Plant Availability of Potassium in Soil Minerals: What’s Happening Near the Roots? (Part 1)

Plants require a lot of potassium (K), and most of what plants need comes from the soil. Soils contain a variety of minerals that contain K in their structures, but that K is not readily available for use by the plant. The traditional view is that K in these minerals comes out in small quantities over long periods of time as soils age. Physical forces and chemical reactions slowly break down these minerals, a process called weathering, releasing K into the soil water where plants roots can take it up. While this traditional view is correct for the bulk soil, weathering can occur quickly in the zone of soil right around and influenced by the root. This zone, called the rhizosphere, has a chemical and biological composition much different from the bulk soil. Bacteria bloom in the rhizosphere. Bacterial populations are often several times those found in the bulk soil. As plant roots grow, they leave behind a lot of carbon. This carbon comes from dead cells and from chemical compounds they release, including mucilage, a gelatinous substance that lubricates the root tip as it extends into the soil. Soil bacteria need carbon, and the rich supply of it in the rhizosphere fuels their growth. Some of the bacteria blooming in the rhizosphere can solubilize a portion of the K in soil minerals. Although there is still a lot to be understood about how they do it, at least some of the bacteria that have been isolated and studied produce acids and other carbon compounds that break the chemical bonds holding mineral structures together. As bonds break, K gets released and becomes available for uptake by roots. Fungi are also abundant in the rhizosphere. They too need carbon. Like bacteria, some fungi exude acids that break down minerals and release K that can be taken up by the plant. In addition, fungi grow mycelia. As these tubular filaments grow, they exert physical forces on soil minerals that can also break their structural bonds and release K.  [International Plant Nutrition Institute. Issue #1 2018]