Abstract: Studies were conducted to investigate the effect of organic matter arid salt concentration on cation exchange equilibria In non-calcareous soils. Various physico-chemical properties like pHs, ECe, soluble ions, exchangeable cations, ESP, SAFI, lime and organic matter were determined. In homovalent (K+-Na+) exchange, the values of Kk increased with increasing organic matter. The results of three levels of salt concentration indicated that equal concentration of both the salts showed less coefficient values es compared to 1:2 K+-Na+ concentration, but this coefficient values markedly increased by 2:1 K+-Na+ concentration. In heterovalent (K+-Ca2+) exchange, selectivity coefficients showed an increase with increasing organic matter, while in case of salt concentration, 2:1 ratio had marked effect on Kk, Ky and KG values.
Introduction
Cation exchange in soli is the second most important phenomenon in nature, it is important in sustaining soil fertility, in causing and correcting soil acidity and alkalinity, in altering soil physical properties and as a mechanism in purifying or altering percolating water. Calcium and sodium are the major competing cations in normal and salt affected soils, Various equations have been proposed to study the non-exchange equilibria in soils. Kerr (1928) used concentration in piece of their activities and was followed for homovalent cation exchange only. Vanselow (1932) used mole fraction in place of activities of cations and this equation holds good for both homovalent and heterovalnt cation exchange. Gapon (1933) used concentration for adsorbed cations and activities for the soluble cations. The current study was planned to investigate the effect of organic matter and salt concentration on monovalent-monovalent (K+-Na+) and monovalent-divalent (K+ -Ca2+) exchange in non-calcareous soils of Pakistan.
Materials and Methods
Three non-calcareous soils samples varying in clay and carbon contents belonging to different soil series (Soil A-Gujrawala soil) series, soil B-Wazirabad soil series and soil C-Pindorian soil series) were collected. The pH of soil saturation electrical conductivity (ECe), soluble ions, exchangeable cations, sodium adsorption ratio (SAR), exchangeable sodium percentage (ESP), basic cation saturation percentage (BCSP) and cation exchange capacity (CEC) were determined by standard methods (Richards, 1954). 250 gm of each soil were saturated with IN NH4CI and allowed to stand overnight. Five gram of soil was taken for three replications of each treatment. The soil win washed with 95% ethanol and made homoionic with K using 1N KCI solution and excess KCI solution were washed with 95% ethanol. The equilibrium suspensions were prepared by shaking K+ saturated soil with a desired salt concentration 1:1, 1:2 and 2:1 (KCI-NaCl) for homovelent and 1:1, 1:2 and :1 (KCI-CaCI2) for heterovalent for half an hour respectively. It was taken allowed to stand overnight and centrifuged. Thia equilibrate extract was analyzed for Na, K and Ca by standard methods.
Exchangeable bases were determined using 1N CH3COONH4 (pH 7.0) as extractants by centrifuging the suspensions and analyzing the supernatant.
To study the effect of organic matter, soils were mixed with well decomposed organic matter to get three levels of organic matter i.e. original, 2 arid 4%. The remaining process was carried out in the same as to estimate the effect of suit concentration. The mineralogical analysis of soil samples was also done. The data obtained was analyzed by ANOVA technique in completely randomized design with two factors (Steel and Torrie, 1992).
Results and Discussion
The physico-chemical analysis (Table 1) of the soils revealed that CEC of soil A is 11.13 cmolc kg–1, soil B is 5.91 cmolc kg–1 and that of soil C is 6.17 cmolc kg–1 with O.46, 0.37 and 0.38% organic matter respectively.
Homovalent Exchange: The K+-Na+ exchange were studied on three non-calcareous soil at three different levels of organic matter i.e. original, 2 and 4%. The selectivity coefficient Kk values ranged from 0.200 to 0.809 (Table 2). In all the three soils, the values of Kk increased with increasing organic matter. There is a little change in Kk values for soils B and C, but for soil A Kk values marked increased with increasing organic matter. This difference of Kk values in all the three soils could be due to difference in CEC of the soils. CEC of soil A Is higher than soil B and C. As organic matter increases, the surface charge density increases, which ultimately increases the CEC of the soils, increase in CEC increased the adsorption of K+ (Murtaza, 1997). The increase in the values of Kk indicates that Na+ was replaced by K+ as greater as the quantity of organic matter increased, showing preferential adsorption of K+ over Na+. It was also noted that soils high in organic carbon, such as soil A, has higher preference for K+ than the soils low in organic carbon such as soils B and C (Mehta et al., 1983; Poonia et al., 1986) Table 2 indicates that equal concentration of both the salts showed less coefficient values as compare to 1:2 K+-Na+ concentration, but this coefficient values markedly increase by 2:1 K+-Na+ concentration. This increase in Kk values for 2:1 K+-Na+ concentration in all the three soils indicates that by doubling the concentration of KCI solution than NaCI there was greater the number of exchangeable K+, therefore sodium was much replaced by potassium. The most important factor determining the relative extent of adsorption or desorption of given ionis its valance. The above results are in accord with Neilson et al. (1972) whom studied the diffuse double layer and stated that within a given valence series, the degree of replaceability of an Ion decreases as its dehydrated radius increases. As we already know that the hydrated size of K+ is small as compared to Na+. The selectivity of K+ over Na+ is in accord with the concept that in homovalent systems, the preferentially adsorbed ion is usually the ion with the smaller hydrated radius (Helfferich, 1962).
| Table 1: | Physics-chemical characteristics of soils |
| Table 2: | Organic matter and salt concentration effect on K+-Na+ exchange in soils |
| Table 3: | Organic matter and salt concentration affect on K+-Ca2+ exchange in soils |
| Table 4: | Mineralogical analysis of soils |
Heterovalent Exchange: Organic matter has favourable effect on K+-Ca2+ exchange. The results of present study revealed that the values of Kk, Ky and KG increase as organic matter increases (Table 3). The increase In CaX2/KX ratio with increasing organic matter indicates the preferential adsorption of Ca2+ over K+. Organic matter in soils has been known to result in a greater preference for Ca2+ than do the day minerals. All The soils and bowed greater preference for Ca2+ over K+ in small quantity at 2% level and more at 4% level.
The suleutivity for the divalent Cations Ca? over K+ can be explained on Mu basis of organic matter role as shown by Ca2+ ions because of their divalent nature are preferentially adsorbed cations then those of K+ particularly on eesily accessible sites.
In case of salt concentration, in all the three soils, the values obtained for Kk, Kv and KG indicated a very large .eckrutivity for Ca2+ Over K+ (Table 3). By increasing salt concentration the CEC also increases which increases the adsorption of Ca2+ over K+. Thus increase In CEC can be best explained as in accord with Overbeek (19 52) who stated that soil colloids of high charge density, that is of high charge or CEC per unit of surface area, generally have the greatest preference for high charged cations. The preferential adsorption of Ca2+ over K+ is also in accord with Neilson et al. (1972) whom reported that divalent ions in general are retained more strouIQly than are monovalent Ions, trivalent ions are retained even more strongly unreplaced by an equivalent amount of KCI.
Mintwalogical Analysis of Soil Samples: The mineralogical analysis (Table 4) of three soil samples showed that all the soils contained almost the same Minerals and the intensity of quartz was high in all the three soils.