What does nitrogen do for plants? Read on to learn how to improve the health of your plants and how to add nitrogen to soil by fixing the nitrogen cycle in your garden.
This article is also in audio form for your listening enjoyment. Scroll down just a bit to find the recording.
Something doesn’t look right in your garden. Maybe your plants don’t have that midsummer vibrancy and look pale or even slightly yellow. Growth seems slow, and the plants seem weak and spindly instead of bushy and vigorous. Older leaves keep dying and falling off.
All of these signs are possible symptoms of a lack of nitrogen. Easy, right? Grab the box of blue powder off the shelf, and mix up some instant problem-solving! Not so fast. That temporary fix isn’t doing the good it should.
Humans aren’t the only organisms that help plants grow, gather nutrients, and survive. Plants are quite capable of providing for their own needs and have developed intricate relationships with other organisms to do just that. Why do heavy feeders like melons and squash grow so well in a compost heap? Because those complex natural relationships of soil biology and plants are working.
The Dance of Sunlight, Water, Carbon, and Nitrogen
Envision a bright, warm, sunny day in midsummer. Your tomato plants bask in the sunlight. It rained overnight — the lovely, gentle, slow dousing that gardeners like to see. In the garden, leaves stretch out to gather sunlight, harnessing its energy via chlorophyll. The stomata in their cells open and collect carbon dioxide. Water is moved up the xylem from the roots to the leaves. The chemical equation you learned in school now plays out in front of you, as carbon dioxide and water molecules are rearranged using solar energy. Sugars, which contain carbon, are created, and meanwhile the plant releases its oxygen “waste.”
You may have noticed that the above process didn’t appear to involve nitrogen, but it did. Nitrogen is behind the scenes of nearly every plant process, a component of the chlorophyll molecule, which is why a lack of nitrogen can appear in a plant as yellowing — it’s unable to make enough chlorophyll. Nitrogen is also used in proteins, protoplasm (the clear goo inside the cells), enzymes, plant hormones, and even DNA. Nitrogen is a part of all living cells. Without nitrogen, we wouldn’t have a plant.
Bacteria and archaea called “diazotrophs” create nitrate and ammonium from atmospheric nitrogen molecules. These organisms can break the triple bond of atmospheric nitrogen molecules and rearrange the nitrogen molecules in ways plants can use; the plants are then able to absorb nitrogen in the form of nitrate and ammonium. Without them, the nitrogen in the atmosphere would be unusable for plants.
Enter Bacteria and Fungi
Plants are pretty much immobile. They can only grow and gather nitrogen, water, and other nutrients where they are. If your tomato plants get up at night and go for a stroll, you have different problems. To some extent, plant roots can “search out” more water or nutrients. Roots can grow toward an area of higher moisture and away from a place of lower moisture via a process called “hydrotropism.” But there are limits.
This is where carbon and nitrogen become dance partners. Plants have developed symbiotic relationships (mutually beneficial processes) with bacteria and fungi. These bacteria and fungi like to eat carbon, so the plant provides it by exuding sugars from its roots. Microbes and fungi also break down organic matter in the soil and leave behind the nutrients they don’t need but plants do. In this way, the plant’s “reach” is significantly increased.
Soil organisms bring new nutrients to the rhizosphere (the area immediately surrounding the fine plant roots) to replace the nutrients already taken up by the plant. Thus, the cycle continues in this beneficial partnership between plants and the bacteria and fungi in the soil.
Audio Article
Nitrogen Storage and Availability
Nitrogen and other nutrients can leach out of the rhizosphere and out of reach of plant roots. Leaching occurs when water carries away water-soluble nitrogen (nitrate). These nutrients can also end up in our aquifers and surface waters, polluting them or causing algal blooms that deplete oxygen from the water. The dead zone in the Gulf of Mexico is an example of problems caused by leaching.
Nitrogen can be held, and therefore prevented from leaching, in several ways. Soil organic matter not only contains nitrogen itself, but also contains locations for nutrient ions to attach. Once attached to a particle, they’re not lost through leaching and are available for the plant or its symbiotic partners.
Because microorganisms are composed of living cells, they’re also stores of nitrogen. When the organism dies and is decomposed by other microbes, that nitrogen is again available for use.
How Artificial Fertilizers Gum Up the Works
When we apply synthetic fertilizers, we attempt to “correct” a perceived nutrient deficiency. We’ve been taught by fertilizer companies that plants “need” regular applications of the stuff. But in most cases, the nutrients are there already, but the soil biology isn’t working. The bacteria and fungi, archaea and nematodes, earthworms and beetles — all those good workers were unemployed, so they moved elsewhere looking for work.
A plant that’s given artificial fertilizer seems to thrive temporarily. It may look green and grow large, but it doesn’t develop the symbiotic relationships necessary to be healthy, resilient, and vigorous. It may get all the nitrogen it needs, but lacks other nutrients the living soil biology would’ve provided.
Curing the Wrong Problem
Artificial fertilizers disturb the natural nitrogen cycle in the soil. Often, too much is applied, and the excess is lost, leading to wasted money and harm to the environment. Carbon exudates aren’t released, so nitrogen-fixing bacteria aren’t attracted to the rhizosphere. Plants that haven’t developed the needed symbiotic relationships are often lacking in the other critical micronutrients that contribute to their ability to resist disease, withstand drought, or even reproduce. They look green, but their health is in jeopardy.
For gardeners, food harvested from plants grown in fertile soil with high levels of soil organic matter and proper symbiotic relationships is more nutrient-dense than food from its artificially nourished counterparts. The produce tastes better, stores better, and contains more benefits, such as antioxidants.
How to Add Nitrogen to Soil
Stop Tilling
Rototilling smashes and bashes the soil. A freshly tilled bed looks loose and inviting, but tilling actually breaks down soil aggregates. Aggregates are groups of individual soil particles held together by bacterial slime (the beneficial kind), fungal hyphae, glomalin, and organic material. Aggregates allow water, air, nutrients, microbes, and larger creatures to move about in the ground. Tillage destroys those aggregates and causes pore space to be lost. It actually increases soil compaction and has little or no beneficial effect on established garden plots.
Potentially more harmful than compaction is the destruction of the soil microbe community and the soil food web feeding your plants. In healthy, living soil, organic matter is steadily decomposed and mixed with soil particles by beetles, worms, beneficial nematodes, archaea, bacteria, and fungi. The tiller upsets this process; breaks fungal hyphae; and kills many of your arthropods, worms, and nematodes.
But tilling kills all the weeds, right? Tilling may well kill or damage the current weed crop, but it also brings dormant annual weed seeds to the surface, where they become viable again and sprout by the thousands. Grass rhizomes are chopped up and stimulated to each grow a new plant. You’re creating more work for yourself with that tiller.
Increase Your Soil Organic Matter
Soil organic matter is magical. It’s composed of material that breaks down relatively quickly (say, a couple of years) and humus, which is resistant to decomposition and can last for decades or longer. Easily decomposed substances are broken down by the soil food web, providing nutrients for plant growth. Humus sticks around practically forever in garden terms and stores charged ions from minerals and nutrients, such as ammonium. Humus also holds water and helps create soil structure. It’s the jack-of-all-trades in soil health. Many of our agricultural soils in the United States have lost most of their soil organic matter. Soils that were 7% to 8% soil organic matter before the industrial age of agriculture are now less than 2% and sometimes less than 1%. Nutrient cycling, water storage, and soil biology have all suffered immensely.
Restoring soil organic matter levels may be the best way to improve your soil and increase your garden’s productivity, resilience, and beauty. Try these three methods for two growing seasons and watch the change.
- Compost, compost, and more compost. Gardeners know compost is good stuff. Not only does it increase soil organic matter, but if it’s fresh and active, it can also supply all those active bacteria, fungi, and other microorganisms that create healthy, fertile soil full of nutrients.
- Plant cover crops. A cover crop is planted to be left in place, not harvested and removed. Cover crops create soil organic matter and pump carbon into the soil to continue feeding the soil food web. Some even fix nitrogen for an added boost. You can plant a cover crop in any season when the ground isn’t frozen. Some common cover plants will winter-kill for ease of spring planting, while others can be crimped or flattened. They also choke out annual weeds.
- Use soil cover. Keeping your garden soil covered is critical to restoring, enhancing, and maintaining your soil’s health. An avid mulcher already knows how beneficial a mulch layer can be for plant growth, since the mulch retains soil moisture and creates more soil organic matter as the layers break down. Even some problems, such as fungal wilts and some tomato diseases, can be reduced by mulch, which prevents soil from splashing onto the lower leaves. Keeping the ground covered reduces erosion, moderates the temperature, and prevents hard soil crusts from forming. Rarely in nature do we see bare, productive soil. Follow nature’s lead. Keep something there.
A Web of Life
Parking your tiller, working to increase soil organic matter, planting cover crops, and quitting artificial fertilizers may sound radical to some. However, these actions will do much to restore your soil food web. Your garden or field plot will begin to cycle nitrogen all on its own. An army of a trillion helpers will work to restore your soil while you sleep, pick beans, read Mother Earth News, or take a vacation.
Our soil can be healed much more quickly than we used to think. Pick a few actions to try and start working toward a healthy soil food web. Once you do, you’ll never go back.
Andy Wilcox is a freelance writer and flower farmer, who’s passionate about gardening, horticulture, and forestry, and who believes healthy soil leads to healthy people. He can be reached at Andy@ThatGardenWriterGuy.com.
Originally published as “Repair the Nitrogen Cycle in Your Garden” in the April/May 2023 issue of Mother Earth News and regularly vetted for accuracy.
