Nitrogen is perhaps the most important plant nutrient. At least 17 other elements are essential for optimal plant growth, but nitrogen is needed in the largest quantity. It is also the nutrient that is most likely to limit plant growth. Most lawns and gardens require synthetic or organic nitrogen supplementation to remain productive, but you can still appreciate the nitrogen added to your soil -- with no effort or expense on your part -- by rain and snow.
The Right Kind of Nitrogen
Numerous forms of nitrogen are present in the environment. The most abundant form is nitrogen gas, which makes up about 78 percent of the Earth's atmosphere. Unfortunately, plants cannot utilize nitrogen in this form. Atmospheric nitrogen must be converted to different molecular compounds that can be absorbed by plant roots, and this conversion requires energy of some sort. The preferred types of nitrogen for plant growth are ammonium and nitrate. Ammonium, even when applied as a synthetic fertilizer, is quickly converted to nitrate in warm, biologically active soil. Nitrates move freely through the soil and are readily available to plants, but they are also subject to leaching by rainfall or irrigation.
Power in the Atmosphere
A violent thunderstorm is an impressive reminder that the atmosphere is a very energetic place. This energy can initiate a reaction that creates nitrate from the two primary constituents of Earth's atmosphere -- nitrogen and oxygen. Through a process known as wet deposition, these nitrates are carried down to the soil by rain or snow. The amount of usable nitrogen added to the soil depends on several factors, one of which is under the gardener's control: soil texture. Soils with crumbly texture and high organic matter content will absorb and retain more precipitation, leading to more nitrogen for your garden.
Quantities of Nitrogen
Precipitation contributes a significant amount of nitrogen in relation to natural ecosystems, but this same amount is usually inadequate for intensively planted gardens and vigorously growing lawns. Rainfall is probably the primary source of atmospheric deposition in areas with frequent thunderstorms, but snowfall contributes more nitrogen in certain mountainous regions. Measurement stations throughout the United States record the actual amount of elemental nitrogen that is added through wet deposition. Quantities range from 0.3 ounces per 1,000 square feet in the western United States to more than 2 ounces per 1,000 square feet in the Midwest.
A Supplement, Not a Replacement
An average garden that receives regular inputs of organic matter needs about 1 pound of supplemental nitrogen per 1,000 square feet per year. A highly productive garden planted in soil that is not naturally fertile might need double this amount. This means that even in the Midwest, wet deposition accounts for only about 5 percent to 10 percent of a garden's annual nitrogen needs. In areas such as the Southwest and Southeast, it might account for less than 1 percent.
- University of Wisconsin Extension: Essential Elements for Plant Growth -- Macronutrients and Micronutrients
- University of Arizona Cooperative Extension: Essential Plant Nutrients
- Colorado State University Extension: Plant Nutrition
- University of Wisconsin Extension: Soil and Applied Nitrogen
- National Atmospheric Deposition Program: Nitrogen in the Nation's Rain
- University of Illinois: Transformations of Nitrogen Species
- University of Montana: Assessment of Nitrogen Deposition and Its Possible Effects on Alpine Vegetation in Grand Teton National Park
- Maine Organic Farmers and Gardeners Association: Providing Nitrogen to Organic Crops
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