Ion Exchange in Soil: Cation and Anion

After reading this article you will learn about the cation and anion exchange in soil.

Cation Exchange:

In a near neutral soil, calcium remains adsorbed on colloidal particle. Hydrogen ion (H⁺ ) generated as organic and mineral acids formed due to decomposition of organic matter. In colloid, hydrogen ion is adsorbed more strongly than is the calcium (Ca⁺⁺). The reaction takes place rapidly and the interchange of calcium and hydrogen is Chemically equivalent.

The reaction is as follows:

This phenomenon of the exchange of cations between soil and salt solution is known as Cation exchange or Base exchange and the cations that take part in this reaction are called exchangeable cations. Cation exchange reactions are reversible.

Hence, if some form of limestone or other basic calcium compound is applied to an acid soil, the reverse of the replacement just given above occurs. The active calcium ions replace the hydrogen and other cations by mass action. As a result, the clay becomes higher in exchangeable calcium and lower in adsorbed hydrogen and aluminium.

If a soil is treated with a liberal application of a fertilizer containing potassium chloride, following reaction may occur:

Some of the added potassium pushes its way into the colloidal complex and forces out equivalent quantities of calcium, hydrogen and other elements (e.g., M) which appear in the soil solution. The adsorption of the added potassium largely in an available condition. Hence, cation exchange is an important consideration for making already present nutrients in soils available to plants. Cation exchange also makes available the nutrients, applied in commercial fertilizers form.

Cation Exchange Capacity (C.E.C.):

The cation exchange capacity of a soil represents the capacity of the colloidal complex to exchange all its cations with the cations of the electrolyte solution (surrounding liquid). It also represents the total cation adsorbing capacity of a soil. Cation exchange in most soils increases with pH. At a very low pH value, C.E.C. is higher and at high pH, C.E.C. is relatively lower.

Factors affecting the Cation Exchange Capacity:

The following factors affect the cation exchange capacity:

(i) Soil texture:

Fine-textured (clay) soils tend to have higher cation exchange capacity (CEC) than sandy soils. Cation exchange capacity for clay soils usually exceeds 30 me/100 gm. while the value ranges from 0 to 5 for sandy soils.

(ii) Organic matter content:

Organic matter content of a soil affects the CEC. Higher organic matter content in a soil have higher CEC.

(iii) Amount and kind of clay:

Montmorillonite has higher CEC in comparison to illite or kaolinite clay.

(iv) pH:

The cation exchange capacity of most soils increases with pH. At very low pH value, the cation exchange capacity is also generally low. As the pH is raised, the negative charges on some 1 : 1 type silicate clay (Kaolinite), humus and Fe, Al oxides increases, thereby increasing the cation exchange capacity.

Milliequivalents:

The cation exchange capacity (C.E.C.) is expressed in terms of equivalents or more specifically, as milliequivalents per 100 grams. The term equivalent is defined as one gram atomic weight of hydrogen (or the amount of any other ion) that will combine with or displace this amount of hydrogen. For monovalent ions such as Na⁺, K⁺, NH₄⁺ and CI^–, the equivalent weight and atomic weight are the same since they can replace one H⁺ ion. Divalent cations such as Ca⁺⁺ and Mg⁺⁺ can take the place of two H⁺ ions.

The milliequivalent weight of a substance is one thousandth of its atomic weight. Since the equivalent weight of hydrogen is about 1 gm., the term milliequivalent (meq) may be defined as 1 milligram of hydrogen. It indicates that other ions also may be expressed in terms of milliequivalents.

Consider calcium, for example. Ca has an atomic weight of 40 compared to 1 for hydrogen. Each Ca⁺⁺ ion has two charges and is thus, equivalent to two H⁺ ions. Therefore, the amount of calcium required to displace 1 mg of hydrogen is 40/2 = 20 mg (atomic wt. divided by 2 to obtain the equivalent wt.). This is the weight of 1 meq of calcium.

If 100 grams of a certain clay is capable of exchanging a total of 250 meq of calcium, the cation exchange capacity is 250/20 = 12.5 meq per 100 gm. The milliequivalent method of expression can be converted easily to practical field terms. For example, 1 meq of hydrogen can be replaced on the colloids by 1 meq of CaCO₃(limestone). The molecular weight of CaCO₃ is 100, it contains 2 equivalent weights (divalent).

Since the amount of CaCC₃ needed is only 1 meq wt., 100/2 = 50 mg will be needed to replace 1 mg of hydrogen (or 1 meq). In view of fact that 1 meq of H⁺ per 100 grams can be expressed as 20 pounds of hydrogen per grams of soil.

Expressed in the metric system this figure is 1100 kilograms per hectare. In general, the more clay there is in a soil, the higher the C.E.C. Sandy soils have, on the average 0.5 m.e. of C.E.C. per 100 gm. of soil, while for clay soils, it usually exceeds 30 m.e./100 gm.

Percentage Base Saturation of Soils:

Hydrogen and aluminium tend to dominate acid soils, both contributing to the concentration of H⁺ ions in the soil solution. Adsorbed hydrogen contributes directly to the H⁺ ion concentration in the soil. A⁺⁺⁺ions do so indirectly through hydrolysis.

Reactions are as follows:

Most of the other cations (Ca⁺⁺, Mg⁺⁺), called exchangeable bases, neutralize soil acidity. The proportion of the cation exchange capacity (C.E.C.) occupied by these bases is called the percentage base saturation. Thus, if the % base saturation is 80 in clay loam soil, 4/5th of the cation exchange capacity (20 meq) is satisfied by bases, the other by hydrogen and aluminium. Same as, 50% base saturation in clay soil having 20 meq C.E.C. x 1/2 (10 meq C.E.C.).of the C.E.C. is satisfied by bases likewise in sandy loam soil with a C.E.C. of only 10 meq, 80% base saturation satisfied, 4/5 of C.E.C.

A definite correlation exists between the percentage base saturation of a soil and its pH. As the base saturation is reduced as a result of loss of calcium in drainage, the pH is also lowered (more acidity)in a definite proportion.

Within the range pH 5 to 6, the ratio for humid temperate region mineral soils is roughly at 5% base saturation, change for every 0.10 change in pH. thus, if the percentage base saturation is 50% at pH 5.5, it should be 25% and 75% at pH 5.0 and 6.0.

Role of Cation Exchange:

Importance of exchangeable cations on plant nutrients is discussed below:

Cation exchange reactions are very important chemical reactions for the availability of plant nutrients in the soil. The capacity of soil to exchange cations is the best single index of soil fertility. Plant roots, when they come in contact with colloidal particles, absorb exchangeable cations directly by inter-exchange or contact exchange between the root hairs and colloidal complex.

(a) Nature and content of exchangeable bases:

The nature and content of exchangeable bases in a soil have an important bearing on its general properties. In all normal fertile soils the total exchangeable bases (Ca, Mg, K, Na) constitute about 80 to 90% of the cation adsorbing capacity. Exchangeable hydrogen is usually under 20%. In these soils, calcium forms the predominant exchangeable base, constituting 60 to 80% of the total exchangeable cation.

The predominance of exchangeable calcium give rise to Ca- clay which imparts a neutral reaction to the soil. The pH value varies from 6.5 to 7.5. When the proportion of exchangeable hydrogen (H) is high it gives rise to acid soil. In such soils, exchangeable calcium is correspondingly low, and in highly acid soils it is almost absent. In such cases the clay is saturated with hydrogen cations (H⁺) and forms H-clay. Acid soils are less fertile. It is called base unsaturated soil.

When exchangeable sodium form more than 10 to 15% of the total exchangeable cation it gives rise to alkaline soils. The pH value of such soils is usually greater than 8.0. When the proportion of exchangeable sodium exceeds these limits (or saturates the colloidal complex), the clay is turned into a Na-clay.

The soil is now highly alkaline and the pH value ranges from 9 to 12 Alkaline soils are also Jess fertile. Soils with a high calcium base saturation are in the most satisfactory physical and nutritional condition. A calcium dominated soil is granular in structure and ensure good drainage and aeration.

(b) Type of colloid:

Type of colloid affects the cation exchange. Montmorillonite colloid hold the calcium ion with greater tenacity than Kaolinite at a given base saturation. As a result, Kaolinite will liberate calcium much more readily than Montmorillonite.

(c) Associated ions:

Presence of exchangeable calcium in excessive quantities in a soil will limit the availability of potassium to plants. In same manner, high-exchangeable potassium may depress the availability of potassium to plants. In same manner, high- exchangeable potassium may depress the availability of magnesium.

(d) Adsorption of cations:

Colloidal clay (humus) hold in varying amount of plant nutrients (calcium, magnesium, potassium, nitrogen, phosphorus and most of the micronutrients) which are available to plant.

(e) Property of base exchange:

Base exchange (cation exchange) property checks leaching losses of available nutrients. On application of potassium sulphate fertilizer in the soil, potassium ions are held on the surface of colloids by cation exchange process. Subsequently, exchangeable potassium ions are directly available to plants.

Cation Exchange and Soil Fertility:

Cation exchange capacity is the best-index of soil fertility. By cation exchange, hydrogen ions from the root hairs and microorganisms replace nutrient cations from the exchange complex. The nutrient cations are forced into the soil solution where they can be assimilated by the adsorptive surface of roots and soil organisms, or they may be removed by drainage water.

(i) Cation saturation and Soil fertility:

Soil with a high calcium base saturation are the most satisfactory physical and nutritional condition. A calcium-dominated soil is granular in structure and porous. Calcium-dominated clay ensures good aeration and good drainage, thus increases fertility of the soils.

Base unsaturated soils are acidic in nature due to exchangeable hydrogen. These soils are less fertile. Base saturated soils with dominant sodium cations are alkaline in nature. Alkaline soils are not fertile due to de-flocculation, stickiness, hard to work, poor drainage and poor aeration.

(ii) Cation exchange and Soil fertility:

Due to the property of cation exchange (base exchange) the soluble inorganic fertilizer nutrients are not washed away from the soil. For example, ammonium sulphate fertilizer is added to the soil, ammonium ions are held on the surface of colloids by cation exchange. Ammonium ions are taken up by plants. This process checks nutrient losses by leaching and make the soil fertile. The cations Ca, Mg, K, and NH₄ are held on the colloidal surfaces and are readily available to plants.

(iii) Influence of complementary adsorbed cations and soil fertility:

The order of strength of adsorption, when the ions are present in equivalent quantities, is as follow:

Al³⁺> Ca²⁺> Mg²⁺> K⁺ = NH₄⁺> Na⁺

Consequently, a nutrient cation such as K⁺ is less tightly held by the colloids if the complementary ions are Al³⁺and H⁺ (acid soils) than if they are Mg⁺⁺ +Na⁺ (neutral to alkaline soils). The loosely held K⁺ ions are more readily available for absorption by plants or for leaching in acid soils.

There are also some nutrient “antagonisms”, which in certain soil cause inhibition of uptake of some cations by plants. Thus, potassium uptake by plants is limited by high levels of calcium in some soils. Likewise, high potassium levels are known to limit the uptake of magnesium even when significant quantities of magnesium are present in the soil.

Anion Exchange:

The process of anion exchange is similar to that of cation exchange. Under certain conditions hydrous oxides of iron and aluminium show evidence of having positive charges on their crystal surfaces. The positive charge of colloids are due to addition of hydrogen (H⁺) in hydroxyl group (OH^–) resulted in net positive charge (OH₂⁺). This + charge will attract anions (—).

The capacity for holding anions increases with the increase in acidity. The lower the pH the greater is the adsorption. All anions are not adsorbed equally readily. Some anions such as H₂ PO₄– are adsorbed very readily (quickly) at all pH values in the acid as well as alkaline range. Cl^– and SO₄– ions are adsorbed slightly at low pH but none at neutral soil, while NO₃– ions are not adsorbed at all. Hence, at the pH commonly prevailing in cultivated soils—nitrate (NO₃), chloride (Cl) and sulphate (SO₄) ions are easily lost by leaching.

In general, the relative order of anion exchange is:

OH > H₂PO_4–>SO_4–>NO_3–

Importance of Anion Exchange:

The phenomenon of anion exchange assumes importance in relation to phosphate ions and their fixation. The exchange is brought about mainly by the replacement of OH ions of the clay mineral.

The reaction is very similar to cation exchange:

The adsorption of phosphate ions by clay particles from soil solution reduces its availability to plants. This is known as phosphate-fixation. As the reaction is reversible, the phosphate ions again become available when they are replaced by OH ions released by substances like lime applied to soil to correct soil acidity.

Hence, the fixation is temporary. The whole of the phosphate adsorbed by clay is, however, not exchangeable, as even at pH, 7.0 and above. So, substantial quantities of phosphate ions are still retained by clay particles.

The OH ions originate not only from silicate clay minerals but also from hydrous oxides of iron and aluminium present in the soil. The phosphate ions, therefore, react with the hydrous oxides also and get fixed as in the case of silicate clay, forming insoluble hydroxy-phosphates of iron and aluminium.

If this reaction takes place under conditions of slight acidity it is reversible, and soluble phosphate is again liberated when hydroxy-phosphate comes in contact with ions. If the reaction takes place at a low pH under strongly acid conditions, the phosphate (ions) are irreversibly fixed and the totally unavailable for the use of plants.