The addition of a lot of brown organic matter (such as autumn leaves and straw; these things have a high carbon to nitrogen ratio) to the soil (by digging it into the soil and adding it as mulch) can alleviate symptoms of nitrogen toxicity because the organisms that break down high carbon materials require a lot of nitrogen. They use up all their nutrients before moving on to other things, so adding more carbon or organic material can help them out. If the problem persists after you've added lots of carbon, then you should probably talk to a soil scientist or plant expert since this indicates that you may have a real issue with nitrogen levels/saturation. Nitrogen is one of those elements that can be harmful in excess, even if it's coming from natural sources such as manure or urine. If your soil tests come back low on nitrogen, you'll need to add some nitrate or ammonium compounds to raise the level again. You can find recipes for homemade fertilizers online or in books about gardening. There are also products available under various names from retailers such as Wal-Mart, Home Depot, and Lowe's that include both carbon and nitrogen ingredients so you don't have to make your own mix. In conclusion, nitrogen toxicity is caused by an excessive amount of nitrogen in relation to other nutrients in the soil. This can happen if natural sources contain too much nitrogen (such as animal manures) or if nitrogen compounds are used as fertilizer without raising other nutrients at the same time.