Features Agronomy
Making sense of the numbers: what is slow release?

By Sean R. Jordan, T.Ag.
Agronomist

To select one fertilizer over another because of a higher analysis and,
in particular, a higher slow release, is only human nature. Are those
numbers alone representative of performance and quality?

As we continue, we will look at the various types of nitrogen that make up the Guaranteed Minimum Analysis, the sources of these types of nitrogen and compare the potential performance of two blends with the same percentage of slow release nitrogen.

As usual, I will provide here some terminology for a better understanding of what we are about to discuss:
Urea Nitrogen:  Nitrogen that is directly supplied by urea, either as urea in the blend or as part of a partially-coated or reacted source.  Sources include: Urea, sulphur-coated nitrogen (SCN) and polymer-coated urea (PCU), polymer-coated, sulphur-coated urea (PCSCU) and stabilized nitrogen*.

* stabilized nitrogen may be noted as a slow-release nitrogen source even though it is included in the urea portion.

Ammoniacal Nitrogen: Nitrogen in the NH+4 (ammonium N) state, typically derived from such sources as monoammonium phosphate, diammonium phosphate and ammonium sulphate.. Ammoniacal nitrogen often causes an intense greening of the turf.  Sources include: mono- and di-ammonium phosphate, ammonium sulphate and ammonium nitrate.

Nitrate Nitrogen : Nitrogen supplied as NO3- (nitrate N). Nitrates are more readily available at colder soil temperatures than any other quick nitrogen source. This form of nitrogen is often used in the shoulder seasons where cooler soil temperatures limit the uptake of urea from urea.  Sources include: Potassium nitrate, calcium nitrate and ammonium nitrate (federal regulation has now made it difficult to include this ingredient in many non-agricultural fertilizer blends).

Water-insoluble nitrogen (WIN): Nitrogen that is slowly made available through microbial degradation or hydrolysis. These products are created by reacting urea with other chemicals to create polymers or are derived from organic sources. In some cases, only a portion of the urea is reacted, leaving some readily available urea nitrogen. The release of nitrogen from these sources is related to soil moisture and temperature which in turn determines the activity levels of the soil microorganisms that break down these products. Sources include: Methylene urea (MU), urea formaldehyde(UF), Isobutylidenediurea (IBDU)*, poultry litters and biosolids.

*released through hydrolysis, regardless of temperature, and is influenced by particle size and soil pH.

Other water-soluble nitrogen (OWSN): Nitrogen from soluble sources that aren’t readily available like the urea, ammoniacal and nitrate types. These sources must first go through a process of microbial breakdown for the nitrogen to be released. This type can further be divided into two categories, hot- and cold-water soluble, that are used to calculate what is known as the Activity Index (AI). The Activity Index describes the reactivity of the fertilizer and furthermore the length of time necessary to become available to the plants.

What you might notice is that there were no release times or longevities related to any of the aforementioned nitrogen sources. There are so many variables that influence release and with so many differing types of each source that pinpointing exact release characteristics is not possible. Release periods are more frequently expressed in terms of a range of weeks or months.

So, here we are, shopping for fertilizer and we have the technical information for two, 20 per cent nitrogen blends showing 75 per cent of the nitrogen is coming from a “slow-release” source. What’s the difference? Let’s look.

First, let’s look at the nitrogen breakdown of the methylene urea-based product.  You’ll notice that the non-urea nitrogen composes 12.7 per cent of the 20 per cent nitrogen. This means that only about 64 per cent of the nitrogen will be slowly available to the plants. Why not 75 per cent? If you look back to the earlier definitions, you will see that not all of the urea in methylene urea is reacted leaving a portion that is readily available.

Now, looking at the 75 per cent polymer-coated, sulphur-coated product, you will notice that all of the nitrogen is listed under “Urea” with an asterisk denoting a comment below stating “*10.5 per cent Slow Release Nitrogen from Polymer- Coated, Sulphur-Coated Urea.” Again, you are probably asking “Why is only 10.5 per cent of the nitrogen slow release and not 15 per cent.” This is due to the fact that the nitrogen is from urea and the mechanism that regulates the release of the nitrogen is physical that are subject to imperfections and breakage. Only around 70 per cent of this theoretical PCSCU product is going to resist releasing the nitrogen when applied with the rest being immediately available urea.

The examples above are fairly straightforward in that they only have two nitrogen sources each. When you get into fertilizer products that use multiple nitrogen sources and derive nitrogen from sources of other nutrients such as phosphorus and potassium, then it gets to be more complicated. To simplify the label a bit, add together everything that is not slowly available like the urea, ammoniacal and nitrate nitrogens and subtract from the total nitrogen; the remainder is the slow release nitrogen. If you divide the amount of slow release nitrogen by the total nitrogen, you will find the actual per cent slow release.

This article has focused on slow release nitrogen as it tends to have the greatest impact on our daily maintenance, but we also have available other nutrients that are reacted or physically coated to control their release and are worth investigating when we have soils with very high leaching potential or where controlling the release of these nutrients will greatly benefit the turf.

So when you are out fertilizer shopping, take a minute to read the Guaranteed Minimum Analysis to get a better picture of what is in the bag.