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Fertilizer Types

Edited by Len Phillips

FERTILIZER LABEL

Fertilizer labels must carry the specific analysis, the weight percentage of the three major nutrients, total nitrogen (N), available phosphate (P₂O₅) and water-soluble potash (K₂O). Total N means the total amount of ammoniacal nitrogen, urea nitrogen, water Insoluble nitrogen, and other recognized and determinable forms of N. The nitrogen breakdown statement is only required when slow release or organic nitrogen properties are claimed or guaranteed.

When reading the label, a 20-10-10 analysis for example, would contain 20% total nitrogen, 10% phosphate, and 10% potash. Therefore, a fifty-pound bag of fertilizer with this analysis would contain ten pounds of total nitrogen, five pounds of phosphate, and five pounds of potash, or a total of twenty pounds of major nutrients. The other 60% would consist of inert material or other minor nutrients. Always read the ingredients and follow all label directions prior to applying fertilizer.

There are many formulations, but the listings are always in the same order, with nitrogen first, followed by phosphate and potash. The specific fertilizer ratio needed will depend on the soil nutrient level. One general rule can be established: the percentage of nitrogen in the formula usually dictates the amount of fertilizer to be applied.

When other plant nutrients in addition to N, P, or K are guaranteed, they must be listed in columnar form immediately below the primary nutrient guarantees and always in the following order with minimum percentages listed: Calcium (Ca); Magnesium (Mg); Sulfur (S); Boron (B); Chlorine (Cl); Cobalt (Co); Copper (Cu); Iron (Fe); Manganese (Mn); Molybdenum (Mo); Sodium (Na); and Zinc (Zn).

There are three types of fertilizers: organic, inorganic, and slow release fertilizer.

ORGANIC FERTILIZERS

The significant difference between natural organics and chemical inorganics is that organisms in the soil must release nitrogen in natural organics before it becomes a food for the tree. Therefore, since they are not soluble in water, they are less likely to be lost through leaching, and they are slow releasing and non-burning. Natural organics are usually low in nitrogen.

Organic fertilizers  are derived from nature, such as manure, sludge, etc., and are marketable in dry form only. All of these materials act more slowly than synthetic fertilizers, thus reducing the danger of over-fertilization. However, those that have a low percentage of nitrogen are bulkier and heavier to handle because they must be applied in much greater quantities. These materials release the fertilizing elements somewhat unpredictably when the soil is warm enough.

Bone meal  decomposes slowly and thus releases phosphate slowly. It is a well-known additive at the bottom of the hole for bulb planting. Another advantage of bone meal is that it helps neutralize the acidity of peat based potting mixes.

Cottonseed meal  is a by-product of cotton manufacturing. It is somewhat acid in reaction as a fertilizer. Formulas vary slightly, but are generally 7- 3- 2. Cottonseed meal is available in warm soils and there is little danger of over-fertilizing. It is used with acid loving plants.

Fish emulsion  is a fertilizer made from fish byproducts. A major disadvantage is the intense odor; however, the smell will dissipate in a day or two. Fish emulsion is high in nitrogen and is a source of several trace elements. It is excellent for use in the late winter to boost the early growth spurt. It is possible to ‘burn’ plants if too strong a solution is applied.

Seaweed  is a source of trace metals and growth hormones. Its greatest disadvantage is the high cost. This form of fertilizer is usually used for houseplants, but experimentation continues for tree and garden use.

Manure  comes in several types including horse, cow, pig, chicken, and sheep products. Fresh manures have the highest concentration of nutrients, but when they are aged or composted, the nutrient content is reduced. Fresh manure should not be used where it will come into contact with tender roots. Composted forms are more commonly used to reduce burning the plants. Manures are useful for increasing the organic content of the soil, improving soil structure, and increasing bacterial activity. Manure should be spread evenly and dug, rototilled, or plowed under. Dried manures from a nursery are a satisfactory substitute for fresh manure.

Sewage sludge  is a recycled product of municipal sewage treatment plants. Activated sludge has higher concentrations of nutrients (approximately 6-3-5) compared to composted sludge. It is usually sold in a dry, granular form for general-purpose use and as a long lasting, non-burning fertilizer. Composted sludge is mainly used as a soil additive and has a lower nutrient content (approx. 2-3-0).

INORGANIC FERTILIZERS

The blended inorganic fertilizers readily break down in the soil and quickly bring nutrients to the plants. These fertilizers are soluble in water. They are often termed "hot fertilizers" because they contain salts and various mineral compounds that can burn the tree roots if not handled properly. The chief advantage is that they contain a high ratio of nitrogen, phosphate, and potash. While inorganic fertilizer supplies nutrients to the soil, it does not feed soil microorganisms. It is the soil microorganisms that are essential for long-term release of nutrients stored in organic matter.

Inorganic fertilizers are chemical or mineral products available in dry or liquid form. The nitrogen becomes available from bacterial action over a longer period of time and thus extends its fertilizing action gradually. Slow release pellets, packets, and sticks are available for tree fertilization. These pellets are an acceptable source of nutrients. However, compared to surface application, they do not provide good lateral movement of nutrients, are somewhat expensive, and have to be reapplied periodically.

Soluble inorganics include urea, ammonium sulfate, ammonium nitrate, potassium nitrate, and sodium nitrate. They are inexpensive and have a very high rate of nitrogen.

SLOW RELEASE FERTILIZERS

Slow release fertilizers are more expensive and thus arborists are reluctant about using them. The three main categories of slow-release technologies are: Synthetic Organics, Coatings, and Urea.

Synthetic Organic Fertilizers  are water-soluble and can be taken up by the plant almost immediately. Thus applying too much of this type of fertilizer can damage plants.

Coatings  are a physical barrier made of sulfur or plastic that surrounds a water-soluble fertilizer core. Sulfur coated urea is coated with plastic. The nitrogen is released when microbes feed on the sulfur. The thickness of the coating determines the longevity of the product. The water-soluble center is released by temperature vaporizing the coating. Polymer-coated fertilizers (PCU) are the most recent advancement in fertilizer technologies. The polymer film, similar to timed-release capsules in drugs, coats the water-soluble urea. Besides urea, these fertilizers may contain muriate of potash, potassium nitrate, and nitrogen-phosphorus-potassium substrates. Osmocote™ is the oldest and best know PCU.

Urea  materials include urea formaldehyde (UF) reaction products. The release of nitrogen from UF-reaction products requires microbial activity. Factors such as temperature and pH can influence the rate of enzymes and microbial activity and the subsequent release of nitrogen. Manufacturing UF materials consists of combining water-soluble urea and formaldehyde in a reactor under controlled conditions. The main components are allowed to react for a period of time and then the reaction stops. A heterogeneous mixture of polymers is produced. The finished product contains some percentage of unreacted urea and varying percentages of UF polymers such as methylene diurea, trimethylene tetraurea and tetramethylene pentaurea. The exact combination of urea and polymers makes a product unique. One of the best-known products is Nitroform® (Blue Chip), with an analysis of 38-0-0.

Poly-methylene urea is a liquid compound that actually delivers its full complement of nitrogen over about 8 months and is completely soluble in water. The fertilizer is non-corrosive, will not settle in the tank, contains no salts, and is completely biodegradable. Additionally, the product is easier on equipment because there is no grit involved. Poly-methylene urea is now commercially available with a premixed complement of micronutrients and some ammonium nitrate, and it can even be used as a foliar fertilizer treatment.

IBDU  are synthetic organics including materials where nitrogen release is determined by a material with low water solubility. IBDU is the predominant product in this group. It is manufactured much like a urea product only with a different aldehyde. The nitrogen release is regulated by water, not microbes. Once the urea is released it is converted to ammoniacal nitrogen by an enzyme and to nitrate nitrogen by bacteria. It is not affected by temperature and is therefore a good fertilizer for fall or spring.

Biofertilizers  represent a new tool for growing trees that are based on biology. Biofertilizers provide slow but lasting fertilizing effects. Biofertilizers are based on living organisms and maintain their beneficial effects as long as the host trees sustain them. Scientists have isolated various mycorrhizal fungi and beneficial rhizosphere bacteria, and they have further devised the means to mass-produce biofertilizer and to formulate new products that contain them. The commercial availability of biofertilizer allows planting sites to possess the sustainable fertility more characteristic of the natural forest.

Sources

• "Basics of Fertilizers", City Trees, The Journal of The Society of Municipal Arborists Vol. 37, Number 3 May/June 2001

• Durden, Carol, “Fertilizer Label”, Minnesota Department of Agriculture, September 5, 2008

• "Fertilizers for the Garden", City Trees, Vol. 37, Number 4 July/August 2001

• Foerster, Vic, "Biostimulants", ArborAge, April 2003.

• Kernan, Michael J. Ph.D., "Biological Treatments for City Trees", City Trees, Vol. 40, Number 3 May/June 2004

• Robbins, Jim, "Fertilizer Facts", Nursery Management and Production, October 2003. P. 30 – 33

• Struve, Daniel K., "Nitrogen Research in the US", Journal of Arboriculture, Vol. 28, No. 6 November 2002

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