In this study, three microorganisms, Bacillus subtilis, Sporosarcina pasteurii, and Bacillus subtilis subsp. subtilis, which are capable of inducing calcite precipitation effectively, were used to investigate possible stabilization and improvement in the compressive strength of the rammed earth. The microorganisms are suspended in a solution of yeast extract and ammonium sulfate. The effects of different concentrations of microorganisms and calcium resources were studied by producing a total of eleven mixes. The concentrations of the bacteria were determined by the optical density method, OD₆₀₀. As calcium resources, low-cost and locally available materials such as air lime and gypsum were used in different amounts. The influence of various microorganisms at different concentrations on earth stabilization was determined by performing uniaxial compression test at different ages, e.g., the 7th and 28th day after production. The cube specimens with a size of 50 x 50 x 50 mm were produced. The obtained results showed that both concentration of the microorganisms and the calcium amount in the medium are the main parameters affecting the mechanical performance of the earth samples. It was found that all microorganisms effectively enhance the compressive strength on and after 28 days. For a definite concentration of the B. subtilis, two times higher values, e.g., 8.4 MPa, were achieved on 28 days than reference mixes, which do not contain any microorganisms. Moreover, macro- and micro-scale observations are done to monitor calcite precipitation. It can be deduced that the preliminary test results are promising to be a feasible alternative to cement and or lime-stabilized rammed earth (SRE).

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