Garden Soil and Nitrogen Fertilizer: Why Choice is Important

Standard fertilizer mixes contain a host of nutrients for vegetation, but especially NPK-"N" for nitrogen, "P" for phosphorous, and "K" for potassium (Latin, kalium). Potassium is always found in ionic salt form. Phosphorous-look for it as "phosphate." Nitrogen? Well, nitrogen is different. Nitrogen is a gas. It can be converted into a variety of forms. The best choice of nitrogen form depends upon what one plants, the nature of the soil, rainfall, and other factors. When it comes to fertilizer, not all forms of nitrogen were "created equal."

General Categories of Nitrogen

There are two general categories of fertilizer nitrogen-organic and inorganic. An example of a commonly used organic compound is urea, H₂N-(CO)-NH₂. For urea, the nitrogen must be broken down by bacteria in the soil to become useful, since urea is not ionic-the form actually used by plants.

Two examples of inorganic nitrogen are ammonium sulfate, (NH₄)₂SO₄, and sodium nitrate, NaNO₃. These are ionic. Ammonium sulfate breaks into its component parts in water, the ammonium ion, NH₄⁺ (two of those) and the sulfate ion, SO₄^-2. Sodium nitrate breaks apart in water to form a sodium ion, Na⁺, and a nitrate ion, NO₃^-.

Inorganic Ionic Nitrogen

We see in the above three examples, the three basic forms of nitrogen. The non-ionic, the positive ionic (cationic) NH₄⁺, and the negative ionic (anionic) NO₃^-. This might not seem important, but it is. For example, the cationic ammonium ion can interfere with the uptake of cationic calcium, Ca⁺², which is essential for the avoidance of blossom end rot in tomatoes. Hence, if the soil is borderline low on calcium, it would be wise to use a form of nitrogen other than cationic ammonium.

So is it wiser to use anionic nitrogen? Often, yes-nitrates absorb readily in water, and are in highly usable form by plants. On the other hand, if the soil packs tightly, oxygen availability within the soil may be severely limited, and bacteria present may strip nitrate of its oxygen atoms, converting the nitrogen to a gaseous form unavailable to the plants-escaping instead to the atmosphere.

Organic Non-Ionic Nitrogen

So then, switch to urea? If the urea is spread and it doesn't rain for some length of time, volatilization occurs, and nitrogen is lost. Since farmers at times place a "residue" atop the surface of the soil before applying fertilizer, absorption into the soil may be somewhat difficult. The residue may contain corncobs or other materials. It may be advisable to fertilize beneath the residue layer. Also, high temperatures and increased pH lead to greater volatilization (Urea should not be used on soil that has just been treated with lime). When appropriate, urea should be applied beneath the residue. If it can be quickly absorbed into the soil, urea is often the superior choice, especially in view of its high nitrogen content.

Efficacy and Cost

To assure success, some household fertilizer brands blend two or more forms of nitrogen. Often the combination is urea plus nitrates. For farmers, weight of available nitrogen per dollar spent is very important. However, cost and efficacy need to be viewed through the light of the advantages and disadvantages discussed above.

References and Resources:

Ohio State University Extension - Selecting Forms of Nitrogen Fertilizer

University of Minnesota Extension - Fertilizer Urea