Fertilizers contain essential elements or nutrients required by plants. These nutrients are important in germination, growth, flowering, and fruiting. When the soil does not provide these nutrients in sufficient quantities, they can be supplied to plants with fertilizer.
Depending on the species, plants require 16 or 17 essential elements. Three of the most abundant elements that comprise about 94% of a plant’s dry weight are carbon, hydrogen, and oxygen, which are acquired from water and air. The remaining 14 elements are absorbed from the soil and are divided into three categories based on the amounts required by plants. The primary nutrients, nitrogen (N), phosphorus (P), and potassium (K), are needed in fairly large quantities. Nitrogen is the most important of the three because it is required in the largest amount and has the greatest impact on growth and development.
Secondary nutrients, calcium (Ca), magnesium (Mg), and sulfur (S), are required by the plant in lesser quantities but are no less essential for plant growth than the primary nutrients. Similarly, plants need micronutrients in very small or trace amounts, which does not diminish their importance in plant growth and development. Micronutrients include zinc (Zn), manganese (Mn), iron (Fe), boron (B), copper (Cu), molybdenum (Mo), chlorine (Cl), and nickel (Ni). A deficiency of any of these 16 or 17 nutrients can impact the appearance, health, flowering, and fruiting of plants.
Determine the Need for Fertilizer
Not all plants require fertilizer. Plants growing in favorable soil conditions, particularly soils that contain adequate levels of organic matter and where organic materials, such as mulch or fallen leaves, decompose and release their nutrients over time, may require little or no fertilizer. Other plants may require fertilizers to replenish the nutrients used in growth and reproduction. They include lawn grasses, bedding plants, vegetables, and fruiting shrubs and trees.
Vegetables growing in garden soil and in containers may require supplemental nutrient applications during the season, especially vegetables. For more information, see HGIC 1254, Recommendations for Liming and Fertilizing Vegetables, and HGIC 1251, Container Vegetable Gardening. Fruiting shrubs and trees, such as blueberries and pawpaws, may require fertilizer to replenish the nutrients used to produce the harvested fruits. In other cases, fertilizer may be applied with the intended purpose of encouraging leaf and shoot growth.
Alternatively, plants that are declining in health due to improper siting or planting, and plants injured by drought or pests, will not benefit from fertilizer applications. In urban landscapes, nutrients may be unavailable or deficient due to disturbed soils that have an inadequate nutrient-holding capacity, soils that are mixed with building debris, or where there is no recycling of fallen leaves to provide organic matter to the soil.
Before you choose a fertilizer, decide if fertilizer will benefit your plants. Make an informed decision by having your soil tested. Refer to HGIC 1652, Soil Testing, for information. A soil test provides information about the soil pH and level of available nutrients. The pH is a measure of the acidity or alkalinity of soil; pH affects the availability of nutrients and the activity of beneficial microorganisms in the soil. Soil pH acts like a window that allows nutrients to pass through it when it’s wide open, which is when the soil pH is between 5.5 and 6.5 for most plants. When the pH is above or below this range, the window closes, affecting the nutrients that can move through the opening. Similarly, nutrients in the soil become unavailable to plant roots regardless of the amount of nutrients present in the soil due to an unfavorable soil pH. For more information, see HGIC 1650, Changing the pH of Your Soil.
When you’ve determined the need to fertilize your plants, use the recommendations in your soil test results to choose the fertilizer that supplies the nutrients required by your plants. With so many types and forms of fertilizers that range from granules to liquids, selecting the right one can be overwhelming. Become familiar with the many options that are available and follow the recommendations in your soil test results.
Fertilizers can be divided into two broad groups: (1) natural and (2) synthetic. Plants cannot recognize the difference between natural and synthetic fertilizers. When the nutrients are dissolved in water, the nutrients must be in a chemical form—inorganic ions—that can be absorbed by roots, regardless of their source.
Natural fertilizers are derived from organic, plant- or animal-derived (herbivores) sources, such as composted manures, seed meals such as cottonseed meal, and animal byproducts such as bone meal, blood meal, and feather meal. A natural fertilizer can also be derived from mined minerals, such as rock phosphate, greensand (glauconite), and gypsum. These minerals are not associated with synthetic chemicals.
Fertilizers derived from natural sources typically have proportionately less N, P, and K than synthetic fertilizers, but naturally occurring fertilizers improve soil structure and support the soil-dwelling organisms, notably bacteria and fungi, which support plant growth. When natural fertilizers are applied to the soil surface or mixed into the soil as an amendment, these natural products improve the physical structure of soil, enhance drainage in clay soils, and water-retention in sandy soils. Organic matter also serves as a reservoir for nutrients, which improves soil fertility. Compared to synthetic fertilizers, natural fertilizers, specifically composted animal manures, contain relatively low amounts of nutrients.
Synthetic fertilizers are products that have been mined, produced through an industrial process, or are the byproducts of an industrial process. These manufactured materials are designed to contain a known amount of nutrients and be easy to handle and apply. Most synthetic fertilizers do not contain carbon (the main element in organic compounds found mainly in living things; urea is an exception) and come from nonliving sources. Examples of synthetic fertilizers include ammonium sulfate, calcium nitrate, and isobutylidene diurea.
Fast- and Slow-Release Fertilizers
Fast-release fertilizers: Natural and synthetic fertilizers that contain nitrogen are classified into two types based on the time it takes for nitrogen to be released and made available to plants. Fast-release fertilizers contain water-soluble nitrogen in the nitrate, ammoniacal, and/or urea forms. The nitrogen in fast-release fertilizers rapidly dissolves in water after application, and the inorganic ions (ammonium—NH4+ and/or nitrate—NO3–) can be absorbed by plant roots.
Virtually all fertilizers are salts. When fertilizer dissolves in water in the soil, the salt concentration increases around the roots. Typically water moves through osmosis from a low salt concentration in the soil solution to a higher salt concentration in the root cells. Applying too much fertilizer, especially a fast-release fertilizer, may injure or kill plants resulting in “fertilizer burn.” The reverse happens where the high concentration of salts in the soil solution draws water from the low concentration in the roots. This water loss causes plants to dry out and exhibit symptoms of drought stress that include leaf-yellowing and leaf scorch, a symptom where the leaf margins or outer edges of the leaves turn brown and appear scorched or burned. Eventually, the leaves turn completely brown and fall, leading to dieback and reduced growth.
Excess fertilizer not absorbed by plant roots has environmental consequences. The nutrients will leach down and away from the root zone, especially on sandy soils. In clay soils, quickly available nutrients may flow across the soil surface and move offsite in runoff. In both soil types, nutrients such as nitrogen and phosphorus that are not taken up by plants can escape and contaminate ground and surface water. See HGIC 1229, Fertilizers and the Environment, for information regarding the appropriate use of fertilizers to protect our natural resources.
When fertilizing plants, apply it at the right time of year and in the right amount when it can be taken up by plants. Too little fertilizer does not satisfy the nutritional requirements of plants. Too much fertilizer may injure or kill plants. When fertilizers are not absorbed by plants, they can harm the environment.
Slow-release Fertilizers: Where it may take a fast-release fertilizer a few days to release nitrogen, it can take weeks or months for a slow-release fertilizer to release its nutrients. Natural fertilizers, such as animal byproducts and manures, rely on soil microbes to breakdown the organic matter to release nitrogen and other nutrients in chemical forms that can be absorbed by plant roots. Microbes are most active with warm soil temperatures, available moisture and oxygen in the soil, and a soil pH greater than 6 (pH of 7 is neutral). However, it is difficult to synchronize the release of nutrients from organic matter with the time when the plant needs it to make the best growth.
Synthetic fertilizers can be manufactured to make nitrogen slowly available to plants. To be considered a slow-release fertilizer, at least one-third of the total amount of nitrogen in the container must be in a water-insoluble or slowly available form. To prevent the nitrogen from readily dissolving in water, some synthetic fertilizers contain nitrogen coated in sulfur. This sulfur coating needs to be consumed by soil microorganisms before it can dissolve in water and become available to plant roots.
Controlled-release fertilizers are another type of slow-release fertilizer where the water-soluble nitrogen is coated or encapsulated in a resin or polymer coating that resists water movement into the granule. Temperature, moisture, and the weathering or wearing away of the coating affects the movement of water through the barrier. As water gradually penetrates the coating, the nitrogen dissolves in the soil solution, where it can be taken up by plant roots. The gradual release of nitrogen from slow- and controlled-release fertilizers lowers the potential of nitrogen losses to the environment.
Some fertilizers are marketed to be used on specific plants, such as azaleas, camellias, rhododendrons, hollies, roses, citrus, fruit trees, and heirloom tomatoes. Avoid the persuasion of having to buy specific fertilizers for the individual plants in your garden and landscape. Read the fertilizer label and compare the nutrients and their amounts in the specialty fertilizer with the recommendations in your soil test report. A specialty fertilizer may be more expensive than a generic one that’s not prescribed for any particular kind of plant, so calculate the cost of nutrients to make the right decision. An exception could be specialty fertilizers for palms, which include magnesium because this element can be deficient in coastal soils. If your soil test results indicate sufficient levels of magnesium, you won’t have to purchase this nutrient.
Fertilizers Mixed with Herbicides
Other specialty fertilizers are combined with herbicides, such as a pre-emergence herbicide that controls summer annual grassy and broadleaf weeds before they emerge or appear in the lawn. Although this fertilizer-herbicide combination offers convenience, the appropriate time for fertilizing the lawn may not coincide with the best time to apply this product to prevent the emergence of weed seeds. For example, a preemergent herbicide that controls crabgrass needs to be applied in early spring when the seeds are germinating and before they appear in the lawn. Unfortunately, this is not the best time to fertilize a warm-season grass such as zoysia- or bermudagrass, which is still dormant and will not absorb the fertilizer. Warm-season lawns are best fertilized at least two weeks after they’ve turned completely green and are actively growing.
Fertilizers can be purchased in a variety of shapes and sizes. The formulation is the type or form the fertilizer comes in. Some common forms include granules, pellets, tablets, liquids, and powders.
Granular fertilizers are scattered or broadcast over the area to be fertilized with a rotary or drop-type spreader. They can also be applied in narrow furrows several inches to the side and below the seeds or transplants called banding. Banding is important on soils that are deficient in phosphorus. Banding allows phosphorus, which readily “sticks” or adsorbs to soil particles, to be in close proximity to the roots. However, application of the fertilizer bands too close to seeds will burn seedling roots. Sidedressing is another method of applying granular fertilizers to growing plants where the fertilizer is scattered along one side of the row about 4 to 6 inches away from the plants and then lightly cultivated into the soil.
Fertilizers can be compressed and molded into pellets, tablets, and spikes. These concentrated forms of fertilizer are inserted into the ground or in containers. These formulations offer convenience, but the nutrients are confined to a small area and not widely distributed over the root zone areas of plants.
Liquid fertilizers can be purchased as a ready-to-use liquid, concentrated liquid that is diluted with water, or water-soluble dry powder or tablet that’s dissolved in water. Liquid fertilizers can be sprayed over the root zone area or injected 4 to 8 inches deep into the soil within proximity to absorbing roots.
Foliar fertilizers, liquid fertilizers labeled for application to the leaves, is a rapid way of supplying nitrogen (urea), iron, and zinc. The longer the nutrient solution is present in a fine liquid film on the leaf surface, the greater the chance of absorption through the cuticle or waxy leaf surface and into the underlying cells. Therefore, when applying a foliar fertilizer, do so on a cool, cloudy day or in the evening to improve its effectiveness.
Foliar fertilization is a temporary, short-term remedy for addressing nutrient deficiencies. Ideally, determine the underlying cause of the deficiency and absence of nutrient uptake by relying on soil test results and consulting with Clemson Extension Home & Garden Information Center staff.
Soil-applied, root absorption of nutrients is the best long-term approach for fertilizing plants, especially when supplying primary and secondary nutrients. Follow soil test results to determine the underlying cause of the deficiency and absence of uptake.
Determine the Cost of Nutrients
If you’re looking for fertilizer, as you shop, study the contents and the cost. To protect the environment and reduce losses to the environment, consider a fertilizer that contains 50% or more of slow-release nitrogen. Once you’ve settled on the nitrogen, examine for the secondary nutrients. If they are necessary, as determined by a soil test, buy it. Some fertilizers contain micronutrients. As before, base your purchase on soil test results. If micronutrients are necessary, you may end up having to purchase them separately to correct a deficiency.
If you’re considering two or three suitable fertilizers, but their prices vary, determine the cost of each fertilizer by calculating the actual cost per pound of primary nutrients. This will help you comparison shop.
- For each fertilizer, add the percentages of nitrogen, phosphorus, and potassium.
- Then multiply the sum of the percentages or total amount of nutrients by the net weight of the package.
- To determine the cost per pound of nutrients in the product, divide the cost of the fertilizer by the weight of nutrients contained in the bag. Use this information to make the right choice.
For example, a 16-4-10 fertilizer contains 16% N + 4% P2O5 + 10% K2O.
1. Add the percentages of N + P2O5 + K2O (16 + 4 + 10) for a total of 30% nutrients in the bag.
- The bag weighs 50 pounds (lbs.), so multiply 30% (0.30) by 50 pounds. This fertilizer bag contains 15 pounds of nutrients.
- If the bag of fertilizer costs $30, then divide its price by its weight ($30 ÷ 15 lbs.) to determine that the cost of nutrients in this 50 lb. bag of 16-4-10 fertilizer is $2.00 per lb.
Slow- or controlled-release fertilizers that contain a high percentage of water-insoluble nitrogen tend to be more expensive per unit of fertilizer than fast-release, water-soluble products. However, slow-release fertilizers are less prone to leaching and are well-suited on fast-draining sandy soils.
High analysis fertilizers that contain 30 percent or more of nutrients, such as 29-0-3, will cost more than a lower analysis fertilizer, such as 5-1-1, but the cost per pound of nutrient is less and it will cover more area. While natural fertilizers typically have a lower analysis than synthetic fertilizers, their cost of nutrients per pound will be less, but more of the natural fertilizer needs to be applied to equal the amount of nutrients provided by synthetic fertilizers. Natural fertilizers improve soil structure and support the subterranean ecosystem of microbes and other beneficial organisms.
- Crouse, D. A. 2018. Soils and plant nutrients, Ch. 1. In: K.A. Moore, and. L.K. Bradley (eds). North Carolina Extension Gardener Handbook. NC State Extension, Raleigh, NC. https://content.ces.ncsu.edu/extension-gardener-handbook/1-soils-and-plant-nutrients
- Gilmer C., A. Werth, D. Park. Increasing soil biodiversity in botanical gardens. Clemson (SC): Clemson Cooperative Extension, Land-Grant Press by Clemson Extension; 2020 May. LGP 1061. http://lgpress.clemson.edu/publication/increasing-soil-biodiversity-in-botanical-gardens/.
- Liu, G., E. H. Simonne, and Y. Li. 2020. Nickel nutrition in plants. HS1191. UF/IFAS Extension <https://edis.ifas.ufl.edu/hs1191>.
Originally published 03/23