Talking here about "earthworm diversity", is another thing that is wrong.
To identify earthworms is difficult even for experienced taxonomists and you need a microscope, a disection table and a lot of training. Polychaetes are much easier by comparison and I can assure you that they aren't easy. You need months of study.
Not, not everybody can do the work of a biologist. Not even the smartest children. Is complex, and hard work. Unless you want to classify the diversity of worms into "small", "medium sized" and "big worm", dig a hole and count isn't enough.
I think the 40% number is correct, the article also contains this line which is oddly specific to be a typo.
> However, 42% of fields had poor earthworm biodiversity – meaning either very few or none of the surface-dwelling and deep-burrowing categorisations of worms were seen.
The title presumably should say 40%