3/27/2018 - By Harrell's

Nitrogen is the most widely used nutrient source that promotes plant growth. The degree of plant response with nitrogen is a function of application rate and the source of nitrogen utilized. Depending on the nitrogen source used, the length of nutrient release can vary from a couple of weeks to several months. There are three basic nitrogen release mechanisms that coincide with the types of nitrogen fertilizers. The release mechanisms include hydrolysis (reacts with water), soil microbial feeding (mineralization), and temperature (Table 1).

Consideration should be given to the type of nitrogen fertilizers that are available for use. Just like the release mechanisms, there are three types of nitrogen fertilizers that include fast release, slow release, and controlled release fertilizers.

Fast Release Nitrogen Sources

Fast release nitrogen sources are also referred to as “soluble” nitrogen sources by many turfgrass managers based on their release mechanism. These fertilizers, such as ammonium nitrate, ammonium sulfate, calcium nitrate, urea, and stabilized urea release their nitrogen “quickly”. Complete release of nitrogen from these fast release sources occurs within one week after application. Many of these release 100% of their nitrogen within 24 hours after application. The typical turfgrass plant response from these fast release nitrogen fertilizers lasts for a couple of weeks after application. Caution should be given to the application rate with these fertilizers due to excessive shoot growth and potential leaching of nitrate and/or urea, which are both very water soluble and susceptible to leaching. Given their short length of release longevity, several applications are required to maintain acceptable turfgrass quality over an extended period of time. Many of the foliar or liquid fertilizers that are utilized contain these fast release nitrogen sources. It is important to read the product label and pay close attention to the “sources of nutrients” or “derived from” section of the label to understand how these fertilizers will perform.

Slow Release Nitrogen Sources

Slow release nitrogen sources are more efficient in their ability to provide nitrogen gradually to the turfgrass plants over time. There are three types of slow release nitrogen sources that include natural organic fertilizers, synthetic reacted organics, and polymer coated sulfur coated urea.

The natural organic fertilizers are produced from waste by-products. These “organics” release nitrogen through soil microbial feeding or mineralization, which is the same process that organic matter and thatch are decomposed. Release of nitrogen is variable and unpredictable and will accumulate over time if applied at higher application rates and/or frequent application timings. Consideration should be given as to the timing of application of organic fertilizer relative to turfgrass type (cool- vs warm-season turfgrass). Most organic fertilizers release more nitrogen in the warmer months due to increased soil microbial feeding, which may not be preferred if applied to a cool-season turfgrass such as creeping bentgrass.

The synthetic reacted organics are also referred to as “urea formaldehyde” fertilizers. The name of these fertilizers offer a lot of insight into their purpose – reacted synthetic organics. These are nitrogen fertilizers that are manufactured from “synthetic” chemical “reactions” to make a fertilizer that releases its nitrogen like “organic” fertilizers. They are manufactured by the reaction of Urea (U) with Formaldehyde (F) to make U:F polymers. The release mechanism is soil microbial feeding or mineralization, which is identical to the organic fertilizers. Some of the recognized brand names for these fertilizers are Nitroform, Nutralene, MethEx, MU, BCMU, Coron, Triazone, and Sazolene. These UF fertilizers are comprised of three nitrogen fractions from the manufacturing process. The balance of urea to the amount of formaldehyde and the length of the reaction determines the balance of the three nitrogen fractions that make up the release characteristics of these products. The three nitrogen fractions are Cold Water Soluble Nitrogen (CWSN), Hot Water Soluble Nitrogen (HWSN), and Hot Water Insoluble Nitrogen (HWIN). On a product label for these UF products, you will note WIN which is water insoluble nitrogen and is comprised of the total amount of nitrogen that is made up from HWSN + HWIN. It is critical to understand the breakdown of WIN to understand how these fertilizers are going to perform. The CWSN (soluble in 70F water) fraction is considered “quick” release as it is completely released within two weeks after application. The HWSN fraction is soluble in hot water (100F) and releases its nitrogen within two to three months after application. Lastly, the HWIN is not soluble in boiling water (212F) and releases over the course of several months to years depending on the climate and environmental conditions that relate to soil microbial feeding activity. Some of these UF fertilizers contain as much as 40% CWSN (Nutralene) and conversely, others contain up to 50% HWIN (Nitroform).

The last type of slow release nitrogen sources is polymer coated sulfur coated urea (PCSCU). These fertilizers release their nitrogen based on water. The actual release mechanism is called “catastrophic”. Fertilizers such as PolyS, PolyPlus, and XCU are PCSCU products. For nitrogen to release, water must be absorbed through the layers of coatings surrounding the urea substrate. The thickness of the coating and amount of water that the PCSCU granule is exposed control the rate of absorption and speed of nitrogen release. As water is absorbed through the coating, it then saturates the urea substrate. Given the water solubility of urea (100,000 ppm at 68 F), it dissolves rather quickly. Once dissolved, it creates an osmotic pressure gradient relative to the concentration of dissolved urea inside the granule versus outside. The osmotic pressure is too excessive for the coating to withstand so the coating fails or explodes, thus the term “catastrophic” release. The slow release properties of PCSCU products are controlled by the variability in coating thicknesses. Not all of the granules are coated with the same thickness; therefore, they release at different times. As a matter of fact, some of the granules are coated with an excessive amount of sulfur and will not release their nitrogen unless the coating is physically damaged from mowing operations or over a long period of time, soil microbes will feed on the coating and create release. These granules that are coated too thickly with sulfur are called “locked off” granules. The amount of lock off can range from 5% (XCU) to as high as 20% (PolyS and PolyPlus) of all of the granules! Normal time for 100% release of the nitrogen from PCSCU products is 6 to 8 weeks. It is important to pay attention to the distribution of particles with PCSCU products to insure even turfgrass response. Many times, turfgrass managers will apply PCSCU in diluted blends (less than 70% PCSCU) and the net result is a “mottling” of the turfgrass due to uneven particle distribution of the PCSCU.

Controlled Release Nitrogen Sources

The controlled release nitrogen sources are also referred to as “resin” coated or “polymer” coated fertilizers. The manufacturing process involves coating a fast release nitrogen substrate such as ammonium sulfate, potassium nitrate, urea, etc with either a resin coating or a polymer coating. Osmocote is a resin coated controlled release fertilizer and releases nitrogen via osmosis, thus the name. Polymer coated products, such as Polyon release nutrients via solute concentration gradient permeation (osmotic diffusion) where the polymer coating behaves like a ‘membrane’. Water is absorbed into the coating as with PCSCU products, then when the water soluble substrate is dissolved, it creates an osmotic gradient and the polymer coating is strong enough to withstand this pressure so the nutrients diffuse out of the granule across the polymer coating. The thickness of the polymer coating determines the length of release, which can range from 8 to 52 weeks after application. The release of nitrogen from Polyon is very predictable and can be evaluated prior to making an application using the PolyGraph software program.