Benefits of worms

Repeat week Golden oldies
This week I am reviving four old posts. In this case to supplement my new post about earthworms published two days ago

Wonderful worms

I am passionate about earthworms. Just ask my former students!  A few years ago, the New Scientist reviewed a delightful book about worms by Amy Stewart. I recommend  it. It’s called ‘The Earth Moved’. Un-be-known to me, Brenda noted it as a potential Christmas present. She went to the bookshop and the assistant checked on her computer. “Oh yes madame, we have it, you will find it in Fiction!”

Minimum cultivation methods are repeatedly shown to increase worm populations. Why are those people who care about worms, not up in arms about soil cultivation which destroys worm habitat? Where are the protests against rotavators that shred worms?

This March, all soil surfaces in my zero cultivated, glyphosate-sprayed, cemetery garden were completely covered in earthworm casts.

Worm populations

These are hugely variable and dependent on a myriad of factors such as  rainfall or drought.The following numbers of worms per square meter are a very rough guide -

• cultivated soil, 150 
• no till management, 300 
• grassland, 500, often considerably more

Counts in excess of a thousand per square meter are not uncommon. Darwin was wrong! Never did I think I would write these words about my all time hero who did more than anyone else to research and promote the merits of worms. It’s just that even he underestimated how many worms there can be. Agricultural grassland can contain a greater weight of worms than the cattle that graze!

Benefits of worms

There are many species of worms. More than 150 of horticultural significance worldwide. All have varying habitats, many in most unlikely places. Perhaps the two most common to gardeners - both usually graced by their latin names - are the night crawler, Lumbricus terrestris and the brandling or tiger worm, Eisenia fetida. The night crawler may be found all the way down to a couple of meters, whereas eisenia is found in decaying vegetation and manure and barely survives in a mineral soil.

The benefits of worms to the soil are huge.

• Their tunnels permeate the soil and provide aeration, drainage and easy root penetration.
• It has been pointed out by Bill Mollison, a founder of Permaculture, that as worms move through the ground they act like pistons sweeping air through their tunnels.
• They redistribute organic matter and in particular pull surface vegetation into the ground.
• They intimately mix soil particles and organic matter in their gut. They create water-stable aggregates (crumbs) of sand, silt and especially clay.
• Nutrients are concentrated in their castings. Compared with figures for surrounding topsoil there are enhanced levels, typically nitrogen x 5, phosphate x 7 and potash x 11.
• They shift soil! A  single worm might cast as much as 5kg of soil in a year!
• Some species destroy harmful soil nematodes in their gut.
• Their casts are rich in biologically active microbes.

It’s not all good

Could it be that worms are ever unwanted?

• Because some worms cast at the soil surface green-keepers do not like them.The same casts are of huge benefit when they occur in the none-digger’s vegetable garden when he wishes to make a tilth. For the domestic lawn, I believe worms to be beneficial. I was a little taken aback at the criticism of my recent  post when I mentioned that my use of iron sulphate as a moss killer also discourages casting worms. My critics have a point, but I would argue that the effect of iron sulphate on overall worm population is small and does little harm to the deep crawlers. The surface casting types - the minority -  just migrate to my borders. Compared to the huge shifts in worm populations caused by the environment and meteorological factors the detrimental effects of a gardener’s activities such as his limited use of fertilizers and weedkillers is small.
• Some gardeners worry that worms might clog the drainage holes in their pots. I have no such concerns and just ignore them.
• The burrows they create has been known to destroy earth dams!
• As described in Amy Stewart’s fine book, introduced none-native worm species, can destroy natural woodland habitat. Worms’ very efficiency in mixing organic matter into the soil removes surface decaying vegetation, the natural niche for many wild plants.
• They bury things! I am being a little frivolous here! My thirty year old rock garden stones in my previous home virtually disappeared! Whenever I shift logs that have been standing on the ground I see evidence of the soil that earthworms move. Could worms be a partial cause of the burial of archeological remains?

The base of my log-pile becomes infiltrated with worm casts. Here an upturned log exposes worm casts on the ground

But some good news for gardeners!  Although many fine public gardens are host to the New Zealand flatworm, (they just don’t tell you), this dreaded nematode predator of worms is not proving to be the scourge we once feared.

What can the gardener do to encourage worms?

• Don’t dig!
• Leave dead weeds, leaves and other  organic debris on the soil surface.
• Mulch.
• Many worms like a neutral or slightly alkaline soil. Their digestive juices need  calcium. On acid soil applying lime may help.
• Grow plants! Perhaps in my post about green manure I neglected to mention how  good it is for worms. And they love clover!
• Add garden compost and well rotted manure.

Worms like untidy gardeners