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) How do chemical reactions involving soil minerals play a crucial role in controlling the availability of essential plant nutrients? 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 Earths 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. 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. 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 Earths 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 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, and such nutrients are not readily available to plants. 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. 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. 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. Read on at nature/scitable/knowledge/library/soil-minerals-and-plant-nutrition-127881474
Posted on: Sun, 18 Jan 2015 23:40:51 +0000
Trending Topics
Recently Viewed Topics
© 2015