Inspired by the Natural Resources Conservation Service Oregon “soil your undies” challenge that helped spark greater interest in soil microbial health in the United States, ECHO Florida tried our own cotton strip trial to approximate soil microbial activity in different natural farming treatments.
Cotton is a carbon source for microbial metabolism and decomposes as bacteria and fungi exude compounds that break down carbon-based materials like cotton. Using cotton strips is a very simple and low-resource way to compare microbial activity in different soils.
The process is as follows:
- Cut cotton strips that are roughly the same size and weight out of the same material. Record the initial weight of each strip.
- Enclose each cotton strip in a mesh material. We used old mosquito netting. This keeps larger soil shredders, such as millepedes, from accessing the cotton. We sowed our mosquito netting, but you could also try heat-sealing.
- Bury the cotton strips 4 to 5 cm below the soil surface. This is the depth at which most of the roots of annual plants are and is therefore the depth with greatest levels of microbial activity. You could bury the cotton strips deeper if you are assessing microbial activity in soil around perennial plants with deeper roots. Make sure to mark the area with a flag or other indicator so that you don’t forget the location. Alternatively, you could leave one corner of the mesh bag out of the soil so that it’s easier to find.
- Wait! You can leave it in for as little as 1 week or as long as 3 months depending on your rainfall and temperature. We found that in Florida, almost all of our 10 g cotton strips were completely decomposed within 1 month, so it was important to dig them up before 1 month had passed.
- Carefully excavate the mesh bags. Gently wash the mesh bags to remove as much of the soil as possible. I found that if I dried the bags first, then rolled something heavy over them (a rolling pin), and shook them, most of the soil (mostly sand where we are) came off.
- Take the cotton strip out of the mesh bag and weigh it.
- Subtract the final weight from the initial weight to obtain the amount of cotton decomposed.
Plots with the highest amounts of cotton decomposition are the ones with the most soil microbial activity. This is in no way a precise, absolute measurement. The test cannot tell you which types of microbes exist in your soil. Rather, the test result provides a relative indicator of overall soil life. You could use the technique to test the same area of your field each year around the same time to see if your soil life is increasing over time.
In our trial, we buried cotton strips in plots with the following treatments:
- Soil drenched biweekly with IMO (Indigenous Microorganisms). The surface was mulched
- Soil drenched biweekly with activated EM (Effective Microorganisms). The soil surface was mulched
- Soil surface mulched
- Soil surface not mulched (control)
Figure 1. Replication D cotton strips in bags after removal of most of the soil upon excavation.
Each treatment was applied to 5 plots, giving us 5 replications for each treatment.
Figure 2. Amount of decomposed cotton for each treatment in the natural farming trial.
The only significant difference in the amount of cotton decomposed was between the EM treatment and the no-mulch control (Figure 2); cotton decomposition was highest and least with EM and the control, respectively. Visually, treatments with mulch were more decomposed than the no-mulch control. This makes biological sense as mulch helps retain moisture and buffer extreme heat, both of which promote soil microbial activity.
Have you ever tried this? Were you surprised by anything you observed?