By Susan Haddock
Turfgrass fertility management is a year round task, so a review of fertilizer basics may be helpful. Generally, fertilizer is defined as any substance containing one or more recognized plant nutrients that promote plant growth. Checking with the Department of Agriculture in a number of states results in a mixed bag of definitions. Many states include any substance that controls soil acidity or alkalinity, provides other soil enrichments or provides other corrective measures to the soil in the fertilizer definition. Most states exclude unmanipulated animal or vegetable manure and some states exclude marl, lime, limestone, wood ashes, peat and/or compost that has not been shown to have value in promoting plant growth.
There can be confusion over the terms natural organic, synthetic organic and organic fertilizer. Natural organic refers to products that are derived from either plant or animal products containing one or more elements, other than carbon, hydrogen and oxygen, which contribute to plant growth. These products are derived from living organisms and may include dried blood, composted manure, fish bi-products, bone meal and cottonseed meal.
Synthetic organic refers to products that are manufactured chemically from elements or other chemicals. Synthetics go through some sort of manufacturing process, although they may come from naturally occurring mineral deposits. Synthetic products may also be referred to as inorganic fertilizer.
Organic fertilizer, in the strict sense of definition, refers to a product containing carbon and one or more elements needed for plant growth, other than hydrogen and oxygen. Organic includes both natural and synthetic products. Hence the confusion, as most who desire an organic fertilizer really want a natural organic or non-manufactured product.
This article will focus on synthetic products.
Important considerations are the expectation on product release rate (how quickly turf greens up), product duration (how long the product produces a greening effect), and environmental impact (product leaching or volatilization). Nitrogen is the most critical nutrient source that promotes turf growth, so turf performance depends on the nitrogen source in the product.
In general, there are three nitrogen release sources: fast release, slow release and controlled release. That being said, there are many environmental and cultural factors that critically affect the ability of turf to uptake nutrients and produce acceptable turf quality. These factors must be evaluated, monitored and corrected, if necessary, before creating a fertilization plan. Soil tests are the basis for developing a fertilization plan and provide information on what the soil needs or does not need to provide for turf nutrient requirements. Additionally, tissue testing can reveal whether the turf is effectively taking up the nutrients.
Fast release sources are referred to as soluble due to their high solubility in water. Fast release nitrogen sources release nitrogen very quickly and may have a quick greening response that lasts just a few weeks. Multiple applications are usually necessary to maintain turfgrass quality. Nitrogen deficiency symptoms may occur between applications due to the cycling between high and low nitrogen levels. Fast release sources may promote excessive shoot growth and readily volatilize or leach. Common examples include urea, ammonium sulfate, ammonium nitrate and calcium nitrate.
Slow release and controlled release sources provide nitrogen gradually over time and promote more consistent turf quality and fewer deficiency symptoms. These sources also minimize losses due to volatilization, leaching and excessive shoot growth. The terms slow release and controlled release are frequently used interchangeably. Here, slow release refers to uncoated products and controlled release refers to coated products. Uncoated products are homogenous, meaning that the composition is the same throughout particles. Examples of uncoated products include ureaform (UF) and methylene urea (MU). These uncoated products contain about 40% nitrogen and rely on microorganisms to mineralize the nitrogen.
Because of this reliance on microorganisms the nitrogen release rate can be quite variable, usually between 8 and 12 weeks, depending upon pH, soil temperature and soil moisture. During cool or cold seasons UF and MU may not be the products of choice, as microorganism inactivity will prevent the nitrogen from being released. Another example of an uncoated product is isobutylidene diurea (IBDU), containing 32% nitrogen. IBDU is soluble and releases nitrogen by hydrolysis, like the fast release nitrogen sources, except that it has low solubility and therefore releases nitrogen very gradually over time depending upon soil moisture. Because of the reliance on soil moisture, dry or drought conditions will delay nitrogen release. Additionally, low pH and high temperature will accelerate the release of nitrogen. Since IBDU is not dependent on microorganism activity it is a preferred product for cool season application.
Coated products include sulfur-coated urea (SCU), polymer-coated fertilizers (PCF) and hybrid polymer-coated sulfur-coated fertilizers (PCSCU). SCU is 30 to 40% urea nitrogen coated with sulfur and sealed with, typically, wax. Imperfections, micro pores and cracks in the coating allow water to enter and dissolve the urea rapidly. Once water enters, the urea can release very quickly and is sometimes referred to as catastrophic release. Coating thickness and imperfections control the release rate, so particles release at different rates, usually between 6 to 16 weeks. Microorganism activity is needed to break the sealant to expose the sulfur coating.
During cool seasons microorganism inactivity and coating variability can cause a mottling effect on turf. When the sulfur coating is too thick, the nitrogen does not release, and is referred to as lock-off. These particles may rely on some physical disturbance, such as mowing, to break the particle and eventually release the nitrogen.
Polymer-coated fertilizers release nitrogen by diffusion through a polymer coating. These products may contain other nitrogen sources such as ammonium nitrate, or other nutrients such as phosphorous and potassium. There are a variety of methods and chemistries used to produce the coating. Regardless, manufacturers are able to produce quite predictable release rates depending upon the number of layers and thickness. Water diffuses through the coating to dissolve fertilizer inside. The fertilizer then moves out of the polymer coating into the soil. Eventually, over 8 to 52 weeks, all of the fertilizer is dissolved from inside the polymer coating.
Polymer-coated sulfur-coated urea is produced with a polymer layer around the sulfur layer instead of a wax sealant. The polymer layer provides more protection and results in even more predictable and uniform nutrient release. Water must diffuse through the polymer coating, dissolve the urea through imperfections, micro pores and cracks in the sulfur coating and then diffuse back through the polymer coating into the soil.
Another brand layers the polymer coating on the urea granule with a sulfur coating and wax sealant on the outside of the particles. With either production method, release rates are longer and less temperature dependent, and surge growth after application is reduced.
Labels
All fertilizer labels will provide a Guaranteed Analysis. The Guaranteed Analysis is the manufacturer’s guarantee for minimum percentage of nutrients claimed for the product. In the guaranteed analysis, nitrogen must be guaranteed as Total Nitrogen (N). If chemical forms of nitrogen are claimed, the forms will be shown in the Guaranteed Analysis. No particular order of forms of nitrogen is required. When a fertilizer contains determinable forms of nutrients that are slowly available and a slowly available claim is made, then the guarantee is shown as a footnote, rather than as a component in the guaranteed analysis. See below. To determine the slow release component or percent divide the slowly available nitrogen by the total nitrogen and multiply by 100.
GUARANTEED ANALYSIS
Total Nitrogen (N) x%
x% Ammoniacal Nitrogen
x% Nitrate Nitrogen
x% Urea Nitrogen*
x% Other Water Soluble Nitrogen
x% Slowly Available Water Soluble Nitrogen
x% Water Insoluble Nitrogen
*x% Slowly available urea nitrogen from _______ (nitrogen source material)
Forms of nitrogen and their sources that may appear in the Guaranteed Analysis include:
- Ammoniacal Nitrogen sources include monoammonium phosphate, diammonium phosphate, ammonium sulfate, ammonium nitrate, urea ammonium nitrate, ammonium polyphosphate, calcium ammonium nitrate and ammonium thiosulfate.
- Nitrate Nitrogen sources include urea ammonium nitrate, ammonium nitrate, potassium nitrate, calcium nitrate and sodium nitrate.
- Urea Nitrogen can come from sources that include urea ammonium nitrate, urea, urea triazone, sulfur coated urea and polymer-coated urea.
- Other Water Soluble Nitrogen can come from sources that include methylene urea, urea triazone, methylene diurea, dimethylenetriurea, dicyandiamide, triazone, ureaform and urea-formaldehyde.
- Slowly Available Water Soluble Nitrogen sources include methylene urea, urea triazone, methylene diurea, dimethylenetriurea, dicyandiamide, triazone, ureaform and urea-formaldehyde.
- Water Insoluble Nitrogen sources that include ureaform, isobutylidene diurea, urea-formaldehyde, feather meal, blood meal, corn gluten meal and other natural organic materials.
So, which fertilizer source is the best?
First and foremost, you must practice sound agronomic practices: perform soil and/or tissue testing, improve soil conditions and correct detrimental cultural practices. Plan nutrient management based on environmental and seasonal influences such as temperature, rainfall and/or irrigation, use intensity and proximity to water bodies. The type of turfgrass, expected quality and budget also influences management strategy.
In addition to fertilization management, you may wonder if the use of biostimulants will produce higher quality turf.
Dr. Keith Karnok from the University of Georgia says turfgrass managers recognize biostimulants to be a product/material that is non-traditional in that it is not a fertilizer or pesticide per se, but is applied to the soil or plant in hopes of improving turfgrass performance (usually roots and shoots). Humic acid, seaweed extract, salicylic acid and plant hormones are commonly used turf biostimulants. Research has shown that biostimulants may improve turfgrass response to environmental or culturally induced stress conditions; however, it needs to be applied before the stress condition. Predicting stress conditions, such as summer stress, drought or disease may be pertinent in the successful use of biostimulants.
Researchers at Virginia Tech (Drs. Xunzhong Zhang and Richard Schmidt) have performed studies on turf response to various biostimulants in regard to enhanced root and shoot development, drought tolerance, salt tolerance, disease resistance, UV light tolerance and heat tolerance showing that biostimulants can improve turf health and stress tolerance. The consensus regarding use of biostimulants is to research products diligently, avoid products that make boastful claims without evidence from scientific studies performed by independent or university backed research, communicate with other turf managers about products that work, test on small areas prior to widespread application and do not substitute biostimulants for a fertilizer management plan.
Susan Haddock, B.S., MBA, is an Agent II for the University of Florida, IFAS Extension, Hillsborough County.