilting point of Ultisols formed on coastal plain sands soils of humid tropical southeastern Nigeria will effectively depend on reliability of determination of clay and coarse sand contents of the soils.