Nitrogen (N) is used by green plants in quantities greater than any other of the 12 essential elements obtained from soil (yes, there are 16 essential nutrients, but three—carbon, hydrogen and oxygen—are not considered mineral nutrients, nor are they ever in short supply). It is most often the mineral nutrient in shortest supply, even though 78 percent of the Earth’s atmosphere is occupied by gaseous N—this is about 36,000 tons over every acre. Atmospheric sources, as plentiful as they are, represent only about 2 percent of the Earth’s total supply of N; the balance is mineralized, at very low concentrations, in virtually all soil particles and rocks.

Nitrogen plays an important role in plant nutrition. It is involved in the formation of proteins, especially functional enzymes that control metabolic activities. It is also a component of chlorophyll, the molecules within which photosynthesis takes place. Not surprisingly, the role of N in plant nutrition has been studied more than any other element. The irony of vast quantities of atmospheric and mineral N, coupled with the fact that it is frequently a growth-limiting factor, makes nitrogen supply and use by plants an interesting dilemma for scientists.

With such a large supply of N in the atmosphere, it is amazing that green plants have not developed adaptations that allow them to assimilate N from the air as they do with gaseous carbon dioxide used in photosynthesis. Although plants are able to absorb nitrogen compounds that come in contact with foliage—usually via rainfall or deliberately with irrigation—they are not capable of absorbing N2, the predominant form of gaseous nitrogen. Trees require forms of N that have combined with hydrogen or with oxygen.

Soil forms of N must exist as salts for uptake by plant roots. There are principally two forms of soil N available to plants: ammonium, which exists as a positively charged cation (NH4+), and nitrate, which exists as a negatively charged anion (NO3). Because of its positive charge, the ammonium cation will mate with the prevailing negative charges on soil particles, provided the soil is not too acidic. The nitrate anion with its single negative charge, however, is free floating and thus subject to leaching with rainfall.

Trees use both forms of N, but when soil conditions are favorable, autotrophic bacteria will convert ammonium to nitrate in a process known as nitrification or mineralization. As a general rule, up to one-third of an N application, regardless of compounds, conditions, species mix or application methods, is lost and not available to the target trees. Nitrogen that is not tied up in minerals exists on Earth as a gas, so it is not surprising that most N losses are the result of volatilization, and a certain amount is lost to nitrification, or the conversion of ammonium to nitrate.

When N is supplied to deficient soils, the response usually takes the form of a rapid and distinct greening of yellow or light green foliage, or a darkening of already green tissue, and an often-dramatic increase in the elongation of cells arising from meristematic tissues. In other words, applications of nitrogenous fertilizers will improve leaf color and increase growth rates almost immediately.

An increase in growth without a concomitant change in color is usually an indication that soils and leaves are already saturated with N. This effect is known as overstimulation. Further applications of N beyond the point of overstimulation can lead to tissue burning, root damage and growth suppression. Excessive amounts of N fertilizers can also pollute groundwater supplies, which is a far greater risk than tissue burning or root damage.

In the “guaranteed analysis” of fertilizer materials that reports the percentage content of the three important macronutrients (nitrogen, phosphorus and potassium), N is the only constituent that is reported as an elemental weight. The other two are reported as oxides. No one is sure why this convention has been allowed to persist, but it does require users to interpret fertilizer recommendations and nutrient content of fertilizer materials very carefully. As a general rule, application rates are almost always reported in elemental forms, not as oxides.

Nitrogen application amounts really depend on needs, but consider annual maintenance N levels of .5 to 2.5 pounds of actual N per 1,000 square feet of area within the drip line. If trees are already showing good color, halve these amounts by fertilizing every other year. For example, if the area within the drip line of a tree is 4,000 square feet, start with 2 pounds of actual N in solution (usually 25 to 30 percent by weight) applied using a pressure wand at depths of 8 to 12 inches in a well-spaced pattern within, and just beyond, the drip line.

For specimens that are showing chorotic symptoms and N is believed to be the culprit, increase the amounts accordingly. Keep good records on trees that you’ve treated so you can identify trends that will help tailor future applications. Finally, if the N is be topdressed in granular form, make sure to do so only when the lawn is dry, then thoroughly water down the treatment area so N can begin to migrate into the rootzone.

Common Nitrogen-Based Fertilizers
Compound Formula Guaranteed Analysis (N – P2O5 – K2O)
Ammonium nitrate NH4NO3 34-0-0
Ammonium sulfate (NH4)2SO4 21-0-0 (24S)
Urea CO(NH2)2 46-0-0
Potassium nitrate KNO3 13-0-45
Nitrogen-phosphate combinations 11-52-0* 18-46-0
* The ammonium phosphates are more often considered a primary source of phosphorus, but they are also an effective source of nitrogen.

The cheapest and most common source of nitrogen is urea (46-0-0). This means that 10 pounds of urea will supply 4.6 pounds of actual N. Urea can be applied during any season as long as the foliage is dry. When applied to wet foliage, there is a risk of burning, and if temperatures are warm enough (anytime during the growing season) urea is quickly converted to gaseous forms that cause N losses through volatilization. If you can smell ammonia within 30 minutes following a topdressing, a significant portion of the N is rising into the atmosphere. So, both urea and ammonium nitrate should be applied when turf is dry. However, both compounds can serve as excellent broadleaf weed killers when topdressed on wet foliage. This is a common trick of Christmas tree growers.

All of the nitrogen salts listed in the table can be dissolved in water and applied as an aqueous solution, but urea is probably the most common product put into solution. It is highly soluble in water and not as corrosive as solutions made from other products. Ammonium nitrate (AN) is an excellent source of nitrogen for trees, but it is also a strong oxidant, which is another way of saying that it is explosive. Oil-soaked bags of AN were commonly used by farmers for blasting purposes—until it was used by terrorists to destroy a federal building in Oklahoma City. Although AN is still available for agricultural and horticultural purposes, it is usually a special-order item, and requesting it will trigger a visit from the Office of Homeland Security.

The most reliable way to assess need for N (short of assuming that it is always in demand by trees) is with foliar testing. Since a generalized chlorosis is a common symptom of many tree maladies, the only way to find out if the problem in N-related is to test for foliar N. Anything less than 2 to 3.5 percent N by dry weight is a candidate for extra N. The lower the test values below 2 percent, the more dramatic the color change one can expect from an N application (especially a foliar application). If you want to wow your clients, simply apply a foliar solution (15 to 20 percent by weight) with N and the color change will occur in 24 hours or less (depending on environmental factors).

On sandy, well-drained soils, figure the total N application required for a season and divide it into three or more applications; the coarser the soil, the more frequent the N applications. On fine-textured soils (clay and clay-loams), divide a season’s requirement into two applications. If labor and equipment costs are too high to warrant more than a single application of N, a slow-release product may be advisable, but, generally, the extra cost of slow-release fertilizers are not worth the extra expense.

The author is a professor and extension forester with the Rubenstein School of Environment and Natural Resources at the University of Vermont.