SOIL DISPERSION AND HYDRAULIC CONDUCTIVITY IN RELATION TO CLAY CONTENT EXCHANGEABLE SODIUM PERCENTAGE AND ELECTROLYTE CONCENTRATION IN SOILS OF SOUTHEASTERN NIGERIA

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

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