An Easy Guide to Garden Fertilizers: Fertilizer Basics (2024)

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Just like vitamin and mineral deficiencies can cause issues like anemia and scurvy in humans, a lack of essential nutrients will lead to issues with plants. Sometimes, the soil provides all the nutrients a plant needs—just think about towering trees and vibrant wildflowers growing in lush forests. However, gardeners often need to supplement the soil to keep their vegetables and flowers healthy.

However, choosing the correct type of fertilizer isn’t always easy. After all, there are hundreds of different products, and plants can’t speak to tell you what they need. Understanding the various plant nutrients and the different types of amendments will help you learn how to apply the correct products. This easy fertilizer guide has all the answers you need for a thriving garden!

What Are Fertilizers?

Fertilizers are substances added to the soil to supply nutrients and improve plant growth. They can be made from naturally occurring materials like manure, bones, and alfalfa, or from substances synthesized in a laboratory.

Sometimes, the soil contains enough minerals to supply plants with what they need. Other times, the soil lacks plant-available nutrients, and gardens benefit from the addition of amendments.

What Nutrients Do They Supply?

As you’ve read above, the goal of fertilizers is to supply plants with nutrients. However, not all fertilizers contain the same types or amounts. That’s because different species have varied nutrient needs, and soils differ in nutrient composition. Therefore, your goal is to add a fertilizer that only supplies your garden with the nutrients it lacks.

When you look at fertilizers, you may see any of the 15 essential plant nutrients listed on the label. Although plants require 18 nutrients, they obtain carbon, oxygen, and hydrogen from air and water.

Looking at the nutrients present and the amount of each nutrient will help you choose a fertilizer that’s right for your garden. Understanding the role of each one makes it all make sense. With that said, here are the nutrients you can look for on fertilizer labels.

Macronutrients

Plants require large amounts of the three macronutrients: nitrogen (N), phosphorus (P), and potassium (K). That’s why all fertilizers contain a label displaying the NPK ratio as three numbers separated by dashes. These numbers show the percent by weight of the three macronutrients.

For example, a fertilizer with an NPK ratio of 12-6-10 contains 12% nitrogen, 6% phosphorus, and 12% potassium. It’s also possible that a fertilizer may display 0-0-0 when it lacks macronutrients but contains other elements plants need.

Nitrogen (N)

One of the most important elements, nitrogen is necessary for protein formation, chlorophyll production, DNA transfer, enzyme formation, and more. Plants require nitrogen throughout their lives, so you should aim to supply them with a continuous supply of this nutrient. Since this element readily moves throughout the environment via the nitrogen cycle, the amount of nitrogen present in the soil varies from one week to the next.

It’s important to recognize that nitrogen takes multiple forms, and not all of them are available for plant uptake. Many sources of organic nitrogen must be broken down by bacteria before the nutrient is available to plants. Therefore, you should aim to foster a healthy soil microbiome whenever you apply organic nitrogen fertilizers.

Phosphorus (P)

This macronutrient helps plants form cells, transfer energy, and synthesize proteins. It’s a main constituent of the energy transfer molecules adenosine diphosphate (ADP) and adenosine triphosphate (ATP), which send energy throughout the plant to complete processes like photosynthesis, respiration, and cell division.

Potassium (K)

Although it’s not a structural component of plant compounds, potassium plays a crucial role within plants. It helps with water movement, thereby maintaining turgor pressure within cells and helping plants deal with drought stress. It also plays a key role in moving nutrients and sugars throughout the plant and activating enzymes. When you put all this together, you’ll find that potassium helps improve both pest and disease resistance.

Secondary Nutrients

Plants require the three secondary nutrients in moderate amounts. Although they won’t be listed on the fertilizer’s NPK ratio, you can look for the percentage of each nutrient on the label.

Magnesium (Mg)

Each chlorophyll molecule contains a magnesium atom in its core. Therefore, magnesium is essential to photosynthesis. Magnesium also helps with cell division, response to temperature and light, fat formation, and enzyme activation.

Sulfur (S)

Sulfur is a component of a handful of amino acids that are used to build structural proteins and enzymes. It also helps plants form fats and starches.

Calcium (Ca)

This element is an important component of plant cell wells. It also moves compounds through cell membranes. Calcium availability depends on water concentration, so calcium deficiency in crops is often caused by insufficient or inconsistent irrigation rather than a lack of calcium in the soil.

Micronutrients

Although plants require only small amounts of the following elements, they’re still essential to plant growth. Sometimes, a sufficient amount already exists in the soil, but if they don’t, you should supplement the soil with an appropriate mineral amendment.

Manganese (Mn)

Manganese helps activate enzymes that drive processes like photosynthesis, pollen formation, and nitrogen metabolism.

Iron (Fe)

Iron is involved in many plant processes, including chlorophyll production and cellular respiration. High pH inhibits plant availability of iron, so it’s possible for soils to be high in iron while plants remain iron deficient.

Chloride (Cl)

Chloride is needed for photosynthesis and enzyme activation. This element also helps regulate water uptake in soils with high salt levels.

Zinc (Zn)

Zinc is a component of multiple enzymes, including those that break down proteins and transfer energy via oxidation. It also helps with starch formation, seed production, and the production of growth hormones.

Boron (B)

This micronutrient is key to cell division, the production of plant hormones and nucleic acids, and the movement of sugar from leaves to other parts of the plant.

Copper (Cu)

Copper is important for photosynthesis and the breakdown of proteins and carbohydrates. It’s also a component of a few important enzymes.

Molybdenum (Mo)

Molybdenum is part of enzymes that fix nitrogen and convert nitrate into other nitrogen-containing molecules. In short, it’s essential for the uptake of nitrogen.

Cobalt (Co)

Although it’s required in small amounts, cobalt is an essential part of vitamin B12 and is also associated with nitrogen fixation.

Nickel (Ni)

Nickel is part of many enzymes, including those that break down urea and oxidase compounds to produce energy. It’s involved in healthy grain production, iron absorption, and nitrogen fixation in legumes.

Types of Fertilizers

While all fertilizers supply nutrients, you can find hundreds of different types of these products! There are multiple ways to classify fertilizers, including whether they are synthetic or organic, liquid or granular, and quick-release or slow-release. I’ll cover a few of the different categorizations to help clear up fertilizer labels.

Organic vs. Synthetic

One way to classify fertilizers is whether they’re organic or synthetic. While all of these fertilizers supply the same types of nutrients, the origin of the products differs.

Organic Fertilizers

Organic fertilizers come from naturally occurring materials, such as manure, bones, plant tissue, and rocks. In short, they are gathered from the earth rather than synthesized in a lab.

The nutrients present in many types of organic fertilizers must be broken down by bacteria before they are available. Therefore, they slowly become available to plants over time. Factors like temperature, soil moisture, and organic matter all impact soil microbes and, therefore, nutrient availability. Maintaining a healthy soil microbiome by adding organic matter to the soil, keeping crops growing at all times, and avoiding unnecessary tillage.

You can also find organic nutrient blends made from multiple materials. These products make it easy to apply the correct ratios of nutrients to plants like tomatoes and leafy greens.

Synthetic

As their name suggests, synthetic fertilizers are synthesized in a laboratory. This process often relies on fossil fuels to complete, which is one reason why these materials are viewed as less environmentally friendly than their organic counterparts.

Synthetic fertilizers often contain higher concentrations than organic products, which means you can apply less of them and still add the same amount of nutrients to the soil. They are also often quick-release and readily available. However, this also makes them susceptible to leaching and runoff. Not only does this make the nutrients unavailable to plants, but it can also lead to environmental problems like eutrophication and fish kill.

Quick-Release vs. Slow-Release

As I mentioned above, many organic fertilizers are considered slow-release, and most synthetic fertilizers are quick-release. These terms only refer to how quickly nutrients are available to plants.

Since quick-release products are readily available for plant uptake, they’re a good solution if your plants need a boost of nitrogen or phosphorus right away. However, this also means they’re in a volatile state that can easily leach out of the soil during a heavy range or exit into the atmosphere as a gas. Therefore, you may need to make repeated applications of these fertilizers to provide your plants with a continuous supply of nutrients.

Although slow-release fertilizers often take a few weeks or months before they are available to plants, they provide nutrition over an extended period. That means you can apply them once and count on them to feed your plants for months at a time.

Since the availability is dependent on environmental factors like temperature and moisture, it’s important to consider the time of year you’re applying them. Crops might not be able to access the nutrients in fall-applied fertilizers until the spring, but spring-applied products are often available within a few weeks.

Forms: Granular, Liquid, and Micronized

Just like you can find different types of fertilizers, you can choose from different forms. One of these forms isn’t necessarily better than the other, but there are some differences to consider.

Granular

Both organic and synthetic fertilizers come in an easy-to-apply granular form. All you have to do is sprinkle the product over the top of the soil or use a broadcast spreader to distribute it. However, it can be difficult to evenly distribute a small amount of these fertilizers.

Liquid

One of the benefits of liquid fertilizers is the multiple application methods they offer. You can dilute the fertilizers and spray them on your plants’ leaves to foliar feed, feed the liquid into a drip irrigation system to fertigate, or spray them onto the soil surface and then rake them into the ground.

Liquid fertilizers also make it possible to evenly apply a small amount of fertilizer over a large area. Therefore, they’re a good option for applying small amounts of micronutrients like boron and copper.

Pelletized

You can find pelletized forms of organic fertilizers like AZOMITE and elemental sulfur, as well as many synthetic fertilizers. These products come in precise pellets that are all the same size. This makes it easy to spread them evenly across an area without worrying about wind drift that may occur with liquid or micronized products.

Micronized

Some fertilizers available in pelletized form also come in this fine powder form. Micronized products make it easy to apply a minuscule amount to your garden. However, since these fine powders are prone to blowing in the wind, make sure to apply them on calm days.

Spikes

Fertilizer spikes are made of slow-release nutrients. You just insert them near the base of your plant, and they feed the soil for multiple weeks or months.

How Do You Determine Which Product to Apply?

A soil test is the best way to determine which nutrients your soil lacks. All you have to do is take a few samples of the top six inches of your garden’s soil and send them to a lab. The soil scientists will run a series of chemical reactions and tell you what is present in the soil, as well as factors like soil pH and the percent of organic matter.

Once you have the test results, you can determine which products to add.. In addition to paying attention to deficient nutrients, you should also note those present in excess. When you choose a product, avoid picking one that contains the nutrients already present in excess.

It’s also important to look at the pH. Soil pH has a big impact on nutrient availability. If the pH is above 7.0 or below 6.0, some nutrients will be unavailable to crops even if they’re in the soil.

With pH and excessive nutrients in mind, you can pick your product. One option is to apply multiple fertilizers that contain a single nutrient. For example, if your soil lacks nitrogen and potassium, you can add blood meal and langbeinite. Or, you can add a product that contains both nutrients, like fish fertilizer or a fertilizer blend.