Why is there soil in my pocket? (poem)

My pocket contains soil, I know that might seem weird
But it’s not as bad as some people may have feared.

Sometimes I might be the home for a mouse or a shrew
Or perhaps I’m home for a caterpillar, or maybe two.

Sometimes my pockets are a den for a tiny ladybird
but personally I don’t think that’s too absurd

My pockets contain soil, for creatures need homes too
I think it also helps that I work in a zoo!