Have you ever noticed that brown spot with a dark green ring around it in your yard that shows up along the sidewalk, or near your mailbox, utility box, or fire hydrant? Your lawn was looking really nice and then this shows up. What a pain. This brown spot is considered dog patch, and by the name, you can tell where is comes from. Here is a quick explanation as to why your lawn is damaged.
Dog urine, just like any urine, has urea in it (after it is broken down from ammonia). Urea is CO(NH2)2. The N (nitrogen) will break off and join with other nitrogen to create N2. N2 is the same nitrogen that is put on through fertilizer. The high concentrations of N2 kill the grass off by nitrogen poisoning which creates the dead spot. The dark green ring around the dead spot is caused by less concentrated amounts of nitrogen, below poisoning levels, which greens up those grass plants to a dark green color.
The unfortunate part of the problem is that there is nothing that can reverse the brown spot in your lawn except for time. Over time, the nitrogen from the urea will break down and leach through the soil. Then the grass plants will have to grow to fill in the damage.
If you notice that a dog has relieved itself on your lawn, and the nothing has dried yet, you can try to lessen the effect of the nitrogen poisoning by spraying that spot and surrounding area with a lot of water. This was you dilute the amount of ammonia and start to wash the urine through the soil past the grass plant. But the key is that it cannot have dried. If the spot is dry, it is going to be too late.
Now there is no reason to freak out about dog patch. Yes it is unsightly to look at, but your lawn will recover as long as it was a healthy turf stand to start. Just remember, time will heal this wound