If the grass looks greener after the skies have cleared, your eyes aren't deceiving you.

There are several reasons rain helps lawns green up, said Jennifer Knoepp, a research soil scientist with the USDA Forest Service, SRS, Coweeta Hydrologic Laboratory in Otto, North Carolina. Both of those reasons involve nitrogen, but one of them might surprise you.

After it rains, there's typically more water available in the soil for plants, Knoepp said. When plants take up that water, they are also taking up nitrogen from the organic matter that's in the soil.

Here's how that works: "As plants grow, their small roots die and new roots grow," Knoepp said. When that happens, soil microbes cause the dead roots to decay. Think of this process as similar to adding compost to your lawn, only this action takes place underground and naturally, without your intervention. The roots are made up of large chemical compounds consisting mostly of carbon but also some nitrogen. Soil microbes use carbon and some of the nitrogen to cause the dead roots to decompose. As this happens, a portion of the nitrogen is released back into the soil as a sort of waste product.

As rain soaks into the soil, it activates the microbes to release more nitrogen, said Knoepp. The grass benefits from the freshly fallen rain because the flush of water allows the roots to take up this "new" nitrogen as well as the nitrogen that the microbes have previously released. At the same time, "the grass is very active with photosynthesis" when the sun returns, Knoepp explained.

Something else happens with nitrogen when it rains. The atmosphere is made up of 78 percent nitrogen gas, which is inert or non-reactive. It also carries particulate nitrogen in the form of ammonium and nitrate. When it rains, the rain brings some of this particulate nitrogen down onto lawns in the form of nitrate and ammonium nitrogen. However, Knoepp said — and this is what might surprise you — only a small amount of the particulate nitrogen that falls directly on grass during rain events is directly absorbed into the leaves.

Monitoring your lawn's nitrogen

Just how much nitrogen falls in the rain depends on several factors, Knoepp said. The factors include where you live (rain in the Northeast contains more particulate nitrogen than rain in the Southeast), how dry it's been and even where the rain that falls in your area is coming from. Particulate nitrogen in the atmosphere can come from various forms and sources, including nitrogen gas that's been oxidized by lightning as well as nitrogen that's the result of emissions from cars or industrial or agricultural inputs. The amounts of particulate nitrogen in the atmosphere have also changed since the mid-1990s, Knoepp pointed out. Since the implementation of the Clean Air Act and the Clean Air Act amendment, nitrate nitrogen has been declining and, more recently, ammonium nitrogen has been increasing.

There's an easy way to find out what kind of nitrogen and how much of it is falling when it rains on your lawn. The National Atmospheric Deposition Program has been monitoring atmospheric chemistry since 1978 and has numerous sampling stations around the country. Their site has an interactive map or handy table to find a sampling location near you. That location will have estimates of nitrogen inputs from rainfall.

Even though rain helps boost the nitrogen that's available to your lawn in several ways, and it remains in water you collect in a rain barrel, you can't count on nitrogen from rain to meet all of the fertilizer needs of your grass or your vegetable garden, Knoepp said. Commercial fertilizers or organic soil amendments are still needed for a balanced fertilizer program, but she urges caution in applying them. While nitrogen is an essential ingredient for good plant growth, be sure to follow package directions. Too much of a good thing can be harmful not only to plants but to nearby ponds, lakes, streams and rivers.

"Nitrate nitrogen is very mobile," Knoepp said. Rain can move it deep into the soil, well below plant root zones, into streams, bodies of water and then aquifers. "You don't want that," Knoepp said. Streams don’t need a lot of nitrogen, and too much of it can lead to problems like the formation of algae.

After all, it's not green streams but green lawns that homeowners want to see when the clouds depart and the sun returns.