Are Clay Minerals a Significant Source of Si for Crops? A Comparison of Amorphous Silica and the Roles of the Mineral Type and pH

Identifying the source(s) of silicon (Si) for plant is a key issue in understanding the terrestrial cycle of Si and for deciphering the reservoir of bioavailable Si to Si accumulating crops. In soils, amorphous Si, one of the most bioavailable source, is mostly present as phytoliths and has been suggested for use as a Si fertilizer by diatomite application. Although clay minerals are known to contribute to plant nutrition, their role as a major source of silica for plants has not been fully addressed. We aim at evaluating the efficiency of clay minerals as a source of Si for crops. We conducted two pot experiments: one wheat-growing experiment to compare a clay (vermiculitic) mineral and amorphous silica particles (diatomite, which is used as a phytolith substitute), and one rice-growing experiment to compare two types of clay (kaolinite vs montmorillonite) common in rice cultivation. We confirmed that the amorphous silica was more efficient than vermiculite for Si uptake by wheat. However, the Si uptake was not significantly different between the 5% diatomite substrate and the 25% vermiculite substrate indicating that clays may challenge amorphous silica, as a source of Si for crops. The kaolinite probably delivered less Si to the rice than the montmorillonite because of the lower specific surface area and lower pH of kaolinite substrates. Because clays are generally much more abundant in soils than amorphous silica, we concluded that clays may be a substantial Si source for plants, depending on the clay mineralogy.