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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.
Mineral constituent |
Igneous rock |
Shale |
Sandstone |
Nutrient element constitutents | |
%
|
Major |
Minor | |||
Feldspars |
59.5 |
30.0 |
11.5 |
K, Ca, Na |
Cu, Mn |
Amphiboles & pyroxenes |
16.8 |
– |
sm |
Mg, Fe, Ca |
Ni, Co, Cu, Mn, Zn, Mo |
Micas |
3.8 |
– |
sm |
K, Ca, Na, Mg, Fe |
Ni, Mn, Co, Zn, Cu |
Titanium minerals |
1.5 |
– |
sm |
Ti, Fe, Ca |
Co, Ni |
Apatite |
0.6 |
– |
sm |
Ca, P |
|
Clay |
– |
25.0 |
6.6 |
K, Mg, Fe, Ca, Na |
|
Iron oxides |
– |
5.6 |
1.8 |
Fe |
Mn, Zn, Ni, Co |
Carbonates |
– |
5.7 |
11.1 |
Ca, Mg, Fe |
|
Other minerals |
– |
11.4 |
2.2 |
– |
– |
Table 1: Average mineralogical and nutrient element composition of common rocks on the Earth's land surface (Klein & Hurlbut 1999, based on data of F. W. Clarke). |
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
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.
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.
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:
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:
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
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
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
References and Recommended Reading
Brady, N. C. & Weil, R. R. The Nature and Properties of Soil, 14th ed. Upper Saddle River, NJ: Prentice Hall, 2008.
Churchman, J. C. & Lowe. D. J. Alteration, formation, and occurrence of minerals in soils. In Handbook of Soil Sciences — Properties and Processes, eds. Huang, P. M., Li, Y. & Sumner, M. E. (Boca Raton: CRC Press, 2012) 20-1-20-72.
Essington, M. E. Soil and Water Chemistry: an Integrative Approach, 1st ed. Boca Raton, FL: CRC Press, 2004.
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.
Goldich, S. S. A study in rock-weathering. Journal of Geology 46, 17-58 (1938).
Havlin, J. L. et al. Soil Fertility and Fertilizers: An Introduction to Nutrient Management, 7th ed. Upper Saddle River, NJ: Prentice Hall, 2005.
Klein, C. & Hurlbut Jr., C. S. Manual of mineralogy (After James D. Dana), 21st revised ed. New York, NY: Wiley, 1999.
Lapidus, D. F. Collins Dictionary of Geology, London, England: HarperCollins, 1990.
Lehmann, J. et al. Subsoil retention of organic and inorganic nitrogen in a Brazilian savanna Oxisol. Soil Use and Management 20, 163-172 (2004). doi: 10.1111/j.1475-2743.2004.tb00352.x.
Nieder, R., Benbi, D. K. & Scherer, H. W. Fixation and defixation of ammonium in soils: A review. Biology and Fertility of Soils 47, 1-14 (2011). doi: 10.1007/s00374-010-0506-4.
Nörmik, H. & Vahtras, K. Retention and fixation of ammonium in soils. In Nitrogen in Agricultural Soils, ed. Stevenson, F. J. (Madison: American Soc Agron, 1982) 123-171.
Parikh, S. J. & James, B. R. Soil: The foundation of agriculture. Nature Education Knowledge 3(10), 2 (2012).
Rasiah V. & Armour J. D. Nitrate accumulation under cropping in the Ferrosols of far north Queensland wet tropics. Australian Journal of Soil Research 39, 329-341 (2001). doi: doi:10.1071/SR99133.
Sparks, D. L. Environmental Soil Chemistry, 2nd ed. San Diego, CA: Academic Press, 2003.
Sparks, D. L. & Huang, P. M. Physical chemistry of soil potassium. In Potassium in Agriculture, ed. Munson, R. D. (Madison: The American Society of Agronomy, Inc., Crop Science Society of America, Inc., and Soil Science Society of America, Inc., 1985) 201-276.
Stevenson, F. J. & Cole, M. A. Cycles of Soil: Carbon, Nitrogen, Phosphorus, Sulfur, Micronutrients, 2nd ed. New York, NY: Wiley, 1999.
Su C. & Suarez D. L. Coordination of adsorbed boron: A FTIR spectroscopic study. Environmental Science and Technology 29, 302-311 (1995). doi: 10.1021/es00002a005.
Thompson, A. & Goyne, K. W. Introduction to the sorption of chemical constituents in soils. Nature Education Knowledge 4(4), 7 (2012).
Wilson, M. J. Weathering of the primary rock-forming minerals processes, products and rates. Clay Minerals 39, 233-66 (2004). doi: http://dx.doi.org/10.1180/0009855043930133.