In a field on a low standing hill outside of Hillsborough, County Down are a criss cross of lines marked on the grass dividing the field into neat rectangles.
Look a little closer and you can see that the grass is greener/yellower or longer/tuftier in some of these rectangles. These are the long term slurry plots at AFBI Hillsborough and represent a unique experiment funded by the Department of Agriculture and Rural Development.
For over forty years, scientists have been routinely adding cattle or pig slurry to these plots and monitoring the effects on soil and grass health. Most recently, this has disclosed that slurry, far from killing earthworms, increase their presence up to five times that of the non-treated plots. “There is a perception that slurry is bad for earthworms because farmers see dead worms on the surface after spreading,” commented Dr Archie Murchie, lead author of the study published in Applied Soil Ecology. But this is just part of the story, it seems that the worms then recover and benefit from the increased food. “It is a bit of a good news story for farming, normally slurry gets a bit of a bad rap,” said Dr Murchie, “and whilst it is true that you will get some dead earthworms, the increased organic matter and nutrients in the soil are good for worms in the long-run.”
What is more, even the inorganic fertiliser was of benefit. The interesting thing was that the deeper dwelling grey worms preferred the inorganic fertiliser plots, whilst the red worms, preferred the slurry. “We don’t fully understand the precise reasons why this is so or the different contributions of each earthworm species to soil fertility yet, but put simply – different worms prefer to eat different food,” said Dr Murchie. Aristotle called earthworms the intestines of the soil, so a healthy earthworm population contributes to nutrient recycling, soil drainage and also provides food for many farmland birds.
There were two downsides though: one was that pig slurry application had increased the level of copper in the soil, which is not good for earthworms. The other observation (unconnected to slurry input), was the presence of the invasive New Zealand flatworm which eat earthworms.
“Although this study showed that cattle slurry in particular benefited earthworms, we should not be complacent about the health of the soil,” said Dr Murchie. “Soil is an important living entity and effective soil management is essential for productivity.”