Bacteria must be present to perform their functions of competing with disease-causing organisms, retaining nutrients and making microaggregates to improve soil structure.
When the total bacterial count is too low, bacteria have to be added back to the soil. This is most commonly done by adding compost or compost tea. Add them back by using a healthy, aerobic compost, compost tea or commercial inoculum
If bacterial activity is too low, the bacteria must be fed. Just like any other creature, bacteria requires food. Plant roots often supply the simple carbon substrates that bacteria require, such as simple sugars, proteins, and carbohydrates. Bacteria need N, P, K, Ca, and all the other nutrients as well, and obtains those from organic matter and from inorganic sources as well.
Various species of bacteria can solubilize mineral elements from the mineral components of soil, but no one species can effectively solubilize ALL minerals. Diversity of species to obtain all the needed nutrients is required.
Often soil tests will indicate that some nutrient is in low supply, but merely by adding the appropriate bacterial or fungal species, these organisms will convert plant unavailable nutrients into plant available forms. Diversity is the key as well as feeding that diverse set of species so they will perform their functions.
If activity is low, then bacterial foods need to be added to increase growth rates and improve numbers. A diversity of foods needs to be added, and thus molasses is a much better choice than white sugar. Fish hydrolysate also adds fungal foods, and N and other micronutrients. Fruit juices can be used as well, but diversity is key.
If activity is higher than the desired, then try to balance the ratios of the organisms by improving the organism group that is too low.
If active bacterial biomass is low, but total bacterial biomass is high, this is a good indicator that anaerobic conditions have occurred. In rare instances, it may be because some environmental disturbance occurred that put the majority of the bacteria to sleep, but did not kill them.