Science Questions

We occasionally look after a dog when our friends go on holiday. We noticed that when it wees on the grass, the grass dies. However, about four or six weeks later, the grass grows back incredibly green and lush. Why? Steve in North London

Your urine contains lots of waste products including a chemical called urea, which is a by-product of protein. Urine is quite concentrated unless you've been drinking a lot of water. When you put concentrated urine on the ground, it essentially dries out the grass roots like a prune and kills it. Urea is also a rich source of nitrogen, which is a good fertiliser and a boost for the grass when new shoots start to grow. So it kills off the grass, and then causes better grass to grow.

February 2006