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How’s Life in the Soil? Ask (Count) the Earthworms.

Updated November 17, 2020
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Anthony Bly

SDSU Extension Soils Field Specialist

Written collaboratively by Debankur Sanyal, Johnathon Wolthuizen and Anthony Bly.

Agriculture in the twenty first century integrated natural resources conservation while pushing the boundaries in global crop production. Building better soil health has become more and more important in the past few decades with regenerative agriculture being promoted by scientists all around the world. Healthy soils are those that are full of optimally functioning natural organisms from the microscopic all the way up to gophers and badgers; earthworms are one of the most important factors in the health of soil.

Earthworms are ‘very special’ creatures on earth and their contribution in soil nutrient cycling and fertility management has been acknowledged from the beginning of agriculture. It is often said that in no-till farming, the tillage is done by earthworms. In search of food and water, earthworms move through the soil while they burrow a series of tunnels, improving soil porosity and ultimately soil health. Their activities above and below ground speed up the breakdown of organic materials in the soil and improve carbon and nutrient status and availability to the crops.

So, the question needs to be asked, how can we help improve earthworm populations? Of course, the best thing we can do to help earthworm populations is to stop doing tillage, but is there more we can do? We chose to look at cover crops as a way to increase earthworm populations as the additional biomass and living roots might improve earthworm population and activity. Therefore, we conducted a study to understand the impact of cover crops on earthworm populations in Eastern S.D. crop lands.

Earthworms on the soil surface inside a white hard-plastic ring, after mustard-vinegar solution was drained down through the soil profile forcing earthworms out onto the surface in order to be counted.
Figure 1. Counting earthworms after pouring mustard-vinegar solution in the specified area.

To conduct our study, we selected fields that were long term no-till, specifically 15 or more years, and selected sites where cover crops were planted in the fall 2019 after a small grain harvest. To incorporate experimental control in our study, we sprayed out 15ft by 30ft plots to kill the cover crops and left cover crops growing in the plots side by side, following a randomized design. We then let the cover crops grow for the rest of the year and came back the following spring and conducted our experiment. To estimate earthworm populations, we used an established protocol where a mustard-vinegar solution poured into a ring which was used to bring earthworms out from the soil profile on to the surface (Chan and Munro, 2000) (Figure 1). Thereafter, we categorized the earthworms into adults or juvenile depending on their sizes and counted them.

Stacked bar diagram where adult and juvenile earthworm counts per acre are stacked on the y axis and categorized under the cover crop and no cover crop treatments on the x axis. The average earthworm population under no cover and cover plots were about 94,000 per acre and 222,000 per acre, respectively. Juevenile earthworms were close to 50% of the count for both the no cover and cover crop plots.
Figure 2. Earthworm population under cover and no cover plots in spring 2019.

In our experiments, earthworm populations varied mostly due to soil conditions from just over 285,000 up to 2,000,000 worms per acre. Our study found that on average, cover crops had more than twice as many earthworms as the no cover crop plots (Figure 2). Therefore, if the goal with no till is to build a healthy soil that promotes earthworms to function to the best of their abilities, then cover crops are essential.

We are extending our study to gather more information on how different cover crop species influence earthworm population and will have a report in the next few years.