21 Jul Many people erroneously think that plants draw their food from soil.
This article appeared in Grounds Maintenance Magazine.
Many people erroneously think that plants draw their food from soil. In reality, plants manufacture their own food through photosynthesis in their green tissue. Soil provides most of the raw materials—mineral nutrients—that plants use as components for the food they produce.
Scientists currently recognize 17 elements as essential for plant growth and reproduction (see table, “Essential plant nutrients,” below). These elements are divided into macronutrients—those that constitute more than 1,000 parts per million (ppm) of plant tissue—and micronutrients— those that account for less than 100 ppm of plant tissue. In the turf and ornamental industry, however, many people use different terminology and refer to N, P and K as the macronutrients or just macros. Ca, S and Mg are the secondary nutrients, and the remainder are the micronutrients, or micros.
All essential elements are necessary to plants in some amount, so a deficiency of any one of them would theoretically produce symptoms. In practice, however, deficiencies of some of the essential elements—Mo, B, Cl and Ni—are virtually unknown because they are present in most soils and plants need very little of them. C, H and O account for more than 95 percent of the dry-tissue weight of plants, but plants obtain these elements from water and air, rather than from mineral soil. Thus, these elements are never limiting to plant growth. That leaves N, P, K and Fe as the nutrients that commonly become deficient, and Ca, S, Mn, Mg, Zn and Cu that occasionally are deficient. Nutrients often become deficient due to conditions that prevent their uptake or use by plants rather than actually being absent from the soil.
Nutrients also are classified as either mobile or immobile, depending on whether the plant can transfer the nutrient from one tissue to another. Deficiencies of mobile nutrients tend to show up first in older tissue, especially leaves, because the plant will withdraw mobile nutrients from these areas to supply the needs of newer growth. Deficiencies of immobile nutrients show up first in new growth because immobile nutrients cannot be transferred within the plant. Thus, new growth suffers if external sources are inadequate. In practice, this knowledge can be quite useful for diagnosis of deficiency symptoms.
You’ll notice in the following discussion that chlorosis— yellowing of normally green tissue—is a symptom common to many deficiencies. Though some clues help you narrow the problem down, it often is necessary to conduct soil and foliar testing to determine the cause of a chlorosis (or other) problem. Both types of testing are sometimes necessary because some deficiencies are caused by an excess of some other nutrient. Thus, adequate soil levels of a nutrient may not necessarily result in adequate tissue levels.
Most landscapes do not experience significant deficiency of nutrients other than N, P, K and Fe. Golf greens are more prone to deficiencies than other turf or ornamental sites due to their sand root zones, which do not retain nutrients well.
• Nitrogen. Plants require N, a component of many necessary compounds within plants, in relatively large quantities. Notably, N is vital for the production of chlorophyll, the green pigment involved in photosynthesis. The two major forms of N that plants obtain from soils are nitrate (NO3-) and ammonium (NH4+). Most other forms of N must undergo transformation (via microbial activity) into these forms before plants can use the N.
Deficiency symptoms include slow growth and chlorosis. Chlorosis occurs because N is a component of chlorophyll, the green photosynthetic pigment in leaves. Chlorophyll production slows or stops when N is inadequate, resulting in yellow coloration. Leaves turn tan and then die in more severe cases. N is a mobile nutrient, so symptoms show up on older leaves first.
Excess N promotes lush foliar growth, often at the expense of flowering, and creates a high shoot:root ratio. This often increases drought susceptibility and may delay dormancy in fall, increasing the chance for frost damage.
Plants perform best with consistent supplies of N. Slow-release fertilizers have become popular for this reason.
• Phosphorus. After N, P is the most frequently deficient nutrient. Like N, P is a component of many necessary compounds within the plant, such as DNA, RNA and energy-rich ATP that drives the synthesis and decomposition of organic compounds. Available forms include the phosphate ions H2PO4- and HPO42-. Soil pH controls which form is present—the latter predominates in pHs above 7.0, the former in pHs below 7.0. P-deficient plants are stunted and become darker colored, sometimes almost black. P is mobile in the plant, so deficiency symptoms show first in older leaves. Excess P promotes additional root growth, which decreases the shoot:root ratio—the opposite effect of N.
Although P traditionally has been promoted as a root-growth enhancer, this benefit is usually overstated where established turf and ornamentals are concerned. However, P does seem to speed establishment of seedlings, sod and herbaceous transplants. This is reflected in the high P content of so-called starter fertilizers.
• Potassium. This nutrient is the third most commonly deficient element. It, too, is mobile in the plant, which uses K in the form of the positive ion K+. K is abundant in plant tissue. It plays a role in the osmotic potential of cells and therefore helps regulate turgor pressure. K also is important as an activator for many enzymes involved in photosynthesis, respiration, and protein and starch synthesis.
In most plants, K deficiency produces slight chlorosis followed by necrotic (dead) lesions. Often, leaf tips and margins are the first parts to die, giving a scorched appearance to the plant. Growth in general is stunted, as well. Excess K can cause deficiency of other nutrients such as Ca and Mg.
• Magnesium. Mg deficiency first appears as interveinal chlorosis (IVC)—yellowing between the leaf veins. Available to plants as Mg2+, deficiency is normally restricted to acidic soils with low cation-exchange capacity (CEC). Other deficiencies can cause IVC but only in neutral to alkaline soils, so a simple pH test often can narrow the problem down to Mg. Mg is mobile.
• Calcium. Ca deficiency, as with Mg, is mostly seen in plants growing in low-pH, low-CEC soils. Ca2+ is the form plants use, and it is immobile in the plant. Calcium plays a role in cell-wall formation as well as cell-division processes. Thus, deficiency often results in twisted or deformed tissues and death in shoot and root tips. Excess Ca can result in a deficiency of Mg or K.
• Sulfur. This element is rarely deficient. However, peculiar soil conditions in a few regions result in inadequate levels. Available to plants as SO42-, deficiency symptoms result in a general chlorosis difficult to distinguish from N deficiency without laboratory analysis. Excess N may cause S deficiency in leaf tissue of trees. S present as an environmental pollutant in rainfall is a significant source of S in many parts of the country.
• Iron. Fe is commonly deficient in turf and ornamentals and—after N, P and K—this element is the most frequent supplemental nutrient that grounds-care professionals apply. Fe is usually present in soil in fair amounts, and deficiencies often result from soil conditions—especially high pH—that restrict Fe uptake by the plant.
Fe deficiency produces pronounced IVC, though this will often spread to the veins as well. Fe is immobile in the plant, so symptoms first occur on younger growth.
• Manganese. Mn is not required in great amounts by plants, and deficiencies—which are not common—are most likely to occur in alkaline soils. The symptoms include IVC and, in severe cases, necrotic margins and spots. Mn is available to plants as Mn2+ and is immobile in plants.
• Zinc. Zn deficiency is rare in turf but occurs occasionally in ornamentals, where it causes IVC and rosette formation. Excessive Zn can reduce Fe levels.
• Boron, chlorine, copper, molybdenum and nickel are rarely or never deficient. If you ever experience a problem with any of these elements, it usually stems from excessive levels, not deficiencies. B and Cl toxicities are not uncommon in some regions, and Cu can reduce Fe levels in plants if present in high amounts.
Several of the micronutrients are available in chelate form. Chelates are much more available to plants than non-chelated forms and are the type you should use, when available, if you need to apply these nutrients.
Fertilizer labels list the fertilizer’s analysis, a three-part designation, such as 20-10-10, representing the percent content (by weight) of N, P and K respectively. Fertilizers that contain these three nutrients are known as complete. Products that contain equal amounts of each are balanced, such as 10-10-10 fertilizer. These three nutrients, because of their importance to plants and their frequent deficiency, are the primary components of commercial fertilizers. However, manufacturers often add other nutrients, as well. Labels usually state how much N is soluble and how much is insoluble, indicating how much is rapidly available and slowly available, respectively.
P content is expressed on fertilizer labels as if it were in the form of P2O5, even though no such compound exists in fertilizers. This is an old convention that the industry has not bothered to change. Because P2O5 is heavier than elemental P, you must multiply the stated content by 0.44 to get the actual content in terms of elemental P. For example, a reported P2O5 content of 20 percent equates to around 9 percent actual P content.
K is expressed similarly—in terms of K2O equivalent—and requires you to multiply by 0.83 to obtain the actual amount of elemental K present in fertilizer.
SOIL CHARACTERISTICS AND NUTRIENT AVAILABILITY
Obviously, if a nutrient is not present in soil, the plant will suffer. Commonly though, as we’ve pointed out, the problem is not actual absence of the nutrient. Rather, soil conditions directly or indirectly prevent plants from utilizing it.
The cation nutrients Mg and Ca are two notable examples that become less available in low-pH soils. Conversely, Fe is less available in many alkaline soils. Sandy soils—putting-green root zones being the extreme—with low CEC values hold fewer nutrients than heavier soils and soils rich in organic matter. Clay and organic matter have excellent abilities to hold nutrients in soil, especially those nutrients that form positive ions, including Ca, Mg and ammonium.
If you have soil conditions that cause nutritional problems, long-term solutions rest with soil modification. In the short-term, you easily can solve most deficiencies with supplemental fertility of the type needed. Chapter 2 discusses soil conditions and amendments in more detail.
One important factor affecting soil fertility is the carbon-to-nitrogen ratio (C:N) of organic amendments. Organic materials high in carbon—especially wood products—require the activity of microorganisms for decomposition. These microbes use N as they act on the organic matter and may withdraw so much from the soil that inadequate amounts remain for plant uptake. This effect is temporary because the N eventually is released as decomposition progresses. In the meantime, which may last a year or two, N deficiency may exist. You must add supplemental N, up to 1 pound per 1,000 square feet for some materials, to counter this effect and to speed up the decomposition of the organic matter in the soil. C:N ratios below 50:1 contain enough N to avoid most problems. Higher values may indicate the need for supplemental N. Wood products such as sawdust may have C:Ns in the range of 400:1 to 500:1 and can tie up large amounts of N. You should have some idea of the C:N of amendments you use in your soil (see table, Chapter 2, for some examples).
Another factor you need to consider, especially for woody ornamentals, is soil mobility of nutrients. Some nutrients are mobile within the soil profile. Others are not. Soil pH also affects soil mobility of nutrients. The practical implication of this is that you must place immobile nutrients directly into the root zone (see “Fertilizing trees and shrubs,” page 78). This is in contrast to soluble nutrients, which you can apply to the surface and water into the root zone. This does not apply so much to turf because turf roots grow so near the surface that surface applications are generally adequate.
Let’s look at some of the types of fertilizer we apply to turf and ornamentals. Quick-release N fertilizers provide N in several forms (see table, “Synthetic N sources,” page 69). These products are the traditional fertilizers and dissolve readily in water. Therefore, they enter the soil solution rapidly and are almost immediately available to plants. They also are quickly depleted.
Slowly available sources are not as immediately available to plants but release N over a longer period and at more consistent rates, which is advantageous for several reasons.
P fertilizers are in the form of superphosphate or treble superphosphate. However, ammonium phosphates also provide P. K fertilizer is derived mainly from potash—KCl. Potassium sulfate and potassium nitrate are also important sources of K.
The secondary and micronutrients are available in forms you can apply separately (see table, “Secondary and minor nutrients,” opposite page) if the need arises. However, special mixes that contain many of the secondary and micronutrients are available, and turf managers often use this “shotgun” approach to ensure that micronutrient deficiency is not a problem.
Various combinations of fertilizer materials can provide an endless array of analyses (see “Example fertilizer label,” right), and manufacturers provide fertilizers blended specifically for almost every type of turf or ornamental in a given climate. Custom blending often is available, too.
Turf and ornamental fertilizer products typically contain N, P, and K and often Fe, as well. However, they may also include many of the other nutrients, too. This usually causes no harm, though the need for these nutrients in many situations is questionable.
• Salinity. Some fertilizers are salty—they will increase the salinity of your soil. This may or may not be a problem depending on a variety of factors, but you should be aware of the salinity of the fertilizer materials you are adding to your soil, especially if your soils are already prone to salt buildup (see table, “Salt index,” at right).
• Acidifying effects. Some fertilizers increase acidity more than others, and some have the opposite effect. You can use this as a means to alter (or maintain) pH levels by choosing a fertilizer with the desired quality (see table, “Acidifying effect,” at right).
Evidence exists that N-deficiency symptoms result not from low N levels, per se, but from unsteady levels of N and cycling between high N and low N levels. This may be one reason for the effectiveness of slow-release fertilizers and their popularity. Their gradual nutrient release helps level out the peaks and valleys of turf growth, resulting in more consistent turf quality and fewer deficiency symptoms.
Manufacturers produce two types of slow-release products: uncoated and coated. Coated products rely on semi-permeable or impermeable coatings to restrict water’s access to soluble fertilizers. Uncoated products take advantage of the low solubility of some N materials to slow their release.
• Uncoated products. Ureaform (UF) and methylene urea (MU) are similar products that result from a reaction of urea with formaldehyde. Both contain about 40 percent N in the form of long chains of molecules. Longer chains are less soluble than shorter chains and so take longer to become available in soil. UF and MU both consist of a variety of chain lengths and so release N over time. UF molecules are generally longer than MU chains, so UF releases N more slowly but over a longer period. These fertilizers ideally release N over 8 to 12 weeks. However, because they rely on microorganisms to attack the molecule and mineralize the N, this release time can vary depending on conditions, such as temperature, pH and soil moisture, that affect microbial activity. Thus, at certain times of the year, soil temperatures may be too low for UF and MU to supply adequate N to turf.
Isobutylidene diurea (IBDU). The other significant uncoated slow-release product available in the United States is IBDU. Formed by a reaction of urea with isobutraldehyde, it contains 32 percent N and is available in granular or powder form.
IBDU slowly releases N by hydrolysis in water, after which it is soon available to plants. IBDU acts as a slow-release product because of its low solubility—only small amounts dissolve and release over time. Moisture levels primarily affect IBDU release—dry conditions delay release. Superintendents also should be aware that powdered forms they apply to greens dissolve more quickly than the granular forms they apply to fairways.
LIQUID SLOW-RELEASE FERTILIZERS
Some products have chemistry and release patterns similar to UF and MU but are liquid-applied. These provide important advantages in certain situations.
For example, liquid fertilizers allow you to tank-mix with pesticides, which is a great advantage in some operations. Plus, it’s easy to customize the mix by altering the rate of each component you use in the mix, something not so simple to achieve with granular products. Gaining these benefits without sacrificing the advantages of a slow-release product make liquid slow-release fertilizers attractive options in many situations.
Another advantage of liquid application is more precise placement of the fertilizer than broadcast spreaders can achieve, making it efficient for areas such as golf greens and tees. Plus, liquids do not leave granular material on the green’s surface and so do not disrupt putting quality.
• Coated fertilizers. Since its introduction nearly 20 years ago, sulfur coated urea (SCU) has enjoyed great success in the turf market. SCU consists of a urea granule with a sealing coat of sulfur plus wax. Thus, SCU supplies turf with S in addition to N, which varies from 30 to 38 percent.
Small cracks and imperfections in the coating layers allow some water to enter the granule and dissolve the urea, which then escapes out into the soil. Plus, microbes attack the wax coating and destroy it over time. Once a granule takes in water, it can release its urea quickly. Coatings vary in thickness and integrity, so the gradual release of urea is a result of some granules releasing urea soon, some later and others only after a long period. Overall release rates vary among products and manufacturers, who can control the coating thicknesses.
• Polymer-coated fertilizers date back to the introduction of Osmocote in the 1960s. However, many of the polymer-coated products available now are relatively recent introductions. Unlike SCU, many of these products contain other sources of N such as ammonium nitrate, as well as other nutrients such as P and K.
Manufacturers are able to manipulate the chemistry of the coatings (wherein lies the main difference between various polymer-coated products) to provide highly predictable release rates. They manufacture these products by applying successive polymer coatings to the fertilizer granule.
When water diffuses across the semipermeable polymer membrane, it dissolves some of the fertilizer inside. This creates a concentrated solution, which then diffuses back out into the soil. This continues over time until the fertilizer is completely released and all that remains is an empty polymer shell.
OTHER FERTILIZER PRODUCTS
• Natural organics. These products are derived from many different substances, such as composted sewage sludge, animal waste, feather meal and others. Nearly all of the N in these products is in organic form and relies on microbial activity for release. These products are considered slow-release fertilizers. Natural organics typically have relatively low N content and, therefore, generally cost more to apply the same amount of N. However, N-use efficiency tends to be high.
• Fertilizer/pesticide combinations. These products combine fertilizer with a pest-control product, often a herbicide or insecticide, for turf use. The economic advantages are obvious—fewer applications, fewer packages to deal with and simpler, less expensive application equipment. Thus, when appropriate, these products may offer significant time and money savings. However, a significant drawback is timing. Obviously, the proper timing for the fertilizer and pesticide must coincide or one of them will have less than maximum effectiveness. Despite this disadvantage, combination products are a popular option with turf managers.
Turf requires nutrient management significantly different than ornamentals, so we’ll discuss turf fertilization separately. N is the most significant nutrient so we’ll start with this nutrient and then touch on the remainder.
To understand turf’s N needs, it helps to understand the nitrogen cycle. The N cycle consists of numerous components and processes, all interconnected and dependent on each other (see Figure 1, page 72). Let’s first discuss the components, or pools, of N in the cycle and then the processes that link them together.
• Pools. N exists in many forms—all are either organic or inorganic. The forms of organic N with which most turf managers are familiar are products such as IBDU, urea, urea formaldehyde, methylene urea or composted sewage sludge. While these are important in turf-management programs, their application represents a small fraction of the total organic N in soil. The bulk of organic N in soil exists in the form of plant material (dead or alive), bacteria, fungi and other soil organisms. Humus also contains organic N. A fertile soil may contain 3,000 to 5,000 pounds of N per acre in the top 6 inches.
By comparison, the inorganic N pool, consisting of nitrate and ammonium, is much smaller—in the range of 10 to 50 pounds per acre. Inorganic N is the critical link between soil organic N and turf growth because inorganic forms are the forms plants can use. N enters the inorganic pool either by fertilizer applications or microbial breakdown of soil organic matter into ammonium by mineralization. Additions of fertilizer are predictable and easy to manage. However, the contribution of mineralization to the inorganic pool is difficult to assess because it depends on the overall size of the organic-N pool, microorganism activity, temperature, pH and other factors.
Further confounding the picture is the fact that the soil organic-N pool is not constant but expands over time, especially in a young turf site. For example, a new golf course built on heavily cultivated agricultural soil may initially have relatively low levels of soil organic N. After several years under turf, however, organic-N levels begin to increase due to fertilizer applications and recycled clippings that ultimately deposit their N in the organic pool. This pool will continue to increase in N content over several decades until it levels off.
During the initial period of rapidly increasing organic N, the flow of N is primarily into the organic pool, with little N flowing back out into the soil. However once the pool is “full,” equal amounts of N flow into and out of the pool. This is important because the flow of N from a full pool back into the pool of inorganic N is significant and can reduce the amount of N fertilizer the turf requires. Thus, a young, high-quality turf may require 6 to 8 pounds of N per 1,000 square feet annually, whereas a mature turf may succeed with 2 or 3 pounds annually. Also, the flow of N from the organic pool is fairly steady through the growing season and will support turf growth without the peaks and valleys associated with applications of quickly available N fertilizers. Superintendents who have managed older courses are well aware of this phenomenon because it’s like having a huge supply of slow-release N “in the bank.”
One of the best ways to increase your N pool is to return clippings. Research has found that you can remove as much as 4 pounds of N per 1,000 square feet annually in clippings.
• Processes. N pools in the soil are in constant flux, with flow from one pool to another. The links or paths between them are processes. We broadly group these into processes that conserve N in the system and those that result in permanent N loss from the system.
• Nitrogen loss. The three basic processes that steal N from turf systems are volatilization, denitrification and leaching (removal of clippings can be considered a fourth). Volatilization is the loss of N from the surface of the system into the atmosphere as ammonia gas (NH3). Depending on conditions, volatilization losses can range as high as 45 percent of the N you apply. Several factors increase this loss, including using ammonium-based or uncoated urea fertilizers, high soil pH, rapid drying conditions, urease activity and failure to irrigate after application. An irrigation of about 0.5 inch of water soon after application greatly reduces volatilization losses.
Denitrification is the second process that steals N from turf. Denitrification also results in the loss of N as a gas. But instead of loss as NH3, N is lost as nitrous oxide (NO2) and nitrogen gas (N2). Microbial activity causes denitrification and can result in 10 to 90 percent of applied N being lost. Conditions that favor microbial activity, such as warm, saturated soil and high fertility, also increase idenitrification. Fortunately, denitrification seems to be fairly insignificant in well-aerated soil, so maintaining good drainage should reduce this avenue of N loss.
Leaching, the third avenue of N loss, is of great concern considering the potential harm of nitrates in drinking water. However, most research indicates that healthy, dense turf loses little nitrate through leaching.
• Nitrogen conservation. Several processes result in changes in the form of N without actual loss from the turf system. To illustrate, consider what happens when you apply urea to turf.
A granule of urea dissolves fairly quickly following application, especially with irrigation. Several processes act on the urea, but the most important is hydrolysis by the enzyme urease, which converts N from the urea form into the ammonium form. Urease is present all through turf systems. It is produced by many living organisms but functions independently in the environment as well.
The ammonium that urease releases can follow several paths. As a cation, it can stick to cation-exchange sites, or soil microorganisms can convert it to nitrate through nitrification. In either case, the N is now available to turfgrass roots. Urea N is too but is used more slowly than ammonium or nitrate.
Turfgrass roots can absorb 50 to 90 percent of applied N and can do so within 2 to 4 days. This is very efficient compared to most plants and demonstrates the value of healthy turf with extensive rooting. Rapid uptake fixes N in the system and prevents loss through other processes. A drawback of rapid uptake is that it causes cycling of N levels that results in inconsistent growth.
Microbial absorption competes with plants for N and may account for 10 to 30 percent of applied N. Plant and microbial assimilation of N into living tissue is called immobilization. This is the opposite of mineralization.
Living organic matter is just a temporary stopping point for N. When organisms die, they release their N back into the pool of soil organic N. Microorganisms convert fresh organic matter into humus through the complex process of decomposition. Microorganisms subsequently process the soil organic N back into NH4 by mineralization, completing the cycle. Remember that all of these pools and processes are interconnected and interdependent.
• Potassium and phosphorus. Turf’s response to P and K is not usually as visible as it is with N. However, these nutrients are just as necessary for good turf health and vigor.
P is important to root growth and therefore affects establishment, both from seed and vegetatively. Deficiencies slow establishment and reduce vigor by reducing root development. Severe deficiencies result in reddish purple leaf blades and poor growth.
K is needed in large quantities by all plants. From a practical standpoint, improved stress tolerance is the most important function of K, so a visible response may not occur if the turf is not under stress at the time. However, adequate levels are vital for good tolerance to environmental stresses and diseases. Visible deficiency symptoms include leaf-tip death and thin turf.
Turf fertilizers usually contain enough P and K to prevent serious visible deficiencies. However, soil conditions— especially low CEC—might cause low levels or reduced plant uptake to the point where vigor or stress tolerance is reduced. Therefore, soil and tissue analysis is warranted to ensure P and K levels are adequate. A lab with turf experience should perform these tests. Not only will the analyses detect deficiencies, they can tell you whether you are applying too much of these nutrients. Doing so is wasteful and could cause deficiencies of other elements such as Fe and Zn.
• Iron. Fe is the micronutrient most likely to be deficient in turf. Its primary function in the plant is in the formation of chlorophyll. Because of this, chlorosis is the main symptom of Fe deficiency. Fe chlorosis is common when soil pH is above 7.0, because alkaline conditions change Fe to a form unavailable to the plant. In low-pH soils, Fe deficiency is relatively uncommon.
Soil tests often are unreliable in reporting Fe levels. The easiest and surest way to test your turf is to apply a dose of iron to a section of turf. Turf’s response to Fe is quite rapid, and you will see a response within 48 hours or less if Fe is deficient. Turf response to Fe often is short lived so periodic applications may be necessary where Fe-availability problems exist.
• Magnesium. As already discussed, low-pH and low-CEC soil can cause Mg deficiency. This is common, therefore, on golf greens. Turf managers often mistake the chlorosis caused by Mg deficiency for Fe or N shortage. If turf does not green up after application of these two nutrients, suspect Mg deficiency. Test spray a small section of turf with Mg to confirm this. You’ll see a response in 24 hours if Mg has been lacking. Epsom salts is a widely available source of Mg you can use for this test. Use 1 teaspoon in 1 pint of water.
• Calcium can become deficient in the same conditions that reduce Mg. However, in practice, low-pH soils usually are limed long before they become Ca deficient. Because Ca is the primary component of lime, Ca deficiency symptoms are rare in the field.
• Manganese. Deficiencies are rare but may occur on golf greens. A simple test application of Mn-containing fertilizer next to a strip of turf fertilized without Mn will confirm any suspected deficiency.
• Sulfur, as already stated, is adequate in nearly all soils and also is supplied by rainfall in many areas. Even so, many fertilizer products include S, making deficiency even less likely. Again, a small test application of elemental S will confirm if S is deficient when you suspect a problem.
The remaining nutrients are not generally deficient in turf. The rare situations where they are inadequate can be dealt with by applying one of the available micronutrient solutions to turf. These products usually contain trace amounts of the micros and easily satisfy the needs of the turf.
It is difficult to generalize about fertilizer rates on turf. Different species require different amounts of N (see table, “Nitrogen needs of turfgrass species,” page 74). Climate and other environmental factors also vary nutrient demand, and intensity of use and level of maintenance are issues as well. Many turf sites, such as home lawns, can perform well with a range of fertilizer rates, depending on the level of quality the owners desire. Thus, no single formula exists for determining how much fertilizer to apply, and no perfect fertility program exists for any turf.
Fertility has many indirect effects, and the interaction between nutrient levels and other factors is often subtle and complex. Peculiar and unexpected problems sometimes arise, and grounds managers may be forced to experiment to find their own solutions to fertility problems. For example, some diseases become more prevalent with high N levels, while others react oppositely. Weed problems change with varying nutrient levels as well. Because so many factors are site specific, turf managers must find what works for their particular situation. Having said that, let’s be more specific.
Fertilizer rates usually are given in terms of pounds of N per 1,000 square feet. Thus, for example, when you hear “2 pounds of N,” it’s implied that this means 2 pounds of N per 1,000 square feet. That is the convention we’ll use here. Fertilizer products often have a nutrient ratio of 3:1:2 for N, P and K. This reflects the needs of turf for these nutrients. By keeping with the desired ratio, varying the level of N you apply also results in proportional variation of the other nutrients present in the formulation, keeping them properly balanced for turf’s needs.
As a general rule, 1 pound of quickly available N should be the maximum you apply at any one time to turf. In hot summer conditions, reduce this maximum to 0.5 pound of N for cool-season turfgrasses. You can safely apply up to 3 pounds of slow-release N at once, but more than this is risky. Close-cut turf such as golf greens should receive no more than 0.5 pounds of N from a quick-release source in one application.
Ideally, N levels should be kept as even as possible. This limits ups and downs in frequency of mowing and reduces deficiency symptoms between fertilization. That is why slow-release products are useful and, in the case of quick-release sources, why it is better to apply less fertilizer more often. However, this must be balanced against the cost of labor, which will be lower with less frequent (and presumably heavier) applications. Applying 1 pound of N at a time is a middle ground that seems to work for many turf managers. Here are some of the major turf uses and some typical N requirements for them. Keep in mind that these are only examples and that rates vary widely according to climate, species, turf use and maintenance intensity.
A reasonable range for golf greens is 0.75 to 1.5 pounds of N every 2 to 6 weeks during the growing season. This is a large range, and the actual rate depends on climate, rainfall and length of growing season. For example, warm, rainy, tropical climates result in nutrient leaching, rapid growth and a long (sometimes continuous) growing season. N demand in this type of situation could be close to 2 pounds of N per month all year long. You should base P and K (and other nutrient) applications on soil tests, but you can expect demand for these nutrients also to be high on golf greens. Temperate climates result in considerably less demand.
Fairway rates vary according to region and species, but 2 to 3 pounds of N annually is a reasonable average figure. The need will be higher in tropical climates, perhaps more than double. The objective is to apply the minimum necessary to maintain a dense, vigorous turf without creating undue mowing requirements.
Depending on many factors, 2 to 6 pounds of N annually are required by lawn turf. Low-N turf such as buffalograss may need no more than 1 pound each year to look its best.
Athletic fields require up to 10 pounds of N annually depending on the level of play. However, if fertilizations result in succulent turf, you should adjust rates or timing: succulent turf is more tender and suffers more from traffic. Conversely, inadequate levels will prevent turf from recovering from damage.
One obvious principle for timing fertilization is that nutrients must be present when turfgrasses are actively growing. For cool-season turfgrasses that is fall and spring, and summer for warm-season species. Conversely, you should withhold fertilizations when turf is dormant. This can be during summer drought or winter dormancy. Nutrients applied at these times will largely be lost by the time the turf becomes active again.
• Cool-season turf. During late fall and early winter, air temperatures may be low enough that shoot growth is minimal or non-existent. However, soil may still be relatively warm and root growth continuing. Fertilization during this time is beneficial because the plants are still photosynthesizing and producing food. However, because it’s too cold for shoot growth to occur, this food is either stored for use the following spring or used for root growth. Thus, fertilizing at this time increases winter hardiness and promotes earlier greenup and spring vigor. If you could make just one application of fertilizer each year, this would be the time. Though N is the main reason for fall applications, K is known to increase low-temperature tolerance.
Usually, you will apply more N than just a fall application. Mid-spring is a typical time to apply additional fertility. This aids the turf during its spring growth push. However, increased N during the warm season increases disease susceptibility in cool-season species. Thus, although you may wish to add additional fertilizer in late spring and summer to turf that is irrigated throughout the season, you must balance the need for nutrients with increased problems you may encounter. Any such applications should be light.
• Warm-season turf. If you had to make just one application to warm-season turf, it would be in late spring, just as turf is starting its annual push. Additional applications are helpful through the summer, as the growth of warm-season species is at its peak during summer months. However, you should avoid late-season fertilizations because this will reduce winter hardiness.
• High-use turf. High-use and close-cut turf require specialized fertilization practices. Golf greens need more consistent and continuous nutrient supplies. Therefore, superintendents fertilize often with smaller nutrient doses. This so-called spoonfeeding takes place throughout the growing season. Athletic fields likewise benefit from numerous lighter applications. Because many sporting events take place during late fall, winter and early spring, it may be beneficial to fertilize earlier in the spring and later in the fall than you would ordinarily.
As you can see, many factors affect turf needs, and your fertilizer choices should reflect those needs. For example, a product with a high acidity rating may be a better choice for alkaline soils. Low-CEC soils benefit from higher fertilizer levels of cation nutrients. Inclusion of many of the secondary or micronutrients should be based on visible symptoms or soil and tissue tests. Of course, price and availability of fertilizers matter as well. You can satisfy N needs with a variety of materials, so other factors may be more critical than N form.
Fertilizers are delivered to turf in two basic forms: liquid and granular. Both have relative advantages and drawbacks.
• Liquid application results in the most rapid uptake of nutrients because of foliar absorption. When you use heavier amounts (3 to 5 gallons per 1,000 square feet) of water, this is referred to as liquid fertilization because much of the solution runs off into the soil where roots can take it up. Under about 0.5 gallons per 1,000 square feet, we call it liquid feeding. In this case, most of the liquid remains on the foliage, and a great proportion of the nutrients are directly absorbed by the leaf blades. Use only low rates (under 0.125 pound of N) for liquid feeding to avoid burning.
Although spray rigs can cover a large amount of turf efficiently, they cannot compare to large spreaders in coverage speed. However, they are an efficient method of spoonfeeding turf that many turf managers use with success. Further, liquid applications allow operators to tank-mix pesticides with the fertilizer, which is a great convenience.
• Fertigation is the practice of applying fertilizers through an irrigation system. Fertigation is problematic without high irrigation uniformity, because varying levels of fertilization can become visible. Even so, this practice is becoming more popular because it reduces spray applications.
• Granular application is the most widely used method in most situations. Granular formulations are easy to handle, and the equipment is simpler to clean and maintain. Further, many pesticide/fertilizer combination products are available for added efficiency.
√Drop spreaders are highly accurate tools for placing fertilizer exactly where you want it without material ending up in non-target areas. However, they cover turf rather slowly and may result in skips if the applicator is not careful. This problem is reduced by halving the application rate and then making two passes at right angles to one another.
√ Broadcast spreaders are used by many turf applicators because of their speed of application. Placement of material is not as precise as with drop spreaders, but their efficiency is high and that’s why they are the preferred type of spreader in most situations.
One problem with broadcast spreaders is differential distribution of different materials in a granular mix. This happens because particles of different size and density travel different distances when the spreader throws them. For this reason, you should apply materials of greatly different size or weight separately or with a drop spreader.
Broadcast spreaders come in a range of sizes and capacities, from tiny models suitable for homeowners to professional models with large-capacity hoppers. Belly grinders are over-the-shoulder types that hang across your chest and have a hand-turned crank that throws the fertilizer.
√ Pendulum spreaders are suitable for large areas such as parks, fairways and athletic fields. They typically are tractor-mounted and PTO-driven, with large-capacity hoppers. They use a discharge spout or tube that swings back and forth to spread the material. Spreading widths as high as 40 to 60 feet allow rapid coverage of large areas.
Follow all fertilizations (except liquid feeding) with irrigation of at least 0.5 inch of water to move the fertilizer into the turf root zone. Further, calibrate application equipment frequently to ensure you’re applying the correct amount of fertilizer.
FERTILIZING TREES AND SHRUBS
If you suspect a tree or shrub nutrient deficiency, remember that woody plant fertilization is not an exact science. Compared to turfgrasses, trees and shrubs do not provide easy-to-read deficiency symptoms. While trees and shrubs need the same essential nutrients as all other plants, deficiencies usually result in simple growth reduction. Specific symptoms, though less common, include:
* Stunted, small leaves
* Leaf distortions
* Dead spots
* Early leaf drop.
Many other factors can produce similar symptoms, so you should not jump to the conclusion that fertility is the problem. Plus, nutrient interactions can be complex and are not well understood. Excess levels of one can result in deficiency of others. In this case, the basic problem is an excess of a nutrient, not a deficiency.
Unfortunately, tissue analysis is not always useful either. Researchers have not documented the relationships between foliar symptoms, tissue-analysis results and fertilizer responses well enough to make meaningful recommendations in many cases. This does not mean tissue analysis is not useful—it often is vital for solving specific problems. However, our knowledge of tree nutrition has not yet evolved to the point where it provides effective practical solutions to all nutrient problems.
Because of the complexity of a plant’s nutrient status, many researchers caution that just-in-case fertilizing is not necessarily a good practice and could cause more harm than good. Many others, however, recommend regular fertilizing and have good results. At this point, no definitive word exists on what is the ideal practical approach to fertilizing trees and shrubs. The discussion below illustrates fertilizing practices according to those that recommend fertilizing annually. Unless symptoms become apparent, micronutrients are not usually necessary, and annual applications should consist of N, P and K. But keep in mind that many authorities discourage the use of fertilizers on a “whether-it-needs-it” basis and recommend it only when trees and shrubs display lack of vigor or specific symptoms (see table, “Nutrient-deficiency symptoms of trees,” page 76).
Soil factors, as discussed, affect nutrient availability. In particular, pH above or below the 6.0 to 7.0 range can create unavailabilities. Another common problem for trees is competition with turfgrasses. If N is in short supply, trees usually suffer more than turfgrass growing in the same site.
Research has repeatedly shown that fertilization at planting time has no effect. Container plants or smaller specimens that can rapidly establish may respond to fertilization within the first year. Larger trees do not respond to fertilizer for a few years, so wait until the third or fourth year after transplanting.
Once established (this takes about 1 year for each inch of caliper), trees grow substantially faster with fertilizer. This is the time to concentrate most on fertility—after establishment, but while the tree is still young. Rapid growth is desirable at this time. As trees become mature, they respond less to heavy fertilization but may still benefit from light amounts. By maintaining reasonable vigor, older trees are less vulnerable to pests and diseases. However, you should remember that fertilizing large trees may encourage growth of specimens that already are as large as the surrounding landscape can safely or aesthetically accommodate.
Trees with restricted roots are special cases. Heavy fertilization may exaggerate their problems by increasing their shoot systems without a similar increase in roots. Unfortunately, fertilizer—including phosphorus—will not directly increase root growth in such trees and shrubs.
Some debate exists about the proper time to fertilize trees. Traditional recommendations suggest spring or early summer applications. However, some research indicates that trees absorb nutrients more readily in the fall. Conversely, fall applications risk some of the N being unused and lost from the root zone while trees are dormant. Apparently, neither time offers dramatically better benefits than the other. If you make fall applications, be careful not to over-apply N, which some arborists feel could delay fall hardening.
When turf covers the root zone, trees may benefit more from spring fertilization, before turf begins rapid growth. Or, with warm-season turf, apply fertilizer in fall after turf has gone dormant. Both of these strategies prevent turf from taking as much of the N before it can move lower into the tree’s root zone.
MATERIALS AND RATES
Some recommendations base rates on trunk diameter, but fail to consider the area of application. If you use a diameter-based rate for a large tree in a restricted space, such as a planter, you may be adding enough salts to the soil to injure the tree.
Tree-fertilizer materials are not fundamentally different than turf fertilizers. Ratios of about 3:1:1 or 3:1:2 are best for tree fertilization. Use at least 25 percent slow-release N. Standard rates suggest 3 pounds of N per 1,000 square feet annually over the root zone. Evergreens should receive about half of that amount.
For the fertilizer to be effective, it must reach the root zone. For most trees in most soils, the root zone is a uniform mat just below the soil surface. The small absorbing roots are in the top 4 to 8 inches of soil and extend from the trunk two or three times as far as the branches. Buildings, pavement and neighboring trees can restrict roots from spreading normally.
For surface applications, spread the fertilizer over the ground out to the drip line and then one-third farther. Afterward, lightly cultivate the soil with a rake. Water the area thoroughly after the application.
Broadcasting soil-mobile elements, such as N, over the soil surface and watering them in supplies the entire root zone. However, you must apply soil-immobile elements such P and K through 8- to 12-inch deep holes.
Drill applications offer several advantages:
They put P and K—nutrients relatively immobile in the soil—deeper into the root zone.
They reduce turf competition for nutrients.
They aerate soil and promote deeper rooting.
Drill applications involve drilling 2-inch-wide holes 8 to 12 inches deep around the tree. You can use an electric drill with an auger bit or a gas-powered auger. Start about 3 to 4 feet from the trunk of large trees and extend to the drip line or slightly further.
Drill applications do not require different rates of N than broadcast applications, but you must do some extra calculations to determine the application patterns (see “Drill-application rates,” at page 77).
Liquid injection is similar to drilling in that you place the fertilizer directly in the root zone. However, the fertilizer is, in this case, a liquid solution, and this method reduces the required time and labor. Use a sturdy gun with side-injection ports on the needle. These help distribute the liquid and prevent clogging. Use any spray rig that develops the necessary pressure—150 to 200 psi.
You can use as few as 8 or 10 gallons of solution or as many as 40 or 50 gallons. It’s more important to know how much fertilizer you’re injecting into each spot (see box, “Liquid soil injections,” below right).
Foliar applications. Applying nutrient solutions to foliage is a rapid, effective way to deliver nutrients to plants, which absorb them directly. However, the effects are relatively short-lived. Therefore, you should use this method in conjunction with soil fertilization for quick effect as well as longer-lasting fertility.
Foliar applications consist of spraying foliage with the same equipment and techniques you’d use for spraying pesticides. You should spray to the point of runoff and ensure complete coverage. The use of surfactants may increase plant uptake. Foliage burn is possible with many products so do not exceed the recommended rate.
Trunk injections. Several techniques allow you to perform trunk injections. One involves drilling holes and inserting injection tees. To these you attach tubes that deliver pressurized nutrient solutions into the sap stream. Another system operates by a similar principle with a small pressurized capsule attached to the feeder tube. A tap with a mallet breaks the seal and starts the injection. A third system also requires you to drill holes. But you then insert small capsules into the holes, tapping them in with a mallet or similar tool. The sap then carries the nutrients to other parts of the tree.
The value of trunk injection is unquestionable where pesticides are concerned—it places them directly into the plant’s tissue to an extent difficult or impossible to achieve with external applications. Although trunk injections also effectively deliver nutrients to the plant, many arborists feel that drilling holes in the trunk is not justified when other effective techniques are available.
TO LEARN MORE
Many situations—golf courses, athletic fields, difficult soil types—demand specialized fertility practices. To learn about specific practices in these situations, talk to other professionals in your area to see what works for them. Extension agents are another valuable source of recommendations and so are manufacturer representatives. Finally, remember that each situation is unique: Tinkering with materials, rates and timing can help you find methods that work best for your site.
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