Elsevier

Geoderma

Volume 145, Issues 3–4, 15 June 2008, Pages 207-215
Geoderma

Effect of sodium and magnesium on kinetics of potassium release in some calcareous soils of western Iran

https://doi.org/10.1016/j.geoderma.2008.03.005Get rights and content

Abstract

The rate of potassium (K) release from soils can significantly influence their K fertility. Sodium (Na), calcium (Ca), and magnesium (Mg) in poor quality (sodic or saline) irrigation water participate in ion-exchange processes resulting in displacement and release of K from minerals into solution. This study determined the effect of sodium adsorption ratio (SAR) and Ca:Mg ratio of water on K release of some calcareous soils in western Iran. Nine different solutions at a total electrolyte concentration of 100 mmolc l 1 and three levels of SAR (5, 15, 45) each with Ca:Mg ratios of 1:3, 1:1, or 3:1, prepared using solutions of NaCl, CaCl2, and MgCl2 were used to extract K from the soils. Significantly different quantities of K were extracted by the solutions. The maximum (average of five soils) (985 mg kg 1) and the minimum (387 mg kg 1) K were extracted by an SAR 5 solution with a Ca:Mg ratio of 1:3 and an SAR 45 solution with Ca:Mg ratio of 3:1, respectively. The importance of Mg versus Ca can be related to the specific ion effect. The kinetics of K release from soils consisted of two phases, an initial rapid phase followed by a slow phase of K release from soils. The two phases of K release are characteristic of a diffusion-controlled process. Based on the correlation coefficients, power function, parabolic diffusion, and Elovich equations adequately described K+ release, whereas a first order equation did not. The K release rate for the soils was estimated by parabolic equation from the above solutions. The constant b (mg kg 1 min 1/2) in the parabolic equation was defined as the release rate and for the Ca:Mg ratio of 1:3 was 96.5 , 55.9, and 35.1.for the SARs of 5, 15, and 45, respectively. The results imply that K extraction from soils could be increased during use of saline irrigation water containing high Mg concentration. The additional K released may be more readily available to plant roots but could also be leached down below the root zone. The results suggest that long-term use of saline irrigation water with a high Mg content could lead to enough leaching of K from soil under saline and sodic conditions that K fertilization management may need modification.

Introduction

Potassium (K) is an element essential for plant growth and its importance in agriculture is well recognized (Sparks and Huang, 1985, Huang, 2005). Soil K is typically divided into four forms: soil solution K, exchangeable K, non-exchangeable K, and K in soil minerals (Sparks, 1987). There are dynamic, equilibrium reactions between these different forms of K. Non-exchangeable K can be an important reservoir of K in soils. Several studies demonstrate that non-exchangeable K from reserves makes an important contribution to plant K supply (Mengel and Uhlenbecker, 1993, Jalali and Zarabi, 2006). For optimal nutrition of a crop, the replenishment of a K-depleted soil solution is affected predominately by the release of non-exchangeable K from clay minerals and organic matter. Therefore, for maximal crop growth, soil solution and exchangeable K need to be replenished continually with K through the release of non-exchangeable K from the weathering of K reserves (Sparks and Huang, 1985, Sparks, 1987) or the addition of K fertilizers.

The amount of non-exchangeable K in soils is greatly affected by the particle size distribution and soil minerals that are present. Vermiculite, mica and illite are the clay minerals that have the greatest capacity to fix K. In addition to the level of K in solution, and the type of clay minerals present in the soil, and wetting and drying, release of K is also dependent on the concentrations of other cations in soil solution, especially Ca, Mg, and Na. The source of these cations for displacing K is either a saline soil solution (Rowell, 1985) or the weathering of soil minerals (Shainberg et al., 1981).

Salinity and sodicity are the principal water quality concerns in irrigated areas of arid and semi-arid regions that use poor water quality for irrigation (Ayars and Tanji, 1999). In arid and semi-arid regions, agriculture is mainly limited by the availability of suitable irrigation water and groundwater is the main source of irrigation. Use of poor quality groundwater has become inevitable for irrigation to compensate rapidly increasing water demands in many arid and semi-arid regions.

Irrigation with water with high concentrations of Ca, Mg, and Na leads to an increase in K desorption and leaching (Meiri et al., 1984, Jalali and Merrikhpour, 2008). This K may be more readily available to plant roots, but it is also easily leached down beyond the root zone. Bar-Tal et al. (1991) showed that irrigation water with high salinity can leach native and applied K from the soil.

Although Ca and Mg are both divalent cations, their effects as the complementary cation to Na on soil stability are often very different. These cations have, for practical purposes, generally been grouped together as similar ions in maintaining soil structure when quantifying sodicity of soils and irrigation waters (Zhang and Norton, 2002). However, it has long been suspected that Mg, as compared to Ca, has deleterious effects on soil structure under certain circumstances. A distinction has been made between a direct effect of Mg on decreasing soil structural stability and an indirect effect of Mg on accumulation of Na on soil exchange sites (Curtin et al., 1994, Yousaf et al., 1987). The nature and extent of these effects may vary with clay type and electrolyte concentration (Zhang and Norton, 2002). The direct effect, also termed a specific effect of Mg, appears to be significant for vermiculitic and illitic soils, but not for montmorillonitic soil (Rahman and Rowell, 1979). The indirect effect is caused by differences in the relative affinity of the clay for Na when Mg, as compared to Ca, is the complementary cation (Curtin et al., 1994). A preference by soils for Ca over Mg could result in a higher exchangeable Na concentration in Na–Mg systems than in Na–Ca systems (Zhang and Norton, 2002). Stigter et al. (1998) demonstrated that irrigation with Na-enriched water results in ion-exchange reactions—specifically, the uptake of Na and release of Ca and Mg. In contrast, irrigation with Ca-enriched water releases Na+ that is bound to the adsorption sites on clay minerals.

The groundwater used for irrigation in the Hamadan area, western Iran has water salinity and sodicity problems (Jalali, 2002). Using groundwater of poor quality for irrigation has further aggravated the problem of soil salinity and sodicity as 8.4% of the groundwater is sodic. Additionally, the water type Na–SO4, which is less distributed in the Bahar area (near the Hamadan), was also recorded (Jalali, 2005b).

The ratio of Ca:Mg in the soil-water may be used to predict a potential Ca deficiency in soil (Ayers and Westcot, 1985). In an Mg dominated water (molar ratio of Ca:Mg < 1) or an Mg soil (soil-water molar ratio of Ca:Mg < 1), the potential effect of Na may be slightly increased. In other words, a given SAR value will show slightly more damage to soil structure if the Ca:Mg molar ratio is less than 1. The lower the ratio, the more damaging the SAR is to soil structure. Jalali (2007) studied salinization of groundwater in arid and semi-arid regions in Tajarak, Hamadan, western Iran and reported that 35% of water samples (28) have a Ca:Mg molar ratio less than 1.

While the specific effect of Mg on structural stability as a result of alkali water irrigation has been the subject of several studies (Rahman and Rowell, 1979, Yousaf et al., 1987, Curtin et al., 1994, Zhang and Norton, 2002), its effects on the kinetics of K release remain relatively unexplored (Rahmatullah et al., 1994). An understanding of the effect of Na–Ca–Mg exchange on the release of exchangeable and non-exchangeable K is necessary to determine how much K will be available for plant uptake and leaching when poor quality waters are applied. Therefore, this study was conducted to evaluate the specific effect of dissolved Mg on release of K from some calcareous soils.

Section snippets

Physicochemical properties of soils

The soil materials used in this study include five surface (0–30 cm) samples selected from several agricultural areas in Hamadan province, western Iran, to provide a range of the calcareous soils normally encountered in the area. They represent soils on which major arable crops (wheat, potato, and barley) are grown. The soil samples were air dried and ground to pass through a 2-mm sieve for laboratory experiments. Particle size distribution was determined by the pipette method; soil pH and EC

Soils

Selected chemical and physical properties of the soils studied are given in Table 2. The soils were neutral to slightly alkaline and low in EC and organic matter. The equivalent calcium carbonate contents varied from 47 to 200 g kg 1. Clay contents in all soils averaged 259 g kg 1 and ranged from 135 to 431 g kg 1. The CEC ranged from 11.3 to 16.8 cmolc kg 1. Exchangeable K ranged from 181 to 561 mg kg 1.

Release of K

Potassium release by successive extraction with nine extraction solutions is shown in Fig. 1

Conclusion

In arid and semi-arid regions, agriculture is mainly limited by the availability of suitable irrigation water. Saline groundwater in these areas is sometimes used for irrigation due to limited fresh water resources. Arid and semi-arid region soils generally contain large quantities of exchangeable and non-exchangeable K. An understanding of how much exchangeable and non-exchangeable K will release due to application of saline water containing Na, Mg, and Ca, especially with a low Ca:Mg ratio is

Acknowledgement

Three anonymous reviewers and editor made valuable comments on the manuscript. The author gratefully expressed his gratitude for their thoughtful and thorough reviews.

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