What is CRN?

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 and/or sulfur and/or 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. 

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Why do CRN "CURVES" matter?

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 coating 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/or multiple coatings, often consisting mainly of polymer.  More coatings and or thicker coatings translate to products with longer dissolution rates.

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Is all CRN the same? What you don't know could BURN your 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 ‘immediate release’ due to the fracturing of about 10% of the SCU granules during storage and transport. Fractured coatings are considered ‘compromised’ and dissolve the same as straight urea.) 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. 

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