What Fertilizer to use?

A few Tips from Jenkins Pest and Lawn in San Antonio, Texas

When nitrogen (N) fertilizers are applied during the growing season, many times the type of fertilizer utilized is determined by cost or convenience. With the variety of nitrogen sources available, proper selection of a type that is compatible with the particular soil pH can reduce overall costs in the long run. Improper use over time can result in the need to amend the root zone to compensate for changes (usually lowering) in the pH.

Soil pH in an orchard is often on the alkaline or acidic end of the pH range. When soils are highly acidic, amendments such as limestone or dolomite are best utilized to raise the pH regardless of the source of nitrogen used. A pH range between 6 and 7 is generally best suited for the maximum availability of most important soil nutrients. If a grower continues to apply strongly acid-forming forms of nitrogen to acidic soils, this only aggravates the problem.

Acid Aggravation

Ammonium sulfate is a common and popular fertilizer used in orchards. It also happens to be the most acidifying form of fertilizer per unit of nitrogen. Each pound of nitrogen as ammonium sulfate has the ability to neutralize 5.3 pounds of free lime. Used consistently over time, this can lead to localized areas in the root zone that are acidic enough to reduce the availability of some nutrients, while freeing up others into solution where they may reach toxic levels.

It is the nitrification process that occurs in the soil in the presence of bacteria that results in the lowering of the pH. When dissolved in water, ammonium sulfate releases ammonium (NH4+) and sulfate (SOa ) ions. It is there that the NH4+ is converted to NOs- by the following reaction:

NH4+ + 202-> N03- + H20 + 2H+ It is the release of the hydrogen ions that causes the soil pH to drop.

Not All Nitrogen Sources Are Created Equal

One might ask why all nitrogen sources that have ammonium or break down into ammonium do not result in the same problem. To a degree they do, but to a much lesser extent. Ammonium nitrate, for example, has ammonium and is 34% N as opposed to 21% N for ammonium sulfate.

Why is it that one pound of nitrogen as ammonium nitrate only neutralizes 1.8 pounds of free lime, compared to 5.3 pounds for ammonium sulfate? The reason is that a large portion of the N in ammonium nitrate is derived from the nitrate, which does not contribute to the acidity. The same applies to other common forms of nitrogen. Urea, anhydrous ammonia, and UAN-32 all contain or break down into ammonic forms which do lower the soil pH. However, they either contain less ammonic N per pound of nitrogen or contain nitrate nitrogen. Each of these three products has a similar effect on soil pH as ammonium nitrate. They all neutralize about 1.8 pounds of free lime per pound of nitrogen.

Keep It Balanced

Does this mean that one should not use acidifying sources of nitrogen? The answer is no. It simply means that the grower needs to consider the existing pH and the effect the fertilizer of choice will have on it. Properly amended soils can tolerate it, while alkaline soils can actually benefit from it. They key to remember is that nitrogen is only one component of a balanced nutritional program.