by Matt Gibson & Erin Marissa Russell
Gardening and maintaining a healthy lawn can come with a long to-do list including mowing, weeding, watering, and fertilizing. Fertilizing is not always the answer. Many plants, including most perennials, as well as bushes, trees, and shrubs, don’t need fertilizer at all. There are situations, however, in which vital nutrients are lacking in the soil and are not available in sufficient levels which allow plants to grow and develop at their full potential. In situations like these, fertilizing your soil is essential.
In this article, we discuss the benefits of fertilizing and illustrate when it is necessary and when it is not. We outline the different types of fertilizer, explain the difference between organic and synthetic fertilizers, define N-P-K ratios (Nitrogen, Phosphorus, and Potassium), and reveal the purpose behind fertilizing the soil directly, as well as foliar feeding techniques. If you are looking to take a crash course on fertilizers, you have come to the right place.
Is Fertilizer Really Necessary?
Fertilizer is often called plant food, but it’s more like a supplement than a meal for plants. Plants create their own food through photosynthesis, harnessing the sun’s energy to create sugar from carbon dioxide and water, whereas fertilizers provide certain nutrients that are typically already present in most soils. Fertilizers provide naturally occurring elements like nitrogen, phosphorus, potassium, iron, calcium, and magnesium. It is only when your soil’s nutrient levels become depleted that fertilizing becomes necessary.
As plants grow and develop, they absorb nutrients from the soil, and a nutrient-rich soil provides a beneficial environment that helps plants thrive. In gardens and farms where plants are being cultivated purposefully, soils can become depleted of their nutrients from season after season of plant growth. In these situations, we might need to provide additional nutrients for some plants by adding fertilizer or other amendments to the soil, but this is not always the case.
Fertilizing when it isn’t needed can have a negative impact, and can actually lead to more work for gardeners. Adding more nitrogen to the soils of hedges, trees, and shrubs pushes the growth of stems and foliage. This means you will need to trim them back more often. Over-fertilizing your lawn will make the grass grow more vigorously, causing you to have to mow more frequently. Ugh, no thank you.
Sometimes new plants might require an application of a slow-release fertilizer in their second or third year. If your plants are struggling and diseases or pests are not the cause, it is possible that you may need to address a deficiency. Testing your soil will provide you with valuable information on whether or not fertilizer is necessary, and what kind of fertilizer you need.
Now that we’ve established that fertilizer is not plant food as most fertilizer companies would have you believe, but more accurately described as a supplement for plants, you may be asking yourself why you need to fertilize your plants at all.
In order to thrive, plants require 16 minerals that are essential to their growth and functionality. Faced with a shortage of any of these 16 elements, your plants will become less productive and can even get sick if the deficiency is especially pronounced.
At the very least, a small amount of each of the 16 mineral compounds are always present in soil. Sometimes, however, one or more essential elements may not be available in adequate amounts for a plant to grow and function at optimal levels. This is when fertilizers are needed.
How Fertilizers Work
Three of the 16 elements, carbon, hydrogen, and oxygen, are provided by water and carbon dioxide, and are always available to plants in abundance in nearly every environmental condition. The remaining 13 essential elements are absorbed through the roots of a plant from the soil. These 13 mineral elements are divided into three categories, and are grouped based on the amounts which are used by plants.
The trace elements, or micronutrients are only used in very small amounts and deficiencies of each of these are highly uncommon. They include: boron, zinc, manganese, chlorine, copper, iron, and molybdenum.
The secondary nutrients are used in much larger amounts. These include: calcium, sulfur, and magnesium. Acidic soils with low pH scores are quite often low in calcium. Lime is often used to raise the pH levels and provide calcium stores, and dolomitic lime is used instead when magnesium levels are insufficient.
Macronutrients, or primary elements are nitrogen, phosphorus, and potassium (the building blocks of most fertilizers), which are used by plants more than any of the other nutrients, and therefore, are most often in short supply. Though these three nutrients are not any more important than any of the other nutritional elements available in the soil, they are used in larger quantities by plants, and are therefore, the primary ingredients of most fertilizers.
The analysis, or the nutritional ratio on most fertilizer labels, is the percentage of nitrogen, phosphorus, and potassium, in that order, that is contained in the fertilizer. To correct nutritional deficiencies in the soil, gardeners use fertilizers to make sure that an adequate amount of each essential element is available for a plant to use to grow to its fullest potential.
Any fertilizer that you purchase will include information about the nutritional elements it is made of. The N-P-K ratio is the percentage of the fertilizer by volume of nitrogen (N), phosphorus (P), and potassium (K). A balanced 16-16-16 fertilizer, for instance, contains 16% of each of the three elements. A 25-4-2 fertilizer formula contains 25% nitrogen, 4% phosphorus, and 2% potassium.
All fertilizers contain at least one of these materials. If one is missing, the ratio will display a zero in its place. For example, a nitrogen only 12% solution is labeled 12-0-0. All fertilizers will display the N-P-K ratio somewhere on the label. If the fertilizers are sold in bulk, be sure to write down the ratio on the containers that you use to carry the fertilizer in.
Organic Vs. Synthetic
Synthetic and organic fertilizers provide plants nutrients in different ways. Synthetic fertilizers are made by chemically processing raw materials, whereas organic fertilizers are made from naturally occurring organic materials and mineral deposits, such as bone or plant meal or composted manure.
The nutrients in organic fertilizers are not water-soluble and will be released over the course of months or even years, so they should be applied in autumn so that the nutrients will be on hand during the spring. Organic fertilizers stimulate beneficial soil microorganisms while improving soil structure. These microbial organisms help convert fertilizers into soluble nutrients that can easily be absorbed by your garden’s plants. Organic fertilizers and compost, in most cases, combine to offer all of the micronutrients and secondary nutrients that your plants require.
Synthetic fertilizers can be absorbed and taken up by the plant almost immediately upon application, as they are water-soluble. Applying excess amounts of synthetic fertilizer can burn foliage and damage your plants.
Though synthetic fertilizers give your plants a quick boost of nutrients, they do very little to stimulate soil life, improve soil texture and structure, or improve the long-term fertility of your soil. Because they are water-soluble, synthetic fertilizers leach out into streams and ponds, polluting the water.
In the early spring, synthetic fertilizers have some significant advantages, as they are available to plants even when the soil is still cold and before soil microbes become active. Because of this benefit, some organically-based fertilizers also contain small amounts of synthetic fertilizers to keep nutrients available before the ground warms up.
For long-term purposes, organic fertilizers and compost are the best fertilizers to add for your garden’s health. Doing so will provide you with a soil rich in organic matter and microbial life.
Complete Vs. Incomplete Fertilizers
A fertilizer with all three major nutrients is referred to as a complete fertilizer, whereas a product that supplies only one or two is considered an incomplete fertilizer. Choosing a complete fertilizer for every garden task seems like a good idea, but it is not always the best pick. If your soil has a good amount of phosphorus and potassium and is deficient only in its nitrogen levels (which is quite common), you can save a bit of money by picking an incomplete fertilizer that only has nitrogen, such as ammonium sulfate. In some situations, complete fertilizers can actually damage a plant. Some plants, such as the exotic proteas, will not tolerate excess phosphorus and will die if given too much.
A home soil test kit, which you can purchase at garden centers or nurseries, will give you a good idea of the nutrients available in your garden’s soil, or you can get a more well rounded evaluation from paying for a professional analysis. Either way, once you know what nutrients are lacking in your garden’s soil, you will be able to better select an appropriate fertilizer for the task.
General & Special Purpose Fertilizers
The fertilizers labeled, “general-purpose,” consist of equal amounts of each major nutrient (such as 12-12-12), or a slightly higher nitrogen level (such as 12-8-6). These fertilizers are designed to meet the needs of most plants throughout the growing season.
Special-purpose fertilizers, in contrast, are formulated to meet specific needs. These fertilizers are made for gardeners who want a specific combination of nitrogen, phosphorus, and potassium for certain plants or soils. There are three different types of special purpose fertilizers.
One special-purpose fertilizer designed for use during the period of active growth, contains a heavy dose of nitrogen for spring use. This type is made to help gardeners encourage lush growth or to help lawns become vibrantly-green.
Another special-purpose fertilizer has low levels of nitrogen and higher levels of phosphorus and potassium in order to stimulate root growth, stem vigor, and flower and fruit production. These may have a ratio of 3-20-20. These special-purpose fertilizers are applied at different times and in different ways depending on the goal. When preparing a new planting area, you will work a dry granular fertilizer like this one deeply into the soil so that the roots can absorb phosphorus and potassium. The nutrients will help to strengthen the new plants growing stems and encourage the vigorous growth of a dense root system.
To promote flower or fruit production, you can apply the same sort of fertilizer to help establish plants after they have produced their first flush of growth, either with dry granules, lightly mixed into the soil, or with a liquid formula applied with a watering can or a garden hose.
A third type of special-purpose fertilizer is formulated for specific plants. These feature N-P-K ratios that are tailored to promote the best performance from a particular plant, as well as other elements that are proven to be valuable to that plant. These fertilizers are titled according to the plant that they are made to nourish.
Most fertilizers are applied directly to the soil, however, plants can absorb nutrients eight to 20 times more easily through the surface of their leaves than through their roots. For this reason, we recommend spraying foliage with liquid nutrients to produce incredible yields. Spray plants during their critical growth stages, such as transplanting time, blooming time, and just after fruit sets for the best results.
The Importance of Soil pH
If soil pH is too high or too low, some nutrients cannot be absorbed by your plants even if proper nutrients are available in the soil. Most plants prefer a soil pH between 6.0 and 7.0. Contact your local extension service for a low-cost soil testing kit to measure the pH of your soil. Send a sample to the lab or purchase a home kit and do one yourself.
It is best to lower or raise soil pH slowly over the course of one to two years as dramatic adjustments can result in the opposite extreme, which could be more problematic than what you started with. Lime or wood ash can be added to the soil to raise the pH and sulfur or aluminum sulfate can be added to lower the pH. Another helpful solution to bring pH levels closer to neutral is to add compost, which moderates the pH, helping you to maintain the ideal level of 6.5
If a soil test reveals specific nutrient deficiencies, or if you would like to tailor your fertilizer to the needs of particular plants, you can select a special formulation. However, in most cases, an all-purpose, 5-5-5 fertilizer provides the nutrients that all plants need for healthy growth. What you choose will depend on your current soil, as well as the preferences of the plants that you are growing.
The three numbers that appear on a fertilizer label tell you the specific proportion of each macronutrient that the fertilizer contains, and reflects the available nutrients by weight. A 100-pound bag of fertilizer with an N-P-K ratio of 5-7-4 contains 5 pounds of nitrate, 7 pounds of phosphate (which contains phosphorus), and 4 pounds of potash (which contains potassium), as well as 84 pounds of filler.
The N-P-K ratio of organic fertilizers are nearly always lower than the ratio of synthetic fertilizers. This is due to the fact that the ratio can only express nutrients that are immediately available, by law. Most organic fertilizers have slow-release nutrients that slowly become available over time and contain many trace elements that might not be supplied by synthetic fertilizers.
We advise using granular organic fertilizers and supplementing with a water-soluble fertilizer. Using granular fertilizers will help build the long-term health and fertility of your soil, while supplementing with a water-soluble fertilizer will ensure that your plants get the nutrients that they need when in active growth.
Common Questions and Answers About Fertilizer Versus No Fertilizer
Can you over fertilize a lawn?
It’s definitely possible to use too much fertilizer on your lawn, with results that can be disastrous, including death of the grass. Other symptoms that your lawn has had too much fertilizer include discoloration, overlapping spreader rows, or stripes in the grass. These things happen because over time, the salts in the fertilizer accumulate in the soil, drying the lawn out in a process called “fertilizer burn.” The more discolored your lawn is and the longer you wait to take action, the less likely it will recover from overfertilization.
Here’s what to do if your lawn is displaying symptoms of overfertilization. If you can see granular fertilizer on the surface of your lawn, use a broom or wet/dry vacuum to remove as much as possible. Then use sprinklers to soak your lawn deeply so the salts can start to flush out of the soil. The first day, water until your lawn simply won’t soak up any more moisture. Continue watering your lawn daily for a week or so, making sure to water in the morning to prevent fungal infections from occurring. It can take some time for your lawn to bounce back from overfertilization, so don’t be discouraged if results aren’t apparent right away.
Do plants grow better with fertilizer or without?
Plants grow best when they get plenty of the nutrients they need to thrive as well as sunlight and water. Sometimes, your soil may have the nutrients your plants need——especially if you’re working with fresh potting soil or a newly dug garden bed. Fertilizer is especially needed as a source of nitrogen, as well as potassium and phosphorous. Plants use up these elements quickly, nitrogen in particular. That’s why fertilizer is needed to replenish them. Without plenty of these elements, plants can grow, but they won’t be as healthy as they are when they have everything they need.
How do I know if my plants need fertilizer?
There are several visual cues that mean your plants need to be fertilized. If the foliage isn’t as dark green as it once was and the older leaves are turning yellow, the plant is low on nitrogen. Pale green leaves with darker veins running through them are a sign that your plant needs potassium. A lack of phosphorus is evident when older foliage begins to turn purple at the base and other leaves look dull. All these missing elements can be provided with fertilizer.
How often should you fertilize your garden?
For a fertilization timeline specific to your garden, it’s best to follow the instructions provided by the manufacturer of your fertilizer when those are available. However, there is a general seasonal timeline you can use to guide you if manufacturer instructions aren’t available. In the winter after the previous growing season’s harvest is complete and leftover plants and debris have been removed from the garden beds, add a slow-release 10-10-10 fertilizer and work it into the soil. (You’ll need to do this when the soil is still warm enough to be worked.)
If you didn’t fertilize in winter before the spring growing season begins, though, it’s not too late. You can use a quick-release 10-10-10 fertilizer in spring, before you sow seeds or add plants to the garden, instead of using the slow-release fertilizer the previous winter.
In the summertime, replenish the nutrients in your soil every three or four weeks with a quick-release fertilizer. Many fertilizers do not contain magnesium, so you may wish to supplement your fertilization regime by adding magnesium in the form of Epsom salts as well. Just mix one tablespoon of Epsom salts with a gallon of water and add it to the base of each plant in your garden.
In autumn, you’ll want to administer fertilizer before you start on your fall garden, if you plan to have one. Use a slow-release fertilizer, and work it into the top few inches of soil after your summer plants have been removed from the garden. Later in the fall gardening season, if your plants aren’t producing as well as you’d like, you can use a quick-release fertilizer monthly. Stop providing fertilizer after the first frost arrives in your area.
Is fertilizer really necessary?
Most of the time, fertilizer is necessary to keep your garden as healthy and productive as possible. Fertilizer is not necessary when you’re using a fresh new batch of potting soil or if you’ve just dug a garden bed in a new area and your soil happens to be perfectly balanced with all the nutrients plants need. You can contact your local Extension office to have your soil tested so you know its nutritional makeup. To find your closest Extension office, use the map on the National Pesticide Information Center website to select your area.
What happens if too much fertilizer is used?
Using too much fertilizer can be harmful for your plants in a few different ways. Administering too much fertilizer can result in fertilizer burn, which dries plants out because the excess fertilizer keeps plants from being able to take in the water they need, even when water is available. If your plants have fertilizer burn, you can flush the excess fertilizer from the soil with plenty of extra water, going above and beyond the moisture your plants need to survive.
What happens if you don’t fertilize plants?
Failing to fertilize your plants has several negative effects in your garden. The most obvious consequence of not fertilizing your plants is that, for vegetables and herbs, your harvest will be reduced. Plants that get enough fertilizer will produce the maximum they’re able to, so failing to fertilize means you’ll see fewer vegetables, fruits, or usable herbs throughout the season. Plants that aren’t fertilized will also be less visually appealing than fertilized plants, because healthy plants produce more foliage and blooms. Plants that don’t get fertilizer may also be malnourished, which makes them susceptible to infestation by garden pests and makes them more likely to catch plant diseases. The eventual repercussions of infestation or disease can range from harmless cosmetic issues to more severe effects—including damage to plants and even their death in some cases.
Which fertilizer makes plants grow faster?
Quick-release fertilizers that are high in nitrogen give plants an immediate boost in their growth, providing gardeners with rapid results. However, slow-release fertilizers dispense the nutrients they contain gradually over a period of weeks.This slow, steady fertilization won’t be as immediately obvious as the quick-release fertilizer, but when gardeners provide their plants with the nutrition they need consistently, the plants show their appreciation by producing more fruit, vegetables, blooms, and foliage that’s both beautiful and healthy. Plants that receive consistent nutrition are also less likely to fall victim to infestations by garden pests and less likely to contract plant diseases. In short, a high-nitrogen quick-release fertilizer will be the quickest, easiest way to make plants grow faster with visible results. But this quick fix should be partnered with a consistent fertilization schedule to make sure plants are performing their best throughout the growing season, resulting in the healthiest plants in the long run.
Why do farmers have to add fertilizers to soil?
Farmers use fertilizer to ensure the soil their plants are growing in has the nutrients plants need to thrive. These nutrients include calcium, nitrogen, phosphorus, and potassium. Especially when fields or garden beds are used year after year and season after season, plants drain the soil of these necessary nutrients as they take them in to help them grow. That’s why fertilizer is needed—to replenish the nutrients plants have taken from the soil in seasons past so those nutrients are available for the next growing season.
Want to learn more about fertilizer versus no fertilizer?
Chicago Tribune covers Fertilize Less: Many Plant Can Do Without
Education.com covers Do Plants Grow Best in Chemical Fertilizer, Organic Fertilizer, or No Fertilizer?
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First Editions covers How Do I Know If My Plant Needs Fertilizing?
Gardner’s Supply Company covers Fertilizer Basics
How Stuff Works covers What is Fertilizer and Why Do Plants Need It?
SFGate Homeguides covers The Best Fertilizer to Grow Faster
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nola.com covers The Myths and Truths About Proper Plant Fertilizing
Quora covers What Are the Disadvantages of Chemical Fertilizers?
UCSB covers ScienceLine covers What Nutrients Are Taken Out of Soil When People Use Too Much Fertilizer?
Soils Matter covers Why Do Farmers Use Fertilizers?
Sunset covers Crash Course in Fertilizers
Today’s Homeowner covers Help for Fertilizer Burn in Lawn Grass
Washington Post covers Why Your Garden Needs Fertilizer- Or Not