What are CRN/SRN Fertilizers and How do Various Coating Technologies Compare?

Spring has finally arrived! Time to revive your turf and supply it with what it needs to assure a lush green appearance all season long. So, what’s the key to healthy green turf? Fertilizer. But when it comes to selecting the right fertilizer for the job, with so many different technologies and analyses to choose from, making a selection can seem overwhelming. Whether a professional landscaper, golf course superintendent or do-it-yourselfer, controlled/slow release fertilizers are typically the best choice for your turf. And economically, are a better investment than conventional fertilizers. 

This Turf[TECH]Report article will be a two-part series. Since there is so much to learn and discuss regarding controlled/slow release fertilizers we have broken it down into two parts to provide a thorough understanding.  

Part 1: What are CRN/SRN Fertilizer and How do Various Coating Technologies Compare?

Part 2: Understanding CRN/SRN Release Label Language and Product Review  (Coming Soon!)

It is our hope that by the end of this extended feature, you’ll have the information you need to determine which slow/controlled release fertilizer is right for you. Let’s get started!   


What are Controlled/Slow Release Fertilizers? 
Controlled/Slow Release fertilizers are uniform granules that are designed to release their nutrients into the soil over an extended period of time. The terms “controlled release” and “slow release” are often used interchangeably. Although many sources claim that “controlled release” fertilizers are capsule-based technologies, and “slow release” fertilizers are biologically and temperature dependent, there is still significant confusion as to how these terms are properly used and defined. Regardless of the confusion, all controlled/slow release fertilizers work to protect nutrients (usually nitrogen) from being lost to the environment (through volatilization, denitrification and leaching).  For the duration of this feature, we’ll be focusing exclusively on nitrogen-based controlled/slow release fertilizers. 

There are two distinct types of controlled/slow release nitrogen technologies (also known as “SRN”): “coated slow release nitrogen” and “reacted slow release nitrogen.”   

Coated slow release nitrogen fertilizers work by encapsulating a nutrient core with a semi-permeable membrane (known as a “coating”) which permits nitrogen-based molecules (such as urea) to trickle out at a slow rate after being dissolved by water. The mode-of-action with coated slow release nitrogen fertilizers is “physical.”   

Reacted slow release nitrogen fertilizers work differently; their modes-of-action are biological. With reacted slow release nitrogen technology, formaldehyde is reacted with urea granules to produce both methylene urea and urea-formaldehyde molecules. These new molecules form chains of varying lengths which are designed to work harmoniously with the biology of soil microbes. Over time, soil microbes slowly digest these chains, breaking off one urea molecule at a time. This biological regulation of urea release leads to decreased nitrogen leaching* and volatilization**, as well as increased nitrogen availability over time. Reacted slow release nitrogen is highly influenced by soil temperature and moisture content (because these factors influence microbial activity). Because much of the nitrogen in these products is not immediately soluble in water, these fertilizers are considered partly “water insoluble.” The amount of nitrogen in a fertilizer that is not immediately available to be dissolved by water is called “Water Insoluble Nitrogen” or “WIN.” 

Example: Nutraline® (Methylene-Urea or MU”) (See example to left)

Why were SR (slow release) Fertilizers Invented? 
Like all organisms, plants thrive when key macro and micronutrients are in abundance, however they can only take in so much nutrition at a time. Unfortunately, dumping a large quantity of conventional fertilizer into the soil is not only wasteful, it can be detrimental to the soil and the plant. This is because most conventional fertilizers are “immediate release,” which means 100% of the nutrition in the fertilizer granule is readily dissolved into the soil by water and can be immediately taken up by the plant. The application of excessive immediately release nitrogen (or “IRN”, also referred to as “quick release nitrogen”) is problematic for two reasons:  

  1. The plant’s inability to assimilate so much nutrition in a short period of time leads to nutrient loss via leaching and volatilization  
  2. Many fertilizers are highly acidic or basic; this results in a radical pH shift in the soil which may end up burning the plant. 

For many decades to avoid these two serious problems, agronomists and turf technicians were forced to apply low-doses of nitrogen frequently to optimize plant nutrient uptake without harming the plant or soil. This process is informally referred to as “spoon-feeding.” Because “spoon-feeding” is economically inefficient, scientists were inspired to create the world’s first coated urea granule. Pioneered by the Tennessee Valley Authority in the late 1960s, the first SRN urea granules were coated with sulfur. Since then, chemists have also devised numerous polymers and waxes to control the release of nitrogen. The innovation of the TVA in the late 60s/early 70s completely revolutionized the fertilizer industries in farming, horticulture and turf. Many of the same fundamental technologies invented by the TVA nearly 50 years ago are still in practice today.   

Coated Slow Release Nitrogen Fertilizers (Coated SRNs) – How do they work? 
Coated SRN fertilizers use a physical barrier to slow the release of nitrogen into the soil. Most SRNs have urea at their core. SRN granule coatings are typically comprised of the following materials: elemental sulfur, polymer, and wax. Some SRNs are only coated with sulfur; some only with polymer; and some are coated with a combination of both sulfur and polymer, in addition to a wax layer. Different combinations of coatings produce different results (in terms of how fast nitrogen is released.)  Economics play a part too; polymer coated urea is generally more expensive than sulfur coated urea.   

All coated SRN granules work on the same principle. Water slowly enters the granule and begins to dissolve the urea inside. The granule slowly expands, and the dissolved urea slowly passes back through cracks in the membrane into the soil, where it is ultimately converted into a form of nitrogen that plants can absorb [ammonium (NH4+) or nitrate (NO3-) through biological or chemical means]. 

This diagram of a basic sulfur coated urea granule shows the movement of water through the sulfur coating and back out again with dissolved urea.


(See diagram above)

What Coating Technologies are Available and How Do They Compare? 
Sulfur Coated Urea (SCU) is the least expensive coated SRN to manufacture. Sulfur is initially very effective at repelling water, however sulfur coatings are extremely brittle. Once water finds its way inside, the granule expands and the sulfur coating factures. Once this occurs, dissolved urea flows out quickly. SCU works on the premise that some granules will take longer to fracture and release their nutrients than others. Over the course of time, this “mix” of release times results in continuous nitrogen feeding until the last granules have fractured. Sometimes wax is applied atop the sulfur layer to slow the rate of nitrogen release. Temperature can also affect SRN granules coated with wax because waxes are slowly broken down by soil microbes (microbes are more active in warmer soil temperatures). 

Polymer (or Plastic) Coated Urea (PCU) is more expensive to manufacture, but generally has longer and more stable release properties. Polymer coatings are less susceptible to fracturing (abrasion) during transport than sulfur coatings thus their release times are more consistent. Through osmosis, water diffuses through the polymer layer and slowly dissolves the urea core. Over the course of many months, urea slowly dissolves through the polymer layer providing an even, controlled distribution of nitrogen over a large portion of the growing season. Once the urea has been depleted, the thin polymer shell is biodegraded naturally by soil microbes, making PCS technology very efficient and environmentally friendly. 

Polymer Coated Sulfur Coated Urea (PCSCU) is exactly what it sounds like; urea coated with sulfur and polymer. PCSCU also has an outer wax layer. PCSCU is essentially the “Cadillac” of coated SRN fertilizers; it is the most expensive to manufacture but provides the longest and most steady nitrogen release properties. XCU® is an example of a PCSCU fertilizer (see diagram above). 

Release Curves – How SRNs are Assessed and Compared 
Whether referencing “coated” or “reacted” SRNs, slow release nitrogen fertilizers are assed by the rate at which straight urea enters the soil and is ready to be converted for plant use. In the case of “coated” SRNs, this is the point at which urea exits the granule. In the c

ase of “reacted” SRNs, this is the point at which microbes digest or “release” single urea molecules from their chain form. In either case, it’s all about documenting when “straight urea” becomes available for conversion.   

Some SRNs have short release curves, meaning their nitrogen is depleted rather quickly (for example: 30-45 days). Other SRNs have long release curves; their nitrogen takes a long time to fully deplete into the soil (for example:  120-180 days). Straight urea has no release curve because it is 100% soluble and immediately available for conversion in the soil. 

In the graph to the right, look at the release curves for 3 generic PCU (polymer coated urea)products. The thicker the polymer coating, the straighter the release curve (this is preferred). Notice that the 45-Day PCU has released about 80% of its urea in 45 days. Conversely, the 120-Day PCU has only released about 38% of its urea in 45 days. SRNs with longer and straighter release curves usually have a thicker polymer coating or multiple polymer/sulfur coatings (with or without wax) and are generally more expensive. Note that over time the curves begin to level off. Some coated urea fertilizers are so effective at repelling water that some of the nitrogen remains trapped in the granule indefinitely. This problem is known as “lock-off.”  Also note that thicker coatings will translate into slightly diminished guaranteed analyses for nitrogen content. (See diagram above)