Nitrogen Fertilizers

In Germany, about 1910, the means for economical chemical fixation of nitrogen into ammonia was worked out, primarily by the scientist Fritz Haber. Later refined by a Frenchman, Georges Claude, the Claude-Haber Process still represents the least expensive effort of technology to “fix” nitrogen.

Simply put, nitrogen and hydrogen are reacted under very high temperatures and pressure to form ammonia. Much of the ammonia so synthesized is liquefied under pressure. Tremendous quantities of the liquid are used by the agricultural industry, applied directly to the soil with the aid of special tools and equipment.

Ammoniacal Nitrogen

Anhydrous ammonia, the product of the Claude-Haber process, is the starting point for the following forms of ammoniacal nitrogen.

Ammonium sulfate (NH4)2SO4 21-0-0
Ammonium nitrate NH4NO3 34-0-0
Urea CO(NH2)2 46-0-0
Monoammonium phosphate NH4H2PO4 11-48-0
Diammonium phosphate (NH4)2HP04 21-53-0

All of these are available to home gardeners either in the forms named above or as sources of “primary nutrients” in many familiar fertilizers.

Ammoniacal fertilizers have a gradual acidifying effect on soils. The nitrogen contained in these fertilizers stays in the root zone longer than nitrate nitrogen because of its chemical composition. It does not readily leach from the soil.

The ammoniacal forms of nitrogen undergo nitrification—they slowly degrade to nitrate—to become the most common form in the soil. This nitrification process is facilitated by soil microorganisms which, in turn, are dependent on warm soil for greatest activity. Ammoniacal nitrogen, therefore, is indirectly dependent on soil temperature for availability to plants. In warm soils, this nitrification will be largely complete within 3 weeks. In cold or sterile soils, it takes much longer.

Ammonium sulfate is very widely used and commonly available. It is the most acidifying and has a fairly high salt index compared with the other common ammonia fertilizers.

Urea is the most concentrated nitrogen fertilizer normally available to home gardeners. Even though urea is a product of ammoniacal nitrogen, because of its chemical composition it has a comparatively low salt index. Some states allow urea to be marketed as an “organic” fertilizer. It is quite acid-forming and, whether in the dry or liquid form, is usually applied with water.

Ammonium nitrate has some of the qualities of both ammoniacal nitrogen and nitrate nitrogen. Because it is basically fast acting, it is grouped with the nitrates.

Ammonium phosphate and diammonium phosphate are also carriers of ammoniacal nitrogen, but because they supply more phosphorus than nitrogen, we have described them with the phosphorus fertilizers.


The first widely available inorganic nitrogen fertilizer was a nitrate. Called Chilean nitrate (16–0–0), it was first mined in the early 1800s in a desert valley in Chile that has vast natural deposits of remarkably pure sodium nitrate (NaNO3).

Chilean or sodium nitrate is used relatively little today. Although a little is commercially synthesized and used to some extent in the southeastern United States, sodium nitrate is now only interesting history in most other areas.

Basically, nitrate fertilizers are made by reacting ammonia (NH3) with oxygen (02) to form nitric acid (HN03). This acid is then chemically manipulated to produce the various nitrate fertilizers.

The more important nitrate fertilizers today are:

Ammonium nitrate NH4NO3 34-0-0
Calcium nitrate Ca(NO)2 15-0-0
Potassium nitrate KNO3 14-0-35

Ammonium nitrate is by far the most widely used of these three.

Nitrate fertilizers are quick to dissolve in water and the nitrate ion (NO3-) is already in a form plants can use—no microbial action is necessary. Therefore nitrate fertilizers are not inhibited by cold or sterile soils.

Unlike ammoniacal nitrogen, nitrate nitrogen does not become attached to soil particles, so it is easily washed by rains or heavy watering deep into the soil, sometimes past the reach of roots.

Nitrate fertilizers are not acid forming. (Ammonium nitrate is the exception, but because of the ammonium, not the nitrate.) In fact, both calcium nitrate and potassium nitrate have an overall basic reaction, slightly raising soil pH in some cases.

The main points to remember about nitrate nitrogen are that it is fast acting, readily soluble, can be easily leached from the soil, does not have an acidifying effect and is not dependent on soil micro-organisms for availability to the plant.

Water-Insoluble Nitrogen (WIN)

Water-insoluble nitrogen fertilizers are, in varying degrees, slow-release fertilizers. Because they make nitrogen available over a comparatively long period of time, efficiency is their virtue. They are applied much less frequently than soluble forms, and supply a slow, steady flow of nitrogen. The ideal fertilizer is one that would release only as much nutrient as necessary when the plant needs it. Slow-release fertilizers seek to do just that.

The rate of nutrient release is slowed in three main ways. Most simply, pellets of relatively insoluble fertilizer are made larger than normal, preventing too rapid a breakdown by soil microorganisms. MagAmp, a popular slow-release fertilizer, is available in a “coarse” grade that will release nutrients for over two years, and in a finer, faster-acting form.

Another way manufacturers slow the release rate of a fertilizer is to change it chemically, rendering a portion insoluble. Urea has been modified in this way to make ureaform (also called urea-formaldehyde and nitroform) and IBDU. Both are variations of the same fertilizer, urea.

Ureaform is 38 percent nitrogen (38–0–0), 70 percent of which is water-insoluble nitrogen (abbreviated WIN). The rate of release of the WIN portion is fastest when microbial activity in the soil is greatest

IBDU (isobutylidene diurea) also contains 38 percent nitrogen, of which 90 percent is WIN. The release of the WIN is affected by the moisture content of the soil and the size of individual fertilizer particles—smaller ones release first. IBDU is less dependent on soil microorganisms than ureaform.

Ureaform and IBDU are used primarily for lawn fertilization. Many lawn fertilizers show on the label “Percent WIN.” Usually ureaform is used for this purpose.

Standard fertilizer classification holds that if less than 15 percent (by weight) of the nitrogen in a fertilizer is water insoluble (WIN), the fertilizer is basically fast-acting. If between 15 and 30 percent is insoluble, it is medium-acting, and any with more than 30 percent insoluble nitrogen is basically a slow-acting fertilizer.

A slow-acting fertilizer is much less likely to burn the lawn after application and is much less subject to being flushed from the soil by water.

The third important type of slow-release fertilizer is essentially a soluble fertilizer wrapped in a small plastic pill. Although these plastic shells are thin and can be easily broken between the fingers, the soluble fertilizers inside are gradually emitted under normal garden circumstances.

This kind of fertilizer has become very popular with growers of potted plants and hanging baskets. Soluble kinds are quickly leached from containers, so need constant replenishment

These also are available as complete fertilizers and in several balanced formulas.

Organic Fertilizers

Organic fertilizers contain much of their nitrogen in the form of protein, which must be broken down by microorganisms into nitrate before it is available to plants. For this reason, it is slow-release, releasing about half of the nutrients they contain through the first growing season. The rest are released over the next several years, as the organic matter slowly degrades.