The chemistry of pedogenic thresholds

Soils formed in volcanic ejecta have many distinctive physical, chemical, and mineralogical properties that are rarely found in soils derived from other parent materials. These distinctive properties are largely attributable to the formation of noncrystalline materials (e.g., allophane, imogolite, ferrihydrite) containing variable charge surfaces, and the accumulation of organic matter. Formation of noncrystalline materials is directly related to the properties of volcanic ejecta as a parent material, namely the rapid weathering of glassy particles. The composition of the colloidal fraction forms a continuum between pure Al–humus complexes and pure allophane/imogolite, depending on the pH and organic matter characteristics of the weathering environment. For soil management purposes, volcanic soils are often divided into two groups based on the colloidal composition of the surface horizons: allophanic soils dominated by allophane and imogolite, and nonallophanic soils dominated by Al–humus complexes and 2:1 layer silicates. Volcanic soils exhibit a wide range of agricultural productivity, depending on the degree or intensity of pedogenic development and the colloidal composition of the rooting zone. The different charge characteristics of allophanic and nonallophanic soils is the most important factor regulating chemical fertility attributes. Phosphorus fixation, strong acidity, and aluminum toxicity are the primary chemical limitations to agricultural productivity. Volcanic soils generally have high physical fertility (tilth) and mature soils are relatively resilient to erosion and compaction. To maximize the productivity of volcanic soils, proper management based on an understanding of the unique physical, chemical, and mineralogical properties of these soils must be practiced.

The Nature, Properties and Management of Volcanic Soils

In this study, an assay to quantify the presence of aluminum ions using a salicylimine-based receptor was developed utilizing turn-on fluorescence enhancement. Upon treatment with aluminum ions, the fluorescence of the sensor was enhanced at 510 nm due to formation of a 1:1 complex between the chemosensor and the aluminum ions at room temperature. As the concentration of Al3+ was increased, the fluorescence gradually increased. Other metal ions, such as Na+, Ag+, K+, Ca2+, Mg2+, Hg2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Pb2+, Cr3+, Fe3+, and In3+, had no such significant effect on the fluorescence. In addition, we show that the probe could be used to map intracellular Al3+ distribution in live cells by confocal microscopy.

Salicylimine-Based Fluorescent Chemosensor for Aluminum Ions and Application to Bioimaging

Pyrrolidine constrained bipyridyl-dansyl click fluoroionophore as selective Al3+sensor

A conformationally constrained (coumarin−pyrrolidinyl−triazolyl−bipyridyl) fluoroionophore conjugate was synthesized through click chemistry. The fluoroionophore serves as a selective chemosensor for Al3+ based on internal charge transfer. The coumarin−bipyridyl chemosensor exhibited a high association constant with submicromolar detection for the aluminum ion.

Conformationally constrained (coumarin-triazolyl-bipyridyl) click fluoroionophore as a selective Al3+ sensor.

Aluminium is a pH-sensitive element that can cause acute toxicity symptoms in some organisms at aqueous activities of 10 µM or less1–3. Scientists working on agricultural systems have long been concerned with the deleterious effects of aluminium on crop roots4,5. More recently, environmental scientists have reported a potentially harmful biogeochemical link between acidic deposition onto forest soils and aluminium toxicity in forest and aquatic communities of northeastern North America and northern Europe6–8. Because of this general interest in aluminium toxicity as an environmental threat, there have been renewed efforts to model the chemistry and transport of aqueous aluminium in soils and surface waters. Here we propose that much of the spatial and temporal variability in aqueous aluminium chemistry can be accounted for by a two-component equilibrium model involving a solid-phase humic adsorbent and an aluminium trihydroxide mineral phase. Inputs for the model are solution pH, copper-extractable organic aluminium and the titratable carboxyl content of soil humus.

Paul R. Bloom

Papers

Predicting aqueous aluminium concentrations in natural waters