What's in your garden soil? - Earthworms

Now that it's spring most gardeners probably have seen worms on sidewalks, driveways, lawns etc. Backyard gardeners probably know that most earthworms are good for their gardens, but exactly why are they good?  Our common earthworms are actually returnees to North America, and in some of our ecological systems they can actually be detrimental.  The following text comes from Cornell University's College of Agricultural and Life Sciences Agronomy Fact Sheet Series - Fact Sheet #102 : Earthworms in Agroecosystems published in 2018 by Madeline Kennear, Quirine Ketterings, Karl Cymmeck, Mike Stanyard, Mark Ross and Kitty O'Neal.  I think it's an excellent, straight forward treatment of what these critters do in farm lands as well as in garden soil and compost piles.

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Introduction 

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Earthworms are soil dwelling invertebrates (animals without a back bone) that were present in the northeastern United States in the past but then got wiped out by ice age glaciers.  Their reintroduction to the moderal agriculatural ecosystem occured a few centuries ago as a result of the European and Asian horticulural trade.  This factsheet provides information about the types of earthworms in North America, impact of earthworms activity on agriculture, a technique that can be used to estimate earthworm populations in a field, and agricultural management pratices that can promote earthworm activities.

Types of Earthworms

Earthworms are classified into three main categories according to their feeding and burrowing habits: endogeic, anecic, and epigeic earthworms. A species may be categorized into more than one of these groups.

•   Endogeic earthworms are deep burrowers that create temporary, horizontally-oriented burrows up to 20 inches below the surface. They consume organic matter from the soil. This organic matter is typically older and not as rich in nutrients as the organic matter consumed by other types of earthworms. Endogeic earthworms typically appear grey in color (little pigment) and range in size from 1 to 5 inches. Species in this group can be found on the soil surface after heavy rain.

•   Anecic earthworms are very deep burrowers. They build permanent vertical burrows and can be found up to 80 inches deep. They feed on soil and leaf litter from the surface, which they pull into the burrows, hereby also supplying “feed” for other organisms at the same soil depth. Anecic earthworms are primarily grey colored as well but they have a red or black head. Earthworms referred to as “nightcrawlers” are in this group.

•   Epigeic earthworms live at the soil surface and feed on leaf litter. They are typically only 0.5 to 3 inches long. This group has pigmented bodies and includes “red worms”.

Benefits of Earthworms

Earthworms are useful in an agricultural system because of their eating habits. They consume large quantities of soil and plant residue and help break down organic matter. Earthworm feces (casts) have high levels of plant-available essential nutrients such as nitrogen and phosphorus, which can be used by plants or other organisms. The casts and burrow walls are hotspots for microbial and enzymatic activity, and in areas where the casts are expelled, the soil pH is often higher.

Earthworms can help with pest management as well. They can control populations of certain nematodes and overwintering fungi populations that reside in the soil. Earthworms eat the eggs and spores of some harmful nematodes and fungi and thereby suppress their population.

Burrowing habits of many of the earthworm species increase soil aeration and tilth of the soil. Burrows allow for easier root growth for crops while also increasing water infiltration. Aggregation that results from burrowing and cast production can increase soil porosity and reduce soil compaction.Through their feeding, burrowing, and the creation of casts, earthworms can contribute toimproved nutrient cycling and availability, and may enhance soil health over time.

Negative Effects of Earthworms

While earthworms can greatly benefit nutrient cycling and soil structure, intense burrowing may result in excessive aeration and nutrient leaching problems. Some anecic species are capable of creating permanent burrows from the soil surface all the way to tile drains, if present. This can be problematic when burrows provide pathways for manure and fertilizer nutrients to quickly reach tile lines and become unavailable for plant uptake. Such issues can be addressed with shallow incorporation of manure and fertilizer which disrupts burrow structures.

Research is currently being done in forest ecosystems to better understand the potential negative effects of the reintroduction of earthworms in these systems.

Population Estimates

Table 1: An example of average earthworm population in fields under various crop 
             management practices that were implemented for 10 years or more.

Tillage	Management	Earthworms/yd2
Corn (continuous)	Plow	8
	No-till	20
Soybean (continuous)	Plow	50
	No-till	120
Bluegrass-clover	Alleyway	365
Dairy pasture	Manure	285
	Manure (heavy)	1100

Source: Earthworms and Crop Management. 
            Purdue University Cooperative Extension Agronomy Guide.

Summary

Earthworms help to improve soil structure and nutrient cycling and are beneficial to agricultural sustainability. The population of earthworms in the soil can be estimated by counting the number of earthworms in small, representative parts of a field. Earthworm abundance, diversity, and their burrowing activity are affected by crop and soil management practices. Practices that improve soil health also support earthworms. These practices include addition of organic materials, leaving residues in fields, manure application, and reduced tillage.

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A Little More About Worms 

• The most recent worm arrival now among us, commonly called either the Jumping or Crazy Worm,  is not so benevolent.   It out competes its more established cousins and other soil critters and easily lives as well in thin forest soils by consuing the relatively scarce hummus on the forest floor.  For more information about this rascal check this blog's POST about it from last year.

• Earthworms are actually more efficient composters than bacteria and fungi and so highly are desirable to have  in your compost pile; but they like cool and moist conditions. Don't let your compost dry out. 

• Most earthworms like neutral or slightly acidic soils. The pH scale goes from 0 → 14 [acidic → alkaline] with 7 being neutral. Most vegetables grow happily between the values of 5.5 →  7.5 which is also pleasant for our familiar earthworms. 

• As acidity increases earthworms may enter diapause, a state of dormancy in hopes of a change in environmental conditions.  They also prefer loamy soils.  Sandy soil can be too abrasive for their skins and it tends to not hold moisture. Clay soils are difficult for earthworms to tunnel and it tends to stay too moist.  

• Earthworms are actually more efficient composers than bacteria and fungi.  They are desirable in your compost pile because they will speed up the production of hummus; but remember to keep your compost pile moist.  If it heats up (an unlikely event for backyard piles) the earthworms will just burrow downward to find cooler temps.

• To attract and keep earthworms in your garden, you also need to keep the soil cool, evenly moist and have compost for them to eat.  Keeping between 2 - 4 inches of compost on top of your soil should do the trick.  

• To check on your garden's worm population do the following:

• Mix 2 tablespoons of mustard powder into 2 liters of water
• Slowly pour this mixture onto a 1 square foot area and wait about 5 minutes
• Then count the number of wigglies that come to the surface

And remember...

"Flies, worms and flowers exceed me still." - Issac Watts