Microbial Biomass vs. Microbial Respiration

What is the difference between microbial biomass (MB) and microbial respiration rate (RR) ?

Both parameters are used to assess soil microbial health. The respiration assay measures the amount of carbon dioxide produced by the microbes in a given weight of soil. The soil is dried and then rewetted and put in an airtight jar that allows measurement of the amount of CO2 produced over 24 hours. The CO2 is produced by the activity of the microbes in the rewetted soil. Between 20% and 70% of the microbes die during drying but their dead bodies often provide nutrition for the survivors to use and regrow the population to its original level.

Respiration reflects the regrowing work that is being done. The respiration level is often mistakenly believed to predict microbial biomass (MB), but it does not. Microbes in a low pH or toxic soil have to work harder, therefore, their respiration rate is higher, just as your respiration rate in the gym is higher than when you are watching TV. Outside of the U.S. the respiration rate (RR) is only considered in relation to MB and this q-value RR/MB is used to determine the level of stress in a soil. If RR is high for the MB, the soil is in trouble.

MB, as measured by microBIOMETER®, correlates with chloroform fumigation—always and microscopic evaluation of soil. It is an excellent predictor of soil health because the size of the microbial population correlates with the nutrition available in the soil. If the soil is deficient in carbon, nitrogen, phosphorus or any other mineral, or contains toxins, MB will be low. In fact, MB is low in any soil that is compacted, has a low pH or is overly dry, because microbes need oxygen and moisture and the correct pH for enzymatic activity.

In nature, the plant uses 30% of its food production to feed a microbial population that will mine the soil for the N, P, K, S etc. that it needs. Interestingly MB is low in soil treated with high levels of mineral fertilizers; researchers have shown that the stimulus for the plant to grow a microbial population is its need for nitrogen and phosphorus. If these are artificially supplied the plant is not stimulated to feed the microbes that usually provide these nutrients to the plant. And since the microbes are at least half of the immune system of the plant, you now need lots of pesticides to protect the plant.

Simple ways to increase the microbial biomass in your soil

Healthy soil is brimming with beneficial microbes, and those microbes are one of the important keys to ensuring the health of your plants. Along with breaking down key nutrients for your plants, they’ll aerate the soil so nutrients are evenly distributed, and fend off parasitic microbes so your garden can grow in peace.

Considering the wealth of benefits, it’s no surprise that it is recommended that you do everything you can to maximize the microbial biomass in your soil. While there’s complicated science behind it, nourishing and increasing the amount of microbes in your soil is simple, and can be accomplished with a few tried and true methods. And  thanks to the microBIOMETER®  soil test, even amateur gardeners can track their microbial biomass levels. 

First, let’s detail how you can take care of those important microbes and enhance their numbers. It’ll involve shedding some old gardening habits, along with taking on some new ones, but we promise the end results will be worth it. 

What To Avoid

Before you start taking extra steps to care for and increase your microbial biomass, you should ensure you’re avoiding certain tactics that are known to hinder their growth.

  • Pesticides

While you might think avoiding pesticides wouldn’t enhance plant health, a close look at the ingredients of most pesticides will show you they do far more harm than good. Amongst a variety of issues, one of the most harmful is the fact they decimate microbial populations in the soil. If you want to ensure pests will stay away in the absence of pesticides, try utilizing companion plants instead.

  • Fungicides

While pesticides are bad, fungicides are even more of a threat. Some of the most vital microbes in your soil, being fungi, would be directly targeted by these treatments. The harshness of these chemicals would also wreak havoc on the non-fungi microbes, all but eliminating any trace of a microbial biomass. Even if you can’t do everything on this list, ensure you at least abide by this particular rule. 

  • Tilling

Lastly, while many gardeners and farmers consider tilling a standard gardening process, you’ll want to abstain from it if you’re focusing on your soil’s microbes. That, of course, is due to the level of soil disturbance that occurs during the process. The process leads to lost microbes (especially fungi), and any benefits gained from additions made to the soil end up being cancelled out. By avoiding tilling, you’ll allow the delicate environment in your soil to function undisturbed and, in turn, at full capacity. 

What To Do

Now that you’ve cut those bad habits out of your gardening routine, you have room for a few that’ll greatly benefit your soil in the long run.

  • Composting

Nothing gets microbes into the soil like a nice big pile of compost! All that food breaking down in one big pile is basically a feast for all the helpful microbes you want around your plants. Once you add it onto your soil, then turn it to make sure air hits every part of it, you’ll be ensuring the microbes have plenty of energy to break down nutrients. To ensure the best compost possible, make sure you add in natural components like grass clippings, fruits, vegetables, wood chips, and straw. There’s no need to exclude other foods, even processed ones, but a healthy blend of green and brown material is a must. 

  • Compost Teas

Following the same logic, compost teas can do wonders for the microbes in your soil. All you have to do is take some compost and put it in a water permeable pouch,  add some microbe feeding nutrients (perhaps like molasses), and let it brew (bubbling air into it) until the microbes in the compost have multiplied and the tea is full of microbes. Once done, pour it all around the base of your plants. One round will do your plants good, but repeating this process a few times during your growing process will really make a difference.

  • Optimize soil moisture, pH, and temperature

This last step is actually three steps and if these conditions aren’t met, virtually nothing else on this list will have a noticeable effect. To start, making sure you have adequate moisture is as simple as regularly watering your plants. You may also want to consider purchasing a moisture meter to assure your levels are ideal. Next, the ideal pH range for soil is between 6.0 and 7.0, so you’ll have to test your soil to see where you’re at. If your soil pH is too low try adding limestone and if your pH is too high you can add aluminum sulfate and sulfur to get things balanced. Lastly, mulching is a great way to help your soil maintain an even temperature. 

Incorporating these simple tactics into your crop management is an important first step to building the microbial biomass in your soil. Another critical step is testing and quantifying the results of these inputs since decision making without data is like driving blindfolded. microBIOMETER® is a rapid, on-site soil test for microbial biomass. Microbes respond very quickly to any changes in the soil, therefore, you can set a baseline then retest within a week to see if you are heading in the right direction.