How to Choose Your CRN - So You Don't Get Burned!

November 7, 2018

What is Controlled Release Nitrogen?
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. 

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

How to read a CRN label

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. 


What is a dissolution rate?

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 the straightest possible release curves, however in reality most dissolution rate curves mimic the shape of a hockey stick.  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 coatings, often consisting of both sulfur and polymer.  More coatings and thicker coatings translate to products with a longer dissolution rate. Not surprisingly, these products take more time to manufacture and use more coating reagents, hence they drive a premium price point.

Take for example XCU, a premium polymer coated, sulfur coated urea product.  This product has a dissolution rate of 45 days.  This may be a little on the short end compared to other PCSCU products, however the trade off is XCU has been engineered to have an incredibly stable and predictable release curve.  This is the perfect CRN for LCOs who fertilizer every other month for their customers.  XCU provides continuous, even feeding during the release period. 

How do I chose the right CRN for me?

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, a do-it-yourselfer homer owner 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, because LCOs generate revenue by cutting lawn and applying fertilizer & control products. 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.

What are the effects of applying low-quality CRN products to turf?

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 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. 

Final Thoughts

Starting with the invention of sulfur-coated urea in the 1960s, Controlled Release Nitrogen continues to play an important role in lawn care through the present.  Knowing how to pick the right CRN will give you a beautiful lawn at the right price point for your situation.

Check out Turf Care Supply Corp's product catalog to see which quality CRN products are available to assure you get results and protection for your turf.

« Back to Blog