Fertilizer is a substance intended to improve the quality or quantity of a plants growth. When it is properly applied, fertilizer can improve a plants vigor, make leaves grow larger, reverse a decline and lessen the chance of insects and disease. Fertilizer does this by providing elements that are essential to the plant's metabolic processes.
Plants are unique in that they derive their energy for growth from the basic elements of soil minerals, light, water and air. When we evaluate the nutrient needs of a turf we must consider factors such as nutrient levels, nutrient availability and interactions between nutrients and most of all we must consider environmental conditions that determine the availability of the nutrients to the grass.
In order to choose the right fertilizer for a particular need, a turf manager must first become familiar with the various elements that plants require for these processes. Two of the most important of these substances, carbon and oxygen, are readily available to grasses from the air. A third vital element, hydrogen, is taken from water in the soil.
Plants obtain nutrients from soil minerals, organic matter, fertilizer and from the atmosphere. Deficiencies of one or more nutrients may occur because:
1. The nutrient may be lacking in the soil or in the environment.
2. The nutrient may be too slowly available.
3. There may be an in balance between nutrients.
For nutrients to be taken up by the grass they must be present in a form that the plant can use. Even if they are available, they may be lost by leaching or volatilization.
Other nutrients which turfgrasses must have are assimilated in the form or ions through the plants' feeder roots. The availability of these substances depends on the soil conditions and the solubility of the elements themselves. They must be dissolved in water for the plants to absorb them.
Grasses may obtain nitrogen from organic matter, but fertilizers provide the major source of nitrogen to turfgrasses. Organic matter, organic fertilizers and some slow release fertilizers must be broken down by soil microorganisms before the nitrogen can be used by the grass. This transformation is called mineralization.
These transformations are dependent upon soil microbes and are sensitive to a number of environmental conditions. They do not occur below freezing temperatures and are slow to take place in poorly aerated soils, very dry soils or overly wet soils. The application of an organic source of nitrogen or ureaformaldehyde does not necessarily provide the grass with its nitrogen requirement.
If fertilizers are applied incorrectly, they can burn or damage the turf.
In choosing a fertilizer, one of the foremost considerations is the compound's content of three essential elements:
Nitrogen (N), Phosphorous (P) and Potassium (K)
The relative proportions of these substances are usually listed as a ratio on fertilizer packages. Thus, a label reading "N-10, P-5, K-5" indicates a compound of ten percent nitrogen, five percent phosphorous and five percent potassium.
NITROGEN (N) - The First Number
The first number to any fertilizer analysis is always Nitrogen. For instance with a 21-7-14 analysis the number 21 is the percent of Nitrogen in this blend. The same applies to a 17-17-17, a 15-5-10 or any other blend.
The importance of nitrogen to plants, including turfgrass, is nearly universal. Since nitrogen is vitally important in the manufacture of chlorophyll, new plant growth is virtually impossible without it. Turfgrass suffering a lack of nitrogen will turn light green or yellow, and leaves will die starting at the tips.
Throughout the years, Nitrogen has been studied and used more than any other nutrient. There are several different types of Nitrogen sources to choose from. Some advantages and disadvantages of each follow:
82-0-0 Anhydrous Ammonia (NH3)
Advantages = Easy to apply and little danger of leaching.
Disadvantages = Uneven distribution and losses in irrigation water with some systems and sprinklers. Possible toxicity to plants and can be very hazardous to the applicator.
20-0-0 Ammonia Solution (aqua ammonia)
Advantages = Easy to apply.
Disadvantages = Same as Anhydrous.
21-0-0 Ammonium Sulfate
Advantages = Minimal leaching loss, easy to use, safe to handle and sulfur boost if needed.
Disadvantages = Delayed availability during nitrification and has a high loss potential on calcareous soils if it is not incorporated into the soil.
33-0-0 Ammonium Nitrate
Advantages = Minimal volatilization loss potential and works well in colder weather.
Disadvantages = Some volatilization loss on calcareous soil if it is not incorporated into the soil.
16-20-0 Ammonium Phosphate-Sulfate
Advantages = Same as with Ammonium Sulfate but also has a Phosphate content where needed.
Disadvantages = Same as with Ammonium Sulfate and carries a higher cost.
15-0-0 Calcium Nitrate
Advantages = Calcium is beneficial to acid and sodic soils. Has immediate availability with little to no volatilization losses.
Disadvantages = Clumps in moist or humid weather and is susceptible to leaching.
Advantages = Easy to use and very quick availability.
Disadvantages = High loss potential if not incorporated or watered in.
16-0-0 Sodium Nitrate
Advantages = Immediate availability.
Disadvantages = Sodium is detrimental to soils.
Formula For Figuring N Rates
Divide the Nitrogen number into 100. The number left is the actual amount of fertilizer needed per 1,000 square feet to get 1 pound of N. For instance:
21-0-0 / 21 divides into 100 5 times. 5 pounds x 21 (N) = 1.05 pounds of N per 1,000 S.F.
Multiplying 5 pounds by 43.56 will tell you how many pounds you will need per acre (217.8 #'s).
Nitrogen is more usable in the nitrate form. At 50 degrees there is little nitrification. The rate of nitrification can double for each 18 degree increase in soil temperature.
PHOSPHORUS (P) The Second Number
The second element listed on a fertilizer package, phosphorous, is an important plant nutrient because it assists in the aging of tissues and stimulates root growth. The element is normally available to plants in the form of phosphates.
A phosphate deficiency will primarily affect the root areas of turfgrass. Unlike nitrogen, this condition does not provide a visual symptom. If a lack of phosphorous is suspected as a source of turf problems, a soil sample may provide confirmation.
There are several factors that can strongly influence the plant uptake of Phosphorus:
*You get more response if the soil is low in Phosphorus. The higher the levels, the less response you will see.
*Phosphorus is taken up easier in moist soils.
*Like Nitrogen, the higher the temperature the quicker it is used up.
*pH of the soil needs to be 6.5 to 7.0.
*High levels of metallic elements will tie up Phosphorus.
POTASSIUM (K) - The Third Number
Potassium, or Potash, is the third of the "big three" nutrients. Potassium is essential in the manufacture of sugar, starches and proteins. When potassium is not available to turfgrass, leaf margins and tips begin to turn brown and curl. However, because a lawn may be mowed regularly, these symptoms are not always visible. A soil analysis is probably the best way to tell if a potassium deficiency is the cause of turf problems.
There are two forms of Potash: 0-0-60 Muriate of Potash (Potassium Chloride) and 0-0-50 Sulfate of Potash (Potassum Sulfate).
Sulfate of Potash is more expensive than Muriate of Potash in spite of its lower K. Some Professionals are hesitant of the chloride in Muriate so they pay more for and use the Sulfate form. In most cases Muriate is just as safe as Sulfate when broadcast.
If your Sulfur is deficient then the Sulfate of Potash form is best.
Fertilizers can also contain small amounts of other elements (trace elements) which are necessary for healthy plant growth. These substances are iron, calcium, magnesium, sulfur, manganese, zinc, boron, copper, molybdenum and chlorine.
In some parts of the country, iron deficiencies (called chlorosis) can cause a yellowing of turf and trees similar to that caused by a lack of nitrogen. A Chlorotic condition can be remedied by applying compounds of iron sulfate or iron chelate.
Because turfgrasses depend on their roots to take in most of their nutritional requirements, soil conditions have an important effect on nutrient availability. Soil is formed by minerals, rocks, organic matter, air and water. Micro-organisms constantly interact with soil particles and decayed organic material to yield simple compounds that can be absorbed and used by plants.
Since different organic materials have different rates of decay, a portion of the matter is usually in a state of partial decomposition. This material, known as humus, represents a "reservoir" of nutrients available for future use. The amount of humus in a soil is a major factor in how much nitrogen is available to the turf.
Temperature has an important effect of the rate of decay of soil humus. Microbial activity, and therefore the rate of organic decomposition, increases with temperature. Conversely, decay is greatly reduced in soil below 45 degrees Fahrenheit.
The amount of nutrients available to turfgrass is affected by other properties of the soil, such as texture. The shape and size of the mineral particles determine the soil texture. Particles are classified wither clay, sand or silt. Clay particles are the smallest, fitting close together resulting in a finely textured soil.
The more finely textured a soil is, the more nutrients and water it is capable of sustaining. Sandy soils contain the largest mineral particles. A sandy soil, for example, cannot hold the water and nutrients that a clay soil can.
Loam soils are an intermediate textural class between clay soils and sandy soils. Loam is a combination of sand, silt and clay particles. It is the best for everyday growing needs. Loam soils have the capacity to drain well, but not too fast, and have enough air pickets to allow plants to develop a good root system.
Soil structure is a factor in the amount of nutrients that a soil may hold. A soil that is compacted or that lacks an open, granular structure can limit water and air movement. This in turn impedes the root activity and plant growth.
Fertilization also has a bearing on the soil's pH level. This term refers to the degree of acidity or alkalinity of a soil, or the relative proportion of hydrogen (acid) and hydroxide (alkaline) ions. A soil that is pH balanced has equal concentrations of the two, or a pH of about 7.0.
A soil's pH level influences the plant growth because of its effect on the solubility of nutrients and the activities of micro-organisms. An acidic soil is characterized by reduced microbial activities, although this is somewhat offset by greater nutrient mobility. Adding lime to the soil will help correct an acid condition.
In an alkaline soil, beneficial micro-organisms are more active, but the mobility of the nutrients is reduced. If a soil has an excessive pH because of alkalinity, it can be adjusted by adding Sulfur or Humic Acid.
How well a soil is aerated is also a factor in its fertility level. An aerated soil will ensure enough oxygen for the plants, as well as for the microscopic life that contributes to the nutrient levels in the soil.
Timing has an important effect of the fertilization process. For cool season grasses, fertilization is most effective in the spring, which gives the grass a head start on summer growth, and the fall, which permits the turf to thrive into the cool season and store up nutrients for early growth the following spring.
Warm season grasses are best fertilized in early spring and midsummer. An early spring application provides nutrients for summer growth, while fertilization in mid-summer enables the turf to sustain itself through the fall months.
Fertilizer can be too much of a good thing, so care should be taken to avoid over-fertilization. Too much nitrogen produces tender, succulent growth that is susceptible to disease and ill-suited to heavy traffic.
Many fertilizers have a relatively high salt content, and their over-use can create saline soil conditions. In applying fertilizer to turf, it is very important that the compound be distributed evenly, otherwise, the response of the grass will be irregular and the lawn will develop unsightly patches and strips. Fortunately, there is a large selection of machines on the market today that can provide such uniform coverage.
One fertilization technique that can help minimize the problems of uneven distribution for dry fertilizers is to apply the compound in two parts. One half of the fertilizer can be distributed in an east-west direction and the other half in a north-south direction.
The environmental issues of fertilization must also be mentioned. Nutrients in a fertilizer are so useful to plant life, runoff from fertilized areas can cause an increase of algae in ponds or lakes. Under such conditions, oxygen levels in the water may sink low enough to be fatal to fish.
Be careful to consider the amount of fertilizer being used and obtain an accurate picture of the soil conditions, so that this kind of pollution can be avoided. One can also prevent pollution by taking care not to over-irrigate immediately after fertilization. This is a practice which may leach large amounts of nutrients into nearby ponds or lakes.
Through a well planned and systematic approach to fertilization, nutrient deficiencies in turfgrass are usually corrected before they can become a serious problem.
The key to proper plant nutrition is knowledge of the plants fertility requirements and environmental influences. Even plants of the same species have different fertility needs based on sunlight, temperature, and soil.
A single landscape has many different types of plants in a wide assortment of environmental conditions. Details make the difference between a grouping of plants and a colorful, effective landscape.
The critical chemical elements required by plants to thrive can be less than a dozen. These elements must be present and available for biochemical reactions to take place properly.
These elements must also be in balance. Too much or too little of one element can reduce the usefulness of other elements. The trick is getting the right amounts of the right elements in the right place at the right time.
First you have to realize that nutrients delivered to a plant through the soil can be rendered useless by the soil. The first soil consideration is moisture. Plant roots accept chemicals and gases in certain forms. Moisture is necessary to convert fertilizers applied by the contractor into the forms that can pass through the root membrane into the plant. Water also carries these nutrients from their point of application to the root zone.
The second soil consideration is acidity and alkalinity (pH), the balance of hydrogen and hydroxyl ions determined by a soil test. A pH of 7 is considered neutral. The soil is categorized as alkaline if it is above 7 and acid if it is below 7. High or low pH can stop the chemical reactions necessary to make nutrients in the soil available to the plant.
Plants should be grouped according to their optimum soil pH range, just as they should be for irrigation and light requirements. This makes fertilization more effective and subsequently provides better plant performance.
Correction of acid soil is usually accomplished over time with Lime (crushed or dolomite). Alkaline soils are corrected with applications of granulated Sulfur and Humic Acid.
Fertilizers and irrigation water can have an impact on soil pH. A variety of fertilizers contain Sulfur and can be used as part of a program to modify alkaline soils. Irrigation water should be tested for pH if fertility problems are diagnosed.
Soil tests also provide another gauge of the chemical reactivity of the soil, cation exchange capacity (CEC). A cation is a positively charged ion. Examples of fertilizer nutrients that are cations are calcium, magnesium, and potassium.
Soils that contain higher levels of organic matter and/or clay, have higher CECs. This means they provide a better environment for chemical reactions. Low CECs mean that chemical reactions are less likely to take place and fertilizer elements might not be available to the plants, even though they are present in the soil.
Salinity is another common cause of poor fertilizer utilization because of the concentration of salts in the soil solution. High salinity is brought about by the use of saline irrigation water or overuse of fertilizer salts. This prevents the plant roots from absorbing the water they require to carry on physiological processes.
The first way to modify saline soils is to over irrigate the area to leach the salts out of the root zone. This requires good drainage. A second way is with a heavy application of Gypsum. The best way is to treat the water with pHairway®. Salinity tends to be a regional problem and requires the attention of an expert using materials such as gypsum and sulfur.
The Key Elements
Most of the research and published work on fertilization has focused on turfgrasses and nursery production. Recent advice that supplemental potassium, iron, and sulfur have a place in annual fertilization programs has some relevance to ornamental shrubs, flowers and trees in the landscape.
Some have suggested that nitrogen rates are too high in many cases and can be reduced by relying more on slow-release synthetic and organic fertilizers and micronutrient supplements. To make this adjustment without harm to the health of landscape plants, you need more than a basic understanding of fertilizers and how they interact with one another. You need to confirm your changes with soil testing.
It is rare, but some soils are well balanced and require only supplemental nitrogen, the element consumed in greatest quantities by plants. Slow-release synthetic and organic forms of nitrogen prevent leaching and help keep the element in the root zone. Remember that yellowing of foliage is the primary sign of nitrogen deficiency.
Phosphorus, important primarily for establishment, is not mobile in the soil and does not leach out to any great degree. Many landscapes have sufficient or excess phosphorus. Excess phosphorus can cause deficiency of some micronutrients, such as manganese.
Potassium, the third and final component of complete fertilizers, is not as stable as phosphorus and is now being recognized for its role in helping plants withstand environmental stresses, such as traffic, heat, and drought. Many complete fertilizers today contain potassium at one-half or more of the amount of nitrogen for this reason. Supplemental potassium and iron can be applied during the summer when high temperatures could cause nitrogen to burn turf and other plants.
Nitrogen is most likely to be deficient in well-drained, sandy soils that are irrigated regularly. Sandy soils can also result in shortages of potassium and micronutrients, especially sulfur and iron. Where sandy soils are needed to solve drainage and compaction problems, fertilizing is a much greater challenge requiring frequent application of low rates of nutrients.
Before applying nitrogen in the summer or during periods of drought, check the salt index of the material. Select nitrogen sources with lower salt indexes, such as organic materials, calcium nitrate or ammonium sulfate.
Ammonium nitrate and sodium nitrate have high salt indexes. Foliar applications of ferrous sulfate can solve summer nitrogen deficiencies while also providing a boost of color from iron.
Phosphorus deficiency is evidenced by a wilted, purple appearance in the foliage. Super-phosphate, diammonium phosphate or monoammonium phosphate can correct the problem. Monoammonium phosphate is recommended for the cure on alkaline soils.
Since Potassium can be leached through the soil, it needs to be applied regularly to most irrigated soils. Poor drought tolerance, increased incidence of disease, and slow grow-in are signs of potassium deficiency. Applications of potassium sulfate offer a low salt index and an acidifying boost from iron during the summer. Potassium chloride is not recommended for summer application due to its higher salt index. Potassium nitrate ranks in the middle for burn potential.
Iron is perhaps the element next likely to be deficient. Chlorosis is the term for the yellow foliage when iron is not available. Iron is easily oxidized and tied up in the soil, rendering it unavailable. Supplements need to be reapplied every few weeks for maximum effect. Applications of granular chelated iron or foliar soluble ferrous ammonium sulfate or ferrous sulfate are the most common corrective solutions. Iron is preferred over nitrogen during the summer to create green foliage for a special event.
Sulfur deficiency is more common than phosphorous deficiency. Not only is sulfur required by plants in significant amounts, it also can improve the availability of other nutrients in alkaline soils. It improves the utilization of nitrogen in the plant and therefore has a greening effect.
Some sources of nitrogen, potassium and iron contain sulfur as well. They include ammonium sulfate, potassium sulfate and ferrous sulfate. Granular sulfur can be applied following soil tests to correct deficiencies. Sulfur can be used to help leach out sodium in saline soils.
Calcium is another element that is part of fertilization programs. Both lime and gypsum provide calcium for the soil. Calcite Calcium is as effective as lime and gypsum but the up front cost may appear to be higher. Since less applications are needed, and a higher percent of the Calcium reaches the plant, this should be the choice between the three. Calcium can also be used to leach out sodium from saline soils. Rarely is it deficient. Dolomitic limestone provides magnesium in addition to calcium.
Micronutrient deficiencies are caused primarily by soil alkalinity or acidity. Alkaline soils reduce the availability of nitrogen, phosphorus and micronutrients, such as iron, manganese, zinc, copper and boron.
Correcting soil pH is much more difficult than supplementing nutrients. Combinations of micronutrients are available to improve plant health while soil problems are being addressed. Foliar applications enable the landscape manager to avoid soil problems, however they need to be repeated every few weeks to maintain adequate levels in the plant tissue.
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