Controlled Release Nitrogen (CRN) consists of uniform granules (usually urea-based) designed to release nitrogen into the soil over an extended time period. This is as opposed to straight urea, which is considered to be ‘immediately available nitrogen.’ The mechanism for controlling the release of nitrogen is a physical barrier. Controlled release nitrogen is typically encapsulated by a combination one or more layers of polymer, sulfur and wax. These layers work in tandem to control the rate that water enters the granule, then control the rate at which dissolved urea nitrogen leaves the granule.  

Sulfur Coated Urea (SCU) was the first type of CRN to hit the market nearly 50 years ago. Sulfur is very effective at repelling water, however sulfur coatings are extremely brittle. The Sulfur coating fractures allowing water to find its way inside, the granule expands, further fracturing the sulfur coating. 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. Most products have a wax that is applied atop the sulfur layer to slow the rate of nitrogen release by protecting the Sulfur layer. 

Temperature can also affect CRN granules coated with wax because waxes are slowly broken down by soil microbes (microbes are more active in warmer soil temperatures). In recent years, SCU has been enhanced by hardeners and or polymers that further protect the inner brittle sulfur layer or protect the water from reaching the inner granular, effectively stabilizing the release curves of SCUs. These enhanced SCU base products which combine polymers are often referred to as Polymer coated sulfur coated ureas. XCU® is an example of an advanced SCU or PCSCU.  

Polymer (or Plastic) Coated Urea (PCU) generally has longer and more stable release properties vs most SCU based products. PCU granules can consist of one or more polymer layers allowing for greater extended release times. 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 weeks or 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 PCU technology very efficient and environmentally friendly. DURATION® is an example of an advanced PCU.

Every fertilizer label contains an N-P-K analysis and a derived-from statement. The analysis shows the percent by weight of each element inside a fertilizer bag. The derived-from statement shows what raw ingredients those elements are derived from. In addition to an analysis and a derived-from statement, every label must also show the guaranteed percent of “slowly available nitrogen,” and the source of that nitrogen.  

Take for example TCS Growstar’s 29-0-4 Professional Lawn Fertilizer + Iron. This blend contains both Urea and Polymer Sulfur-Coated Urea (PSCU). 29% of the entire bag by weight is nitrogen. 9.1% of the entire bag by weight is nitrogen derived from PSCU. That means that 31.3% of the total nitrogen is CRN (from PSCU) and 68.7% of the total nitrogen is urea.  

Every controlled-release fertilizer has a dissolution rate associated with it. The dissolution rate refers to how much dissolved urea has been depleted (or released) over a time period. In general, the industry goal is to manufacture CRNs with minimal to no release in the first 3 days, because urea is providing the Nitrogen during the first 3 days. Then from day 4 to the desired days of release (typically 45,90,120,180) a consistent release on nitrogen each week (see chart 1) is optimal, however in reality most dissolution rate curves varying by CRN type. CRN products can last anywhere from 30 days to up to 180 days. In general, 30-day products have thinner coatings, so they deplete their nitrogen faster. 180-day products have thicker and multiple coatings, often consisting mainly of polymer. More coatings and or thicker coatings translate to products with longer dissolution rates. Not surprisingly, these products take more time to manufacture and use more coating components, hence they drive a premium price point.

Take a look at the dissolution rate curves in the graph above. These curves do not represent specific CRN products, but rather demonstrate the evolution of classes of CRN products over time. First, note that straight urea (46-0-0) releases nearly all of its nitrogen within 3 days under normal conditions (chart 1). Urea is the best source of nitrogen on a cost per ton basis, hence urea is the base raw ingredient for nearly all CRN products.

Next, look at (chart 2) basic SCU (invented in the late 1960s). Note that basic SCU releases nearly 30% of its nitrogen within the first day or two. Its release curve mimics a standard logarithmic curve, meaning compared to other CRNs, it releases more of its nitrogen earlier in the curve. Given this challenge (too much nitrogen release within the first few days), manufacturers and agronomists were determined to reduce this initial nitrogen load into the ground. This led to the development of polymer coated urea (PCU) several decades later. In addition to quelling the early nitrogen release problem associated with first generation SCU, PCU also brought straighter, more consistent release curves to the industry (chart 3). Today’s improved PCU products have release curves that mimic a shape of a hockey stick, resulting in more gentle transitional nitrogen feeding for turf. Improved PCU products have also been engineered to reduce lock-off issues, and last longer in the field.

  

Last, the higher costs associated with components and manufacturing improved PCU products has led to the development of improved or “enhanced” SCU products. Improved SCU uses hardeners and or polymers and or waxes to mitigate the early nitrogen release issues associated with 1stgeneration SCU products. Improved SCU products are also more resistant to abrasion and have longer release windows. Some improved SCU products now last as long as 90 days in the field.  

Choosing the right CRN comes from an understanding the dissolution rate (or release curve profile) of each CRN, and then deciding which CRN gives you the nitrogen you need when you need it. For example, an applicator who only likes to drop fertilizer in the spring may want to pay a premium for a 180-day CRN, that way he or she only has to fertilizer once per year. In most cool season grass climates, a 180-day product will make it through most of the growing season (year to year variations in weather will affect the performance of a 180-day product). Conversely, a lawn care business might prefer a 45-day product such as XCU, due to sporadic weather conditions, financial factors, specific agronomic / control programs, etc. This isn’t necessarily a bad thing for lawn health either; more nitrogen is making into soil and being taken up by the plant per year than dropping a 180-day product only once per year. You should pick your CRN product based on how it fits into your turf maintenance program. For example, if you only fertilizer twice per year, two 90-day products might be right for your situation. If you’re a golf course superintendent, you might prefer to “spoon-feed” your turf, which means you might want to use 30-day products 5-8 times per year. It all depends on your climate and the nature of your turf maintenance program.

The whole concept behind CRN products is that they release nitrogen slowly and evenly over time. Low quality coatings are more susceptible to fracturing through abrasion (the colliding of CRN granules against each other during storage and transport). This is why most CRN products guarantee up to about 90% of their slow-release integrity on the bag. (What this means is, if you had a 50 LB bag of pure 39-0-0 SCU, about 10% of the bag would be considered ‘immediate release’ due to the fracturing of about 10% of the SCU granules during storage and transport. Fractured coatings are considered ‘compromised’ and are treated the same as straight urea from a labeling standpoint.) Low-quality CRN products run the risk of behaving like straight urea, which means they have the potential to burn or even kill a lawn. Before purchasing any CRN, ask for dissolution rate data and don’t hesitate to dig into that product’s industry reputation.