After reading this article you will learn about the potassium and ammonium fixation in soils.
Cation fixation in soils occurs when exchangeable or water soluble cations are converted to a form that cannot be readily extracted with a neutral solution of a salt. Potassium (K+) and ammonium (NH4+) ions have been primarily involved in such reactions.
Fixation of NH4+ takes place in the crystal lattices of different clay minerals viz. montmorillonite, illite and vermiculite. Amorphous oxides of Fe and Al and allophane also adsorb NH4 in an unavailable form, leading ultimately to formation of taranakite.
The mechanism of NH4+ fixation is similar to that of K+ fixation because of almost the same ionic sizes and polarizabilities. As depletion of K from the interlayers of minerals continues, the rate of K+ release becomes progressively slower although they still retain a very high selectivity to K+ in comparison to divalent ions.
Addition of K+ to such minerals leads to a strong K+ adsorption to these positions causing a contraction of the mineral, to a unit distance of about 1 nm against 1.4 nm in weathered biotite and muscovite. This process is called K+ fixation.
Cations such as Ca2+, Mg2+, Na+ and H+ ions which cause lattice expansion replace fixed NH4+ and thereby releases in the soil solution. On the other hand, K+, Rb+ and Cs+ which cause lattice contraction are unable to do so.
In a study with soils of West Bengal, it was reported that fixation of ammonium (NH4) increased when K+ was applied as its hydroxide with the level of saturation being 10 per cent of CEC and thereafter it decreased as the degree of saturation of CEC by K+ was raised to 50 per cent.
The crystal chemistry of potassium (K+) fixation by soil colloids is governed by the fact that the size of potassium (K+) ion is very close to that of the hole (hexagonal cavity) created by the juxtaposition of two tetrahedral sheets of three-layer silicate minerals. This hole surrounded by twelve oxygen atoms each carrying one-twelfth unit of negative charge holds one potassium ion.
The primary requirement for potassium fixation is that the mineral should be in the expanded state before addition of K+ so that it gets entry. When the lattice collapses the K+ ions which enter are trapped in the hole. In addition to the layer silicates, potash- feldspars and amorphous minerals in soil also fix potassium. Under specific conditions, the fixed or trapped potassium may be released in the soil solution.
Fixation by Clay Minerals:
A “lattice-hole-theory” was proposed by Page and Baver (1940) that gave consideration to the ionic size of the un-hydrated ions, the expanding contracting nature of the montmorillonite type of minerals and the geometric arrangement of the oxygen ions at the surface of the crystal units.
The essential elements of their theory were as follows:
(a) The exposed surface between the layers of the 2: 1 expanding-lattice clays consists of a sheet of oxygen ions arranged hexagonally, the opening within the hexagon being 2.8 Ã… in diameter.
(b) As the clay is dehydrated, the layers contract and the ions lose their hulls or oriented water molecules approaching the un-hydrated ionic diameter in size. These diameters are respectively: Li, 1.20 Ã…; Na, 1.90 Ã…; K, 2.66 Ã…; NH4, 2.86 Ã…; Rb. 2.96 Ã…; Cs, 3.38 Ã…; Mg, 1.30 Ã…; Ca, 1.98 Ã… and Ba, 2.70 Ã….
(c) Ions whose diameters allow them to fit snugly into the lattice “holes” should be held very tightly, because they are closer to the negative electrical charges within the crystal, and by fitting into the “hole” they would allow the layers to come together and be locked against rehydration and re-expansion.
Large cations, which could not enter the “holes” would remain more loosely held between the layers rather than within the layer, and would be more accessible for rehydration. Smaller cations would be able to enter the “holes” but would be too small to contact and bind the two layers together. The relationship between ionic radii of different cations and their percentage fixation is presented below (Fig. 17.6)
Fixation by Organic Matter:
Fertilizers containing ammonia when applied to the soil react with soil organic matter to form compounds which resist decomposition and so it is said to be “fixed” by the organic matter.
The exact mechanism of such fixation is not known but it is found that aromatic compounds, quinones and â€”OH groups in the organic matter can fix NH4+. The fixation of added NH4+ was linearly related to the percentage of carbon in the organic matter.
Release of Fixed NH4+and K+in Soils:
From the fixation mechanism of both NH4 and K+ in soils it is found that there is equilibrium between fixation and release or liberation. So fixation is not entirely an irreversible phenomenon. Fixation and release can proceed simultaneously in a system containing heterogeneous minerals of 2: 1 type not in equilibrium.
The release of fixed K+ by montmorillonite and vermiculite is easier than that by illite. Although the release of fixed K+ is a very slow process.
However, there are various factors to be considered for the release of K+ from micas either by cation exchange reactions or through dissolution:
(i) Tetrahedral rotation and cell dimensions,
(ii) Degree of tetrahedral tilting,
(iii) Hydroxyl orientation,
(iv) Chemical composition,
(v) Particle size,
(vi) Structural Imperfections,
(vii) Degree of potassium depletion,
(viii) Layer charge alterations and associated reactions,
(ix) Hydronium (H3O+) ions,
(x) Biological activity and complexing organic acids,
(xi) Inorganic cations,
(xii) Wetting and drying,
(xiv) Redox potential and
Since continuous and adequate K+ nutrition of plants depends not only on the level of exchangeable K+ in soils but also on the rapid renewal of its supply, an understanding of the factors whose measurements can lead to a fair estimation of the rate of K+ release or which can be manipulated to enhance K+ release, offers exciting possibilities.
Thus, chemical composition of octahedral sheets of micas controls tetrahedral rotation, whereas tilting and the orientation of OH groups in turn control the stability and rate of release of K+ from micas. Also, pH has a distinct influence on the release of K+ from the micas.
There are many sources of hydronium (H3O+) ions in soils, the most important one being water, the dissolution of CO2 from the air and soil atmosphere, strong mineral acids arising from weathering reactions and low molecular weight organic acids.
In growing plants, the root lets are surrounded by an ionic atmosphere consisting mainly of (H3O+) ions. Thus, hydronium (H3O+) ions are probably the cause of major release of structural K+ upon alteration of minerals during weathering.
The activity of K+ ions in soil solution around mica particles is a factor in determining the release of K+ from micas. When the K+ activity is less than the critical, K+ is replaced from the interlayer by other cations from the solution. However, if the K level is greater than the critical value, the expanding 2: 1 mica mineral takes K+ from the solution.
Leaching enhances K+ release from micas by removing the reaction products. Hence, leaching accelerates the transformation of micas to expansible 2: 1 layer silicates and other weathering products.
The mechanisms of the release of fixed K+ by clay minerals are shown in Fig. 17.7.
Majority of the above factors are also responsible for the release of fixed NH4+ into the soil solution. The rate of release of fixed NH4+ is also very slow like that of release of fixed K+ in soils. Some recently fixed form of NH4+ is slowly replaced by cations such as Ca2+, Mg2+ and Na+, but not by K+.
The release of clay fixed NH4+ may occur more rapidly under flooding. When the soils did not contain large amounts of exchangeable NH4+ or exchangeable K+, the uptake of fixed NH4+ by crops was as rapid as that of exchangeable NH4+.
Factors Affecting K+ and NH4+Fixation in Soils:
Various factors affect fixation, the most important of those are discussed here:
(i) Nature and Amount of Clay:
The ability of the various soil colloids to fix NH4 and K+ varies widely. Illite, weathered mica, vermiculite, smectite and interstratified minerals take part in fixation reactions to a great extent whereas 1: 1 type minerals such as Kaolinite fix NH4+ and K+ in very small amounts.
(ii) Soil Reaction or pH:
The K+ and NH4+ fixing capacity can be reduced by the presence of Al3+ and Al-hydroxide cations and their polymers which form under acid conditions. These Al3+ cations will occupy the NH4+ and K+ selective binding sites. A decrease in soil pH reduces K+ and NH4+ fixation either as a result of competition of hydronium (H3O+) ions for the inter layer exchange position or by the destruction of the lattice surface.
(iii) Concentration of Added NH4+ and K+:
Increasing concentration of these ions (NH4+ and K+) up to a certain level in soils of high fixing capacity will obviously encourage greater fixation.
(iv) Wetting and Drying:
Air drying of some soils high in exchangeable K+ and NH4+ will result in fixation and a decline in exchangeable K+ and NH4+. The K+ fixation under moist conditions is associated with illite or others containing mica. Sometimes, the release of K+ and upon drying as being caused by cracking of edge-weathered micas and exposing inter layer K+ and NH4+.
(v) Freezing and Thawing:
The freezing and thawing of moist soils may also be important in the release of fixed K+ and NH4+. When soils containing high in exchangeable NH4+ and K+, some of these exchangeable ions become fixed.
Liming favours the fixation of both K+ and NH4+ due to an increase in pH and by the replacement of NH4+ and K+ from the inter layer positions. For example, in certain soils K+ deficiency apparently results from the presence of excess of calcium carbonate.
Soils saturated with different ions have the K+ fixing power in the order, Na+> Mg2+> Ca2+> NH4+> H+. The order of competitive ability of ions in reducing K+ fixation was NH4+> Ca2+> Mg2+> Na+.
The nature of anions associated with NH4+ ion also influence NH4+ fixation. This may be due to the effect of the anion on pH of the soil. The effect of the anion on the adsorption capacity of the associated cation is also to be considered in influencing the NH4 fixation in soils. It is expected that an anion which is strongly adsorbed by the soil may increase the adsorption of the cation (Paneth-Fajans-Hahn’s rule).
The fixation of NH4+ is usually increased by drying, especially when low concentrations are involved. The amount of added NH4+ fixed was found to be appreciably decreased with an increase in water: soil ratio.
(x) Cation-Exchange-Capacity (CEC):
With the increase in CEC and percentage base saturation of a soil, the fixation of both K+ and NH4+ will increase.
(xi) Sequence of K+ and NH4+ Application:
The sequence of K+ and NH4+ application in soil significantly influences their availability in soils as well as fixation. The highest availability of added NH4+ was found in soils where K+ was applied 7 days before the addition of NH4+ and least when K+ was applied 7 days after the addition of NH4+.
(xii) Organic Matter:
The amount of NH4+ fixation increases with the large quantity of organic matter present in soils. Generally it is observed that at least 50 per cent of the amount of ammonium (NH4+) fixed by surface soils is attributed to some reaction of ammonia with the soil organic matter.
The process of NH4+ fixation by organic matter was more rapid in alkaline than that of acid soils and pure polyphenolic compounds fix NH4+ only in presence of oxygen.
Within the temperature interval of 0-60Â°C, the fixation of both K+ and NH4+ in soils increases with an increase in temperature. It has been found that the fixation of NH4+ increased by heating the soil at 100Â°C.
Practical Implications of NH4+ Fixation:
Although the agricultural significance of NH4+ fixation is not generally considered to be great, it can be of importance in certain soils, and clay-fixed NH4+ has received attention in many countries. Native fixed NH4+ is significant in many soils and it can amount to about 10-31 per cent of the total fixation capacity.
In light textured and waterlogged rice soils, most of the applied nitrogen undergone losses through leaching and de-nitrification processes. Under such conditions crops will suffer due to nitrogen deficiency and in such situation fixation of nitrogen as NH4+ is desirable because it protects nitrogen from such losses and subsequently supply nitrogen slowly as per requirement of the crop.