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 biomass1, 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. A study in Ohio found that a population of about 100 individuals per m2 of Lumbricus terrestris could ingest 840 kg/ha (750 lbs/ac) per year of corn residue2 . Earthworm populations in agricultural soils commonly range from 100-350 individuals per m2 3. Crop residue, soil organic matter, and mineral soil get mixed in the earthworm gut to produce casts.

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 casts4. 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.

There are also transformations that happen in the earthworm gut and in microbially-rich casts that affect nutrient availability. On average, mineral N is 241% higher, and available P is 84% higher in casts than in the rest of the soil4. The increase in mineral (plant-available) N is due to decomposition of organic matter, but the story with P is more complicated. In a 2019 study, Vos and collaborators5 found significant differences in available P in casts between earthworm species compared to the soil, but in the end all earthworms increased P availability (Figure 1). Considering the large amounts of legacy phosphorous in many Ontario soils, earthworm-induced P availability could be significant where populations are high enough. The same study also showed that pH was significantly increased in worm casts, potentially offsetting part of the need for lime application (Figure 2).

Figure 1. Available P in earthworm casts and bulk soil. Colours indicate earthworm functional groups (light grey: epigeic; grey: en-dogeic; dark grey: anecic). From Vos et al, 2019.
Figure 2. pH in earthworm casts and bulk soil. Colours indicate earthworm functional groups (light grey: epigeic; grey: endogeic; dark grey: anecic). From Vos et al, 2019.

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 fields3. 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 tillage6. Most of the increases come from the re-establishment of deep-burrowing anecic worms7, whose major benefit to soil porosity and drainage will be the subject of another article.

One interesting finding of the meta-analysis of relative cast fertility was that it increases in the presence of plants. The relative increase in concentration of TOC, for example, was more than double when living roots were present. Relative enrichment of plant-available N was also significantly higher at 385%. In addition to increasing the fertility of earthworm casts, maximizing living roots over time increases earthworm numbers. A study comparing worm populations in different cropping systems found an average of over 1000 worms per square meter in the plots with a living mulch of clover8!

Additions of organic matter such as manure or compost also stimulate earthworm populations and their benefits. A study comparing a range of organic amendments found that after two and a half years, manure treatments had the highest worm populations (800-900 individuals per m2 ), while control plots with no amendments had the lowest abundance (about 150 individuals per m2 ). Compost treatments had intermediate values (400-500 individuals per m2 )9.

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.
Any reduction in tillage depth, intensity or frequency will benefit worm populations, as will adding organic amendments, especially manure. Like many other aspects of soil life, recent research suggests that maximizing living roots is the best thing you can do for our humble helpers.


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