IMPROVEMENTS TO THE USE OF BENTONITE CLAY AS A PROTECTIVE AGENT, INCREASING SURVIVAL LEVELS OF BACTERIA INTRODUCED INTO SOIL

Bentonite clay was used to improve the survival of Rhizobium leguminosarum biovar trifolii cells introduced into soil. Adding 5% bentonite clay to a loamy sand before inoculation markedly improved bacterial survival through the creation of large amounts of protective microhabitats. However, amending loamy sand with 5% bentonite clay also had a strong influence on soil physical properties. We therefore studied different introduction methods, with the aim of applying less bentonite, but maintaining high bacterial survival levels similar to those found with 5% bentonite. Using starved cells instead of a freshly grown culture slightly increased survival in a loamy sand in the absence of bentonite. In the presence of bentonite, no positive effects on survival due to the use of starved cells could be found. Soil-adapted rhizobia survived better in unamended and bentonite-amended loamy sand than freshly cultured rhizobia. However, this was only the case in soil containing up to 1% bentonite. The previously found high survival levels using 5% bentonite could not be reached. Mixing a fresh rhizobial culture or freeze-dried cells with bentonite clay before introduction into soil was very successful in enhancing bacterial survival. Only 1% of bentonite was necessary to reach survival levels comparable to those found previously, after mixing 5% bentonite with loamy sand before inoculation. It was concluded that when bacteria and bentonite were mixed prior to inoculation, the clay offered protection at the site of introduction, resulting in a more efficient use of bentonite for enhancing bacterial survival.