Proper nutrient management in sports field management plays a key role in plant health and stress resistance, as well as overall aesthetics and playability (plant density, recovery, and wear tolerance). However, improperly applied nutrients can result in wasteful use of natural resources and nutrients. Thus, nutrient use should be undertaken with care and consider the impact of nutrient applications with respect to the environment, economy, and society.
Therefore, the goal of the nutrient program should be to achieve an acceptable, safe playing surface that maximizes plant nutrient uptake while applying a minimum of nutrients to achieve these results.
Essential Mineral Nutrients
Essential mineral elements are required for turfgrass growth. Phosphorus, potassium, sulfur, and, especially, nitrogen are most commonly deficient (Table 3).
1 Values shown are not intended to represent optimal ranges, but rather are what is commonly measured. Optimal levels vary by species, variety, use, and environment. 2 The soil test values shown for the primary macronutrients have good confidence due to significant research, but the other nutrients have relatively less scientific backing and, instead, are based largely on observations and extrapolations with other species. The excessive soil test level shown is not meant to be a “sufficiency level”, but rather the point at which there is virtually no chance that a fertilizer response would be likely.
By law in most countries, anything sold as fertilizer must list the percentage in the following order: N, P, K. The phosphorus is expressed as P2O5 and potassium as K2O. For example, a 20-2- 5 fertilizer has 20% N, 2% P2O5, and 5% K2O1. The fertilizer label often also includes the percentages of other nutrients and/or the materials from which the nutrients are derived.
Nitrogen is the nutrient that has the greatest impact on plants. Turfgrass has variable nitrogen requirements based on the species and usage, fertilizer source and timing, seasonal evapotranspiration rates, precipitation, and soil properties. Turfgrass requires nitrogen in greater quantities than all but the non-mineral nutrients that come from air and water (carbon, hydrogen, and oxygen).
Nitrogen plays a role in nearly all plant functions and is an essential component of amino acids, proteins, nucleic acids, etc. It is vital to understand the nitrogen cycle in order to maximize uptake by plants and minimize losses to the environment.
Understanding which nitrogen sources should be used is an essential component in an efficient nutrient management program. In many cases, nitrogen sources are applied without regard to their release characteristics. This increases the risk of negative environmental impacts as well as management costs. Each nitrogen source is unique and therefore should be managed accordingly.
The first selection criterion in choosing a nitrogen fertilizer source is the rate at which it becomes plant available. Some sources are immediately plant available (quick release/water soluble nitrogen) and others become so over time (slow or controlled release/water insoluble nitrogen). The most common dry fertilizer sources that are readily plant available and dissolve into the soil solution are urea (46-0-0) and ammonium sulfate (21-0-0). The most common liquids that are readily plant available are urea (various concentrations) and urea ammonium nitrate (28-0-0; 32- 0-0). These readily available sources provide quick uptake and rapid greening, which can be especially important during cool times of the growing season and just before/after sporting events. However, high rates applied at any one time result in excessive shoot growth and increased mowing requirements at the expense of decreased root growth and increased probability of infection of some pathogens. The risk from fertilizer burn is relatively high with these quick release sources, especially when applied at high rates and/or when weather is hot and/or dry. Additionally, these rapid release sources are more likely to be lost to the environment. Within this group of “quick release” fertilizers, urea molecules rapidly convert to ammonia gas and then ammonium. Left on the surface, the ammonia can be volatilized – potentially losing much of the applied nitrogen. Additionally, the ammonium converted from urea or applied as a fertilizer can revert back to ammonia and be volatilized, especially in alkaline soils common in arid regions. Volatilization potential can be reduced by avoiding urea application under hot, humid, and/or windy conditions. After application, watering with 0.25” irrigation water reduces volatilization potential.
Ammonium converted from urea or applied as a fertilizer converts to nitrate within a few hours/days. The nitrate is prone to leaching below the root zone with high precipitation and irrigation rates, particularly in sand-based soils. Nitrate is further lost to the environment due to emissions of nitrification/denitrification gases (e.g., nitrous oxide), especially with prolonged soil saturation. Thus, proper irrigation and drainage can help minimize losses of nitrate.
Use of fertilizers that are not “quick release” and become available over time can result in increasing nitrogen uptake and reducing losses. These fertilizers can effectively “spoon-feed” plants by releasing or converting nitrogen in a steady manner rather than a flood of it entering the soil solution. These nitrogen sources can reduce losses to the environment, decrease foliar burn potential, and reduce labor with fewer required applications. Although complex, understanding these sources of nitrogen fertilizers can be simplified by separating into two types: - Slow/Control Release - Nitrogen is released slowly or, in some cases, engineered to release in a controlled rate. For example, long-chain molecules containing nitrogen (e.g., methylene urea and urea formaldehyde) are broken down through microbial degradation—eventually resulting in ammonium and nitrate as breakdown products. Another example are the coated fertilizers, such as polymer coated urea. Composted animal and plant biosolids and similar sources are included in this category as well.
- Stabilized – Inhibitors are added to water soluble nitrogen products and slow down the nitrogen cycle to decrease the chance of loss and increase the window of when the plant available forms of nitrogen (ammonium and nitrate) are available for uptake. Inhibitors include:
o Urease Inhibitors – The conversion of urea to ammonium is slowed as the enzyme that catalyzes this reaction is temporarily inhibited.
o Nitrification Inhibitors – The conversion of ammonium to nitrate is slowed as microbes responsible for this conversion are temporarily inhibited in their activity.
While these enhanced efficiency fertilizers are generally more expensive on a cost per pound of nitrogen basis as compared with quick release materials, their benefits include increased efficiency (lower rates can be applied), reduced costs (fewer applications; reduced mowing needs; reduced clippings), reduced risk for nitrogen-related diseases, and reduced environmental impacts. Often, a blend including 30-50% of these sources along with quick release nitrogen is affordable and effective.
Proper nutrition is imperative as plants are preparing for dormancy late in the season. Therefore, nitrogen fertilization is often necessary. As always, any local regulations should be followed when applying fertilizers late in the year. It is also noteworthy that some irrigation waters, especially waste waters, can be high in nitrogen and should be tested and accounted for to avoid excessive nitrogen applications.
As with nitrogen, phosphorus deficiencies or excesses are detrimental to plants and excesses are harmful to the environment. Phosphorus plays important roles in cell structures and in energy transformations. It is especially important for root development with newly established sod and seedlings. However, excessively high phosphorus can result in poor plant health and encourage weed infestation, particularly annual bluegrass.
However, in contrast to nitrogen, phosphorus is rarely deficient in well-maintained and established turfgrass and is readily managed through soil testing. Plant response is well correlated to the proven soil tests, with no benefit to applying phosphorus when soil test values are sufficient. It is noteworthy that sports turfgrass tends to need relatively higher concentrations of phosphorus due to frequent overseeding and clipping removal. Despite this, there is no proven benefit, even in sports turfgrass, to continue to add phosphorus fertilizer when the soil test levels are very high. Doing so is a concern for water quality as it contributes to eutrophication. For this reason, fertilizer applications are regulated by some states/counties. In cases where phosphorus is not needed due to high soil test values and/or when prohibited, “phosphorus-free” fertilizer sources should be used. In most cases, uncoated/coated ureas, ammonium sulfate, and potassium chloride/sulfate suffice to provide the needed nutrients for turfgrass without application of any phosphorus.
More so than nitrogen, phosphorus fertilizer accumulates in the soil. Phosphorus is poorly soluble, especially at extreme alkaline and acid soil pH levels. Shortly after fertilizer is applied, the majority of the phosphorus precipitates into a solid form. Nutrients need to be dissolved into the soil solution for plant uptake and this solid phase phosphorus is temporarily not available for plants until it slowly solubilizes over time. This is not a concern if enough of these precipitates exist in the soil in proximity to the roots of each plant.
The forming of these solid phosphorus precipitates greatly minimizes phosphorus leaching, especially as compared with nitrate. However, phosphorus can be leached when soil test concentrations are high. Of greater concern is phosphorus loss due to surface water runoff when soil test values are excessive, especially in close proximity to surface water bodies. Fertilizer that lands on impervious surfaces (e.g., sidewalks) that lead to stormwater drains should be minimized and removed.
The most common forms of phosphorus fertilizer are the ammonium phosphates. However, there are a wide variety of phosphorus fertilizers that can generally be categorized as follows:
Traditional inorganic phosphates – These are in granular (such as monoammonium or diammonium phosphate) or liquid (ammonium polyphosphate) form that quickly react in the soil to form precipitates.
Coated phosphates – These are similar to the coatings for nitrogen that are released slowly over time.
Organic complexed phosphates – These products have been reacted with organic acids or are bound in various plant and animal biosolids (e.g., animal or plant manures or treated sewage sludge). These release nutrients into the soil as they are decomposed by microbes or chemicals. (Note: The phosphorus described here is potentially different than the phosphorus found in “organically certified” fertilizers, which can include these materials or most other sources listed here.)
Specialty products – These include a wide range of products, which are primarily used for pathogen control and improvement of stress tolerance.
Recommended rates of phosphorus, when needed, are 1-4 lbs/1,000 ft2, with the rate proportional to soil test values. Timing of phosphorus applications is not as critical as nitrogen. Typically, a single annual application is adequate, although more may be needed if the soil test is very low and/or with new sod/seed. It is recommended to apply phosphorus and incorporate into the soil ahead of establishing turfgrass if soil test values warrant its use.
Potassium is essential for proper water relations in plants, as well as other functions as it supports stress resistance. The overapplication of potassium is wasteful of maintenance costs and natural resources. As with phosphorus, there are good correlations with plant response and soil test values. Potassium is intermediary compared with nitrogen and phosphorus with regard to soil holding capacity. It is held loosely in the soil by clay and organic matter, which means that it is not easily leached in soils with higher levels of these soil components, but is readily leached in sandy, low organic matter soils. In these, it tends to need careful management similar to nitrogen to provide for season-long availability. Otherwise, a single annual application is generally adequate.
Rates of potassium, when needed, should be based on soil test results. The most common forms of potassium fertilizer sources are potassium chloride and potassium sulfate, although other sources are available and potentially useful (e.g., potassium nitrate, potassium thiosulfate, etc.). Coated sources are available and can be helpful, especially in sandy, low organic matter soils.
As with primary macronutrients, secondary macronutrients are found in plants at percent levels (>0.1%). However, they were classified as “secondary” because they are historically less commonly deficient in crop plants.
Sulfur deficiencies have become relatively more common, especially in turfgrasses, due to reductions in acid rain pollutants and increasingly pure fertilizer materials. Predicting sulfur deficiencies is difficult as the soil test is not well correlated to plant response. Rather, organic matter is a somewhat better predictor, with the likelihood of response diminishing as organic matter levels increase above ~2%. As with nitrogen, sulfur is prone to leaching losses. As such, sulfur is more likely to be needed on high sand, low organic matter soils, especially on those that receive high precipitation/irrigation rates. Irrigation water should be tested because many sources, especially greywater, can be high in sulfur.
When sulfur is likely to result in improved plant health, it is commonly added in conjunction with nitrogen as ammonium sulfate and/or with potassium as potassium sulfate. Micronutrients (zinc, iron, manganese, and copper) are also often applied as sulfate salts, although the rates may not supply enough sulfur to meet all needs. Gypsum (calcium sulfate) and Epsom salt (magnesium sulfate) also contain sulfur, although these are usually applied for reasons other than sulfur nutrition. Sulfur coated urea or elemental sulfur are good sources for steady release of sulfur over the growing season, which is especially helpful to soils prone to leaching. Excesses are not typically environmental or plant health concerns, although these are wasteful of resources and, as with all soluble fertilizers, can be a contributor to excessive salts/fertilizer burn.
Calcium and Magnesium
Although calcium and magnesium are essential to plant function, they are ubiquitous in the environment and, thus, rarely have documented deficiencies. Soil and irrigation water tends to be very high in these nutrients. Although much of the calcium and magnesium is found in solid form in soils, equilibrium chemistry assures that there is ample found dissolved in soil solution. When deficiencies do occur, they are typically on acidic sandy soils with no or minimal irrigation or with very pure irrigation water. Testing for and maintaining an appropriate pH with dolomitic limestone, which contains both of these nutrients, is generally enough to provide for healthy plant growth as both pH and nutrition is managed. Excess amounts of these nutrients are common and not typically concerning, although unwarranted applications are wasteful and potentially detrimental due to excessive salts.
Micronutrients are typically found in relatively low concentrations in plant tissues, although they are just as essential for proper turfgrass health as macronutrients. They play a variety of roles in turfgrass biology, including photosynthesis, enzyme catalysis, protein synthesis, and a wide variety of other physiological activities and structural components. However, they are often found in ample concentrations in soils and turfgrass rarely shows response to their application. For example, nickel and molybdenum are needed in extremely minute quantities and there are no documented deficiencies in field grown turfgrass. Although rare, deficiencies of the other micronutrients have been documented. These are far more likely to occur in sand-based fields with low organic matter.
Generally, there is ample chloride in irrigation water and soils. It is also included with the most common source of potassium fertilizer which is “potash” (potassium chloride). Chloride deficiencies are more likely to occur in non-irrigated, high rainfall areas when potassium chloride fertilizer is not utilized.
Boron, zinc, manganese, copper, and iron are more likely to be deficient in alkaline soils due to poor solubility. In the past, iron chlorosis (yellowing) was somewhat common. However, modern varieties have been bred to mostly avoid chlorosis, especially with Kentucky bluegrass. Regardless, it is common to do a foliar iron spray a few days before high visibility sporting events. This doesn’t necessarily improve plant health, but the practice typically results in visual response of greening.
Rates of micronutrient fertilizers are relatively low and should follow label recommendations. It is relatively easy to cross over from deficient to toxic given the fact that these are needed in such low quantities. This is especially true for copper and boron. In most cases when one or more micronutrients are needed, a single application annually will suffice. However, in severely deficient situations more frequent, generally foliar, applications are warranted. This is especially true for newly established sand-based fields.
“Natural” and “Organically Certified” Fertilizers
In some cases, communities or organizations require/prefer to use “natural” and/or “organically certified” fertilizers. However, these terms are often the subject of misinformation. It is important to realize that, despite popular opinion, these are not necessarily healthier with respect to human health. For example, arsenic is a natural compound, yet is highly toxic to humans.
In terms of plant nutrition, an atom of a nutrient is chemically identical regardless of source. For example, the fertilizer with greatest volume of use is urea. It is manufactured using nitrogen gas from the atmosphere that is converted to ammonia using natural gas in the Haber-Bosch process, which is then combined with carbon dioxide. The nitrogen in this and the urea molecule itself are identical in every way to the urea that is naturally produced in animal livers. Either source is beneficial to plants and pose no risk to plants or animals (including humans) when used properly. However, manufactured urea requires the use of non-renewable resources. Conversely, low nitrogen analysis fertilizers require more fossil fuel use for transportation in order to supply the same amount of nitrogen (e.g., urea is 46% nitrogen, whereas most of these alternative fertilizers are less than 10% nitrogen). Regardless, demand for these products exists and it is important to understand their properties and the management practices needed for their proper use.
The nutrients in any fertilizer, including natural and organically certified fertilizers, must be factored into the overall nutrient management planning. In addition, some regulatory requirements (e.g., phosphorus prohibitions) must be adhered to regardless of fertilizer source.
Labeling of fertilizers as “natural” is not subject to regulatory oversight. The definition is “existing in or caused by nature.” In reasonable consideration, so-called natural fertilizers tend to include protein-rich plant or animal wastes. These tend to have the benefit of including a broad spectrum of nutrients that are generally released slowly, mostly during the summer when temperatures drive high decomposition rates. These materials tend to have high carbon content, which can be beneficial if organic matter building of the soil is desired. This is typically helpful, although not in sand-based fields where excessive organic matter can result in reductions in drainage and increased compaction potential. Some of these materials, especially those with high fiber content, can be a source of pathogen stimulation. Typically, the main disadvantage is that the low concentration of mineral nutrients in these sources correlates to higher costs of the fertilizer, as well as transportation, storage, and application.
Many sources of these materials exist, such as:
Animal manures (uncomposted and composted wastes).
Animal industry by-products (bone, blood, feather, fish, etc. meals).
Green manures (plant-based composts).
Liquid cocktails (manure extracts, seaweed extracts, compost teas, etc.).
By contrast, “organically certified” fertilizers are any materials approved by the Organic Materials Review Institute (OMRI). These can include any of the products listed previously, including those that are derived from carbon-based materials, typically animal and plant waste materials, but also can include inorganic salts (e.g., calcium carbonate, calcium sulfate, potassium sulfate) and many other materials. OMRI certifies products rather than providing generic certifications for chemicals. For example, one potash source may be certified for organic use after the review and labeling process while another, despite being chemically identical, will not be certified for organic use if it has not gone through the certification process.
Sports field managers should carefully review and evaluate each commercial product before use. Because considerable variation exists in the physical and chemical properties of the various fertilizers, they should be carefully evaluated when used as part of a nutrient management program. The evaluation criteria should include nutrient content and quality, release rates, cost, ease of handling and distribution, offensive smell or odor, infiltration rate, and any tendency to stain shoes and clothing.
Predicting/Identifying Nutrient Deficiencies
Predicting or identifying nutrient deficiencies can be done using the following tools:
Plant tissue analysis
Fertilizer response evaluation
Visual assessment is a valuable first step for identifying nutrient deficiencies in plants However, these symptoms are less specific in turfgrass compared with most other types of plants. It is also noteworthy that when avoiding urea application under hot, humid, and/or windy conditions there is often “hidden hunger” with no visible symptoms.
In general, nutrient deficiencies cause a reduction in chlorophyll, which results in chlorosis (yellowing) that can progress to necrosis (dead tissue). In many plants, the various nutrients show distinct patterns in terms of age of tissue and type of chlorosis that help in deficiency identification. However, in turfgrass, mowing and relatively thin shoots can make it difficult to see these patterns.
In most instances, when chlorosis occurs, it is usually a result of nitrogen deficiency, although sulfur, iron, and potassium deficiencies are also relatively common causes. Deficiencies in most of the other nutrients can also cause chlorosis, although these instances are rare. It is somewhat common for soils that are excessively wet for prolonged periods to exhibit chlorosis due to problems in soil chemistry. Phosphorus deficiencies are an exception. If severe, these deficiencies result in plant tissues turning dark green or even red/purple. Visual assessment needs to be coupled with the other assessment tools to effectively diagnose deficiencies.
Soil analysis is a tool that can help customize fertilizer needs in turfgrass with estimates of nutrient availability predicting plant response to an applied nutrient. Although a reliable tool, soil testing is not a perfect science. Some nutrients have been more thoroughly researched than others and some tests are more highly correlated to plant response than others (Table 3). The most reliable tests indicate native soils high in silt and clay are somewhat resistant to change in nutrient and pH levels, and therefore soil testing may only need to be conducted every one to two years (and no less than every five years), unless monitoring corrective action (such as liming an acid soil). Sand-based soils are less resistant to change in nutrient status or pH, and thus may require relatively more frequent sampling such as once per year.
For soil analysis to be effective, accurate and representative samples are needed. Each field should be sampled separately, with about 12-15 cores per sample (typically about a pint in total volume). Within a field, if there are areas that are behaving differently, these samples should be segregated. Laboratories and other organizations/businesses dealing with soils can provide sampling instructions. Sampling depth for turfgrass is generally recommended at 3” to 4”. It is imperative to take separate samples from areas with varying soils and/or management. In addition, soil samples should not be collected following fertilization.
A laboratory with a record of sound QA/QC should be selected to conduct soil testing. Laboratories can provide documentation of their data quality, such as participation in proficiency testing. (See the North American Testing Proficiency Program for more details.) It is a good management practice to track data trends over time, which is difficult to do if switching soil test methods or laboratories because they often use different methods thus make comparisons difficult. In addition, it is important that the same phosphorus extraction method – the most common are the Sodium Bicarbonate, Bray P1, and Mehlich 3 – is used for consistency in soil test interpretations relative to nutrient concentrations.
In general, soil testing is not extremely helpful for nitrogen and sulfur because the inorganic forms tested for (nitrate and, in some cases, ammonium for nitrogen and sulfate for sulfur) are very transient in their soil concentrations as they are regularly changing between plant available and unavailable forms due to rapid plant/microbial chemical transformations. Additionally, the amount of these nutrients released from soil organic matter is very difficult to predict. As such, it is generally best to develop a nitrogen and sulfur management plan based on reliable research studies and previous results and then use soil and plant tissue analysis to fine-tune the recommendations. For example, adjustments to the fertilization plans can be made if unusually high concentrations exist in the soil, plant tissue, and/or irrigation water.
The remaining nutrients are managed mostly by soil test values as the starting point. For most of these, a correlation exists between the soil test and probability of a positive plant response – with high likelihood of response at low soil test values with decreasing likelihood of response as soil test values increase. These correlations are relatively good and significant research exists for phosphorus and potassium. However, minimal data is available in turfgrass for the other nutrients, though there are reasonable correlations for calcium, magnesium, zinc, manganese, copper, boron, and chloride for other plants (mostly crops). These give us some basis for judgment, although the confidence in interpreting results is not as high as with phosphorus and potassium. Nevertheless, there is a slim chance of a positive response to these nutrients if the soil test values are high.
Plant tissue analysis, as discussed below, is an additional tool that can be used to make decisions on these nutrients but it is rare to see responses in sandy soils with low organic matter. Iron is unique, as the correlations for soil testing are very poor. It can also be difficult to obtain clean tissue samples, as dust is very high in iron concentration. Rather, soil pH and plant species/variety selection are used to help manage for iron.
Soil pH is a measure of hydrogen ion (H+) activity ("active acidity). The pH scale is 0 to 14 with 7 being neutral. Values below 7 are acidic, and values above 7 are alkaline. Soils tend to range from pH 4 to 8. The optimal soil pH for nutrient solubility is approximately 6 to 7. However, turfgrass is commonly grown successfully from pH 5.5 to 8.4.
Alkaline soil can result in poor solubility of plant nutrients. It is generally not practical or affordable to lower the pH as these systems are highly resistant to change due to carbonates in irrigation water (hard water) and in soil (limestone). Rather, the nutrient requirement is slightly higher and managers need to be aware to watch closely for deficiencies of these other nutrients.
Acidic soil also has nutrient solubility issues, as well as toxicities of aluminum and/or manganese (see Table 3). These results are variable by soil, with some worse than others. Acidic soils can be neutralized with limestone (calcium and/or magnesium carbonate), burnt lime (calcium oxide), hydrated lime (calcium hydroxide), or similar, based on a Buffer pH soil test. The quality of the liming materials (calcium carbonate equivalent and fineness of grind) also needs to be factored in, as well as ease of handling and cost. Whenever possible, soil pH should be adjusted prior to establishment, as preplant incorporation greatly accelerates the neutralization of the acidity throughout the root zone. Once turfgrass is established, the ideal time to apply lime is in conjunction with core cultivation, which helps to move the liming material into the soil. Cooler temperatures help to minimize risk of foliar burn. Standard lime applications are usually suitable just about any time of year as long as they do not exceed 50 lbs/1,000 ft2. Extremely acidic soils may require multiple applications over multiple seasons to sufficiently raise the pH. It is best to have a liming program with smaller annual applications to maintain pH at a reasonable level rather than waiting until it drops to a toxic level and then attempting a rescue.
Soil tests may include the following: organic matter, salinity, sodicity, texture, cation exchange capacity, and sand size distribution. A soil textural analysis (percentages of sand, silt, and clay) is important as soil texture can impact water and nutrient holding capacity, as well as irrigation, drainage, and cultivation. Both texture and sand size distribution are vital for proper construction and maintenance to meet ASTM F2396 specifications of sand-based root zones.
Organic matter (OM) is not only a source of nitrogen and sulfur, but also all other nutrients. In addition, organic matter increases nutrient and water holding capacity. Organic matter is often one of the main measures of soil health. Turfgrass is relatively efficient at creating organic matter over time, which is beneficial for the reasons above and as it stores carbon away from the atmosphere. Despite these benefits, organic matter can be detrimental to compaction potential and infiltration rate in sand-based fields.
Salinity and sodicity are important considerations where greywater is used for irrigation, as well as in certain arid zones where irrigation waters/soils can be natively high in salts. It is important to understand that there can be an overall salt problem (salinity) and/or specific ion toxicities (sodium, chloride, and boron are the most common) when irrigating with greywater.
When a soil test shows that the electrical conductivity used to measure salts is above 4.0 dS/m, the soil is considered “saline” although plants can experience stress before the salt concentration in soil gets this high. In this case, it does not matter which salts are present – as all contribute to the overall salt effect. Plants need salts for their metabolic processes and while all fertilizers are salts, excessive salts in direct contact with plant tissues will burn the foliage. In the soil, salts bind to water so strongly that plants can desiccate even when there is ample soil moisture. Saline soils are corrected by ensuring adequate drainage followed by irrigating to excess with reasonable quality water to move the salts below the root zone.
Specific ion toxicities occur when nutrients and other chemical elements are excessively high. Chloride and boron are both essential plant nutrients, but they are sometimes present in excessive amounts, usually in the irrigation water, which can kill plants. Again, soil testing can identify these toxicities. These are potentially corrected through leaching below the root zone.
Similarly, sodium is a beneficial nutrient (not essential) found in all soils and most irrigation waters. It can become a problem when its ratio relative to calcium and magnesium is high, creating a “sodic” soil. This is relatively more common in arid regions, but also with some reclaimed irrigation waters. Sodic soils have an Exchangeable Sodium Percentage >15% and/or a Sodium Adsorption Ratio >13, but preventative action should be taken before reaching these levels. Sodicity results in the soil structure being destroyed as clay-based aggregates disintegrate, which is not a problem in sand-based fields. Sodic soils are remediated similar to saline soils except that a soluble calcium source (most commonly gypsum; limestone should not be used in alkaline soils) needs to be applied prior to leaching.
Plant Tissue Analysis
Visible plant symptoms and soil testing can offer helpful clues in diagnosing nutrient deficiencies but can also be confusing and misinterpreted. Tissue testing is an effective way to determine precisely what nutrients are in plant tissue at a particular point in time. While that data is beneficial, it does not necessarily reflect why the nutrient is at a deficient or excessive level. It is important to pair tissue testing with soil testing data to best determine nutrient management strategies and closely follow the lab’s guidelines for how to sample and prepare the tissue samples to get meaningful results.
Tissue testing can help to adjust nutrient management programs in these ways:
Confirm a suspected nutrient deficiency or toxicity.
Monitor plant nutrient concentrations for sufficiency.
Pair tissue tests with soil tests for troubleshooting.
Plant tissue samples can be easily taken in turfgrass from fresh mowed clippings – taking them from multiple locations throughout the field. However, careful cutting with clippers may be needed when sampling small areas with visual symptoms for comparison to areas that appear to be healthy. (Soil samples should be taken from the same areas for comparison.) Samples should not be taken within a few days of a fertilizer or amendment application. For micronutrient analysis, plant samples should be rinsed lightly and quickly to remove any dust or soil particles and then air dried or oven dried at temperatures below 150oF) before being placed in clean paper bags and sent to the laboratory.
For diagnostic samples, plant tissue samples should be collected as soon as symptoms appear. Plants showing symptoms of severe deficiency are often the most difficult to interpret correctly, since a deficiency of one element may result in deficiencies or excess accumulation of other elements if uncorrected. Plants under prolonged stress of any kind (temperature or moisture extremes, pests, flooding, mechanical damage, etc.) can have unexpectedly high or low nutrient levels due to the stress. As with soil testing, plant tissue analysis is a useful tool but is not always certain in its findings.
Fertilizer Response Evaluation
Another tool for managing nutrients, especially for correcting suspected, but unconfirmed deficiencies is the application of fertilizers to small test areas to observe whether greenup occurs. Application of a complete fertilizer containing all of the nutrients can help determine whether the problem is nutritional or related to some other stress. Application of individual or paired nutrients can help isolate which nutrient is deficient.
Liquid vs. Dry
Turfgrass is unique among most fertilized plants. Each individual plant has a very narrow cylinder root contact with soil. As such, uniformity of fertilizer application is very important.
Dry fertilizers are relatively inexpensive, especially when purchasing common forms in bulk from agricultural suppliers. However, these can result in poor nutrient uniformity. Fertilizers with a high size guide number (SGN) (i.e., larger particle size) result in some plants getting excess fertilizer and others getting none. Conversely, fertilizers with a smaller SGN more uniformly deliver nutrients to all plants.
Liquid fertilizers, when properly applied using an accurately calibrated sprayer, can provide improved nutrient distribution – with every plant receiving nearly identical rates. However,liquid fertilizers do have some disadvantages. The liquid has direct and immediate contact with the shoots, which can have a high burn potential if the rate is high and/or the environmental conditions are hot, dry, and/or windy. Liquids can also have chemical reactions in spray tanks, hoses, nozzles, and other equipment. This can result in plugging that can be very costly to clean. This is particularly a problem with phosphorus due to its low solubility and highly reactive nature with calcium, magnesium, and other cations in the water and/or fertilizer blend. If the liquid is being injected into the irrigation system, distribution uniformity problems related to irrigation water distribution may arise. Finally, liquid fertilizers tend to be more costly if they are shipped already mixed, although some products, such as urea, ammonium sulfate, potassium chloride, etc. can be purchased inexpensively in dry form and then dissolved.
Nutrient Application Programs and Strategies
Stewardship that considers the impact of nutrient applications with respect to the environment, economy, and society, includes the following “4R’s”:
Right fertilizer sources
Applying a quick-release fertilizer at high rates on a hot, windy day near impervious surfaces is a good example of ignoring the 4R’s rule. This example represents a waste of natural and facility resources, results in contamination to the environment, and may result in poor plant health as well. Poorly managed fertilizer usage has resulted in some instances of serious environmental contaminations and adoption of regulatory requirements by many state or local agencies. Sports field managers need to be good environmental stewards to avoid further problems and additional regulation. A one-size-fits-all approach to nutrient application is not possible, given all the variables of turfgrass species, sports, traffic intensity, soil, climate, budget and equipment available. Some strategies, such as Minimal Level of Sustainable Nutrition strategy, can be used. Using the information presented in this chapter and consultation with Extension specialists can help managers develop an appropriate site-specific nutrient management plan.
Fertilizer Application Equipment and Calibration
Dry fertilizers are typically spread with a broadcast spreader (or a drop spreader in rare situations). Liquid fertilizers are applied with a sprayer or injected into the irrigation system.
The selection and calibration of application equipment is an important aspect of nutrient management, as not all fertilizers can be applied with every spreader. For example, coated fertilizers can crack, and their control release properties can be destroyed when handled roughly, such as with certain drop spreaders equipped with an agitator.
Accurately calibrated sprayers and spreaders are essential for proper fertilizer applications. Incorrectly calibrated equipment can result in an application of too little or too much fertilizer, resulting in deficiencies or toxicities, excess costs, and greater potential for nutrient pollution. In keeping with the BMPs for equipment washing, spreaders should always be thoroughly cleaned after each use to remove salt residue that corrodes metal parts of the spreader. Many universities have publications on proper calibration methods.
Nutrient Management Best Management Practices
Soil test every one to two years (and no less than every five years).
Collect representative soil and plant tissue samples, and separate samples with varying soils and/or management. Send to a reliable laboratory for analysis. Track soil test results over time and identify any trends.
Fertilizers should be spread evenly using a turf-grade blend of uniform particle size with a low SGN number or with liquid applications with good uniformity.
Calibrate fertilizer application equipment regularly.
Apply nitrogen according to the sources, rate, and timings found as needed and following regulatory requirements.
Any late season applications should be made at an appropriate time such that the fertilizer can be taken up before the end of the growing season and in keeping with any state or local regulations.
Account for nutrients in some irrigation waters, especially greywater.
Use a fertilizer containing phosphorus only when indicated by a soil test or during turfgrass establishment, following any applicable phosphorus fertilizer regulations.
Phosphorus and potassium should be applied based on soil test recommendations.
Apply sulfur at 10-30% of the nitrogen rate unless it is natively high in the soil and/or irrigation water.
If using organically certified fertilizers, ensure that the nutrients are factored into the overall nutrient management planning, in addition to adhering to any nutrient regulations.
Use plant tissue testing to confirm a suspected nutrient deficiency or toxicity and to monitor plant nutrient concentrations.
Pair tissue tests with soil tests for troubleshooting.
Conduct fertilizer response evaluations if needed to help determine if turfgrass issues are nutritional or related to some other stress.
Keep accurate nutrient application records and conform to any local or state record-keeping regulations.
Prevent fertilizers from being deposited onto impervious surfaces.
Do not apply fertilizer when heavy rains are likely.
Do not apply fertilizer on established turfgrass stands until greenup and active growth begins.
Do not apply fertilizer to severely stressed turfgrass.
Do not apply fertilizer to frozen ground.
Follow label guidelines for the need and amount of irrigation following fertilization.