The Garden Digs

Plant Nutrients: Macronutrients

Plant Nutrients: Macronutrients

Let’s get real here and talk about plant nutrition. You’re probably somewhat familiar with human nutrition, but plant nutrition may seem like a foreign language. Throw in words like potassium, sulfur, and carbon dioxide, and it can seem like you in a chemistry class rather than discussing a vegetable garden. In this post, I want to break down some of the most important nutrients for your plants. Understanding how they are used will help you to grow vigorous, healthy plants.

What are Macronutrients?

This entire post is dedicated to macronutrients. The word itself can be broken down into two parts: ‘macro-‘ meaning large, and nutrients. Nutrients are chemicals that plants need in order to survive. In people, some common nutrients include carbohydrates, fats, vitamins, and minerals. In plants, we’re going to talk about nutrients like nitrogen, boron, and calcium.

So, what exactly is a macronutrient?

A macronutrient is a nutrient that a plant needs in large quantities. Don’t get confused and think that macronutrients are large nutrients. They’re small in size; it’s the amount needed by the plant that is large.

We can break macronutrients into two main groups: primary and secondary. Primary macronutrients are nutrients that the plant needs huge amounts of. Secondary macronutrients are still needed in large quantities, but not as much as the primary macronutrients. Both groups of macronutrients are important and crucial for plant growth and development.

The other type of nutrient is a micronutrient. These are nutrients that plants need, but they only need in small quantities. Read more about micronutrients in this post.

The Six Macronutrients

There are six macronutrients: nitrogen, phosphorus, potassium, magnesium, sulfur, and calcium. The first three, nitrogen, phosphorus, and potassium, are primary macronutrients. These nutrients are needed in the largest amounts and are critical for your plant’s health. Without these three nutrients, plants will die quickly.

Nitrogen, phosphorus, and potassium are also the three nutrients that are found in almost all fertilizers. If you’ve grabbed a bag of fertilizer and noticed the three numbers on the bag, you’ve seen what’s called the NPK value. These three numbers represent the percentage of nitrogen, phosphorus, and potassium in the fertilizer. The first number tells you how much nitrogen is present, the second number tells you how much phosphorus is present and the last number indicates how much potassium is present.

For example, let’s say that you pick up a bag of 10-0-4 fertilizer. The fertilizer contains 10% nitrogen, 0% phosphorus and 4% potassium.

Keep in mind that while nitrogen, phosphorus, and potassium are crucial for plant health, they aren’t the only nutrients that your plants will need in large amounts. The secondary macronutrients are also key to keeping your plants in tip-top shape. The secondary macronutrients include magnesium, sulfur and calcium. When your plant doesn’t get enough of a secondary macronutrient, it will show some tell-tale signs to let you know that it needs a little bit of help.

What Macronutrients Do

Each macronutrient has certain roles that it plays in plant health.


Plants use a form of nitrogen that is called nitrate. It’s extremely important in plant health since it helps to create strong leaves and foliage. It also helps to produce chlorophyll. Chlorophyll is the pigment that absorbs sunlight, providing a source of energy for the plant to make food.


Phosphorus is used in the growth and development of flowers and roots. A strong root system is key to plant health since water and nutrients are absorbed primarily through the roots. Without phosphorus, plants cannot develop flowers properly and fruit will not set. It’s also been shown to increase the robustness of your plants by helping them cope with environmental stress and cold winters.


Potassium helps to strengthen the plant overall. During early growth, potassium is used to boost plant growth. It also plays a role in helping the plant to retain water and increases drought resistance. Plants with adequate levels of potassium are less susceptible to disease and parasites. Potassium is used to open and close stomata. Stomata are small pores on the undersides of leaves where carbon dioxide is absorbed.


Magnesium is also involved in providing the green coloring in leaves. Magnesium is a building block for chlorophyll, the green pigment in leaves that absorbs sunlight. Without magnesium, plants could not produce this sunlight-absorbing pigment and would not be able to produce their own food.


Sulfur plays several roles in plant health. It can help the plant be more resistant to diseases. It is also involved in plant growth through the production of amino acids, proteins, enzymes, and vitamins. Sulfur is actively involved in the process of seed formation.


Calcium is crucial for developing strong plants. Calcium helps create cell walls around plant cells. Plants don’t have bones to hold them up, so they rely on rigid cell walls to help keep them upright. Strong cell walls also help to protect the plant from disease. Calcium is also required for plant metabolism. Interestingly, plants also use calcium to help them absorb nitrogen from the soil. Without calcium, plants may not be able to take in enough nitrogen.

Signs of a Problem

A healthy plant will have deeply colored foliage, upright stems, and will put on blooms or fruit as expected. Plants that are lacking in macronutrients will usually show you signs that there is something going on. If you know what to look for, you can apply the proper fertilizer or adjust the soil to ensure that the plant can get the nutrients it needs.

There are two issues that can crop up with macronutrients: deficiencies and toxicities. A deficiency is caused by a lack of a nutrient, while a toxicity is caused by too much of a nutrient.


Nitrogen, along with potassium, is considered a very limiting nutrient. Without these two nutrients, plant growth is severely limited or nonexistent. Plants that have nitrogen deficiencies are often smaller, have leaf chlorosis, and a significant reduction in crop production.

Leaf chlorosis is one of the most common signs of a nitrogen deficiency. The older leaves (leaves towards the bottom of stems) will yellow in color. Eventually, if the problem isn’t fixed, the entire plant’s foliage can turn yellow.

Too much nitrogen can burn the roots and tissues of the plant.


A plant that is lacking in phosphorus may not develop many flowers and fruit may not set properly. When the plant is severely lacking phosphorus, it will develop dark purple coloring in older leaves. Older leaves that turn dark purple are an indication that the plant is dealing with a critical deficiency.

Although too much phosphorus isn’t desired, it’s not likely to kill your plant. It will interfere with the absorption of other essential nutrients though, like iron, manganese, and zinc.


Potassium is critical for plant growth and is often referred to as the ‘quality’ nutrient. Potassium can affect the plant’s size, shape, color, and crop taste. Potassium deficiency can show up in a few different ways. Plant growth, root development, and seed or fruit development is reduced in plants with potassium deficiency.

The first signs of plant deficiency usually show up in older leaves. The leaves may look brown and scorched on the edges. It can also cause the leaves to yellow in between the veins. Occasionally, when you flip the leaves over, purple spots can be seen.

Plants aren’t likely to absorb too much potassium, but it is possible. When it occurs, it can interfere with the absorption of magnesium, manganese, zinc, and iron.


Magnesium deficiency shows up in older leaves first. It causes the tissue between the leaf veins to turn yellow. However, it does so in a way that distinguishes it from other types of interveinal chlorosis. A magnesium deficiency will cause the leaf to yellow starting at the tip and working back towards the stem. It can also cause brown scorching on the leaf edges. Plants that are in full sunlight will often have worse symptoms.

Magnesium toxicity is very rare and doesn’t show visible signs.


Sulfur deficiency is another nutrient deficiency that causes leaves to turn yellow. With a sulfur deficiency, the entire leaf will turn yellow, including the leaf veins. Younger leaves are the first to yellow. You may also notice some curling of the leaves.

Sulfur toxicities cause plants to have smaller leaves and stunted growth. It can also cause some leaves to turn yellow and have scorched edges.


Calcium is an immobile nutrient in plants, meaning that it can’t be moved around the plant if needed. One of the most common signs of calcium deficiency is blossom end rot. Blossom end rot is seen in vegetable plants like tomatoes, squash, or peppers. It can be seen in any fruit that develops after a bloom. The fruit will develop a wet, brown spot on the blossom end of the fruit.

Other signs of calcium deficiency include browning and yellowing of leaf tips on younger leaves. If you are transplanting a plant, you may notice a poor root system. The roots may be slimy and dark brown or black.

Proper plant nutrition is key to overall plant health. It’s a good idea to consider the type of plant that you’re growing before choosing a fertilizer. Also, look out for signs of deficiencies or toxicities and remedy them as soon as possible to prevent further disruption.

Written by Shelby of Farminence 

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