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NEW ADVANCES IN BORON SOIL CHEMISTRY
Citation:
GOLDBERG, S. AND C. SU. NEW ADVANCES IN BORON SOIL CHEMISTRY. Presented at 3rd International Symposium on All Aspects of Plant and Animal Boron Nutrition, Wuhan, CHINA, September 09 - 13, 2005.
Impact/Purpose:
To inform the public.
Description:
Boron is an essential plant micronutrient for which the range between deficiency and toxicity is narrower than for any other nutrient element. Plants respond directly to the amount of B in soil solution and only indirectly to the amount of B adsorbed on soil particle surfaces. Therefore the adsorption complex acts as both a source and a sink for B and can attenuate phytotoxic soil solution B concentrations. In this presentation we will discuss various chemical aspects of B soil chemistry that play an important role in governing plant B response. These include: methodologies for determining the mechanisms of B attachment to soil particle surfaces, kinetics of B desorption reactions, description and prediction of B adsorption reactions using chemical models, use of B soil tests to predict plant response in field situations. To accurately describe the adsorption behavior of B in soils the mode of attachment of B to soil particles must be known. Infrared spectroscopy is an in situ technique that can provide insight into the type and number of bonds that an adsorbed ion forms with the solid surface. Boron was found to form strong inner-sphere complexes containing no water between the adsorbed B and the surface functional groups on the soil minerals: amorphous iron oxide, amorphous aluminum oxide, and allophane. Boron has also been found to form weak outer-sphere complexes containing at least some water between the adsorbed B and the surface functional groups on amorphous iron oxide. This physically bound B could be readily leached and would be available for plant uptake. Excess soluble B in arid land soils is often attributed to the weathering of B containing minerals. Reclamation of high B soils may be followed by continued release of sparingly soluble sources of B from the presence of illite, chlorite, and palygorskite minerals in some soils. Weathering of such soils may produce toxic levels of B within a few years following reclamation unless continued leaching is maintained. Chemical surface complexation models describe ion adsorption both as a function of solution ion concentration and solution pH. A general regression model has been developed to obtain soil B surface complexation constants to allow prediction of B adsorption using this approach. The model parameters are obtained from easily measured soil chemical properties: surface area, organic carbon content, inorganic carbon content, and free aluminum oxide content. The regression model was well able to predict B adsorption by 37 diverse soils from both the Southwestern and Midwestern parts of the USA. These results suggest widespread applicability of the prediction approach for describing B adsorption behavior in soils both as a function of solution B concentration and solution pH. Historically, evaluations of the ability of B soil tests to predict plant B content have been conducted in greenhouse studies. These conditions provide a much more controlled environment than the field, where clay and water content and root distribution vary considerably. It has been shown that B soil tests that provided extremely high correlation with plant B content in container studies provided only poor predictability of B content in field grown plants despite statistically significant relationships. These results would suggest that B soil tests may not be able to represent spatial and temporal distribution of soil solution B experienced by plants in the field.