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Turfgrass Fertilizers: N P K

Turfgrass fertilizers usually contain three plant nutrients: nitrogen, phosphorus (designated on labels as available phosphate, or P 2 O 5 ), and potassium (designated as water soluble potash, or K 2 O). These three nutrients are represented on the fertilizer container as three numbers, indicating the percentages by weight of nitrogen, phosphate, and potash—always in that order (Figure 3). The three numbers are referred to as the fertilizer grade. When nitrogen, phosphorus, and potassium are all present in the container, the fertilizer is called a complete fertilizer. Sometimes one or two of these nutrients are not present, and the missing nutrient(s) are simply listed as “0” in the grade. Occasionally, turfgrass fertilizers contain other nutrients such as sulfur, iron, and/or calcium. These are usually listed on the label but are not part of the fertilizer grade. A fertilizer grade is used to determine the percentage by weight of plant nutrients in the product. For example, a 100-lb bag of fertilizer with a grade of 30-0-10 contains 30 lb of nitrogen, no phosphate, and 10 lb of potash. A 50-lb bag of the same product would yield 15 lb nitrogen, no phosphate, and 5 lb of potash. Knowing the fertilizer grade is important in determining how much fertilizer to apply to your turf. Sometimes, a fertilizer ratio is specified on soil test reports or in fertilizer recommendation sheets. The fertilizer ratio indicates the proportion of nitrogen, phosphate, and potash in the product. For example, an 18-6-6 fertilizer contains three parts nitrogen to one part phosphate to one part potash. Thus, this fertilizer has a 3-1-1 fertilizer ratio.

Reference: Penn State College of Agricultural Sciences

Phosphorus Fertilizer

Phosphorus is one of three primary nutrients needed by turfgrasses as a regular fertilizer addition. Although it is present in small amounts in turfgrass tissues (0.3–0.55 percent on a dry weight basis), phosphorus is extremely important for rooting, seedling development, cell division, and the synthesis of various compounds used by plants. Phosphorus is available to turfgrasses as H 2 PO 4 - and HPO 4 = and is mobile in plants (meaning that it can move from one portion of the plant to another). Phosphorus deficiencies in turf are usually expressed in the early stages of seedling development, appearing as a purple or red coloring of leaf blades and as reduced growth and tillering. Research at Penn State has shown that at least 60 lb of plant-available phosphorus per acre is required for normal growth and development of lawn grasses. Phosphorus is present in inorganic and organic forms in mineral soils, and both are important sources for plants. Although the total amount of phosphorus in soils can be large, much is unavailable to turf because it forms insoluble complexes with other elements and/or because it is “fixed” to clay particles. The most important factors affecting phosphorus availability to turfgrasses are soil pH and concentrations of iron, aluminum, manganese, and calcium in soils. In acid soils, the H 2 PO 4 -form of phosphorus predominates and combines with iron, aluminum, or manganese to form insoluble compounds that are unavailable to turfgrasses. When the soil pH drops to 5.5 and below, enough phosphorus can be rendered unavailable to cause deficiencies in turf. Also, under acid conditions, some phosphorus can be “fixed” by silicate clays, resulting in reduced availability to plants. In high-pH soils, HPO 4 = is the most common form of phosphorus. In these soils phosphorus combines with calcium to form insoluble calcium phosphates. As the soil pH approaches 8.0 or above, significant amounts of phosphorus are unavailable to turfgrasses. Maximum amounts of plant-available phosphorus (both inorganic and organic forms) are obtained by keeping the soil pH between 6.0 and 7.0. Reference: Penn State College of Agricultural Sciences

Reference: Penn State College of Agricultural Sciences

Nitrogen Fertilizer

Nitrogen is an essential element for all living things and the mineral element needed in the largest amounts by turfgrasses. Although nitrogen is abundant in the atmosphere (about 80 percent of the air surrounding us is nitrogen gas), it is in limited supply in soils and available to plants only after it has been converted to nitrate (NO 3 - )or ammonium (NH 4 + ) by microorganisms or industrial processes. In most cases, nitrogen fertilizer must be applied regularly to maintain high quality turf. The source of nitrogen in a fertilizer is important for determining your turf’s growth rate, density, and color. Nitrogen fertilizers can be divided into two categories—quick release and slow release. Quick-release nitrogen sources are soluble in water; hence, nitrogen is available to plants immediately. They also can burn turf more easily than slow-release sources. Slow-release nitrogen sources typically release a portion of their nitrogen over relatively long periods (several weeks to several months).

Quick Release Nitrogen:

Quick-release nitrogen sources are also called “quickly available,” “fast-acting,” “soluble,” “readily available,” and other terms that indicate rapid availability of nitrogen to turf after application. This group includes compounds containing ammonium, nitrate, or urea.  Being water soluble, they may be applied in liquid as well as in dry form. They give a rapid green-up response, and frequent applications at low rates are suggested for reducing excessive growth and fertilizer burn.

Slow Release Nitrogen:

Slow-release nitrogen sources, also called “controlled-release,” “slowly available,” “slow acting,” and “water insoluble,” are an important part of turfgrass fertility programs. They provide a longer duration of nitrogen release than the quick-release sources and are safer to use on turf because of their lower burn potential. Recent studies have shown that under certain conditions, slow-release nitrogen sources are less likely to leach into groundwater than quick-release sources. Disadvantages of slow-release nitrogen sources include their high price per unit of nitrogen and reduced efficiency (a lower percentage of the applied nitrogen is used by turf in the first year or two of use) compared to quick-release sources. The higher cost and low efficiency have prompted many manufacturers and turf managers to mix or blend both slow- and quick-release sources. Slow-release nitrogen sources can be grouped into several categories, including the natural organics, ureaform, urea-formaldehyde products, triazones, IBDU, sulfur-coated urea, and polymer-coated nitrogen.

Reference: Penn State College of Agricultural Sciences

Potassium Fertilizer

Potassium is a primary turfgrass nutrient and is usually supplied annually as fertilizer to lawns. It makes up about 1.0–2.5 percent of the plant’s dry weight, and its primary role involves regulating several important physiological processes. Potassium activates plant enzymes used in protein, sugar, and starch synthesis. It also plays a key role in maintaining turgor pressure in plants. Thus, it has a strong influence on drought tolerance, cold hardiness, and disease resistance of turfgrasses. Deficiencies of potassium in turf may be expressed as increased susceptibility to drought, winter injury, and disease. Although large quantities of potassium are present in soils, only a small fraction is available to plants. Most soil potassium is in unavailable forms as feldspar, muscovite, and biotite minerals. Potassium is available to turfgrasses in the ionic form (K + ) and occurs in the soil solution and on negatively charged soil particles. In general, more plant-available potassium is present in fine-textured mineral soils (soils that contain high amounts of clay) than in sandy soils, especially in areas that receive high amounts of rainfall or are regularly irrigated. The best way to determine potassium needs for turfgrass is through soil testing. Reference: Penn State College of Agricultural Sciences.

Reference: Penn State College of Agricultural Sciences