Lecture 3

Soil Microorganisms• Bacteria are classified as either autotrophs or heterotrophs, depending on theirsources of energy and C. They can be further grouped as aerobes (require O2),anaerobes (do not require O2), and facultative anaerobes (grow in the presence orabsence of O2).• The type and abundance of any one group of bacteria depend on such factors asavailable nutrients and soil environmental conditions. Some of the aerobic andanaerobic bacteria form spores that allow them to survive under adverseenvironmental conditions during dry and high temperature periods.• Soil conditions that affect the growth of bacteria are many; however, the mostimportant factors are the O2, temperature, moisture, acidity, and inorganic andorganic nutrient status of the soil. Bacteria and fungi are usually active in aeratedsoils, but in ecosystems containing little or no O2, most of the biological and chemicaltransformations are governed by bacteria.• Optimum temperature and moisture requirements for most bacteria are 20 to 40°Cand 50 to 75% of the moisture-holding capacity of the soil, although specializedgroups are active at higher and lower temperatures and moisture contents. Extremelyhigh and low pH are generally not suitable for most bacteria, but there are specializedbacteria that are active in soils with pH levels less than 3 and others above pH 10.