We first had the pleasure of working with Briana Alfaro and Soul Fire Farm in 2021 when they began testing soil with farmers in their network as part of their SARE research project, Soil Carbon Capture for Diverse Farmers; Black, Latinx, Asian, Indigenous and other farmers and farm workers of color take the lead in testing soil carbon sequestration strategies and measurement protocols and disseminate those findings to the farming community in both English and Spanish.
Every two years the Soul Fire Farm team takes a closer look at the soil ecosystem and assesses how healthy their soil is. They do this by performing a series of in-field tests. Long before the western study of soil science, Indigenous communities practiced–and still practice–methods of evaluating soil health using characteristics such as color or the presence of specific plants or insects that tell us something about the system as a whole. On their soil testing days, they count the number of earthworms, perform a slake test to observe aggregate stability, look at soil color as an indicator of organic matter, and use the microBIOMETER® field kit to assess soil biology.
You can learn more in their Guide to In-Field Soil Health Measurement Protocols: How Alive is My Soil (English) & ¿Qué tan vivo está mi suelo? (Español), and by watching their Liberation on Land skill share videos: Soil Carbon part 1, Soil Carbon part 2 & Investigating Soil with an Auger.
Both microbial biomass and respiration are parameters used to assess soil health. Soil respiration is the measure of the carbon dioxide produced by the microbes in a given weight of soil while microbial biomass is the measure of the mass of microbes- both active and dormant.
Microbial biomass (MB) is an excellent predictor of soil health because the size of the microbial population correlates with the available nutrients in the soil. Interestingly, MB is low in soil treated with high levels of mineral fertilizers. Research has shown that the stimulus for the plant to grow a microbial population is its need for nitrogen and phosphorus. If these nutrients are artificially supplied, the plant is not being stimulated to feed the microbes that usually provide these nutrients to the plant. This can alter plant-microbe interactions and cause an increased need for pesticides in order to protect the plant, as microbes play a fundamental role in the function of the plant’s immune system.
Microbial respiration measures the amount of carbon dioxide (CO2) 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, though it doesn’t.
People often assume a high respiration rate is good because it means there is a lot of microbial activity occurring. However, it doesn’t necessarily mean the soil is healthy. Microbes in a low pH or toxic soil have to work harder, and therefore their respiration rate is higher, just as your respiration rate in the gym is higher than when you are watching TV. High respiration rates can indicate an unstable microbial population, which, for example, can be seen after excessive tillage occurs. Tillage aerates the soil, so right after there is often a boost of microbial respiration. That increased activity however does not always last, as the other damage done by tillage – disruption of microbial life and destruction of existing plants- can lead to a decreased soil microbial population over time.
The use of soil primers stimulates an increase in soil organic matter (SOM) decomposition, which temporarily increases microbial respiration. Excessive decomposition of SOM can cause a loss of stored soil carbon and other mineral nutrients, allowing for the increased production of CO2. Basically, when you stimulate the soil using a fertilizer or biostimulant, it’s an all-you-can-eat buffet for the microbes. It wakes them up and they start growing and reproducing. But whether they can continue to grow depends on the continual supply of existing nutrients and plant life in the soil. It’s very important that there be sufficient food for the microbes after stimulation. For most soils, this requires that the fertilizer have the correct C:N ratio for the soil and crop. A fertilizer with too high a C:N ratio will cause the microbes to harvest some of the stored carbon, nitrogen and other nutrients in the soil, boosting respiration. This means the stored carbon is being depleted and released into the atmosphere as CO2, the microbes won’t be able to nourish the plant and build soil structure as needed. Adoption of less invasive management practices, such as select-till and reduced chemical fertilizers can reduce CO2 emissions from agricultural soils by retaining soil organic matter.
Priming can be a good way to understand the difference between and uses of respiration data and microbial biomass data. Testing for both initial respiration and long term microbial biomass population can tell you if the priming worked and if the increase in microbial activity led to increased soil microbial biomass and therefore increased soil health and fertility.
Increasing your soil microbes increases carbon sequestration. Carbon is stored in the soil as “humic materials” i.e. C,N,P,K etc.; rich organic matter which is the soil organic carbon or sequestered carbon in the soil.
The formation of humus, the final stable carbon, is a stepwise process. All organic carbon in soil comes from plants, either directly or via digested plant material. It starts with plant material being digested by soil microbes, or in the case of brown manure, being predigested by animals and further digested by microbes. The breakdown process begins with soil fungi and bacteria. As these microbes are fed carbon, they multiply. If fresh carbon stores are not utilized, they become attached to soil particles and become stored, therefore, less available as food sources. As microbes die, if they are not immediately cannibalized, their remains also become part of the more recalcitrant humic material.
Slowly, this humic material, which is as much as 80% the bodies of dead microbes, builds up. We measure it as soil organic carbon (SOC) and it reflects the carbon sequestered in the soil, but it also contains all the minerals and other plant nutrients. To increase SOC, the fresh organic matter required to feed the microbes and in turn the plant via the microbes, there needs to be an excess of the minimum required for a low microbial population. If there is an excess, the microbial population increases, and their dead bodies will increase the humic matter, in return increasing carbon sequestration. If it is not adequate, the soil microbes will be stimulated by the plant to mine the stored organic matter, which will decrease the stored carbon. It is not surprising that scientists have compared the plant/microbe/soil fertility index to economic models. A rich soil, like a rich man, has money in his pocket and money in the bank, for soil the currency is carbon.
This system is very much like our agricultural complex. There is fresh food, which we utilize within days, food we freeze or can, which requires freezers and can openers to access, and food stores (our sequestered carbon) that we maintain in silos as protection against disaster.
Understanding Soil Organic Matter and its impact on soil health and microbial biomass.
We are often asked what is a good level of microbial biomass (MB). There is no one answer. The level of MB you can reach is dependent on soil organic matter (SOM.) Soil organic carbon (SOC) is a large part of soil organic matter but SOM is a mixture of Carbon (C), Nitrogen (N), Phosphorus (P), Sulfur (S) and all the other minerals that microbes and plants need.
There are 2 types of SOM: Stable SOM, often referred to as humic matter; and Fresh SOM. Fresh SOM is composed of SOM material recently released from Stable SOM and any fertilizers, amendments or litter. You can compensate for low stable SOM by providing lots of fresh SOM. The key to the efficacy of fresh SOM is that it needs to be nutrient balanced*, i.e. it needs the correct balance of C,N,P, and S. That is where understanding soil chemistry and using the right additives comes in.
Think of SOM as your credit reserve. In spring, the plant starts to grow and puts out exudates that stimulate the microbes to multiply. But these multiplying microbes need more than the sugars that the plant supplies, they need the N, P, S and micro nutrients that are in SOM.
Agronomists often cultivate soil for intensive organic agriculture and those soils contain lots of fresh organic matter. The microbial biomass of these mixtures can read as high as 2000 ug MBC/gram of dry soil. As the microbes and plants in this rich soil die, they become fresh SOM. The amount of stable SOM that soil can store depends to a large degree on the type of soil because storage requires mineral surfaces for attachment and aggregates for protection. If your soil is inherently poor at storing SOM, you will need to rely on fresh SOM to feed your microbes and plants.
We highly recommend that you read the review referenced below to better understand SOM.
Coonan, E.C., Kirkby, C.A., Kirkegaard, J.A. et al. Microorganisms and nutrient stoichiometry as mediators of soil organic matter dynamics. Nutr Cycl Agroecosyst 117, 273–298 (2020). https://doi.org/10.1007/s10705-020-10076-8
