Release of plant-available silicon from various silicon-rich amendments into soil solutions and leachates

Abstract

This study assessed the efficacy of selected silicon-rich soil amendments in terms of their ability to provide dissolved silicon (Si) to soil solutions during a 15-month experiment. Four inorganic amendments (peat ash, coal ash, rice husk ash and ground granulated blast furnace slag (GGBS)) and one organic amendment (rice husks) were mixed with a natural field soil. An un-amended control and two liquid-based Si-positive controls (sodium metasilicate and a commercial Si fertiliser (Pro-Tekt)) were also included. Soil-filled 4-l pots (n = 5 per treatment) were placed outdoors, over funnels to collect soil leachates, while soil solution samplers were installed in the pots to collect in situ soil solutions. Plant available Si concentrations were measured in the soil solutions and leachates using colourimetric heteropoly blue method. Rice husks were found to be most effective soil amendment as they provided the highest release of dissolved Si into the soil solutions (22.3 to 47.9 mg Si l−1) and leachates (10.9 to 31.0 mg Si l−1). Si release from rice husks was found to be greater than rice husk ash and either of the two liquid-based amendments. Si release from the peat ash, coal ash and GGBS amendments was found to be very low for both soil solutions (0 to 1.8 mg Si l−1) and leachates (0 to 2.0 mg Si l−1), suggesting that these are poor choices for Si soil amendments. Release of dissolved, plant available Si from rice husks and rice husk ash was found to be consistently high for the duration of the experiment, making them the most attractive candidates for slow release Si-rich fertilisers for crop production. By contrast, despite being used previously as a soil amendment, coal ash and GGBS were found to be less suitable as they released little soluble Si. Evaluation of the release rates of soluble Si from any proposed soil amendments prior to their application to soils is therefore essential.