Fertilizer: The Role of Nitrogen in the Garden

Just like humans need a wide range of vitamins and minerals to thrive, plants require diverse nutrients to remain healthy. One of the most essential plant nutrients is nitrogen. Without enough nitrogen, plants can’t form proteins, complete photosynthesis, or use carbohydrates.

Your goal as a gardener is to apply just enough nitrogen for your plants. Too little nitrogen leads to weak plants, while too much can cause other plant problems and environmental harm. So, how do you find the sweet spot when applying nitrogen fertilizer? By examining factors like the amount of nitrogen in the soil and your plants’ nitrogen needs!

In this article, I’ll explain how plants use nitrogen so you know why it’s essential. Then, I’ll cover types of nitrogen-rich fertilizers, how nitrogen moves through the environment, and how to determine your plants’ nitrogen needs. By the end of the article, you’ll have a whole new understanding of this important plant nutrient.

Nitrogen Basics

Nitrogen, symbolized by the letter N, is one of three plant macronutrients. The other two macronutrients are phosphorus (P) and potassium (K). Plants require large amounts of these three nutrients to thrive, which is why fertilizers list the amount of N, P, and K on their labels.

While nitrogen is a single element, it often combines with other elements to form larger compounds. Some common types of nitrogen particles in the air, water, and soil include nitrogen gas (N₂), nitrate (NO₃-), ammonium (NH₄+), and larger, carbon-containing organic compounds. Nitrogen frequently cycles between these different forms and moves throughout the environment.

It’s important to note that plants mainly take up nitrogen in the form of nitrate and ammonium. Therefore, the nitrogen in atmospheric nitrogen gas and complex organic compounds (i.e., those found in compost) is not available to plants.

However, microbes can convert these unavailable nitrogen forms into plant-available compounds through nitrogen fixation and mineralization. This is why a healthy soil microbiome is so important!

Nitrogen Fixation

Perhaps you’ve heard that legumes like peas, beans, and clover can “fix” nitrogen, but what does that really mean? It’s time to clear up any confusion.

During nitrogen fixation, certain bacteria convert atmospheric nitrogen into ammonia. Some of these nitrogen-fixing bacteria, such as Rhizobacteria, form symbiotic relationships with leguminous plants.

The bacteria live in nodules in the plant’s roots, where they receive energy-rich carbohydrates from the plants. In turn, the bacteria use these carbohydrates to complete the energy-intensive nitrogen fixation process and supply the plants with nitrogen they can use.

Nitrogen Mineralization

Mineralization is the process in which bacteria convert large, nitrogen-containing compounds—like those found in compost and decaying plant material— into smaller compounds. Eventually, this breakdown process releases plant-available ammonium into the soil. Because mineralization relies on soil bacteria, aim to maintain a healthy soil community by applying organic fertilizers and avoiding fumigants or synthetic fertilizers.

Since mineralization relies on living creatures, factors like temperature and moisture significantly impact the process. That’s why plants can’t access nitrogen from organic sources in the winter but they can in the spring.

The Role of Nitrogen in Plant Growth and Health

Understanding how plants use nitrogen can help you diagnose and understand deficiencies. When you realize just how essential nitrogen is to plants, you’ll be motivated to supply your plants with the right dose of this macronutrient.

Protein Formation

Nitrogen is an essential component of all amino acids, which combine to form proteins. Some proteins are structural components, while others are enzymes that trigger or speed up critical plant processes.

Chlorophyll

You may know that chlorophyll is the molecule that drives photosynthesis. But did you know that nitrogen is a crucial component of chlorophyll? Therefore, if plants don’t have enough chlorophyll, they can’t use energy from the sun to produce carbohydrates. A lack of carbohydrates leads to problems with everything from growth to flowering to disease resistance.

Genetic Code

Nitrogen is also a key part of the nucleic acids that form DNA. A lack of protein can lead to issues with reproduction and seed production.

Energy Transfer

Like all living creatures, plants contain two energy transfer molecules known as ATP and ADP. Plants can capture and release energy by transferring electrons between these two molecules. Since nitrogen is a crucial part of these molecules, it’s essential to energy transfer.

Types of Nitrogen Fertilizer

Now that you know the importance of nitrogen in plants, let’s cover how you apply it to soil. In general, there are two types of nitrogen fertilizer: organic and inorganic. 

Naturally occurring substances like manure, blood, and feathers make up organic fertilizers. Most of the nitrogen in these fertilizers exists in complex molecules. Bacteria must first break down these large molecules before plants can access the nitrogen they contain. This means a rich microbial life is essential for creating plant-available nitrogen, and nutrients are less susceptible to leaching.

Humans create inorganic nitrogen fertilizers by combining nitrogen gas with hydrogen to form ammonia. While ammonia is more readily available to plants than organic sources of nitrogen, it’s important to note synthetic nitrogen fertilizer production emits sizable amounts of carbon dioxide and uses lots of energy. Synthetic inorganic nitrogen is also more prone to leaching into waterways and harming the ecosystem.

Now that you understand the differences between organic and inorganic nitrogen fertilizers, check out these common organic nitrogen sources.

Blood Meal

Blood meal is a solid organic fertilizer loaded with nitrogen and not much else. As you might have guessed, it consists of dried blood, a meat industry byproduct. Blood meal typically has an NPK ratio of 13-0-0 or 12-0-0. Since blood meal contains such a high amount of nitrogen, it’s more expensive than many other organic fertilizers.

Feather Meal

Another byproduct of the meat industry, feather meal consists of ground chicken feathers and has an NPK ratio of 13-0-0 or 12-0-0. Since it’s a good nitrogen source but doesn’t add much phosphorus or potassium, it’s useful when you only need to add nitrogen.

Cottonseed Meal

As its name suggests, cottonseed meal consists of ground cotton seeds. It has an NPK ratio of 6-2-1. Since cottonseed meal naturally lowers soil pH, consider using it for plants that grow well in acidic soils, like rhododendrons, blueberry bushes, and azaleas.

Alfalfa Meal

Alfalfa meal consists of ground alfalfa plants. You can use it in multiple ways on the homestead, from feeding animals to boosting soil health.

The exact nutrient composition depends on the product you choose, but most types of alfalfa meal have an NPK ratio close to 3-0.5-3. Not only does alfalfa meal supply major nutrients, but it also provides micronutrients and natural growth hormones.

Determining How Much Nitrogen to Apply

Since plants use large amounts of nitrogen throughout the year, you may plan on regular nitrogen fertilizer applications. The amount of nitrogen you should apply depends on the soil organic matter, types of plants, and time of year.

Although nitrogen regularly moves through the soil and changes forms, I recommend conducting a soil test before applying fertilizer. Not only will this let you know the nutrient composition of your soil, but it will also give you recommendations for how much fertilizer to apply. I recommend conducting a soil test in the late fall or early spring and submitting the sample to your local agricultural extension office.

Example Calculation

Once you have your results, you can use these as a guide. Let’s say your results recommend applying four pounds of nitrogen for every 1,000 square feet.

First, measure the area you plan to fertilize to determine the square footage. If your garden is 15×20 feet, it’s 300 square feet.

Next, follow these calculations to determine how much nitrogen to apply.

Now that you know you have to apply 1.2 pounds of nitrogen, it’s time to determine how much fertilizer to apply. Let’s say you want to apply blood meal with an NPK ratio of 13-0-0. This product contains 13% nitrogen, so you can use this to calculate how much you need to apply.

1.2 lb x 0.13= 9.2 lb

Apply 9.2 pounds of blood meal to supply your garden with the nitrogen it needs!

Once you apply the initial dose of nitrogen fertilizer in the spring, keep an eye on your plants. While many plants will grow well on this initial fertilization application, heavy-feeding and long-season crops like broccoli, corn, and tomatoes benefit from a midsummer nitrogen application.

Problems with Deficiency and Excess

Overapplying and underapplying nitrogen can both lead to problems in plants and the surrounding environment. Look out for the following to determine if you should adjust your fertilizer applications.

Signs of Too Little Nitrogen

If your plants can’t access enough nitrogen, you may notice yellowing leaves, stunted growth, and decreased disease resistance. Since this is a mobile nutrient, yellowing will appear in older leaves before new growth.

By the time your plants start showing signs of deficiency, they’re severely lacking in this vital nutrient. Therefore, apply a fast-acting source of nitrogen ASAP, such as liquid fish emulsion fertilizer. However, avoid over-applying fertilizer since this can lead to just as many problems.

Signs of Too Much Nitrogen

While plants need nitrogen to thrive, too much can cause serious problems. If your plants have access to lots of nitrogen but not enough phosphorus and potassium, they’ll often develop luscious green growth but few flowers or roots. That’s why you see spindly carrot roots topped with large greens and flowering plants without blooms.

Too much nitrogen can also lead to grain crops like corn and wheat falling over, a phenomenon known as lodging. Plants may also develop dark green, succulent leaves that succumb to pests like aphids and thrips.

Not only does too much fertilizer negatively impact plants, but it also leads to unnecessary costs and environmental harm. Excess nitrogen readily leaches out of the soil and into surrounding waterways, causing algae growth, a lack of oxygen, and dead fish. While applying too much nitrogen to your garden is unlikely to cause serious environmental harm, don’t assume more nitrogen is better.

Finally, if your soil is not rich in micronutrients and microorganisms, nitrogen will not be able to bond with other molecules. This makes it inaccessible to plant roots. Healthy soil should contain everything you need for proper collaboration between soil organisms, plants, and supplemental nutrients.