Soil Productivity - Plate
The soil is one of the main reservoirs of microbial life. Typical garden soil has millions of bacteria in each gram. The most numerous microbes in soil are bacteria (Table 1). Although actinomycetes are bacteria, they are listed separately because conidiospores make their dry, powdery colonies easily recognizable. Soil bacteria include aerobes and anaerobes with a wide range of nutritional requirements, from photoautotrophs to chemoheterotrophs. As usable nutrients and suitable environmental conditions (such as light, aeration, temperature) become available, the microbial populations and their metabolic activity rapidly increase until the nutrients are depleted or physical conditions change, and they then return to lower levels.
Human pathogens, with the exception of endospore-formed bacteria, are uncommon in the soil. Soil microorganisms are responsible for recycling elements so they can be used over and over again. The numbers of bacteria and fungi in soil are usually estimated by the plate count method. The actual number of organisms is probably much higher than the estimate, however, because a plate count only detects microbes that will grow under the conditions provided (such as nutrients and temperature).
In a plate count, the number of colony-forming units (c.f.u.) are determined. Each colony may arise from a group of cells rather than from one individual cell. The initial soil sample is diluted through serial dilutions in order to obtain a small number of colonies on each plate. A known volume of the diluted sample is plated on sterile nutrient agar. After incubation, the number of colonies is counted. Plates with between 25 and 250 colonies are suitable for counting. A plate with fewer than 25 colonies is inaccurate because a single contaminant could influence the results. A plate with greater than 250 colonies is extremely difficult to count. The microbial population in the original soil sample can then be calculated. For example, if 232 bacterial colonies were present on the plate containing 0.01 mL of the 1:1,000,000 dilution, the calculation would be
|Distribution of microorganisms in numbers per gram of typical garden soil at various depths|
99 mL sterile water in a bottle (3)
Pipettes or calibrated one-piece plastic droppers
Spreading rod (L-shaped glass rod or place of piece of No. 16 shrinkable Teflon tubing over a large, straightened (L-shaped) paper clip or metal spreaders can be purchased from Carolina Biological Supply)
Alcohol (rubbing alcohol, 70% isopropyl alcohol)
This procedure can also be used with food, milk, and water.
Figure 2. Use of a spreading rod.
Figure 1. Serial dilutions