Abstract: Three calcareous soils were taken to investigate the effect of salt concentration and organic matter on their exchange selectivity coefficients. In homovalent exchange (K+-Na+) all the three soils showed an increase in the values of selectivity coefficient with increasing salt concentration and organic matter. The maximum value of Kk was observed at 4.00% level of organic matter and 2:1 salt concentration. In case of heterovalent exchange (K+-Ca2+) there was also an increasing trend in Kerr (Kk) Vanselow (Kv) and Gapons (KG) selectivity coefficients values with the increase in salt concentration and organic matter. Moreover the results indicated preferential adsorption of K+ over Na+ in case of homovalent exchange and Ca2+ over K+ in case of heterovalent exchange.
Introduction
Knowledge of cation exchange chemistry provides valuable clues about plant nutrient deficiencies, toxicity and about exchangeable sodium. Kerr (1928), Vanselow (1932) and Gapon (1933) used concentration of cations mole fraction and activities respectively for the soluble cations while studying the cation exchange equalibria in soils. The value of Kk increased with an increase in organic matter. The soils show preferential adsorption of K+ over Na+ and Ca2+ over K+ (Zia et al., 1999). The value of exchange selectivity coefficients increased with an increase in pH and organic matter in both the soils (Samin et al., 1999). The present project was planned to determine the suitability of existing cation equilibria in calcareous soils of Pakistan and cation selectivity of different soils.
Materials and Methods
Three calcareous soil samples of different soil texture varying in clay and carbon contents belonging to different soil series, soil A-"Rasulpur soil B-"Kotli and soil C- "Hafazabad" series. Various Physico-chemical properties of the soil samples (1) pHs, (2) electrical conductivity (ECe), (3) soluble ions (4) exchangeable cation, (5) sodium adsorption ratio (SARI, (6) exchangeable sodium percentage (ESP), (7) cation exchange capacity (CEC), (8) lime and (9) organic matter were determined by standard methods (Anonymous, 1954; Moodie et al., 1959). Particle size was determined by Bouyoucos hydrometer method (Bouyoucos, 1962). Then 5 gm of each soil sample was treated with 1N NH4CI solution to remove all the cations present in soils after drying, grinding and sieving the soil samples. After washing with 95% ethanol the soils were made homoionic by saturating with 1N KCI. The excess of KCI was removed by washing the samples with ethanol.
The equilibrium suspensions were prepared by shaking K+ saturated soil with different salt concentrations 1:1, 1:2 and 2:1 for homovalent (KCI: NaCI) and 1:1, 1:2 and 2:1 for heterovalent (KCI: CaCl2) for half an hour respectively. These were then allowed to stand overnight and centrifuged. The equilibrium extract was analyzed for Ca, K and Na by standard methods.
The exchangeable bases were determined using IN CH3COONH4 (pH 7.0) as extratants by centrifuging the suspensions and analyzing the supernatants.
To study the effect of organic matter soil samples were mixed with well decomposed organic matter to get three levels of organic matter i.e., original, 2.00% and 4.00%. The remaining process was carried out in the same way as to estimate the effect of salt concentration. The data obtained was analyzed by the ANOVA techniques following complete randomized design (CRID) with two factors (Steel and Torrie, 1992).
Results and Discussion
The results of physico-chemical analysis (Table 1) showed that soil A sandy loam soil B clay and soil C is loamy clay with organic matter 0.67, 0.531 and 0.81% with pH 7.8, 8.5 and 7.50 and CEC 11.13 16.11 and 12.08 cmolc Kg–1 respectively.
Hornovalent system: In homovalent the K+-Na+ exchange (1:1, 1:2 and 2:1 KCI: NaCl) the value of Kk increased by increasing the salt concentration. In the present investigation the Kk value ranged from 0.374 to 3.818 (Table 2). The value of Kk is markedly increased with 1:2 and 2:1 K+-Na+ exchange as compared to the 1:1 K+-Na+ exchange. The three soils showed preferential adsorption of K+ over the Na+ and is in accord with the concept that most important factor in determining the relative extent of adsorption and desorption of given ion is its valency, within a given valency series the degree of replaceability of an ion decreases as its hydrated radius increases (Neilson et al. 1972). In case of organic matter, the value of Kk increase with an increase in organic matter (Table 3). The Kk value of all the three soils showed similar trend but soil A sowed maximum adsorption at 2% level as compared to 4% level. The increase in Kerr coefficient indicates that Na+ was easily replaced by 10- as the quantity of organic matter increased. The difference in behaviour of three soils could be due to difference in CEC of three soils. External sites are easily accessible to all competing cations for adsorption while internal sites are accessible only to the cations with their hydrated sizes compatible to the cations with the geometry of exchange sites (Mehta et al., 1983; Poonia et al., 1986). By increasing the organic matter contents, the internal exchange sites are increased which lead to higher preference for K+ in all the three soils. By increasing the organic matter the negative charges increases CEC also increases which leads to the increase in adsorption of K+ over Na+ (Murtaza 1997; Chaudhry et al., 1999; Samin et al., 1999).
| Table 1: | Physical and chemical characteristics of soils |
| Table 2: | Effect of organic matter on lc in K+ - Na+ exchange in soils |
| M = Organic matter | |
| Table 3: | Effect of salt concentration on K in K+-Na+ exchange in soils |
| SC = Salt concentration | |
Hstsrovalent System: The effect of salt concentration on K+-Ca2+ exchange equilibria showed that by increasing the salt concentration there was an increase in selectivity coefficients Kk, Kv and KG values. (Table 4, 5). All the three soils showed preferential adsorption of Ca2+ over K+. This selectivity for Ca2+ over K+ is in accord with concept that in heterovalent system the preferentially adsorbed cation are those with high charge density per unit surface area. (Overbeek, 1952). Moreover Neilson et al. (1972) stated that divalent ions, in general are retained more strongly than monovalent ions wherever trivalent ions are unreplaceable by an equivalent amount of KCI.
By increasing the organic matter contents the values of selectivity coefficients Kk, Kv and KG increase. The soils showed minimum preferential adsorption of Ca2+ over K+ at 2% level and maximum at 4% level of organic matter. By increasing the organic matter the CaX2/KX ratios increase which is indication of preferential adsorption of Ca2+ over K+. Organic matter in soils has been known to result in a greater preference for CO+ than do the clay minerals (Black, 1968).
| Table 4: | Effect of organic matter on Kk Kv and Ky in K+-Ca+ exchange in soils |
| Table 5: | Effect of organic matter on Kk Kv and Ky in K+-Ca exchange in soils |
Particularly hydrous mica like clays (Bohn et al., 1985 which is dominant mineral in Pakistan soils. In heterovalent (K+-Ca+2) exchange both soils showed an increased in Kerr, Vanselow and Gapon selectivity coefficients with an increase in organic matter and pH showing greater affinity for Ca+ 2 than for K+ (Zia et al., 1999).
From the above discussion it can be concluded that in homovalent exchange soils showed greater selectivity of K+ over Na+ by increasing the salt concentration and organic matter.
In heterovalent exchange soils showed greater selectivity of divalent cation over monovalent cations by increasing organic matter and salt concentration.
Selectivity coefficients used for both homovalent exchange (K+-Na+) and heterovalent exchange (K+-Ca2+) equlibria not remained constant at different levels of organic matter and salt concentration.
Kerr, Vanselow and Gapons equations were found better to predict the K+-Na+ and K+-Ca2+ exchange equilibria in soils of Pakistan.