Sports Field
NUTRITION
Written by Stuart Cameron-Lee
Turfgrass Nutrient Requirements
Turfgrasses require at least 16 nutrients for optimal growth and development. Some nutrients are needed in large amounts, other nutrients only in minute quantities. Regardless of the amount required, a deficiency of any of these nutrients will limit the growth and development of your turf.
- 9 of the 16 required nutrients (carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, and sulphur) are needed in much larger quantities and are called macronutrients.
- Carbon, hydrogen, and oxygen make up about 90-95% of the turf grass plant’s dry weight. They are never deficient in turfgrasses because they are derived from carbon dioxide (CO2) and water (H2O).
- The micronutrients iron, manganese, zinc, boron, copper, molybdenum, and chlorine are required only in minute amounts and are less likely to be supplied as part of a turf grass nutrition programme.
Each of the 16 essential nutrients has specific roles or functions in turfgrass plants.
Managing Nitrogen in Sports Turf
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 (approximately 80%), it is in limited supply in soils and available to plants only after it has been converted to nitrate (NO3–) or ammonium (NH4+) by microorganisms or industrial processes.
Generally, nitrogen fertiliser must be applied regularly to maintain high quality turf.
Although soil testing can provide guidelines for levels of phosphorus, potassium and other nutrients, it does not give reliable information about nitrogen due to its volatility and free movement in the soil or sand profile.
The quantity of nitrogen required in a turf nutrition programme also depends on the turf grass species, the soil conditions, how the turf is managed, and how the sports field is used. The amount of nitrogen that turfgrasses take up is influenced by application timing, the source(s) of nitrogen, and the amount of nitrogen applied per application.
The goal of a nitrogen fertility program is to optimise plant uptake while minimising leaching, runoff, and gaseous losses.
Nitrogen can contaminate ground water through leaching and runoff. Excessive nitrate concentrations in drinking water present a health risk. Nitrogen movement into water can also accelerate degradation of water bodies through a process called eutrophication, resulting in algae blooms, dense aquatic plant growth, depletion of oxygen, and, in advanced stages, fish death.
a) Leaching of Nitrogen
Leaching occurs when irrigation or rainfall carries nitrogen, primarily in the nitrate form, downward through the soil profile, beyond the plant root zone into the groundwater. How much nitrogen is leached from a sports field depends on the soil type; the amount and rate of precipitation; and the nitrogen source, rate, and timing of application.
The greatest potential for leaching is in sandy soils during periods of wet weather or under excessive irrigation or following applications of quick-release nitrogen at high rates. Leaching can be reduced by:
Using slow-release nitrogen sources, using low-rate applications of quick-release nitrogen sources - ‘less, more often’
Restricting nitrogen applications when plants are not actively growing (during midsummer and winter) or during extremely wet periods of the year.
b) Runoff
When nitrogen is applied to turf, some may be lost in runoff from the surface depending on the rate of precipitation (e.g., storm events), slope, infiltration capacity of soil, and turf cover. A dense cover of leaves, stems, and thatch will significantly slow the rate of surface flow.
c) Atmospheric Losses: Volatilization and Denitrification
Volatilisation and denitrification can cause atmospheric losses of nitrogen fertiliser. Volatilisation occurs when nitrogen is converted to ammonia gas (NH3) and escapes to the atmosphere. It is more likely to occur following surface applications of urea or ammonium-containing fertilisers. Losses are favoured by high soil pH (basic or alkaline conditions), high temperatures, sandy soils, and thatch. Watering-in applications of urea and ammonium-containing fertilisers will reduce volatilisation in turfgrass.
Denitrification takes place in saturated soils when anaerobic bacteria (bacteria that survive in the absence of oxygen) convert nitrate to N2, a gaseous form of nitrogen that escapes into the atmosphere. Turf that survives in poorly drained soils often turns yellow in wet weather owing to denitrification. Improved drainage at these sites will reduce N2 losses.
- Actively growing turf uses nitrogen more effectively so ensure other nutrients are not deficient through soil testing.
- Apply nitrogen in amounts needed by the turf plant - match nitrogen application to turfgrass growth profiles and the specific needs of the species being grown.
- Apply nitrogen fertiliser in multiple applications over the growing season so that application is matched to plant growth needs.
- Returning clippings can cut nitrogen fertiliser requirements by approximately 30%.
- Don’t overwater and cause leaching.
- Use slow-release fertilisers when making infrequent, high-rate applications in areas where soils are prone to leaching e.g., sand profiles.
- Do not apply nitrogen when the turf is not actively growing.
- Water-in urea or ammonium fertilizers, especially when applications are made in warm weather.
Applying Nitrogen to Sports Fields
When evaluating and selecting nitrogen products it is important to understand how quickly the nitrogen in the product is released and under what conditions this occurs. It is also helpful to know how the product is formulated and its potential for burning turf.
In soils, bacteria convert ammonium into nitrate through a process called nitrification. Plants may use nitrogen in either the ammonium or the nitrate form, but most nitrogen is taken up as nitrate.
Quick-release nitrogen sources are also called ‘fast-acting’, ‘soluble’, or ‘readily available’, indicating rapid availability of nitrogen to turf after application. This group includes compounds containing ammonium, nitrate, or urea. Quick-release sources are generally less expensive than slow-release sources and being water soluble, they may be applied in liquid as well as in solid form.
They give a rapid green-up response, and generally require frequent applications at low rates to minimise excessive growth, fertiliser burn, and nitrogen loss to the environment.
Quick-release nitrogen sources are often combined with slow-release sources to provide a fast but moderate green-up and growth response while extending the duration of the response.
Some types of quick release nitrogen products are shown below.
*Salt index is a relative measure of the salinity of fertilisers and indicates the relative burn potential of nitrogen sources (a high salt index indicates a high potential to burn turf). Sodium nitrate is the benchmark value against which all other materials are compared, with a salt index of 100.
Fertiliser products may contain urease and nitrification inhibitors. Sometimes referred to as ‘stabilised’ nitrogen fertilisers, these products are essentially quick-release urea nitrogen sources that can, under certain circumstances, improve fertiliser efficiency by reducing nitrogen losses via ammonia volatilisation and/or nitrate leaching.
When urea is applied to turf and watered into the soil, it undergoes hydrolysis and is rapidly converted to ammonia and then ammonium, which is a relatively stable form of nitrogen that can be taken up by the plant. If the urea is not watered-in, some of the hydrolysed urea-nitrogen can be lost to the atmosphere through ammonia volatilisation. The urea hydrolysis process is hastened by the urease enzyme, which is abundant in soil and thatch.
a) Urease Inhibitors
Chemical additives called urease inhibitors block urease enzyme activity and dramatically slow the conversion from urea to ammonia, thereby reducing volatilisation. The most common urease inhibitor is N-(nbutyl) thiophosphoric triamide, commonly referred to as NBPT.
Urea products containing urease inhibitors are not slow-release fertilisers, and any potential improvement in nitrogen efficiency is a result of reduced ammonia volatilisation. If fertiliser application occurs during hot days and or the product is left on the soil surface or in thatch with no follow-up irrigation or rainfall, using urease inhibitors will likely result in more nitrogen uptake and less volatilisation.
b) Nitrification Inhibitors
Nitrification inhibitors can, in some circumstances, reduce nitrogen leaching through their inhibitory effects on specific soil microorganisms. When nitrogen fertiliser is applied to turf, it is converted from ammonium to nitrate through a process called nitrification. Nitrification is a natural process in soils that is mediated by two specialised soil bacteria. One of these bacteria, Nitrosomonas spp., transforms ammonium to an intermediate nitrogen compound called nitrite, while the other (Nitrobacter spp.) converts nitrite to nitrate. Nitrification inhibitors are designed to specifically target Nitrosomonas bacteria so that the nitrogen fertiliser remains in the more stable ammonium form. Ammonium tends to be stable because it is positively charged and adheres to clay and organic matter. Nitrate is negatively charged and more susceptible to leaching during rainy periods and in sandy soils.
Only one nitrification inhibitor, dicyandiamide (DCD), is currently used in turfgrass fertilisers. If application conditions favour leaching after application, there is a higher probability of improved nitrogen retention in the soil using a nitrification inhibitor.
Slow-release nitrogen sources provide a longer duration of nitrogen release, are safer to use on turf because of their lower burn potential and are less likely to leach into groundwater.
Disadvantages of slow-release nitrogen sources include their higher product price per unit of nitrogen and reduced efficiency (lower proportion of the applied nitrogen is used by the turf grass plant in first 2 years due to rate of breakdown and release) compared to quick-release sources. The higher cost and low efficiency have resulted in products that blend both slow release and upfront nitrogen sources.
Slow-release nitrogen sources can be grouped into several categories, including the natural organics, ureaform, urea-formaldehyde products, triazones, IBDU, sulphur-coated urea, and polymer-coated nitrogen.
a) Ureaform
Ureaform is made by reacting urea with formaldehyde in ratios of about 1.3 to 1. These products typically contain at least 35% nitrogen, with at least 60% of the total nitrogen being WIN (water insoluble nitrogen).
Urea-formaldehyde products not falling within these guidelines are referred to by other terms such as methylene urea.
Ureaform is divided into 3, almost equal fractions based on solubility. Fraction I is soluble in cold water and contains urea, methylene diurea, and dimethylene triurea. Nitrogen availability in this fraction is similar to that of quick-release nitrogen sources, but the nitrogen is not as quickly available. Fraction II is insoluble in cold water but soluble in hot water; it is made up of the slow-release compounds trimethylene tetraurea and tetramethylene pentaurea. Fraction III, the most slowly available, is insoluble in both hot and cold water and is made up of pentamethylene hexaurea and longer chain polymers. Studies have shown that over a 6-7 month period about 4% of Fraction I, 25% of Fraction II, and 84% of Fraction III remain in the soil. The slow breakdown of Fractions II and III accounts for the low efficiency of ureaform during the first years of use. With continued use and buildup of ureaform, recovery of applied nitrogen improves.
Release of nitrogen from ureaform depends on microbial activity, and the same environmental factors that affect release from natural organics also affect release from ureaform. Because of low nitrogen recovery (efficiency) in the first years of use, you will usually need to use higher rates or supplement ureaform with soluble sources in these years. This low recovery and slow response during cool periods support the concept of fertilisation with combinations of ureaform and quick-release nitrogen sources.
b) Other Urea-Formaldehyde Products
These are also reaction products of urea and formaldehyde but are made with wider ratios of urea to formaldehyde (more urea) than ureaform; thus, they release nitrogen faster.
These products contain 30-35% nitrogen and are classified ‘slowly available’. However, some contain enough water-soluble nitrogen to give a response closer to quick-release nitrogen sources, such as urea, than to slow-release nitrogen sources. Others can be expected to give a quick initial response, but they have a slightly slower release rate than the quick-release nitrogen sources. Any urea-formaldehyde product that does not claim WIN (water insoluble nitrogen) or claims CRN (controlled-release nitrogen) and not WIN as a percentage of the total nitrogen, will release nitrogen quickly (similar to urea).
Most urea-formaldehyde products are available as granular fertilisers. They contain mostly water-soluble compounds such as unreacted urea, methylol urea, and short-polymer methylene ureas (methylene diurea and dimethylene triurea). The amount of each compound in a product is largely dependent on the urea/formaldehyde ratio and the conditions under which the reaction takes place during manufacture. These nitrogen sources are typically more expensive than urea and ammonium and nitrate products, but they are safer since they have reduced fertiliser burn potential.
c) Triazones
Triazones are water-soluble compounds produced through a reaction involving urea, formaldehyde, and ammonia. On a dry weight basis, triazone products are about 30-36% triazones, about 40-50% urea, and the remainder, methylol and methylene ureas.
Triazones are classified as slow-release nitrogen sources, even though their nitrogen-releasing properties are closer to those of urea than to slow-release nitrogen sources. Although more expensive than urea, triazone products are safer because of their reduced burn potential. Products containing triazones are liquids.
d) IBDU
IBDU is made by reacting isobutyraldehyde and urea. It contains 31% nitrogen, with 90% of the total nitrogen being WIN (water insoluble nitrogen) in the coarse (0.7-2.5 mm) product and 85% WIN in the fine (0.5-1.0 mm) product.
IBDU breaks down slowly in soils because of low solubility, but once in solution, it is hydrolysed and releases nitrogen. Particle size has a large effect on the release of nitrogen, with smaller particles releasing more quickly. The release rate is faster with higher soil-water content and, to a limited extent, higher temperatures.
There may be a application response delay of up to 3-4 weeks so application may be supplemented by a soluble nitrogen source if an ‘upfront’ growth response is required.
e) Sulphur-Coated Urea
Sulphur-coated urea (SCU) is made by spraying preheated urea prills or granules with molten sulfur. A sealant, such as wax or a mixture of oil and polyethylene, is often applied to seal pores and imperfections in the sulphur. Nitrogen content is usually in the range of 32-43% and depends on coating thickness. Increasing the thickness lowers the nitrogen content.
Nitrogen is released from SCU by microbial degradation of the sealant and diffusion of soluble nitrogen through pores and cracks in the sulphur coating. The release rate quickens as coating thickness decreases and as temperature increases. Also, breakage of the coating as a result of mechanical damage or aging enhances the release of nitrogen.
Particles within a SCU product are not identical. If they were, one might expect all of them to release nitrogen at the same time. Quick release occurs with imperfectly coated particles; an intermediate rate of release takes place with particles in which the sealant has covered imperfections; and the greatest delay in release occurs with the more thickly and more perfectly coated particles. Once release begins from a given particle, it is quite rapid. Thus, the slow-release properties of SCU come from the variability in coatings among the individual particles.
Sealant-free SCU products typically release nitrogen at a slower rate since they have thicker sulphur coatings.
f) Polymer-coated nitrogen
Polymer-coated nitrogen fertilisers consist of urea, SCU, or other nitrogen sources coated with a thin layer of polymer (plastic) resin.
For nitrogen release to occur from polymer-coated urea, water is absorbed through the coating and dissolves the nitrogen. Nitrogen is then gradually released through the coating by osmosis. Different coating thicknesses may be used to obtain different nitrogen release rates. The thicker the coating, the slower the release. Release increases with a higher temperature and is not significantly influenced by soil moisture levels, volume of water applied, soil pH, or microbial activity.
Organic sources are often by-products from the plant and animal processing industries or waste products. Examples include bone, blood, and feather meal; fish scrap and meal; seed meals; dried and composted manures; activated and composted sewage sludges.
Considerable variation exists in the physical and chemical properties of different natural organic fertilisers.
The natural organics can be characterised by relatively low nitrogen contents (usually below 10%), the presence of water insoluble nitrogen (WIN), and nitrogen release intermediate between that of quick-release nitrogen sources and extremely slow-release nitrogen sources such as ureaform.
Release of nitrogen is dependent on microbial activity and is highly variable among products. Factors influencing nitrogen release are the chemical composition of the material and environmental conditions that influence microbial activity. Environmental conditions affecting breakdown include temperature, soil moisture and oxygen, and soil pH.
Phosphorus in Turf
Phosphorus is the second primary nutrient needed by turfgrasses and may require fertiliser addition.
Although it is present in small amounts in turfgrass tissues (0.3-0.55% on a dry weight basis). Phosphorus is a key component of ATP and promotes cell division, root development, seedling development, maturation, establishment and tillering. It is also plays an important role in the synthesis of various compounds used by turfgrass plants. Phosphorus is available to turfgrasses as H2PO4- and HPO42- 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 colouring of leaf blades and as reduced growth and tillering. 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, aluminium, manganese, and calcium in soils. In acid soils, the H2PO4- form of phosphorus predominates and combines with iron, aluminium, 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, HPO42- 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.
Phosphorus can be supplied to turf as inorganic and/or natural organic fertilisers. Inorganic phosphorus fertilisers include superphosphates and ammonium phosphates and are manufactured by treating rock phosphate with various acids. Natural organic fertilisers typically contain phosphorus derived from plant or animal by-products.
Phosphorus is largely immobile in soils, meaning that it takes a long time to move from the turf surface into the root zone. Phosphorus may take weeks or months to move just a few centimetres in soil. Because of its poor mobility, phosphorus should be incorporated into the soil prior to seeding if identified as deficient after soil testing. Phosphorus may be applied to the surface, then incorporate 100-150mm deep with a rototiller (or similar) so that developing roots can use the fertiliser. On established turf, some phosphorus can be incorporated into soil either just before or just after coring. Phosphorus, along with nitrogen, is one of the major nutrient sources contributing to surface water and groundwater pollution - often via runoff.
Phosphorus is present….
Potassium in Turf
Potassium is the third primary turfgrass nutrient needed by turfgrasses and may require fertiliser addition. It makes up about 1.0-2.5% 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 promotes root growth and storage of carbohydrates. It promotes meristematic tissue and increases thickness of outer cell walls improving abiotic and biotic stress tolerance and also plays a key role in maintaining turgor pressure in plants, regulating water loss via transpiration. 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.
Potassium is mobile in plants and sometimes can be taken up in amounts greater than needed for optimum growth. This phenomenon, called ‘luxury consumption’ is generally considered inefficient use of the nutrient. Potassium can be supplied to turf using inorganic fertilisers or natural organic fertilisers, or both. However, most fertiliser potassium is derived from inorganic sources, in particular, muriate of potash (potassium chloride - 50-52% K; 60-63% available K2O) and sulphate of potash (potassium sulphate - 42-44% K; 50-53% available K2O), both of which are water soluble.
Although it is readily leached into groundwater, potassium is typically not a major pollutant in surface water and groundwater. It rarely is present in concentrations toxic to people or aquatic life, and it does not deplete water of oxygen
Secondary Nutrients in Turf: Calcium, Magnesium, and Sulphur
Calcium, magnesium, and sulphur are considered secondary nutrients because in most cases they only occasionally need to be supplied to turf in the form of fertiliser. Applications of calcium and magnesium are usually only necessary when the soil pH is below optimum for turfgrass growth.
Liming soil supplies turf with calcium or calcium and magnesium-containing limestone. When a soil test indicates a need for calcium but not magnesium, a lime source containing only calcium carbonate can be used. If the soil is low in magnesium, dolomitic limestone is a good option since it contains both calcium carbonate and magnesium carbonate.
Some common sources of Calcium, Magnesium and Sulphur are:
- Calcium carbonate (agricultural limestone) - 32% Calcium
- Magnesium/calcium carbonate (dolomitic limestone) - 22% Calcium; 12% Magnesium
- Gypsum - 22% Calcium; 19% Sulphur
- Calcium nitrate - 19% Calcium
- Magnesium sulphate (Epsom Salt) - 10% Magnesium
- Ammonium sulphate - 24% Sulphur
- Ferrous sulphate - 19% Sulphur
- Potassium sulphate - 18% Sulphur
- Elemental Sulphur - 90% Sulphur
If calcium is recommended for turfgrass growing in soil with an adequate pH, gypsum can be used as a source of calcium, noting that gypsum is not a liming source. Also, despite claims, gypsum only improves soil structure in soils with high sodium concentration.
Sulphur is sometimes used to lower soil pH.
Micronutrients in Turf
The micronutrients (trace elements) required by turfgrasses include iron, manganese, zinc, copper, molybdenum, boron, and chlorine. Micronutrients are needed by turfgrasses only in minute amounts and rarely need to be supplied to turfgrasses growing in mineral soils. However, when turfgrasses are grown in soils with high sand content or high in pH, micronutrient applications can be beneficial.
Iron is an important component of plant enzymes and proteins involved in respiration, nitrogen metabolism, and chlorophyll synthesis. In individual turfgrass plants iron deficiencies appear as chlorosis (yellowing) of the youngest leaves. Turf iron-deficiency symptoms show up as yellow mottling, as opposed to the uniform yellowing observed in nitrogen-deficient turf.
In rare instances where excessive liming has occurred or irrigation water with a high pH is used in large volumes, the uptake and/or translocation of iron by turf may be reduced. This problem, sometimes referred to as lime-induced chlorosis, can be corrected by acidifying the soil and by supplying iron-containing fertilisers.
Application of iron fertiliser can be used to enhance turf colour without stimulating excessive leaf growth. Iron applications can produce darker green turf even when levels are adequate in plant tissues before applications are made. By reducing the rate of nitrogen fertiliser and supplementing with small amounts of iron, a noticeable turf green-up can be achieved with fewer of the negative aspects associated with excessive nitrogen fertilisation, such as frequent mowing and outbreaks of disease.
The most common forms of iron fertiliser for turfgrasses are inorganic iron salts and organic iron chelates (chelated iron).
An inorganic iron salt is a water-soluble form of iron that contains iron or iron, and ammonium paired with sulphate (e.g., ferrous sulphate, ferric sulphate, or ferrous ammonium sulphate). These products are typically applied as foliar sprays as the turfgrass plant can absorb the iron through the leaves. In soil applications, much of the iron from inorganic sources is converted to insoluble iron hydroxides, iron phosphates, or iron carbonates all of which are unavailable to turfgrasses.
Chelated iron sources are usually more efficient at supplying plants with iron than inorganic iron salts and lower rates present less chance of injuring turfgrass with an iron application.
Rates of iron fertiliser can vary depending on the source, time of year, and number of applications.
Excessive amounts of iron can cause noticeable discoloration (a black-green colour) in turfgrasses and, in some cases, may injure them. The degree of injury depends on the type of turf, the rate of iron, and the environmental and management conditions at the time of application.
Iron, zinc, manganese, and/or copper often occur in forms that are not taken up by plants. This problem is especially marked if the soil has a high pH (8.0 or above). One way of correcting this problem is to apply the nutrient as a chelate. Chelate comes from the Greek word ‘clawlike’ and denotes a soluble and stable product formed when an organic compound called a chelating agent bonds to the nutrient. The chelating agent keeps the nutrient in solution and releases it at the root surface where it is absorbed into the plant. Chelated nutrients can also be absorbed through turf foliage.
The most common commercial chelating agents used in the turfgrass industry are EDTA (ethylenediaminetetraacetic acid) and DTPA (diethylenetriaminepentaacetic acid). EDTA chelates iron at a pH of less than 6.3; above a pH of 6.8, it reacts with calcium, rendering it ineffective. DTPA chelates iron up to a pH of 7.5; above 7.5, calcium interferes with solubility, making it ineffective.
The cost of chelated micronutrients is typically higher than that of inorganic sources.
Unless the soil has a high pH (greater than 8.0) and the texture is extremely sandy, micronutrient fertiliser applications are generally not needed. In fact, micronutrients other than iron are rarely beneficial and are sometimes harmful when applied to turfgrasses. Boron, for example, is toxic to turfgrasses even when applied in small amounts. Indiscriminate use of copper can lead to deficiencies of iron in turfgrasses.
If the need micronutrient supplementation is identified by soil testing a fertiliser product containing only the micronutrients that are required to correct the deficiency is recommended.
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