Micronutrient Solubility and Availability in Soils

By the term ‘micronutrient availability’ we mean the total micronutrient forms in soils, which are available for plants (i.e., all the soluble forms that can be taken up by plants). Availability depends on solubility in soils, i.e., the solubility of micronutrients in soils determines their availability for plants (thus their uptake) and their downward mobility. Understanding the factors controlling trace element solubility allows the selection of soil amendments that promote or reduce their availability and of course the selection of the suitable plant species (those that are optimal for the desired goal of managing trace element influx in the soil-plant system). There are many soil factors influencing micronutrient solubility and availability for plants. The most important of these factors are pH, organic matter content, CaCO3, soil texture, cation exchange capacity (C.E.C.), erosion, soil moisture and temperature etc. Generally, trace element solubility in soils depends on a plethora of physical (e.g., water retention capacity), chemical (e.g., pH), microbial (e.g., mycorrhiza) and plant factors, as well as on the properties of each nutrient, which are fully analyzed below. There are many extractants (like DTPA, EDTA, Mehlich-1, Mehlich-3) used to determine available for plants quantities of micronutrients in soils; since great differences exist in the extractability of these chemical solutions, in order to estimate plant available micronutrient concentrations it is absolutely necessary to study micronutrient extractable concentrations in soils, in relation to their uptake by plants. All the topics concerning micronutrient solubility in soils, together with the factors influencing micronutrient availability and uptake by plants, are analyzed in detail in this chapter. In addition, a comparison between the extractant solutions used to estimate plant available quantities of micronutrients in soils is included in the second part of this chapter.

Keywords: CaCO3, cation exchange capacity (C.E.C.), micronutrient availability, micronutrient solubility, organic matter, pH, soil moisture, soil texture, trace elements.