The word ‘credit’, as used in the passage, means

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When a field gives a good crop, the farmer, the rain and the seed usually share the credit. Hardly anyone thinks of the earthworm. It lives out of sight, it is slimy, and most people meet it only on a wet path after a storm. Yet much of the work that keeps soil alive goes on in its dark tunnels.

An earthworm eats its way through the ground, swallowing soil along with bits of dead leaves and roots. What comes out at the other end, called a cast, is richer in plant food than the soil around it. Its burrows matter just as much. They let air reach the roots, and they help rainwater soak in instead of running off the surface.

Charles Darwin watched these animals on and off for about forty years. His last book, published in 1881, was about earthworms. He estimated how fast their casts piled up, and he showed that stones and even old ruins left on the ground slowly sink under it, buried a little at a time by the soil the worms bring up.

Today many gardeners keep worms to turn kitchen waste into vermicompost. On farms, though, the story often runs the other way. Repeated deep ploughing tears up their burrows, and some pesticides kill them or bring their numbers down. The loss does not show at once. It shows later, in soil that packs hard and lets water stand on top after rain.