How does ammonia effect the growth of plants?

Ammonia is a source of nitrogen for plants that can digest it. Many plants cannot digest it directly, by which I mean absorb it and use it in making plant tissue and for similar needs, but many cannot. However, there are many microbes in the soil that can turn ammonia into nitrates and similar chemical compounds that plants can absorb and use.

If the ammonia is not very dilute, it will kill most things, including any plant. However, many large-scale agriculturalists use ammonia for fertiliser because it is a very cheap source of nitrogen.