Inoculating Soil with Beneficial Bacteria

Lebanon Seaboard produces several Roots products that contain beneficial rhizosphere bacteria to promote plant growth. Recently, some of you have encountered researchers who contend that the use of such products is meaningless, since even heavily managed soils have high bacteria counts already, so that application of limited numbers of selected microbes is “like throwing water in the ocean.” How do you respond to this argument?

Numbers Don’t Tell the Whole Story

Such an argument can be very compelling, especially when microbial counts are provided, showing many thousands of counts per gram of soil prior to any treatment. This is no surprise, since all soils are exposed to the environment, and are susceptible to natural, random inoculation. However, the mere presence of microbes does not characterize a functioning ecosystem. Herein lies the difference. Consider two battleships. Both were manned with 5000 sailors. Ship A was manned by taking a list of rookie sailors who were recently conscripted, and the first 5000 names were assigned to this ship. Ship B was manned by selecting sailors based on their skills. A certain number of cooks, officers, weapons technicians, medical personnel, etc., were selected to fill the various occupations needed to run a ship. At first glance, both ships appear to be equally manned with 5000 personnel. But which ship is ready to sail? So, is it adequate to merely count microbes to compare soils?

Established Industrial Practice

This same concept has been demonstrated repeatedly in industrial microbiology. One of the most established industrial uses of microbes involves the sewage industry. The sewage treatment plant receives raw sewage for microbial digestion prior to release into open waters. Raw sewage is jam- packed with microbes—some of the highest counts imaginable. And yet, sewage treatment plants routinely inoculate their primary treatment process using sludge taken from the last run. By doing so, they achieve rapid, efficient cleaning of the sewage. In cases of newly built plants, where there is no sludge from a prior run, they will acquire inoculum either commercially, or by borrowing a sample of sludge from a neighboring plant to use as their inoculum. Why is this necessary, since raw sewage is already jam-packed with microbes? Apparently, the counting of microbes is not a legitimate measure of their efficacy in performing a particular task. The same is true when studying soil microbial activity. Bacterial counts are not a reliable measure of plant growth-promoting activity. There is definite value to inoculating with at least some of the microbes needed to achieve specific valuable results.

Established Agricultural Practice

Another example involves leguminous crops. Legumes like peas and soy beans, associate with Rhizobium bacteria that live in nodules formed on the plant roots. Within these nodules, the bacteria fix nitrogen, allowing legumes to grow even in nitrogen-deficient soils. Since the discovery of this association, farmers have been routinely inoculating their leguminous crops with Rhizobium to achieve higher yields. Why would this work if inoculating soils with bacteria were tantamount to throwing water in the ocean?

Function, Not Numbers, Is the Key

This same principal applies throughout microbiology. A high count of microbes may comprise a population on paper. But it will not necessarily function as an ecosystem. That is the fundamental difference between soil taken from a forest, and soil taken from a golf course green. One has an interconnected population of microbes functioning efficiently as an ecosystem, while the other is often a hodgepodge mix of various microbes competing in the face of typically harsh management practices. By inoculating the green with selected microbes designed to provide specific services, we are attempting to insert particular activities to the green that will improve the interactions among microbes and plants.

The act of inoculating specific microbes into an existing population is an important step in converting an unstructured microbial population into an efficient ecological machine. This has been proven repeatedly in various different applications such as those cited above. Apparently, the microbial inoculation of selected species into areas where high populations of microbes may already exist does not correspond to “throwing water in the ocean.”