(News article for June 19, 2021. This article is part of a series. Click to see the introductory article and ones about phosphorus and potassium fertilizers; calcium, magnesium, and sulfur fertilizers; and micronutrients.)

Nitrogen is perhaps the plant nutrient people most often think of when they think about fertilizers. Plants often need supplemental nitrogen, in addition to what they would naturally be able to obtain from the soil, for optimal growth.

At the same time, excessive amounts of nitrogen can cause problems, such as a reduction in or delay of flower or fruit and seed production, salt burn of plants, increased problems with certain insects and diseases, and pollution of waterways leading to algal blooms and low oxygen conditions for fish and other aquatic creatures.

Nitrogen tends to leach out of soil with rainfall or irrigation water if not taken up by plants. We cannot count on nitrogen that was in the soil at the time of a soil test being present when plants need it, so soil testing laboratories that serve people in our high rainfall climate typically do not test for nitrogen in routine soil samples. Instead, nitrogen recommendations are made on a default basis for the type of plant.

Another consequence of nitrogen’s leachability is that it’s often recommended that applications be split up over the course of a plant’s growing season. Alternatively, a slow-release fertilizer is sometimes a good way to spread out the provision of nitrogen.

The concentration of nitrogen in fertilizer is indicated by the first number in the fertilizer “grade.” For example, 33-0-0 is 33% nitrogen. If recommendations call for 1 pound of nitrogen per 1000 square feet, 3 pounds of 33-0-0 would be needed to provide 1 pound of actual nitrogen.

Besides being important to apply appropriate rates of nitrogen-containing fertilizers, it’s also important to apply them at appropriate times. If nitrogen is applied too late in the growing season, it can result in plants not being as cold hardy as they normally would when freezing temperatures arrive. Nitrogen applied to warm season turfgrasses in too early or too late in the year can make grass more susceptible to the fungal disease large patch.

Plants take up most of their nitrogen in either the nitrate (NO₃^-) or ammonium (NH₄⁺) form. If nitrogen is supplied in other forms (urea, organic forms of nitrogen, etc.), it’s typically converted to one of these before being taken up by roots.

Some plants perform better when receiving nitrogen in one or the other of these two nitrogen forms. For example, we typically avoid applying nitrate forms of nitrogen to blueberries.

Ammonium nitrate (33-0-0 or 34-0-0) was a common nitrogen source for many years, but its availability is now limited. Many places sell a combination of ammonium sulfate and urea as a substitute for ammonium nitrate. This blend is typically 33% nitrogen and is sometimes called “urea ammonium sulfate” (UAS), “urea sulfate,” or just 33-0-0. Because of the ammonium sulfate component, it also provides sulfur.

Ammonium sulfate (21-0-0) is also sold alone as a fertilizer. It has a higher sulfur concentration than UAS. Ammonium sulfate acidifies the soil. UAS and urea do, as well, but to a lesser extent, per pound of product.

Urea (45-0-0 or 46-0-0) is a relatively inexpensive nitrogen source, per unit of nitrogen. However, if urea is not mechanically incorporated into the soil or watered in with rainfall or irrigation, a large proportion of the nitrogen can be lost as a gas (ammonia). Some urea fertilizers are coated to reduce volatilization. In the soil, urea is converted to ammonium before being taken up by roots.

Calcium nitrate (15.5-0-0) is typically more expensive, per unit of nitrogen, than the previously mentioned fertilizers but can be a good option when supplemental calcium is desired. I often recommend using calcium nitrate to side-dress vegetables like tomatoes that are susceptible to blossom end rot.

Of course, people often use fertilizers that also contain nutrients other than nitrogen.

Let me know if you have questions.

Contact Mary Helen Ferguson.