Thursday, 28 September 2017 09:54

The humble yet great earthworm

Written by  Tessa Nicholson

Charles Darwin once wrote of the earthworm; “It may be doubted whether there are many other animals which have played so important a part in the history of the world.”

After listening to Nicole Schon from Ag Research in Lincoln discuss earthworms at the recent Organic and Biodynamic Winegrowing conference, I tend to agree. For one I had no idea there were so many different species. Or that the ones that are important to our vineyards (and gardens and pasture) are not native but exotic. Or that there are earthworms in New Zealand that grow to 150cm. Okay I could do without actually knowing about that last fact. It gives me the heebie geebies.

Anyway back to the humble earthworm and its importance to our soils. That very fact is pretty well acknowledged, but the whys and wherefores maybe not so.

Currently in New Zealand we have around 180 native earthworms, that have been here for a very long time. They reside mostly in our bush where they are unlikely to be disturbed.

There are around 20 exotic earthworms, that arrived in the soil of fruit trees brought to this country back in the 1800s. These are the ones that are important to viticulture.

There are three specific worms that help in aerating soil, improving water holding capacity and delivering nutrients to the soil for the plants to feed on. They are the dung worm, which lives very close to the soil surface, feeding on organic matter. By eating that organic matter and then excreting in the form of casts, they deliver the nutrients from the soil surface to below.

The grey worm, which is the most common type in New Zealand, Schon says, doesn’t feed so much on organic matter.

“But it does burrow quite extensively and does a very good job of reworking that soil structure.”

There are a few important factors in that. Not only do the burrows created by these worms free up the soil, allowing better water absorption below the surface, those burrows also provide nutrient rich areas for fine roots to grow down.

The third is the deep burrowing earthworm. This is the biggest of all three, and found mostly in the North Island. Known as the blackhead worm, it feeds on more of the dung or organic matter but takes it far deeper into the soil than the dung worm.

“In an ideal system, we want all types of these worms working together to get the most out of the soil,” Schon said.

In terms of increasing and maintaining the numbers of earthworms, she said growers need to be feeding, watering and caring for the worm’s habitat. Given worms tend to hibernate during the summer months, (as they don’t like dry conditions) irrigation can help to keep the number of active worms up. Feeding organic matter into the soil, whether that be in mulch, compost or fertilizer, (the higher the quality the better apparently, worms can be fussy) will help. And try not to undertake too many of the tasks that earthworms dislike. The compaction of soils is a turn off for worms. So think carefully about how many times you have to drive close to the vines over those summer months.

Earthworms also do not like cultivation.

“Regular cultivation to control weeds will not be good for earthworm numbers,” Schon said. “As the frequency of cropping increases, the abundance of earthworms decrease as their habitat gets broken up and they get chopped up.” said.

Bear in mind the role earthworms play in improving growth. Schon said in terms of pasture, an area that has plentiful earthworms will have about 20 percent more growth, than an area without worms.

And never underestimate how many worms can survive if your soil is providing the right ingredients. Schon said while a spade of soil may deliver just 15 earthworms, when you multiply that into a square meter, the number rises to 375. What does that mean the numbers are per hectare? A massive 3.75 million!

Something to think about.


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