ABSTRACT
The objective of the study was to investigate the influence of some soil properties on dispersion and hydraulic conductivity of soils. Twenty soil samples collected from a depth of
0-20 cm were analyzed for their physical and chemical properties. The total clay  fraction (clay) of the particle size distribution ranged from 80 to 380 g/kg with a mean of 203 g/kg and a coefficient of variation (CV %) of 47.5%.  Total silt was between 20 and 400 g/kg with a mean of 129g/kg and a CV of 86.9%. Water-dispersible clay (WDC) varied from 60 to 160 g/kg, with an average WDC value of 95.5g/kg and a CV of  30.1%. The values of water- dispersible silt (WDSi) ranged between 10 and 380 g/kg with a mean value of 101g/kg and a coefficient of variation of 109.7%. The electrical conductivity of the soils ranged from 16 to 22 μS/cm with a mean of 17.95 μS/cm and a coefficient of variation (CV %) of 9.30%. The exchangeable sodium percentage (ESP) of the soils varied from 0.43 to 2.76% with a mean of 1.1% and CV of 46.36%. The soil organic carbon content of the soils ranged from 0.8g/kg to 27.2g/kg. Total nitrogen content of the soils ranged  from 1.0 to 3.4g/kg with a mean  of 2.4g/kg and a coefficient of variation (CV %) of 20.83%. The clay dispersion ratio (CDR) of the soils varied between 0.16 and 0.92 with an average CDR of 0.58 and 46.6% coefficient of variation (CV). The total clay content (clay) had highly significant negative correlations with dispersion ratio (DR), clay dispersion ratio (CDR) and clay dispersion index (CDI) ‘r’ = – 0.84**,  –  0.91**  and  –  0.91**  respectively,  but  positively,  it  had  highly  significant correlations  with clay flocculation  index (CFI)  and aggregated  silt and  clay  (ASC) ‘r’ =
0.91** and 0.96** respectively. The total clay content correlated negatively and significantly with exchangeable sodium percentage (ESP) ‘r’ = – 0.49*. Exchangeable sodium percentage had significant and positive correlations with exchangeable sodium (Na+), electrical conductivity (EC) and bulk density (BD) (r = 0.52*, 0.48* and 0.46* respectively). Soil
organic carbon (SOC) correlated positively and significantly with hydraulic conductivity (Ksat) ‘r’ = 0.54*. Dispersion ratio (DR) positively and highly significantly correlated with CDR and CDI (r = 0.86** and 0.87**) respectively but negatively and highly significantly, it correlated with CFI and ASC (r = -0.87** and -0.93**). Clay dispersion ratio (CDR) had a positive and highly significant correlation with CDI (r = 0.99**). Water- dispersible clay (WDC) had a negative and significant correlation with BD (r = -0.53*). Water-dispersible silt (WDSi) also had a negative and significant correlation with BD (r = -0.54*). Dispersion ratio (DR) had positive and significant correlations with pH both in water and in KCl (r = 0.46* and 0.56*) respectively. The clay contents had positive and highly significant correlations with the levels of dispersion in all the soils. As the amount of 0.1N NaOH used for dispersion increased, the amount of dispersed clay increased while hydraulic conductivity of the soils decreased correspondingly.
CHAPTER ONE INTRODUCTION
Clay dispersion leads to soil erosion which is a major environmental problem in several parts
of southeastern Nigeria. Soil erosion has been directly linked to the rate and volume of water- dispersible clay in a soil. Potential soil erosion in areas of high rainfall has been estimated using water-dispersible clay and its indices (Amezketa et al., 1996; Igwe 2001; 2003; 2005; Igwe and Agbatah 2008; Calero et al., 2008). Soil dispersion hardens soil and blocks water infiltration, making it difficult for plants to establish and grow. The major implications associated with decreased infiltration due to sodium-induced dispersion include; reduced plant available water and increased runoff and soil erosion (Warrence et al., 2003). Dispersion causes a reduction in macro porosity and therefore, lowers infiltration rates and hydraulic conductivities as well as an increase in soil strength and other undesirable soil physical properties.
Exchangeable sodium percentage (ESP) is the amount of sodium adsorbed to soil particles and it is a measure of soil sodicity. Sodic soils contain a large amount of exchangeable sodium and low levels of soluble salts. Gopali et al., 2007 stated that sodic soils are associated with structural changes that principally affect permeability of soils and that ESP is the major influence in the dispersibility of soils. It is generally recognized that high levels of exchangeable sodium (Na+) lead to soil structural deterioration, which is accompanied by a reduction in water movement through the soil profile. According to Hiebert et al., (2010),
elevated levels of sodium in soil can result in deleterious sodic effects in soil. These effects include swelling and dispersion of clay, which can lead to decreased infiltration capacity and lower hydraulic conductivity, producing poorly drained soils. Irrigation with sodic waters may cause soil structure to deteriorate which can result in a loss of soil productivity because,
permeability to water and air is reduced, and the soil offers greater resistance to root penetration (Curtin et al., 1991). The dispersing effect of sodium ion (Na+) according to Franzmeier et al., (1996) leads to reduced hydraulic conductivity and high water dispersible
clay, resulting in slaking, and eventual hardening of the soil. The hydraulic conductivity of a soil which is a measure of the soil’s ability to transmit water when submitted to a hydraulic gradient determines the behavior of the soil fluid within the soil system under specified conditions. More specifically, the hydraulic conductivity determines the ability of the soil fluid to flow through the soil matrix system under a specified hydraulic gradient. It is dependent on soil properties as dispersion, swelling behavior, and exchangeable sodium percentage of the soil (Gopali et al., 2007). The hydraulic conductivity of a given soil depends on size and shape of the particles, void ratio, arrangement of the pores and soil particles, properties of the pore fluid and the amount of undissolved gas in the pore water (Rao and Mathew, 1995).
The influence of sodicity on soil physical properties varies with clay content and clay mineralogy. In soils with higher contents of swelling/shrinking clay minerals, lower exchangeable sodium percentage can cause a more significant physical effect. According to Warrence et al., (2003), particle size distribution and clay content play an important role in all aspects of irrigated agriculture, and the role of soil texture with respect to effects of salinity and sodicity is no exception. Soil texture helps determine how much water will be able to pass through the soil, how much water the soil can store, and the ability of sodium to bind to the soil. In fine grained soils, hydraulic conductivity under saturation conditions is controlled by the microstructure of the soil matrix which in turn depends on the type of the clay mineral present in the soil, the composition of the exchangeable cations and the electrolyte concentration in the pore water system (Rao and Mathew, 1995). The ease with which deflocculation, and swelling of a clay mineral take place affects the structure of the
pores and, hence, the hydraulic conductivity. Dispersion and swelling of clays within the soil matrix are interrelated phenomena, and these will change macropores to micropores which will reduce soil hydraulic conductivity. Swelling reduces pore sizes and dispersion blocks pores. Plugging of soil pores by dispersed clay particles is the major cause of reduced hydraulic conductivity (Rao and Mathew, 1995).
Exchangeable sodium percentage (ESP) and electrolyte concentration (C) of the soil solution play a significant role in determining soil physical properties and the response of soil clays to dispersion and swelling. Hydraulic conductivity decreases with an increase in exchangeable sodium percentage (ESP) and a decrease in the total electrolyte concentration of the soil solution (Gopali et al., 2007). The reduction in the hydraulic conductivity has been attributed mainly to swelling and dispersion of the soil clays (Gopali et al., 2007). Excess exchangeable sodium has an adverse effect on plant growth, soil structure and results in reduction in crop use. Sodicity is a problem because, high sodicity causes clay to swell excessively when wet. The clay particles move so far apart that they separate (disperse) and this weakens the aggregates in the soil, causing structural collapse and closing-off of pores, reducing the internal drainage of the soil. This results in water logging and hydraulic conductivity is consequently affected and this may subsequently cause soil degradation. Continued degradation of agricultural lands is leading to decreased productivity and increased environmental risks (Igwe 2005).
Inherent  soil  properties  influence  the  behaviour  of  soils.  Therefore,  knowledge  of  soil properties is important in determining the use to which a soil may be put (Amusan et al., 2006).  Studies on effects of clay content, exchangeable sodium percentage, and electrolyte concentration on dispersion and hydraulic conductivity of soils are few and are restricted to certain areas. Clay dispersion and hydraulic conductivity of soils are very important subjects
to be studied since they affect soil structure, and studies on them will add to knowledge on environmental degradation, land resources, and land use of soils of southeastern Nigeria. Therefore, in highly erodible soils such as the ones in south eastern Nigeria, there is a need to monitor the clay dispersion characteristics to direct and modify soil conservation strategies (Igwe, 2005).
In view of these, the major objective of this work was to study the effects of clay content, exchangeable sodium percentage and electrolyte concentration on dispersion and hydraulic conductivity of some soils in southeastern Nigeria. The specific objectives were to:
(i) determine the particle size distribution, exchangeable sodium percentage, electrical and hydraulic conductivities of these soils.
(ii) ascertain the effects of clay content on hydraulic conductivity and dispersivity of the soils.
(iii) determine the effects of different levels of dispersant (NaOH) on the soils
(iv) measure the conductivity of water through the soils when treated with different levels of dispersant and
(v) evaluate the relationship between some soil properties and clay dispersibility
This material content is developed to serve as a GUIDE for students to conduct academic research
SOIL DISPERSION AND HYDRAULIC CONDUCTIVITY IN RELATION TO CLAY CONTENT EXCHANGEABLE SODIUM PERCENTAGE AND ELECTROLYTE CONCENTRATION IN SOILS OF SOUTHEASTERN NIGERIA
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