The present invention provides a method for preparing a biological fertilizer composition that comprises yeast cells and poultry manure. The yeast cells of the invention have an enhanced ability to fix atmospheric nitrogen, decompose phosphorus minerals and compounds, decompose potassium minerals and compounds, decompose complex carbon compounds, overproduce growth factors, overproduce ATP, decompose undesirable chemicals, suppress growth of pathogenic microorganisms, or reduce undesirable odor.