Fertilizer vs. Plant Food: What’s the Difference?

by Matt Gibson and Erin Marissa Russell

The words fertilizer and plant food are often used interchangeably, but they are not technically the same thing. Gardeners use fertilizers to help enrich their soil, supplying it with the essential nutrients that plants need to grow and bloom correctly. Aside from hydrogen, oxygen, and carbon, plants require 13 other nutrients that they typically get from the soil. The most important of these nutrients, or the, “big three,” are nitrogen, phosphorous, and potassium, often referred to on fertilizer labels as N-P-K, for their periodic table abbreviations.

These three macronutrients are essential to the growth and health of all plants. Fertilizers namely contain these three nutrients, as well as other nutrients which help plants thrive, plus a few fillers. Gardeners add fertilizers to enrich the soil when it has become depleted.  Fertilizers work to enrich the soil, while plants use the nutrients found in the soil, and in the environment, to create their own food.

To put it simply, fertilizers and products that are labeled, “plant food,” are really just soil additives that contain lots of nutrients. If the soil in your garden beds contain the proper nutrients needed for healthy plant growth, then your soil is providing your plants with everything they need to make their own food.

What is Fertilizer?

Plant fertilizers are a combination of macronutrients, micronutrients, and fillers, or ballast. Some types of fertilizers are comprised of equal amounts of the, “big three,” macronutrients nitrogen, phosphorous, and potassium. These are commonly labeled as N-P-K 10-10-10 or 20-20-20. The numbers represent the percentage of each nutrient in the fertilizer. The first number is for nitrogen, the second for phosphorous, and the third represents potassium.

Some fertilizers contain a larger amount of one of the nutrients than the other two. Nitrogen is used to promote foliage growth, so a fertilizer that is made specifically for foliage plants may be composed of 20-5-5, for example. Certain plants require more of one nutrient than the others, so there are fertilizers with all different ratios of N-P-K in order to meet specific needs.

Fertilizers are also made up of micronutrients like calcium and iron. Organic fertilizers often contain the micronutrients boron, copper, iron, chlorine, manganese, molybdenum, calcium, and zinc.  A balanced fertilizer, for example, one that is labeled 10-10-10 is comprised of 10 percent of each macronutrient for a total of 30 percent macronutrient content. Another 10 to 20 percent of the fertilizer is made up of various micronutrients, while the remaining 50 percent is filler. The majority of all fertilizers are made up of micro and macro nutrients, but the bulk of the content within most fertilizers are fillers. Fillers are not just there to take up space, but are actually designed to help distribute the valuable nutrients and increase their absorption capability.

What is Plant Food?

Fertilizers are made for the sole purpose of revitalizing soil in order to provide plants with nutrients, but it is up to the plants themselves to concoct their own meals. Plants make their food with the nutrients that they absorb from the soil in combination with a special blend of air, water, and sunlight. The air provides the plant with carbon dioxide which enters through its leaves.

As the carbon dioxide comes in through the plant’s foliage, it meets chlorophyll, which absorbs and stores the sun’s energy, resulting in chloroplasts. The chloroplasts inside chlorophyll combine with the carbon dioxide to create a simple sugar. This sugar spreads out with the help of absorbed water traveling through the entire plant.

Water moves up through the roots and into the plant, taking the sugar with it, as well as minerals and nutrients taken from the soil that are vital for the process of photosynthesis to function as needed. The presence of water is also essential to maintaining the turgidity of the plant’s cells. If the plant is not getting sufficient water, the cells will not be as turgid, resulting in wilt.

Fertilizers are added to the soil in order to help provide the elements needed for plants to create their own food. Plant food is made from nutrients in the soil as well as other essential elements, like air, water, and sunlight. When fertilizers contain high levels of nitrogen, phosphorus, and potassium, but lack the other needed micronutrients, plants will receive inadequate nutrition. Plants need all 13 nutrients to grow well and provide balanced nutrition.

Synthetic Versus All-Natural Fertilizer

Different fertilizers get their nutrients from either organic or chemical forms. Organic fertilizers are made from manure, compost, or fish meal. Chemical nutrients are purer in form but can be rather costly. Chemical fertilizers are usually water-soluble and are often added to the plants directly during irrigation by diluting them into water and then using that water to irrigate your plants so that the nutrients can be absorbed by the plants immediately. Organic nutrients take a bit longer to break down into the soil, but if you are a home gardener with your own compost pile, using organic fertilizer can save you a lot of money.

Chemical fertilizer, or synthetic fertilizer is made from liquid ammonia. Liquid ammonia is quite cheap to produce, and its impact on American agriculture has been massive. Between the years 1950 and 1975, the output of production from American farms has increased by over 50 percent while farm labor hours decreased by an astounding 60 percent.

The increased use of chemical pesticides and fertilizers, as well as genetic improvement and mechanized labor, all combined to create this change which has revolutionized agriculture in America. Unfortunately, the boost in output from American farms comes at a steep cost. Atmospheric nitrogen overload from synthetic fertilizers has been credited by environmental scientists, as the primary cause of global pollution, according to a report by the World Resources Institute.

Natural fertilizer, or fertilizer made from all-natural sources, is a much more environmentally friendly way to provide our garden plants with the nutrients they need. Cottonseed meal, feather meal, seaweed, fish waste, bone and blood meal, and poultry manure are all common ingredients in organic fertilizer. All-natural fertilizers require the presence of soil microorganisms in order to be effective. According to the Colorado State University Extension website, natural fertilizers require soil microorganisms, which are dependent on sufficient moisture and temperatures above 50 degrees Fahrenheit. 

Compost is an all-natural alternative to fertilizer that you can make at home in a compost bin or a simple pile. Compost contains all of the 13 needed nutrients that are vital to plant growth, as well as oxygen and water. Compost can be started in your own backyard by gathering yard trimmings, grass clippings, kitchen waste, shredded newspaper, and dried leaves. Layer these ingredients in a compost bin with layers of soil, water it regularly to keep the contents moist and leave it to decay over time, turning the pile over occasionally to help speed up the decomposition process. Compost will become mature and ready to use as plant food in 30 days to three months time. 

Soil pH

Another important factor when it comes to the ability of plants to absorb fertilizers and create their own food is the pH level of the soil they are grown in. Soils with excessively high pH (7 or higher) or low pH (5.5 and below) are not welcoming to nutrients. In soils like these, the nutrients present in fertilizers are either too soluble or not soluble at all, and the plants can’t absorb them, or they become toxic to the plants. Soil with too high or low pH levels can be amended with lime or elemental sulfur to lower or raise the pH.

Testing and Dilution

Nutrient intake of garden plants is a finely-tuned system. Too much or too little of any one nutrient can upset the system. A soil test will help you to determine the content of your soil and the type and amount of fertilizer your soil needs. If either inorganic or organic fertilizers are applied too heavily to the soil, they can cause plant tissue to burn or become chlorotic, resulting in unhealthy or dead plants. Without a soil test, it can be very difficult to figure out what fertilizer is best and what amount of fertilizer is needed to balance out your garden soil. Once you have determined what your soil needs, read product labels carefully and follow directions to avoid toxicity problems. Many gardeners recommend diluting fertilizers to half strength before adding them to the soil to avoid overfertilization issues.

Common Questions and Answers About Fertilizer Versus Plant Food

Can fertilizer hurt plants?

Too much fertilizer can hurt plants as a result of them getting too much of the nutrients they need to survive. Over fertilization is harmful to plants because it causes them to grow faster than their roots can develop to support the new foliage. Too much fertilizer is also harmful to microorganisms in the soil and deposits excessive amounts of salt in the soil. Over fertilization also leads to illnesses such as iron chlorosis and root rot, as well as leaving plants more susceptible to illness and infestation in general. You can flush extra fertilizer out of your garden’s soil by giving your plants lots of fresh water to wash the fertilizer away.

Can fertilizer kill plants?

Fertilizers contain salts that, in excessive amounts, can be harmful to plants and even kill them. Plants that get too much fertilizer can also grow faster than their root systems can support their growth. Over fertilization also leaves plants susceptible to infestation by garden pests and infection by plant diseases, especially root rot and iron chlorosis. If your plants are experiencing harmful effects as a result of over fertilization, give them plenty of fresh clean water to flush out the buildup of salts and excess nutrients in the soil.

Can I make my own liquid fertilizer?

You can make your own liquid fertilizer out of seaweed, vegetable scraps, manure, or garden weeds by allowing materials that are high in nitrogen to soak in water. The amount of time your homemade liquid fertilizer will need to soak ranges from just one night to several weeks, depending on the material you’re using. Supplies you’ll need include a garden hose, bucket or other large container, and kitchen as well as a blender for some recipes. Your homemade liquid fertilizer should be prepared outdoors, as the mixtures can produce offensive smells.

• Vegetable scrap fertilizer: Save the scraps and ends of vegetables you’d otherwise throw away in your freezer until you’ve collected a few quarts to use in homemade liquid fertilizer. Thaw the scraps and puree them in the blender with water until they’re a smooth liquid. Empty the blender into your bucket along with half a teaspoon Epsom salt and a capful of ammonia for each blender load you add. Continue until you’ve blended all of the vegetable scraps you saved. Stir the mixture in your bucket and allow it to soak overnight. This mixture is a liquid fertilizer concentrate. To make it ready to use, mix one quart of puree with a gallon of warm water in a spray bottle, and shake well. Apply this fertilizer to the base of plants.

• Weed and grass clipping fertilizer: Save the weeds you pull from your garden or use clipped grass from mowing the lawn. In a five-gallon bucket, add a few handfuls of grass clippings or pulled weeds, then fill the bucket with water. Allow the mixture to steep outdoors for four weeks. When it’s ready, apply your homemade liquid fertilizer to the base of your plants.

• Liquid manure fertilizer: Add a shovel full of the manure of your choice to a five-gallon bucket, and combine with water until the bucket is full. Allow this mixture to steep for four weeks, then apply to the soil at the base of your plants.

• Compost tea: Mix a shovel full of finished compost with water in a five-gallon bucket. Let the mixture steep for four weeks, then it is ready to be applied to the base of plants.

• Seaweed fertilizer: Add a few handfuls of seaweed to a five-gallon bucket filled with water. Let this combination steep for four weeks. When ready, apply to the base of your plants.

Do I need plant food?

If you are planting in a new garden bed in soil that has not been used before and is fertile and rich, you will not need to use plant food for the first season. You will also not need to use plant food at first when planting in commercial potting soil. However, after plants have been growing for a while in either new fertile soil or in commercial potting soil, they will take in the nutrients the soil contains, and plant food will become necessary to replace those nutrients.

How do I know if my plants need fertilizer?

Plants will show signs of malnutrition when fertilizer is needed. These signs include pale green or yellow foliage when nitrogen levels are low, chlorosis (dark green veins on pale green leaves) when potassium is low, and dull, dark green foliage with purple leaves at the base of the plant or reduced flowering when phosphorus is low. Blossom-end rot can indicate a deficiency of calcium. Ensure that foliage discoloration is not due to overwatering (for yellow leaves) or underwatering (if foliage looks dead or crisp) before applying fertilizer.

How do you fertilize a garden plant?

Fertilize garden soil in the spring before planting annual flowers and vegetables, while perennials are just beginning their growth for the season. Incorporate a general-purpose fertilizer into the soil at a depth of six inches where annuals and vegetables are growing. Where perennials are growing, work the fertilizer gently into the soil around the plants. Apply fertilizer again when plants are growing the quickest. This period is early in the spring for lettuce and other salad greens and the middle of summer for corn, tomatoes, potatoes, or squash. When growing long-season crops, use a small amount of fertilizer when you set seed, then apply more at the beginning of summer just before the plants are growing their quickest. When growing blueberries, apply fertilizer early in the season when buds are breaking. Fertilize strawberries after the first harvest. For ornamental trees, shrubs, or perennial plants, apply fertilizer when plants come out of dormancy at the beginning of their growing season.

Dry or granular fertilizers can be spread over a large area using a spreader or by hand, or they can be applied along the rows of your plants and seeds as a side dressing. Work dry fertilizer into the top four to six inches of soil using a hoe or spade, then water the fertilizer in to help it soak into the soil. Subsequent applications later in the season can be made just to the top inch of soil in garden beds or where plants grow in rows or at the drip line around trees and shrubs.

Liquid fertilizers are used by combining the fertilizer with the water you normally give your plants. Water-soluble fertilizers should be applied to the base of plants. Apply liquid fertilizer two to three weeks after planting. Before applying liquid fertilizer, water plants well with untreated water so that the roots will not be burned with fertilizer. Ensure that liquid fertilizers are diluted according to their package directions, as a too-strong mixture can also burn plants.

How long does fertilizer last in soil?

Different types of fertilizer take varying periods of time to break down in soil, making them appropriate for different uses. The nutrients in liquid fertilizers are available for plants to use immediately after application, and the fertilizer remains available in soil for only a short period of one or two weeks. Dry or granular fertilizer blends remain active in soil for six to eight weeks, after which period they should be reapplied.

How long does it take for granular fertilizer to work?

Quick-release fertilizers begin working within a few days, but their effects only last a short period of time before they must be reapplied. Plants begin taking in nutrients from quick-release fertilizer within 15 to 24 hours. With slow-release fertilizers, plants don’t begin to see effects for three to 10 weeks. However, the slower release time means these fertilizers are available for longer, and their effects continue much longer than quick-release fertilizers, meaning they don’t need to be reapplied as often. Consult the packaging of the particular fertilizer you’re using in your garden to find out how often your fertilizer should be reapplied.

How much liquid fertilizer does a plant need?

Liquid fertilizer should be diluted in water as directed on the packaging, and then that water should be given to plants as usual for their hydration. Once the liquid fertilizer is diluted, it should be distributed to plants at the same dosage as untreated water is normally given.

How often should I apply slow release fertilizer?

Slow-release fertilizers should be applied to the garden every six to eight weeks, unless the instructions given on their packaging indicate otherwise.

How often should I fertilize my tomato plants?

Fertilize tomato plants once just after planting them in the garden. Give tomatoes a second dose of fertilizer once they begin to set fruit. After tomato plants begin to develop fruit, nourish them with a light fertilizer every one or two weeks until the plant is killed by frost.

How often should you fertilize flowers?

Different types of fertilizer have different timelines for application, so always follow the guidelines provided by your fertilizer’s manufacturer as indicated on the packaging. Liquid or water-soluble fertilizers are normally applied every one or two weeks, and are always given mixed in with the water a plant normally receives. Slow-release fertilizers last several months once they’ve been applied. A slow-release fertilizer should be given at the beginning  of the season just as plants start to grow. One dose is sufficient in northern areas, but in southern regions, a second dose may be needed when plants are growing at their fastest later in the season. Granular fertilizers should be used as a soil amendment mixed into the top four to six inches of soil just before planting.

Is granular fertilizer better than liquid?

Granular and liquid fertilizers have different benefits, so which is better will depend on what is important to each individual gardener and the specifics of their situation. Liquid fertilizers are better able to reach plants, as the nutrients in granular fertilizers stay located in the granule whereas liquid fertilizers deliver nutrients to plants through the movement of water underground. Granular fertilizers can contain more nutrients, so the danger of “burning” plants through over fertilization is more prevalent than with gentler liquid fertilizers. Liquid fertilizers have a uniform makeup that is the same throughout the mixture, while the nutritional makeup of granular fertilizers varies among granules. Liquid can be easier to apply than granular, though there may be some initial cost when transitioning to a liquid fertilizer if new equipment is needed. Granular fertilizer does not need to be applied as often as liquid and is cheaper when purchased in bulk.

Should I feed my plants every time I water?

Houseplants should be given water-soluble fertilizer once a week, while outdoor container plants should be fed with water-soluble fertilizer twice a week. Garden plants should get water-soluble fertilizer once every two or three weeks. Landscaping plants should receive water-soluble fertilizer once a month.

Should I water plants before fertilizing?

Before giving plants fertilizer, water them well so that the roots aren’t coming into contact with water containing the fertilizer when they’re dry. Applying fertilizer after watering your plants will help prevent damage from “burning” plants when they’re exposed to too much fertilizer.

What are some examples of natural fertilizers?

Natural fertilizers include manure, worm castings, peat, seaweed, and compost. These natural fertilizers can be used as a soil amendment, applied alone as a fertilizer, or can be included in homemade fertilizer mixtures.

What are the three fertilizer numbers?

The three numbers on fertilizer packages that are separated by hyphens provide the percentage of nitrogen, phosphorus, and potassium the fertilizer contains (in that order).

What fertilizer helps flowers bloom?

Fertilizers that are high in phosphorus aid in flower production. To increase blooming, look for fertilizers with a high second number, because the second number indicates the percentage of phosphorus the fertilizer contains.

What is a good fertilizer for orchids?

Gardeners should give orchids a balanced fertilizer such as a 20-20-20 blend that does not contain any urea weekly. Experts recommend giving a small amount of fertilizer at a time in an approach called fertilizing “weakly, weekly.” They suggest giving fertilizer at a quarter strength on a weekly basis.

What time of day should I fertilize my plants or flowers?

Plants should ideally be fertilized at the same time as you provide them with water, and when you water more than once per day, you should provide fertilizer in the morning. Plants are able to take in nutrients better before they’ve become stressed by the midday heat.

Which fertilizer makes plants grow faster?

To make plants grow faster, look for a high-nitrogen fertilizer. Fertilizers with a high first number will be high in nitrogen, because the first number indicates the percentage of nitrogen the fertilizer contains.

Want to learn about using fertilizer versus plant food?

Basics of Gardening covers Is Using Plant Food Really Necessary?

Penn State Extension covers Over-Fertilization of Potted Plants

Fine Gardening covers Fertilizing Basics

UBC Botanical Garden covers Fertilizer Vs Plant Food

National Gardening Association covers Plant Food Vs Fertilizer

Garden Guides covers Difference Between Plant Food and Fertilizer

SFGate Homeguides covers How Long Does it Take for Fertilizer to Decompose?

UCCE El Dorado County Master Gardener covers Perils of Over-Fertilizing Plants and Trees

My Plant Place covers Best Garden Plant Food and Fertilizer

NCAGR covers A Homeowner’s Guide to Fertilizer

NOLA covers Fertilizer is Not Plant Food