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
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.
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
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
- 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
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
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
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
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
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
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
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
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
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
What are some examples of natural
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
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
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
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
“Fertilizer vs. Plant Food: What’s the Difference?” was first posted here