Plant Answers

Lawn Fertilization

It is time to fertilize your lawn.

Homeowners will be bombarded with fertilizer advertisements, smooth talking salesmen and information they do not understand. Some homeowners throw up their hands in disgust and hire a professional lawn service to care for the lawn.

Regardless of your situation, you should try to understand fertilizer—how it works and what causes problems. All plants, regardless of the name of the plant printed on the fertilizer bag, require the same 16 elements for proper growth and development. Three of the elements—carbon , hydrogen, and oxygen—are provided by air and water; the other essential elements are obtained from the soil.

The macro-nutrients—nitrogen, phosphorus, potassium, calcium, sulfur and magnesium—are used in greater quantities than the other mineral elements absorbed from the soil. Nitrogen, phosphorus and potassium are often called the primary nutrients because of the amount used by the plants and their importance in supplemental fertilizers.

The micronutrients—iron, manganese, copper, zinc, boron, molybdenum and chlorine—are required in smaller quantities, but are no less important. The so?called "acid?loving" plants have a relatively high requirement for certain micronutrients, and chlorosis caused by an iron deficiency is a common ailment when these plants are grown in alkaline soils (over pH 7.0).

Fertilizer is defined as any material that supplements the soil's supply of elements required for plant growth and development. Fertilizers may be categorized as natural organic, synthetic organic, or inorganic based on their source and chemical structure.

Organic fertilizer consists of nutrient elements derived from compounds with a carbon structure. The term organic, when applied to fertilizer, should include only organic materials that are insoluble in water.
All living matter—plant or animal—is composed of compounds with a carbon structure. Proteins, fats, carbohydrates and other compounds synthesized by an organism have one common factor—a carbon structure. Any of these materials could be considered as organic fertilizers when placed in the soil. Common examples of natural organic fertilizers are animal manure, bone meal, sewage sludge and plant refuse.

Inorganic fertilizers are nutrient elements derived from sources which are not organic, those which have neither a carbon structure nor which have been derived from living matter. Examples of inorganic fertilizers are ammonium nitrate, ammonium phosphate, potassium nitrate and potassium chloride.

A complete fertilizer contains sources of nitrogen, phosphorus, and potassium. An incomplete fertilizer contains one or two of these elements in any combination, but never all three. Other fertilizer nutrients such as iron or magnesium may be present, but are not considered in the definition of "complete" and "incomplete" fertilizers.

Fertilizer analysis or grade is the minimum guaranteed percentage by weight of nitrogen (N), phosphorus (expressed as P205 equivalent) and potassium (expressed as K2 0 equivalent), and is printed on the container in that order.

For example, a 100-pound bag of 19?5?9 fertilizer is formulated from a nitrogen source(s) that contains 19 pounds of elemental nitrogen, a phosphorus source(s) that contains the equivalent of 5 pounds of P20 , and a potassium source(s) that contains the equivalent of 9 pounds of K20. If any of these elements that are missing from the formulation it would be represented by a zero in the analysis. Ammonium sulfate for example, which does not contain phosphorus or potassium, has an analysis of 21?0?0.

In addition to the total nitrogen, water insoluble nitrogen (WIN), if present, is also printed on the label as a percent of the total weight. For example, if half of the nitrogen of a 20?10?5 fertilizer is in a water soluble form, the WIN content is 10 percent. Although WIN indicates the portion of nitrogen in a controlled?release fertilizer that is slowly soluble, it is not appropriate for coated fertilizers that encapsulate soluble nitrogen. In this case, the controlled?release nitrogen may be expressed in terms of dissolution rate.

Fertilizer ratio is the relative amounts of nitrogen, phosphorus and potassium. A fertilizer with an analysis of 20?10?5 would contain 4 times as much nitrogen as potassium and twice as much phosphorus as potassium. The ratio then would be 4:2:1.

Regardless of their source, all fertilizer nutrients are absorbed by plant roots as charged atoms, or groups of atom ions or nutrient salts. These ions exhibit either a positive or a negative charge that is essential for root absorption by electrical attraction.

Inorganic fertilizers form ions readily when dissolved in water and therefore are quickly available for root absorption. Organic fertilizers—both natural and synthetic—must be hydrolyzed (decomposed) by soil micro?organisms from complex compounds to the same nutrient salts provided by inorganic fertilizers. The rate of decomposition is dependent upon soil factors such as temperature, moisture and pH.

Fertilizer burn is the visible symptom of insufficient water in a plant associated with an over application of fertilizer salts. The movement of water across the root cell membrane is regulated by the concentration of dissolved fertilizer salts in soil solution relative to the dissolved salts within the cell. As fertilizer salts dissolve in water, they raise the osmotic pressure of the solution. Water always moves from the side of the membrane with the low osmotic pressure to the side with higher osmotic pressure. Root cells actively absorb fertilizer salts from soil solution, and under normal conditions, maintain a higher osmotic pressure.

If excess fertilizer salts are applied and raise the osmotic pressure of the soil solution, water cannot enter the cell and may actively move out of it. The resulting injury is known as fertilizer burn or physiological drought.

Salt index values are a measure of a fertilizer's relative tendency to increase the osmotic pressure of the soil solution. Sodium nitrate has been give a salt index value of 100 and the value for all other fertilizers is relative to an equal weight of sodium nitrate. The higher the salt index, the greater the potential for a fertilizer to raise the osmotic pressure of soil solution and, thus, cause burn.

Because some nutrient sources are more concentrated than others (have higher percentages of N, P, or K2O), the actual increase in burn potential is affected by the application rate as well as the salt index. The partial salt index is calculated per unit of each nutrient and compares the relative burn potential of fertilizers based on equal amounts of nitrogen or equivalents of P2O5 or K2O.

The term pH expresses the relative concentration of hydrogen (H+) and hydroxyl (OH?) ions in solution. A pH of 7.0 means the hydrogen and hydroxyl ions are equal and the solution is said to be neutral. A pH below 7.0 means the solution contains more hydrogen ions than hydroxyl ions and is said to be acid. Similarly, a pH above 7.0 means the solution contains more hydroxyl ions than hydrogen and is alkaline. Soil pH may influence nutrient absorption and plant growth through the effect of hydrogen ions and their indirect influence on nutrient availability.

Nitrogen, phosphorus and potassium are the 3 nutrients required in the greatest quantity from the soil. In addition to these, iron is most likely to be found deficient in soils. Soil and tissue analysis can be used to determine the deficiency of any nutrient.

Nitrogen is required in larger amounts than other elements supplied by the soil. Compounds formed by the plant from nitrogen comprise up to 50 %
of the dry weight of the plant. Nitrogen is a component of proteins, chlorophyll, amino acids and enzymes. Nitrogen is more often deficient in soils than any of the other essential nutrients.

Plants can absorb nitrogen as either the ammonium (NH4+) or nitrate (NO3?). Urea or inorganic forms of nitrogen are converted to ammonium which is subject to volatilization when surface applied. More than 25 % can be lost to volatilization under certain conditions. The ammonium form of nitrogen may be taken up by plant roots or transformed to nitrate, which is the form most nitrogen is absorbed by plants.

Since nitrate ions are negatively charged, they are not absorbed by soil colloids (negatively charged) and readily move with soil water. Thus, heavy rainfall or irrigation may move nitrate below the root zone.

Because of the transitory nature of nitrogen in mineral soils, soil analysis is not as useful in determining deficiencies as an observation of symptoms. Nitrogen deficiencies are observed as uniformly yellowish?green leaves or needles that are more pronounced in older tissue. Leaves are small, thin and may start dying at the tips. The growth rate is reduced.

Nitrogen sources used for horticultural fertilization are often categorized as quick?release or controlled?release, based upon the rate nitrogen becomes available to the plant. Controlled?release nitrogen sources include both slowly soluble nitrogen, which is an inherent characteristic of the fertilizer, and slow?release nitrogen that is imparted to a soluble fertilizer by an artificial coating.

In general, both types cost more per unit of nitrogen than quick?release sources and provide the following advantages: a gradual supply of nitrogen which reduces the number of necessary applications; reduced leaching and reduced volatilization and a lower risk of burning which allows higher application rates.

Slowly soluble nitrogen sources release nitrogen as their chemical structure slowly breaks down. Both natural and synthetic organic fertilizers can be classified as slowly soluble and are broken down by hydrolysis and/or microbial activity into soluble forms of nitrogen. Natural organics include sewage sludge and plant and animal wastes, generally low in nutrient content.

Because of the bulk required to provide sufficient nutrients, and storage and odor problems, natural organics are being replaced by synthetic organic nitrogen in many fertilization programs.

Slow?release nitrogen is produced by encapsulating quick?release nitrogen with an insoluble coating. The soluble nitrogen is released through tiny pores as the coating is broken down in the soil. A mixture of variable coating thicknesses provides continuous release of soluble nitrogen for a controlled period of time. Slow?release nitrogen sources that are commercially available include sulfur-coated urea, plastic-covered urea and Osmocote.

Sulfur?coated urea is produced by coating urea with molten sulfur and then sealing the granule with oil or wax. The soluble nitrogen is released through tiny pores or imperfections in the coating. No 2 particles are coated the same.

The nitrogen release rate or dissolution rate is determined by placing sulfur?coated urea in water at 100 degrees F. for a 7?day period and is expressed as percent dissolved at the end of that time. Most SCU products have a dissolution rate between 20 and 30 %. Factors that increase the release rate of nitrogen from SCU include increasing soil temperature and increasing soil moisture.

Osmocote is manufactured by encapsulating soluble fertilizer with a plastic, semi-porous coating. Water enters the capsule, dissolves the nutrients, and then diffuses out into the soil for plant uptake.

Quick?release nitrogen sources are all soluble in water and are either available for root uptake in their present form or are readily converted to available forms in the soil. Inorganic nitrogen fertilizers (those that do not contain carbon), such as ammonium nitrate and ammonium sulfate, are quick?release. Urea, although technically an organic source of nitrogen, is soluble and possesses many of the same characteristics as the inorganics. Organic doesn't always mean slow?release.

In general, the quickly available nitrogen sources are less expensive than controlled?release sources and have the following characteristics: 1) readily soluble in water, 2) immediately available for absorption, 3) can cause growth flushes, 4) short soil residual, 5) leach and/or volatilize, and 6) high burn potential.

Recent developments in urea formaldehyde reaction products have provided quick?release nitrogen with a burn potential much lower than for other soluble nitrogen sources.

Phosphorous is especially important in seedling growth. It is utilized in carbohydrate conversions, energy transfer, and is a component of nucleoproteins and phospholipids. Phosphorous helps maintain a desirable pH in cells and contributes to root development.

Phosphorous deficiencies are most often encountered in seedlings. Leaves or needles turn a dull green becoming reddish?bronze to purple, especially along margins in cold weather.

Some phosphorous is provided by soil minerals and soil organic matter, but it is very slowly available from these sources. Since phosphorous moves very little through soil, supplemental phosphorous tends to accumulate near the application site, moving only a few inches in 50 years.

Plants take up phosphorous primarily in the orthophosphate form
(H2PO4 ?). Although soils normally contain relatively large amounts of phosphorous, much of it is in forms not available to plants.

Phosphorous availability is influenced by soil pH. At a pH below 5.5, iron and aluminum form an insoluble complex with phosphorous that is not available to plants. At a pH above 7.5, calcium combines with phosphorous to form insoluble compounds. Phosphorous is most available between pH 6.0 and 7.0.

The most common phosphorous sources for granule application are the super-phosphates with a P2O5 equivalent of 20 to 48 percent.

Potassium can be found in all plant parts in relatively large quantities. Potassium functions as a catalyst in plant reactions, a regulator of transpiration and an aid in the translocation of materials between cells. Although more research needs to be done with turf grasses and woody plants, potassium is thought to influence rooting and increase heat, cold and drought resistance as well as enhancing resistance to disease.

The effects of potassium on plants are more subtle than the effects of nitrogen because they are not normally expressed visually in terms of growth rate or leaf color. Potassium deficiencies may restrict the translocation of carbohydrates and nitrogen metabolism and are evidenced first as marginal and intervenal yellowing of older leaves. Leaf tips may roll, turn brown and wither. Growth is often stunted.

Potassium mobility in soils is less than that of nitrate, but greater than that of phosphates. The available form of potassium (K+) is strongly absorbed by clay particles that prevent excessive leaching except on sandy soils.

The most common potassium fertilizer is potassium chloride (0?0?62), although potassium sulfate (0?0?14) is often used in arid regions where chloride is a problem, or in commercial lawn care programs because of its lower burn potential.

Potassium sulfate has a lower solubility and may contain insoluble silica fractions. Potassium nitrate (13-0-44) is an excellent fertilizer but generally is not priced competitively with the chloride or sulfate forms. Monopotassium phosphate (0-52-34), as mentioned earlier, has excellent potential as a fertilizer but its use is limited because of its high cost.

Deficiencies of micronutrients such as iron, zinc, manganese, copper and boron are sometimes found in certain plant species, especially when grown in alkaline or sandy soils. Iron is the micronutrient most likely to be deficient throughout much of the United States and Canada.

Iron is essential for the formation of chlorophyll and its deficiency is initially expressed as interveinal and marginal yellowing of the youngest leaves. Prolonged iron deficiency can result in decreased shoot and root growth because of a lack of chlorophyll to maintain photosynthesis.

Iron deficiencies do not usually result from a lack of iron but rather because the iron is tied up or "fixed" in insoluble compounds. Iron is most commonly deficient in alkaline soils although excessive levels of phosphate, manganese, zinc and copper can produce iron deficiency. Waterlogged soils can also reduce the availability of iron.

Since iron deficiency is often the result of alkaline soil reactions, acidifying soils would appear to be a practical solution. Calcareous soils, however, may have large reserves of calcium to buffer attempts to lower the pH, particularly if the soil is fine textured.

Compounds containing iron can be applied to the foliage, soil, or, for trees, injected or implanted into the trunk.

Materials for foliar and soil application include inorganic salts, such as ferrous sulfate (copperas) and chelated forms, such as Sequestrene.

Iron from inorganic salts is quickly combined into insoluble forms in alkaline soils and little remains available for plant use. Chelated irons react slower with soil components and improve the continued availability of iron. Now that we have discussed fertilizer elements, let us consider actual lawn fertilization. Obviously nitrogen is the major element of concern for proper lawn fertilization.

Cultural practices, such as irrigation and removing clippings may create a need for additional nitrogen. Supplemental watering of turf grasses will increase the rate at which nitrogen is leached from the turf grass root zone. Losses of nitrogen are substantial when quick?release sources are applied to soils high in sand content.

Collection of clippings following mowing has been estimated to remove approximately 20 % of the nitrogen applied to the turf grass. As a result, additional nitrogen should be applied to maintain the same quality as where the clippings are not removed.

Phosphorous and potassium have been routinely applied along with nitrogen in fertilizers with ratios such as 3:1:2, 5:1:2, or 4:1:1. Applying phosphorous and potassium each time that nitrogen is applied is not necessary.

Since many soils contain high levels of phosphorous, after several years of fertilizing has occurred, little, if any, response may be obtained when phosphorous is applied to established turf.

The rate of nitrogen applied also depends upon the time of application and the nitrogen source.

Applications of quick?release nitrogen sources in spring or fall are commonly limited to no more than 1.0 pound nitrogen per 1,000 square feet. Summer applications of quick-release sources are frequently limited to no more than 0.5 pound nitrogen per 1,000 square feet.

In contrast, applications of nitrogen using controlled-release sources are generally made at rates from one to three pounds nitrogen per 1,000 square feet.

The longer residual of controlled?release nitrogen sources reduces the need for more frequent applications required when using quick?release sources. The extra cost of controlled?release products can be balanced by savings in time and labor. Actually, St. Augustine lawns need only 2 fertilizer applications per year—a slow-release formulation in the spring after the second cutting has been made and a “winterizer” (4-1-2 or 3-1-2 ratio) type in the fall (October-November).

Fertilizers can be applied in either dry or liquid forms. The choice of either liquid or dry equipment for fertilizer application has been the subject of great controversy, particularly in the lawn care industry. Research has shown turf responds equally regardless of the method of application.

Two types of spreaders are used to apply granular (dry) fertilizer—gravity and centrifugal. Always use a fertilizer spreader to insure even distribution. Even distribution avoids grass damage and insures that you get more for the money you spent on the fertilizer.

Gravity or drop spreaders drop the fertilizer, agitated by a mixing bar inside a trough, through a series of slots to the turf below. The centrifugal or broadcast spreader drops the fertilizer from a hopper onto a spinning disk that propels the fertilizer ahead and to the sides of the spreader. The centrifugal spreader applies a wider swath of material allowing the turf manager to fertilize large areas more quickly than with drop spreaders.