Soil Minerals And Plant Nutrition

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Soil Minerals and Plant Nutrition
By: Balwant Singh, Ph.D. (Department of Environmental Sciences, The University of Sydney) & Darrell G. Schulze, Ph.D. (Department of Agronomy, Purdue University) © 2015 Nature Education
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Citation: Singh, B. & Schulze, D. G. (2015) Soil Minerals and Plant Nutrition. Nature Education Knowledge 6(1):1

How do chemical reactions involving soil minerals play a crucial role in controlling the availability of essential plant nutrients?
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All plants require 17 elements to complete their life cycle, and an additional four elements have been identified as essential for some plants (Havlin et al. 2005). With the exception of C, H, and O, which plants obtain from air and water, plants derive the remaining 14 elements from the soil or through fertilizers, manures, and amendments (Parikh & James 2012). The bulk of the soil solid fraction is constituted by soil minerals, which exert significant direct and indirect influences on the supply and availability of most nutrient elements. The main processes involved in the release and fixation of nutrient elements in soils include dissolution-precipitation and adsorption-desorption. We will discuss these processes and how they impact macronutrients and micronutrients.



Primary Minerals and Soil Fertility

Sedimentary rock covers 75-80% of the Earth's crust, and it forms parent materials for a large majority of soils. Soil parent material has a significant direct influence on the nutrient element contents of the soil; this influence is more pronounced in young soils and diminishes somewhat with increasing soil age and soil weathering. In order to better understand the effect of soil parent materials on the soil elemental composition, it is useful to review the mineralogical composition of common rocks that make up the soil parent material (Table 1). Primary minerals form at elevated temperatures from cooling magma during the original solidification of rock or during metamorphism, and they are usually derived from igneous and metamorphic rocks in soil (Lapidus 1987). In most soils, feldspars, micas, and quartz are the main primary mineral constituents, and pyroxenes and hornblendes are present in smaller amounts.

Primary minerals — including K-feldspars (orthoclase, sanidine, and microcline), micas (muscovite, biotite, and phlogopite), and clay-size micas (illite) — are widely distributed in most soil types, except in highly weathered and sandy soils. These primary minerals act as an important reservoir for K, with over 90% of K in soils existing in the structure of these minerals. Significant amounts of Ca, Na, and Si and smaller amounts of Cu and Mn are also present in the feldspars. Micas and illite are the most important source of K in many soils, and they also contain Mg, Fe, Ca, Na, Si, and a number of micronutrients. Amphiboles and pyroxenes are vital reservoirs of Mg, Fe, Ca, Si, and most of the micronutrients. Carbonate minerals, including those derived from soil parent material and those formed in soil through pedogenic processes, serve as both a source and a sink for Ca and Mg in soils.

The physical, chemical, and biological weathering of primary minerals releases a number of nutrient elements into the soil solution. Weathering rates and pathways of primary minerals are highly variable and depend on several factors, including mineral properties and climatic conditions. Although the weathering rates of primary minerals for certain elements may not be fast enough to meet plant nutrient requirements on a short-term basis, particularly in managed cropping systems, mineral weathering is an important and long-term source of several geochemically derived nutrients. The nutrient supply capacity of a soil through weathering of primary minerals diminishes as the extent of soil weathering increases.



Secondary Minerals and Soil Fertility

In contrast to the primary minerals, secondary minerals in soils are usually formed by low-temperature reactions during the weathering of primary minerals in the aqueous environment at the Earth's surface. Secondary minerals primarily control nutrients through adsorption-desorption, dissolution-precipitation, and oxidation-reduction reactions.


Adsorption reactions involving minerals are often more important in controlling plant nutrient element availability than the release of nutrient elements by mineral weathering. Phyllosilicates with a permanent charge (e.g., vermiculite and smectite) offer exchange sites that hold a number of essential nutrients in their cationic form (cation exchange capacity), such as Ca2+, Mg2+, K+, and Na+; the nutrients are retained by outer-sphere complex formation (Figure 1) and may be easily taken up by plant roots. On the other hand, variable charge minerals (e.g., Fe oxides) retain some nutrients (P, Zn) by forming inner-sphere complexes (Figure 2), and such nutrients are not readily available to plants. Important reactions relevant to specific nutrient elements are discussed below.
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Figure 1: Hydrated exchangeable cations within the interlayer region of the clay mineral smectite.
© 2014 Nature Education Courtesy of Balwant Singh and Darrell G. Schulze. All rights reserved.
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Figure 2: Schematic representation of phosphate adsorption by forming inner-sphere complexes involving monodentate and bidentate bonding on a goethite surface.
© 2014 Nature Education Courtesy of Balwant Singh and Darrell G. Schulze. All rights reserved.
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Nitrogen: Plants usually take up the nitrate (NO3-) and ammonium (NH4+) forms of soil nitrogen. In soils, N applied through fertilizers and mineralized N from organic matter mostly ends up in the NO3- form. Due to the limited anion exchange capacity of most soils, leaching of applied N in the form of NO3- ions is a common water quality problem, particularly in agricultural regions. It also represents an important economic inefficiency, because producers apply excessive amounts of fertilizer to compensate for the leaching. Highly weathered soils, such as oxisols and ultisols, are the exception. The mineralogy of oxisols and ultisols is dominated by minerals with variable surface charge, mainly kaolinite and Fe and Al oxides, which provide these soils with the capacity to retain large amounts of NO3-N, particularly in the subsoil horizons. For example, Lehmann et al. (2004) observed 150-300 kg NO3-N ha-1 (up to a depth of 2 m) in a Brazilian oxisol in a maize-soybean cropping system. Additionally, Rasiah & Armour (2001) estimated between 17,000-32,000 kg NO3-N ha-1 to a depth of 10 m under different land uses in oxisols from northern Queensland in Australia. The anion exchange capacity of the Australian oxisols was large, with values as high as 41 mmolc kg-1. The adsorbed nitrate is too deep and is likely inaccessible to most field crops, nevertheless, it does not leach into groundwater.

In contrast to highly weathered oxisols and ultisols with variable charge minerals, soils in temperate regions generally have permanent charge minerals (e.g., smectite and vermiculite) with high cation exchange capacity and the ability to retain ammonium (NH4+) ions. Indeed, a large proportion of the NH4-N is retained in the interlayers of 2:1 phyllosilicates and is not readily exchangeable, causing it to be referred to as fixed NH4. The process of NH4-fixation is similar to that of K-fixation, which is demonstrated in Figure 3. Vermiculite, illite, and interstratified minerals with 2:1 layers are involved in the fixation of NH4+ ions in soils. With the exception of sandy soils, the amount of fixed NH4+ in the soil ranges from 350-3,800 kg NH4-N ha-1 in the top 30 cm of soil; vermiculite and partially weathered illite generally have a greater capacity to fix NH4+ in soils than the smectite group of minerals (Nieder et al. 2011; Nörmik & Vahtras 1982; Stevenson & Cole 1999). The different behavior and capacity of 2:1 phyllosilicates in fixing NH4+ ions is related to the magnitude and origin of negative charge in these minerals. NH4-fixation generally increases with the increasing amount of layer charge in the 2:1 phyllosilicates, and the fixation is greater in minerals with charge originating in the tetrahedral sheet than in minerals with charge originating in the octahedral sheet.

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Figure 3: Fixation of K+ and NH4+ results when these monvalent cations from the soil solution (top left) displace hydrated cations (shown as Ca2+ and Mg2+) from the interlayer of vermiculite (top right). The K+ and/or NH4+ cations dehydrate and are held tightly within cavities of opposing 2:1 layers to form a structure analogous to mica (bottom right), while the displaced cations move to the soil solution (bottom left). The reverse reaction results in release of the fixed cations.
© 2014 Nature Education Courtesy of Balwant Singh and Darrell G. Schulze. All rights reserved.
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Phosphorus: P is primarily taken up by plants in the form of phosphate ions (HPO42- and HPO4-) from the soil solution. The concentration of P in soil water is generally very low (< 0.01% of the total soil P), with the bulk of the soil P existing as organic P, insoluble compounds of P with Al, Fe, and Ca, and phosphate adsorbed to Fe and Al oxides and phyllosilicates (Stevenson & Cole 1999; Brady & Weil 2008). Phosphate ions from dissolved chemical fertilizers react rapidly in most soils, resulting in P fixation in the soil. These soil reactions involve both adsorption and precipitation processes.

(i) Adsorption Reactions

Adsorption reactions of phosphate ions on mineral surfaces predominantly involve the formation of inner-sphere complexes on the variable charge surfaces of Fe and Al oxides and kaolinite. An example is provided in Figure 2, where phosphate ions are adsorbed on goethite surfaces by forming monodentate and bidentate bonds. Phosphate ions adsorbed by such processes are only slowly available to plants. Phosphate is also known to be sorbed by calcite in calcareous soils, with the sorption occurring via the replacement of CO32- on the calcite surfaces.

(ii) Precipitation Reactions

In strongly acidic soils, the precipitation reactions involving soluble phosphate from fertilizer results in the formation of insoluble Al, Fe, or Mn phosphates. In contrast, in calcareous soils, insoluble Ca phosphates are formed, which are gradually converted to insoluble carbonated hydroxyapatite. General chemical reactions of phosphate in acidic and calcareous soils are shown below:

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Potassium: Among the essential elements, K is usually the most abundant in soils. Total K in soils varies from 0.5-2.5% of the soil mass, and most of the K exists in mineral form (K-feldspars and micas). Potassium is released following the weathering or dissolution of K minerals in soils, as shown in the following examples:

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Of these two reactions, K release by the weathering of mica is generally more important in supplying K to plants in unfertilized soils.

Phyllosilicates retain and release K for plants from non-exchangeable or fixed (i.e., exchanged very slowly and only when the K concentration in soil water drops below a threshold value) and exchangeable forms. Potassium ions present on the exchange sites are adsorbed by outer-sphere complexation and are readily available for plant uptake (Figure 1). On the other hand, illite, vermiculite, and interstratified 2:1 clay minerals release fixed or non-exchangeable K from interlayer sites through cation exchange and diffusion processes at slower rates than the exchangeable K (Figure 3). Similar to NH4+ ions, K supplied through fertilizers or other amendments can be fixed in the interlayers of the 2:1 minerals (Figure 3). The non-exchangeable or fixed K can be potentially released back into soil solution if the solution K concentration falls below a certain threshold value.



Secondary Nutrients

Among the secondary nutrients, Ca and Mg are taken up by plants in their cationic forms, Ca2+ and Mg2+. These cations are retained at negatively charged sites of phyllosilicates via electrostatic attraction (outer-sphere complexation) (Figure 1). The precipitation of secondary carbonates, such as calcite (CaCO3), magnesium calcite (Ca1-xMgxCO3), and gypsum (CaSO4.2H2O), is common in soils of arid and semi-arid environments. Secondary carbonates are considered to be important scavengers of some nutrients through incorporation in the mineral structure (e.g., Mn) or inner-sphere complexation (e.g., P and Zn) at the mineral surface.


Sulfur is taken up by plants as sulfate (SO42-), and this is the most common inorganic S form in soils. Fe and Al oxides and kaolinite provide adsorption sites for SO42- in most soils, even if these minerals are present in small amounts. Sulfate ions are believed to be adsorbed by these minerals by forming both inner- and outer-sphere complexes. In calcareous soils, SO42- may be sorbed on CaCO3 by forming a CaCO3-CaSO4 co-precipitate, which renders SO42- unavailable to plants. Sulfide (S-, S2-) minerals form under reducing environments (e.g., freshwater and tidal marshes) where SO42- ions are reduced to form minerals such as pyrite (FeS2). Such reduced forms of S are oxidized following exposure to air, releasing SO42-, H+, and Fe3+ ions to the soil solution.



Micronutrients

Among the micronutrients, Fe, Mn, Cu, Zn, and Ni are taken up by plants in their cationic forms, and B, Mo, and Cl are taken up by plants in their anionic forms. Fe and Mn are often present in large quantities in most soils, and adsorption reactions play little role in controlling their plant availability in soils. Oxidation and precipitation reactions predominantly control the soil solution concentration of Fe and Mn. Goethite, hematite, and ferrihydrite are the most commonly occurring secondary Fe oxides in soils. Due to the microcrystalline size of Fe oxides, these minerals possess high specific surface areas and provide numerous adsorption sites for both cationic and anionic elements in all varieties of soils. The two most stable Fe oxides, goethite and hematite, are known to have substantial structural substitution of trace elements, including Mn, Ni, Zn, and Cu. Manganese minerals are not as abundant and common as Fe oxides. Often, they exist in soils as mineral coatings, as nodules, or as finely dispersed particles in the soil matrix. Both Fe and Mn oxides are common mineral constituents in many soils and are important substrates for the retention of many macronutrients and micronutrients. Plant availability of both Fe and Mn is greatly reduced in calcareous soils due to the extremely low solubility of Fe and Mn oxides and of Mn carbonates. In such situations, plants induce biochemical responses, such as release of reducing and chelating compounds and acidification of rhizosphere, which can increase the availability of Fe, Mn, and other micronutrients.


Copper, Zn, and Ni are adsorbed by Fe and Al oxides by forming inner-sphere complexes at low solution concentrations. However, at higher solution concentrations, precipitation of metal hydroxides occurs (Ginder-Vogel & Sparks 2010). Adsorption of Cu2+, Zn2+, and Ni2+ occurs by outer-sphere complex formation on negatively charged surfaces of 2:1 phyllosilicates and perhaps by inner-surface complex formation on kaolinite surfaces. In alkaline soils, adsorption of Zn on calcite and co-precipitation of Cu in calcite may also occur.

B and Mo are taken up by plants as H3BO3 and MoO42-. Limited evidence suggests that B species (i.e., B(OH)3 and B(OH)4-) in soils are adsorbed by forming inner-sphere complexes on the surfaces of Fe and Al oxides (Su & Suarez 1995). Similarly, MoO42- is strongly adsorbed by metal oxides. Chlorine is taken up by plants in the chloride (Cl-) form, and adsorption reactions involving Cl‑ ions are similar to those involving NO3- ions. Therefore, a relatively high potential for exchange-based adsorption of Cl‑ ions occurs in highly weathered soils because their mineralogy is dominated by variable-charge minerals, such as kaolinite and Fe and Al oxides. In certain soil environments, such as those with restricted leaching or those with low-lying areas in arid climates, Cl may exist in precipitated mineral forms, such as NaCl, CaCl2, and MgCl2.



Conclusions

Soil minerals serve as both sources and sinks of essential plant nutrients. As primary minerals that originally formed at high temperatures and pressures in igneous and metamorphic rocks are weathered in soils, they release plant nutrients into the soil solution. New minerals form in the aqueous phase of soil environments. These secondary minerals serve as sources of nutrients themselves, or they precipitate or adsorb essential elements, keeping them from being taken up readily by plants. In many cases, secondary minerals serve as important reservoirs where nutrients are held strongly enough to prevent leaching, yet weakly enough to allow plants to draw on them to meet their nutritional needs. In some soils and in certain topsoils, the soil organic matter contains and releases plant nutrient elements.

link to source materials https://www.nature.com/scitable/knowledge/library/soil-minerals-and-plant-nutrition-127881474


References and Recommended Reading
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Ginder-Vogel, M. & Sparks, D. L. The impact of X-ray absorption spectroscopy on understanding soil processes and reaction mechanisms. In Synchrotron-Based Techniques in Soils and Sediments, eds. Singh, B. & Gräfe, M. (Burlington: Elsevier 2010) 1-26.

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