University of Maryland Extension

Fertilizer

Why Fertilize?     
Fertilizer Basics
Plant or Soil Nutrients
Fertilizing Tips
Ways to Apply Fertilizer
Types of Fertilizers
Synthetic (Chemical) Fertilizers -What to do if you can't find the fertilizer from your soil test report
Organic Fertilizers - Converting a synthetic fertilizer recommendation to an organic fertilizer one
Protect the Environment: Fertilize Wisely
Fertilizing Plant Groups  

Key Points

  • The three numbers on fertilizer products (e.g., 3-4-3) represent the percentage, by weight, of N (nitrogen), P (phosphorus) expressed as P2O(phosphate), and K (potassium) expressed as K2O(potash). For example, a 5 lb. bag of 10-5-5 fertilizer contains 0.5 lbs. of N, 0.25 lb. of P2O5, and 0.25 lb. of K2O.
  • Plants should be fertilized based on soil test results, UME recommendations, and plant needs. Most trees, shrubs, and perennials grow well without supplemental fertilizer
  • Fertilizers won't help plants injured by insect pests and diseases, or site, and weather problems unless soil nutrients are deficient
  • The amount of fertilizer needed by plants decreases as the organic matter content of the soil increases
  • Overuse of fertilizers can lead to weak, succulent growth, encourage insect pests and disease problems, and contribute to water pollution.
  • Read and follow fertilizer labels. When appropriate, substitute slow-release fertilizers for those that are highly soluble and substitute locally available organic fertilizers and soil amendments (e.g., backyard or municipal compost) for synthetic fertilizers.
  • Most garden and landscape plants grow best in a soil pH range of 6.0-7.0. Many nutrients become either unavailable or overly-abundant outside this range. Pay close attention to your soil pH level and be prepared to adjust them according to soil testing lab results and recommendations.
  • UME's residential lawn fertilization guidelines were developed based on research and in response to the Maryland Lawn Fertilizer Law. Out-of-state soil test labs may provide fertilizer recommendations that conflict with Maryland guidelines and laws. Maryland residents should strictly follow UME guidelines and the Maryland law when fertilizing lawns.

Why Fertilize?

  • Plants growing in the wild get their nutrients from air, water, soil minerals, and soil organic matter, the plants and fallen leaves, microbes, and animals that die and decompose. Wild plants don't need fertilizer or our help to grow and reproduce!
  • Our gardens and landscapes are unlike wild areas in some key respects:
    • Soils around homes have been disturbed and topsoil removed making the soils less fertile and plant-friendly. Also, organic matter is less abundant (leaves and other yard waste are often sent to the landfill) so there is less recycling of nutrients that could benefit garden and landscape plants.
    • Many native and non-native trees and shrubs thrive in home landscapes without fertilizers, while other plants such as vegetables, fruits, and flowers won't reach their full potential without additions of organic matter and/or fertilizer.
  • Good plant health depends on a continuous supply of available nutrients from the soil or, in the case of container plants, the growing media.
  • Plants get their nutrients from the minerals and organic matter in the soil or from added fertilizers and compost. Nutrient needs vary from plant to plant and the ability of the soil to supply those nutrients varies by site, season, and weather condition.

Fertilizer Basics

  • Fertilizers contain nutrients, they are not “plant foods.”  Nutrients are directly absorbed by plant roots, and in some cases by foliage. Plants use nutrients to make carbohydrates, proteins, defense compounds, and other compounds.
  • Fertilizers are regulated materials that contain at least one plant nutrient. The nutrient content is guaranteed by the three numbers (e.g., 3-4-3) found on a fertilizer bag or container. Also known as the nutrient analysis, the numbers represent the percentage, by weight, of N (nitrogen), P (phosphorus) expressed as P2O5 (phosphate), and K (potassium) expressed as K2O (potash). For example, a 5 lb. bag of 10-5-5 fertilizer contains 0.5 lbs. of N, 0.25 lb. of P2O5 , and 0.25 lb. of K2O.
  • The P and K fertilizer recommendations made by soil testing labs are usually given as pounds of phosphate and potash. If needed you can convert the oxide form to the elemental form
                P x 2.3 = P2O5                            K x 1.2 = K2O
                P2O5 x 0.44 = P                          K2O x 0.83 = K
  • Fillers that prevent caking, reduce dust, and allow for smooth pouring make up the remaining weight of fertilizer products.

  • Soil amendments are materials applied to or mixed into, topsoil to change or improve soil properties in an effort to improve plant growth. For example, compost, shredded leaves, and manure improve soil structure, water-holding capacity, and biological activity. Lime is a soil amendment that increases soil pH. Although they may also supply a wide range of plant nutrients, their nutrient content is variable. However, some commercial composts and manures are carefully managed, processed, tested, and labeled as fertilizers. Organic Matter and Soil Amendments provides detailed information. 

Plant or Soil Nutrients

  • There are Seventeen (17) elements considered essential for plant growth. Three of these are non-mineral elements acquired from the air and water: carbon (C), oxygen (O), and hydrogen (H).
  • The other fourteen (14) are considered mineral nutrients. Six are termed macronutrients because relatively large amounts are needed by plants. They are further divided into primary macronutrients (nitrogen, phosphorus, potassium) and secondary macronutrients (calcium, magnesium, and sulfur). 
  • The trace elements, required in very small amounts, are boron, copper, chlorine, iron, manganese, molybdenum, nickel, and zinc.
  • Nitrogen (N) is required in the largest quantity after carbon, oxygen, and hydrogen. Unlike other mineral nutrients it is mostly derived from organic matter, lightning (nitrates come down in rainfall during electrical storms), purchased fertilizers, and legumes, such as clovers (special bacteria in their roots converts atmospheric nitrogen to usable forms). 
    • Nitrogen is available to plants in two forms- ammonium and nitrate. Nitrate is very soluble and prone to run-off and leaching. N levels can change rapidly depending on environmental and biological factors; that’s why soil testing labs don’t include it as part of their basic test. Soil testing labs almost always recommend N fertilizer because there is often not enough N at any given time to meet plant needs. Nitrogen gives plants their green color and is an essential element for protein synthesis. N deficiency causes older lower leaves to become pale and yellow.
  • Phosphorous (P) is highly insoluble; chemically bound to calcium, aluminum, and other elements also very soil pH sensitive. P levels can build up in soils where it can wash into and pollute waterways, especially from sloped land. P is important for cell division and root, flower, and fruit growth. Phosphate fertilizers originate from rock phosphate mines that are a dwindling world resource. Phosphorus is also supplied by organic matter.
  • Potassium (K) is used in quantities as large as nitrogen. It is mobile in plants, unlike N, P, and calcium (Ca).  Coastal Plain soils tend to be lower in K than soils in Central or Western Maryland. Does not move as much in soil as N but more than P.  K is involved in plant metabolism, regulates water status, and aids in carbohydrate transport and cellulose production. Potassium fertilizers originate from ancient salt deposits. Potassium is also supplied with organic matter.
  • Magnesium (Mg) and calcium (Ca) are abundant in most Maryland soils. Mg is more soluble and may be deficient in Coastal Plain soils.
  • Sulfur (S) is highly mobile in soil and leaches out easily. Sulfur levels can change considerably over short time periods making it a difficult element for soil testing labs to accurately measure. Reduced soil deposition of sulfur from coal-fired power plants may result in the need to add sulfur to deficient soils.
  • Boron, zinc, and manganese are trace elements that can sometimes be deficient in Maryland, especially in Coastal Plain soils.

Fertilizing Tips

  • Lawns, gardens, and landscape plants should be fertilized based on soil test results and UME recommendations. This will vary by the type of plant, type of soil, time of the year, and other factors.
  • Get baseline information on the levels of major nutrients, organic matter, and soil pH by submitting a soil sample to a soil testing lab for analysis and recommendations. Do this every 3-4 years depending on circumstances.
  • If soil test results are not available, follow fertilizer label instructions.
  • The nutrients in synthetic and organic fertilizers come in the form of salts. If a large amount of any fertilizer remains in direct contact with plant roots, or other plant parts, you may see some injury symptoms. This "burning" is known as phytotoxicity.
  • Don't fertilize drought-stressed plants.
  • Water-in fertilizers applied to turf, garden, and landscape plants to make the nutrients soluble and available to plants and prevent salt burn of roots.
  • Late summer and early fall fertilization can interfere with plant winterization and encourage succulent late growth that is easily killed in winter.
  • Fertilizers (synthetic or organic) won't reverse plant problems caused by insect pests, diseases, weather, or poor site conditions.
  • Mix fertilizers into the top few inches of soil when possible.
  • High phosphorous "starter" fertilizers (the middle number is highest) are widely available but research studies on container nursery plants do not support their use. Plants need 4X more nitrogen than phosphorus. If a soil test indicates that your phosphorus level is low, the lab will recommend a phosphate fertilizer.
  • As the soil organic matter level increases the amount of fertilizer required decreases.

Ways to Apply Fertilizer   

  • Broadcasting- spreading fertilizer (usually sprinkling by hand) over an area where plant roots will be growing, either before or after planting.
  • Banding- applying fertilizer in a narrow band next to a line or furrow where seeds or seedlings will be planted.
  • Side-dressing- applying fertilizer around individual plants or along the sides of plants in a row, after plants have become established. Fruiting vegetable plants, like tomato, pepper, and eggplant, are often side-dressed when first fruits start to form.
  • Foliar- soluble fertilizers mixed with water and sprayed on foliage 

Types of Fertilizer

Complete fertilizers contain all three of the primary macronutrients. Some fertilizers contain only one or two of the three major nutrients, such as nitrate of soda (16-0-0), a good choice when your soil test indicates high levels of P and K. Cottonseed meal (6-2-1) is a complete organic fertilizer and 10-10-10 is a complete synthetic fertilizer.

 sample fertilizer label
Example of a complete organic fertilizer made from poultry manure with a guaranteed 5-4-3 analysis. A 10 lb. bag contains 0.50 lbs. nitrogen.

  • Additional nutrients contained in the fertilizer will also be listed in the nutrient guarantee. For example, a bag of potassium-magnesium sulfate contains sulfur (S) and magnesium (Mg) in addition to potassium. Its guarantee is 0-0-22-22S-11Mg.
  • Granular or dry fertilizers are the most common types of fertilizer. They are easy to apply by hand or with a spreader.

 granular or dry fertilizer examples
Dry fertilizers can be applied by hand or shaken from a small container

  • Quick-release" or highly soluble fertilizers come in liquid or powder form, are mixed with water, and applied at the base of plants or sprayed directly on foliage. They are frequently used in container gardening and on newly transplanted annuals and perennials to promote more rapid root growth. This may be especially helpful in early spring when soil temperatures are cool. Foliar fertilizers are applied directly to the upper and lower leaf surfaces. Plants take up nutrients more efficiently through leaves than through roots. But this method is less practical, requiring frequent applications, and more likely to cause leaf burn.
  • Slow-release fertilizers make nutrients available in small amounts over an extended period and are less likely to leach out of the soil. They may have a clay or polymer coating like sulfur-coated urea. Many turf, tree, and shrub fertilizers provide part or all of their nitrogen in a slow-release form.
  • Fertilizer spikes or tablets are inserted in soil and in the root zone soil area and often comes in a slow-release formulation. 

Synthetic (Chemical) Fertilizers

  • Synthetic fertilizers are typically referred to as "inorganic" or "chemical" fertilizers. Superphosphate, produced by treating rock phosphate (a mineral) with sulfuric acid, is an example of a synthetic fertilizer. The production of synthetic nitrogen fertilizers requires large amounts of natural gas, releasing carbon dioxide into the atmosphere, and contributing to climate change.
  • Synthetic fertilizers are typically less expensive (per pound of nutrient) and usually contain relatively high concentrations of nutrients in a readily available (soluble) form for plant uptake, although some, like sulfur-coated urea, are designed to release nutrients slowly. Relatively small quantities are needed and the exact amount of nutrients applied is known and adjustable.
  • Synthetic fertilizers do not burn plants if applied according to label instructions.

Examples of Synthetic Fertilizers and Their Nutrient Content (Analysis)

  • Ammonium sulfate-20-0-0-24S (sulfur) -  

Ammonium sulfate is a dry fertilizer which is 21% N, plus sulfur. Very acidic, especially suitable for blueberries, which require the ammonium form of nitrogen. Mix into soil to prevent loss of nitrogen into the atmosphere. 

  • Calcium nitrate - 15-0-0-22 Ca (calcium)

  • Muriate of potash - 0-0-60

  • Potassium nitrate - 13-0-44

  • Sulfur-coated urea - 46-0-0 (slow-release)

  • Superphosphate - 0-18-0; triple superphosphate - 0-45-0

  • Urea-46-0-0 

The nitrogen in urea releases rapidly with a high “burn potential”. Handle and use with care. Must mix into soil to prevent conversion to ammonia and subsequent escape into the air. Sulfur-coated urea is a slow-release formulation.

What to do if you can't find the fertilizer recommended in my soil test report?

Organic Fertilizers

  • Commercial organic fertilizers are relatively low and variable in nutrient content, and typically release nutrients more slowly than synthetic fertilizers, although this varies. For example, compost releases 5-10% of its nutrients during the season it's applied, compared to 85% for fish emulsion. Organic fertilizers are more expensive to buy on a per pound of nutrient basis than synthetic fertilizers.
  • Many are made from composted or processed animal and plant waste products, such as fish fertilizers, bloodmeal, bonemeal, composted manure, and alfalfa meal. A number of products are blends of several organic ingredients and some products contain a combination of organic and synthetic fertilizers. A few organic fertilizers are inorganic materials (lack a carbon backbone), such as rock phosphate, greensand, and sodium nitrate.
  • Vegetarian or vegan fertilizers are plant-based, like alfalfa meal, cottonseed meal, and soybean meal (7-2-1). However, research studies have shown that soybean meal fertilizer applied and incorporated at planting can inhibit the germination of small seeds like kale and lettuce.
  • Some organic fertilizers sold to home gardeners carry OMRI (Organic Materials Review Institute) certification. Farmers participating in the USDA's National Organic Program can only use OMRI-certified fertilizers. Gardeners are free to use organic fertilizers that are not OMRI-certified.
  • There can be variations of the nutrient content or nutrient analysis of organic fertilizers.  

Some Examples of Organic Fertilizers

  • Alfalfa meal - 5-1-1 (fast release) - 

May contain ethoxyquin, a preservative, to keep it green.

  • Blood Meal - 15-1-0 (medium-fast release) - 

Nitrogen availability lasts about 2 months. May help repel deer and rabbits when top-dressed around plants.

Is a relatively high N organic fertilizer also labeled as a preemergent for annual weeds. It is not recommended for use on Maryland lawns as an organic preemergent herbicide because the recommended rate for weed control would exceed the amount of nitrogen allowed by Maryland's Lawn Fertilizer Law.

  • Cottonseed Meal - 6-2-1 (medium release)

  • Dried Poultry Litter - 4-3-3 (medium-fast release)

  • Fish Products - (fast release) 

Formulations range from fish powder (9- 1-1) to fishmeal emulsion (5-1-1). May have a strong fishy smell. The highest quality products are enzymatically digested.

  • Greensand - (slow release) 

A naturally occurring iron-potassium silicate (also called glauconite). Potassium (5-7 %) released very slowly over 4 to 5 years. 

  • Kelp and Seaweed Products - 

Most are relatively low in macronutrients but high in micronutrients and plant growth hormones.  Available in meal, powder, and liquid forms and often used for seedlings and transplants. The highest quality products are enzymatically digested.

  • Nitrate of Soda - 16-0-0 (medium-fast release)

  • Rock Products (slow to fast release)

(Not considered organic if treated with a chemical to increase nutrient solubility.)
Soft rock or colloidal phosphate— phosphate clay with 18-22% phosphate; 2% phosphate immediately available, the rest slow-release over 3-5 years. Has half the liming value of ground lime.

 How to convert a synthetic (chemical) fertilizer recommendation for an organic fertilizer.

Additional Resources

Protect the Environment: Fertilize Wisely

Nutrients in fertilizers can contribute to surface water and groundwater pollution. Excess nitrate and phosphate leach through soils and move off-site in stormwater and eroded soil. These nutrients may eventually be transported to large estuaries such as the Chesapeake Bay and the Gulf of Mexico where they create large algal blooms that block sunlight needed by aquatic plants. Oxygen for water plants and organisms is depleted when the algae die and microorganisms use the available oxygen to consume the dead algae, leading to dead zones and fish kills. This process is called eutrophication.

Maryland farmers, nursery and greenhouse growers, lawn maintenance companies, and residents must follow state regulations regarding fertilizer applications to reduce the risks of nutrient pollution. Tips for fertilizing wisely:

  • Overuse of fertilizers can lead to weak, succulent growth, encourage insect pests and disease problems, and contribute to water pollution.
  • Landscape and garden plants very often get adequate nutrients from minerals in soil and from lawn clippings, tree leaves, and compost. (You can reduce your lawn fertilization by 25% by leaving grass clippings on the lawn.)
  • Read and follow fertilizer labels. When appropriate, substitute slow-release fertilizers for those that are highly soluble and substitute locally available organic fertilizers and soil amendments (e.g., backyard or municipal compost) for manufactured chemical fertilizers.
  • Maryland's Lawn Fertilizer Law prohibits using fertilizers to deice sidewalks and driveways. They are very corrosive to concrete and metal, can burn plants, and contribute to waterway pollution. Select alternative materials.
  • Sweep granular fertilizers from paved surfaces to prevent them from washing into storm drains and waterways.
  • UME's residential turf fertilization guidelines were developed based on research and in response to the 2011 Maryland Fertilizer Use Act. Soil test labs may provide fertilizer recommendations that conflict with Maryland guidelines and laws. Maryland residents should strictly follow UME guidelines and the Maryland law when fertilizing lawns.

 Fertilizing Plants According to Groups 

 

 Author: Jon Traunfeld, Director HGIC, Extension Specialist, Fruits and Vegetables.
10/2020

 

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