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Nitrogen is one of three major macronutrients, and one of six total macronutrients, in the soil that are used by plants for healthy, normal growth. Soil organisms like bacteria, algae, fungi, (and on up the soil food web chain) assist in sequestering nitrogen into the soil or fixing it from the atmosphere. The more microorganisms in the soil, the more nitrogen is held where plants can access it. Bacteria also help convert the organic form of nitrogen into an inorganic form accessible to plants in a process known as mineralization. In gas form nitrogen can be converted to nitrate through fixation; first in the combustion of lightning and second through the symbiotic relationship between bacteria and legumes (view source). These are just two examples of the many different transformations nitrogen can make throughout the nitrogen cycle.
"The plant-available forms of nitrogen are ammonium-N (NH4 -N) and nitrate-N (NO3 -N). The abbreviation NH4 -N means nitrogen in the ammonium form, and NO3-N means nitrogen in the nitrate form. Soil concentrations of (NO3 -N) and NH4 -N depend on biological activity and therefore fluctuate with changes in conditions such as temperature and moisture. Nitrate is easily leached from the soil with high rainfall or excessive irrigation. Soil tests can determine NO3 -N and NH4 -N concentrations at the time of sampling but do not reflect future conditions.
- OSU Extension Soil Test Interpretation Guide
Nitrogen deficiency will affect the entire plant, resulting in numerous visible symptoms like:
- Yellowing leaves
- Yellow leaves that turn tan as they fall from the plant
- Stunted or slow growth
- Pale green foliage
While nitrogen plays an important role in leaf and tissue development, over fertilizing can be equally as troublesome as deficiency. Over fertilization can stimulate the growth of foliage but then inhibit the growth of flowers. Fertilizing correctly is imperative to healthy plant growth, healthy soil, and environmental stewardship. A low-cost soil test is invaluable in determining correct fertilizer applications for the needs of the soil in its current, analyzed state. When you submit a soil test they generally ask what it is you're going to plant in the samples area which helps them guide you on amendments and inputs, including nitrogen.
Organic fertilizers are derived from the by-products of once-living organisms and when packaged will include N-P-K (nitrogen-phosphorous-potassium, the three major macronutrients) on the label. The ratio of NPK you need will depend on the current state of your soil and the crops you intend to plant. Some examples of organic fertilizers are:
- Compost - a great organic fertilizer but depending on how it is made may vary in nutrient density and/or quality.
- Blood meal - very high in nitrogen, must be applied cautiously
- Feather meal
- Cotton meal
- Manure - generally 1-1-1; actual nutrient content depends on the animal producing it, what the animal is fed, and how it is managed. Should be aged appropriately to limit distribution of pathogens.
- Fish fertilizer - a byproduct of commercial fishing; very high in nitrogen.
Raw manure supplies two forms of N to the soil: organic-N and ammonium, an inorganic form.
Ammonium is immediately available to plants, while organic-N needs to mineralize into ammonium to be useful. Ammonium can easily convert to ammonia gas in a process called volatilization, and be lost to the atmosphere if not incorporated into the soil.
—University of Minnesota, Manure Characteristics
How accessible nitrogen and any applied amendments are to the plants depends many different things - microbial presence in the soil, overall soil health, pH, water quality and frequency, and more. The best place to start is with a soil test which will illuminate the state of your soil and help you to determine the steps you can take toward building healthy, balanced soil for resilient plants.
- What Happens When Nitrogen is Applied to the Soil
- Estimating Plant-available Nitrogen from Manure
- What is the Soil Food Web?
- A Soil Science Masterclass with Dr. Elaine Ingham
- Cornell Small Farms - BF 110: Soil Health
- Understanding Soil Microbes and Nutrient Recycling
- The ABCs of NPK: A fertilizer guide