OMAFRA Report: What’s a worm worth?

GUELPH – This is about the benefits of earthworms in agricultural soils and how to reap them.

Earthworms are the ecosystem engineers of the soil. In most terrestrial ecosystems they represent the most abundant belowground biomass, and in the same way beavers create habitat for a host of other species to thrive, earthworms make the soil ecosystem work.

They help to break down and incorporate plant residues, bringing them in close contact with microbes that further decompose them and release their nutrients, and with soil particles to which that organic matter can be adsorbed and incorporated into microaggregates. Their burrowing creates channels in the soil that improve aeration and water movement and makes room for plant roots and other organisms to grow.

Earthworms have been shown to improve soil structure (increasing stability and reducing runoff), mineralize and stabilize organic matter, increase nutrient availability, and even affect plant health by inducing the production of hormone-like substances. But by far their most well-established benefit to crop production is through their impact on nutrients, so let’s start there.

Nutrient cyclers

Earthworms release nutrients tied up in plant residues and soil organic matter. Surface-living worms (epigeic) and deep-burrowing worms (anecic) feed almost exclusively on surface litter, while topsoil worms (endogeic) feed on soil organic matter at various stages of decomposition. Deep-burrowing worms in particular play a huge role in residue incorporation.

How fertile are earthworm casts?

Earthworm casts (worm poop) are hot spots of fertility in the soil. Through their feeding, worms concentrate elements from organic matter and the bulk soil. After analyzing 81 studies totaling 405 observations in a meta-analysis of the relative fertility of casts and bulk soil, researchers from Wageningen University found that total organic carbon (TOC), total phosphorus (P), and total nitrogen (N) concentrations were between 40 and 48% higher in worm casts. Cation exchange capacity (CEC) was on average 38% higher, similar to TOC and underscoring the important effect of organic matter on CEC. These are concentration effects – worms concentrate elements from residue and soil into their casts.

Capturing Cast Benefits

Realizing the potential benefits of earthworms on nutrient cycling and fertility requires nurturing their populations. One of the most well-established facts about soil management is that tillage reduces worm populations. Intensive tillage can kill worms directly, expose them to drying and predators, destroy their burrows, and remove their food source. This is especially true for surface-dwelling and deep-burrowing worms, which can be completely absent from conventionally-tilled fields. A recent meta-analysis of the effect of tillage on worm populations found that earthworm abundance and biomass were 137% and 196% higher, respectively, in no-till systems compared to conventional tillage. Most of the increases come from the re-establishment of deep-burrowing anecic worms, whose major benefit to soil porosity and drainage will be the subject of another article.

The End of the Wormhole

How do your earthworm numbers stack up? There’s no need to excavate a whole meter of soil to find out. Take a shovel to the field and dig up one square foot of topsoil (6-8 inches deep). If you want to convert to the square meters used in scientific papers, multiply your number by 11 (or 10.76 if you want to be exact). Earthworm abundance is a great, simple measure of soil health and an indicator of how much your soil can benefit your crops.

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Written by Sebastian Belliard, soil management specialist, OMAFRA.