Fertilizer Basics

Revision Date: 11/19/2012
Susan S. Barton, Extension Specialist
Ornamental Horticulture


Water is a precious resource in Delaware. This series of publications addresses how homeowners can do their part to protect ground and surface water by following proper fertilization procedures. The Pennsylvania Safe Drinking Water committee found that surface sources of pollutants, such as fertilizer, insecticides, accidental spills at airports and highway de-icing salts, represent a problem of growing intensity.

Individual lawns and planting beds are small but the total area of lawns and planting beds in urban environments is significant. Proper fertilization will enhance plant growth without polluting the environment. However, misuse of fertilizer can harm the environment and injure landscape plants by causing fertilizer burn to leaves and/or roots.

The pollutants people can control and manage effectively can be divided into four categories.

1) Domestic pollutants include animal wastes, household chemicals, waste oil, septic systems, pesticides and fertilizer. 2) Agricultural pollutants include eroded soil, animal wastes, fertilizer and agrichemicals. 3) Urban pollutants include all the domestic pollutants, landfills, leaking sewage pipes, storm water catchments, chemicals from streets and parking lots and sediment from construction and sewage treatment plants. 4) Industrial pollutants are usually treated and discharged directly to streams.

Nutrients that affect water quality:  Nutrients, essential for plant growth, may be present in a form or an overabundant supply that cannot be used by plants. Excess nutrients that enter the surface or ground water may reduce water quality. Phosphorus is an important nutrient necessary for plant growth. It is also the primary cause of algae and weed growth in lakes and ponds. The over-abundance of decaying algae depletes the water oxygen supply and can kill fish and desirable vegetation. When phosphorus is applied as a fertilizer, it is quickly bound to soil particles or taken up by the plant. Therefore, the major source of phosphorus in runoff into surface water is phosphorus bound to eroding soil particles or clippings and leaves containing organic phosphorus that are left in streets and gutters.

Nitrogen is the nutrient that produces the greatest growth response in plants. Nitrogen is usually present in the soil as nitrate ions. Nitrate ions are soluble and not held by soil particles. Excess nitrate that is not taken up by plants will leach downward with percolating water and may enter the ground water supply.

Nitrogen is particularly dangerous to the very young. Infants less than six months old do not yet have stomach acid strong enough to prevent the growth of certain bacteria in their intestinal tract. Those bacteria then convert nitrate to nitrite. Nitrites oxidize hemoglobin in the blood, making it unable to carry oxygen. This condition is called methemoglobinemia or "blue baby syndrome." Without oxygen in the blood, brain damage or suffocation can occur. The allowable level of nitrogen in the water for infants under six months is 10 ppm (10 mg/l) as nitrate nitrogen or 45 ppm (45 mg/l) as nitrate. Older children and adults can probably tolerate much higher levels of nitrates, but the primary drinking water standard has been set at the more conservative level.

Nitrate contamination is most commonly caused by over-application of nitrogen fertilizers, animal manures and improperly designed or installed septic tanks. Sandy, coarse-textured soils, common in southern Delaware are most susceptible to nitrate pollution.

To avoid phosphorus and nitrogen loss to surface and ground water:

•    Remove clippings and leaves from streets and gutters.

•    Cover the soil with healthy plants or mulch to reduce soil erosion.

•    Apply the proper amount of fertilizer

•    Be sure that all septic tanks are properly designed and installed (see Brochure # ).

Soil testing:  Begin your safe fertilization practices by taking a soil test. Soil testing is a service provided by the University of Delaware. Soil test bags can be purchased at your county Extension office. A soil test will tell you the levels of available phosphorus and potassium in the soil as well as the soil pH. Because nitrogen is so soluble, it is of no value to test for the level of nitrogen. To take a soil sample, collect 6-inch-deep cores from eight to ten locations in the front yard. Mix them in a plastic bucket and take about 1 pint from the bucket for testing. Repeat this procedure for the back yard or any other area that will be treated differently. A fertilizer recommendation will be included with your soil test results. If levels of phosphorus and potassium are sufficient, there is no need to apply those nutrients.

The other important piece of information on your soil test results is the soil pH. Turfgrasses grow best in a pH range from 6.0 to 7.0. In this range, nutrients are most available and microorganism populations necessary for decay are active. Most trees and shrubs grow best in a pH range from 5.0 to 7.0. A good compromise for landscapes that include trees, shrubs and turf is to maintain the pH between 5.5 and 6.5. To increase the pH, add lime and to decrease the pH add elemental sulfur, aluminum sulfate or an acidifying fertilizer. Certain acid-loving plants such as azaleas and rhododendrons can be grown in beds that are maintained between 4.5 and 5.0. When the ground is covered and the soil stabilized by healthy turf, groundcover, shrubs or trees, soil runoff is greatly reduced. By maintaining the proper pH, you are helping to insure that the nutrients you apply will be taken up by your plants rather than lost by leaching or surface runoff.

•    Take a soil test to determine levels of phosphorus and potassium present in the soil.

•    Maintain pH between 5.5 and 6.5 for most trees, shrubs and turf.

Fertilizer types:  Fertilizer can be purchased as "single ingredient" fertilizers such as ammonium nitrate and urea for nitrogen, triple super phosphate for phosphorus or muriate of potash for potassium. Combination fertilizers such as 10-10-10 or 5-10-10 include all three nutrients. The three figures or the fertilizer analysis represent the percentage of each element in the fertilizer. For example a 5-10-10 fertilizer contains 5 percent nitrogen, 10 percent phosphorus (as P2O5) and 10 percent potassium (as K2O). The 5-10-10 fertilizer is a 1-2-2 ratio. An 8-16-16 fertilizer is also a 1-2-2 ratio, but it contains a greater percentage of each of the fertilizer elements. Smaller amounts of the 8-16-16 should be used in place of the 5-10-10.

Fertilizer can also be classified by their solubility. Soluble fertilizers release their nutrients rapidly, whereas organic and slow-release fertilizers release nutrients over time. The fertilizer in the slow-release formulations is released by water penetration, weathering or microbial action depending on the type of slow-release fertilizer. With one application of a soluble fertilizer, plants may not be able take up all the available fertilizer and some nitrogen may leach below the plant rooting depth with percolating water. Slow-release fertilizers are designed to release at a rate more consistent with plant needs. Slow-release fertilizers are usually more expensive than soluble fertilizers. The choice of soluble or slow-release nitrogen demands on the reason for fertilization. Soluble fertilizer may be required when a quick response is desired and slow-release fertilizers are excellent for regular maintenance. Fertilizers are also formulated with a combination of soluble and slow-release nitrogen.

Fertilizer is also available in dry and liquid forms. There is no difference in uptake from these forms. Both require irrigation or rainfall to distribute nutrients in the soil-water solution so they are available for uptake.

•    Learn to read the fertilizer analysis--a 5-10-10 fertilizer contains 5 percent nitrogen, 10 percent phosphorus and 10 percent potassium.

•    Use soluble fertilizers for a quick response.

•    Use slow-release fertilizers for regular maintenance.

Alternate nutrient sources: Many organic materials provide excellent sources of nutrients. Release of nutrients from organic materials, applied in proper quantities, is usually slow so little nitrogen is leached from the soil (although some "raw manure" sources such as poultry

manure are "fast release"). Composted sewage sludge, poultry manure, cow manure, composted leaves and other vegetation, peat moss, sea weed and fish emulsion are all organic sources of nutrients. Prepared organic fertilizers include leather dust (10-0-0) and cotton seed meal (7-2-1).

The problem with using organic materials as sources of nutrients is that the content of available nitrogen and other nutrients is often unknown. If you purchase packaged manure or composted sewage sludge at a garden supply store, the "total" fertilizer analysis should be on the package. This does not mean that all those nutrients will be "available" to the plants. The level of nutrients in unprocessed organic materials, bulk sources or organic material produced at home is anyone's guess. You may not apply enough fertilizer to get the desired result or you may over-fertilize and nitrate leaching, salt toxicities or excessive vegetative growth will occur. It is a common misconception that using organic fertilizers is beneficial to the environment and "safe." Organic fertilizers are if used properly but over-application can be just as detrimental to groundwater as over-application of inorganic fertilizers.

•    Organic materials can be excellent sources of nutrients.

•    Be careful applying organic materials when the fertilizer analysis is unknown.

•    Remember that organic fertilizer are not necessarily beneficial to the environment and "safe."

Purchasing fertilizer: In order to make a decision about which fertilizer to purchase and how much to apply, it is important to determine why you are fertilizing. For shade trees, you may want to promote rapid growth so that young trees quickly become large and functional. Or you may want to maintain the health and appearance of a mature tree. An additional reason for fertilization is to rescue a declining tree. Different amounts of fertilizer are required to fulfill each of these goals.

Before fertilizing your lawn, decide whether you would like to:

1.  promote a thick, lush, green lawn that requires a lot of maintenance;

2.  maintain a healthy and persistent lawn with less mowing and maintenance needed; or

3.  have a grassy area for erosion control or utility requiring little upkeep.

The following chart gives general recommendations for fertilizer required for different plants under different circumstances.


Fertilizer recommendation

lb N/1000 ft2 amount suggested
Shade trees
   promote active growth 2 11 lb 18-6-12/1000 ft2
   maintain mature tree 1 5 ½ lb 18-6-12/1000 ft2
Conifers 2 11 lb 18-6-12/1000 ft2
   deciduous shrubs 1 1/2 ¼ cup (2 oz) 10-6-4/yd2
   evergreens 3/4 1/8 cup (1 oz) 10-6-4/yd2
Groundcovers 1 2 lb 5-10-10/100 ft2
   early spring 1 3/8 cup (3 oz) 5-10-10/ yd2
   June 3/4 ¼ cup (2 oz) 5-10-10/ yd2
   before planting 1 1/2 ½ cup (4 oz) 5-10-10/ yd2
   after first flowering 1 T 5-10-10/per plant
   before planting 1 1 lb 10-10-10/ 100 ft2
   at transplanting ¼ cup (4 oz) 10-10-10 plant
   sidedressing 1 ¼ cup (4 oz) 10-10-10 plant
   before planting 1/2 5 lb 12-4-8/1000 ft2
   March – April 1/2 5 lb 12-4-8/1000 ft2
   September - October 1-2 10-20 lb 12-4-8/1000 ft2


Credits: EPA, DNREC, New Castle-Kent-Sussex Conservation Districts, Delaware Nature

Society, Delaware Cooperative Extension, University of Delaware.

Write to the New Castle County Cooperative Extension office, University of Delaware, 910 S. Chapel St. Newark, DE 19717-1303 for a copy of any publication you need.


Original Publication Date:

Cooperative Extension Education in Agriculture and Home Economics, University of Delaware, Delaware State University and the United States Department of Agriculture cooperating. Distributed in furtherance of Acts of Congress of May 8 and June 30, 1914. It is the policy of the Delaware Cooperative Extension System that no person shall be subjected to discrimination on the grounds of race, color, sex, disability, age, or national origin.

Disclaimer: Reference to commercial products or trade names does not imply endorsement by University of Delaware Cooperative Extension or bias against those not mentioned.

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