If you are looking to replace nitrogen in your garden soil, there are plenty of plants to choose from. Some of them fix nitrogen by the action of bacteria, and others add nitrogen to the soil when they die and are tilled into the soil.
So, what plants add nitrogen to soil? Legumes such as peas, peanuts, beans, clover, and alfalfa are all plants that will add nitrogen to soil. On the other hand, grasses and brassicas will absorb nitrogen from the soil, which will prevent it from being leached away by rain or irrigation. Cutting down these plants and tilling them into the soil will return the nitrogen to the soil.
Let’s get into our list of plants that add nitrogen to soil. We’ll also discuss when you might (or might not) want to use each one.
What Plants Add Nitrogen To Soil?
Legumes such as peas, peanuts, beans, clover, and alfalfa are the best plants for adding nitrogen to soil. According to Wikipedia, a legume is a plant that has “symbiotic nitrogen-fixing bacteria in structures called root nodules.” (The specific type of bacteria is called Rhizobia).
In simpler terms, this means that bacteria grow on the roots of legumes. These bacteria convert nitrogen from the air into ammonia. Ammonia can then be converted into ammonium, and these contain nitrogen in a form that is useful to plants.
For more information on the nitrogen cycle, check out this article from the University of Delaware.
For more information on legumes, check out this article on legumes from Wikipedia.
Now let’s look at some of the best plants for adding nitrogen to soil.
Peas are an annual legume (they live only one year), and they technically grow as a fruit above ground on tall vines. There are a few different varieties of peas, such as snow, snap, and sugar peas.
Peas, like all legumes, are a good source of plant protein. You can feed peas to livestock after growing them to add nitrogen back to your soil. You can also till them into the soil to provide additional nitrogen and other nutrients to the soil.
Peas are a cool-weather crop, and will not grow well in the hot summer months. This makes them ideal for growing in the northern U.S., including New England where I live.
Your best bet is to plant peas in the spring before temperatures get too warm. You can start peas in the soil outside as soon as temperatures reach 50 degrees Fahrenheit (10 degrees Celsius). Peas prefer a soil pH of 6.0 to 7.5.
If you have trouble germinating your peas, check out my article on how to get peas to germinate faster.
You can also check out this article on peas from Wikipedia.
Peanuts are a legume, and they grow underground. They prefer a soil pH between 5.9 and 7.0.
They are often grown in tropical and subtropical regions. This makes them ideal for warmer regions in the southern U.S., including some parts of Georgia, Texas, Alabama, Virginia, and the Carolinas.
Peanuts are used for both their seeds (peanuts) and oil (peanut oil) for human consumption. The rest of the plant can also be used as feed for livestock.
For more information, check out this article on peanuts from Wikipedia.
Beans are a legume, and they grow above ground on tall vines. Bush beans sprawl out and cover the ground, not getting much taller than 1 or 2 feet.
Pole beans can grow to 10 feet or taller, and do well when supported by tall stakes or trellises. You can also grow beans together with corn, using the corn stalks as support for the beans to grow.
Beans prefer a soil pH of 6.0 to 6.5, and need warm temperatures to grow, making them an ideal summer crop for many areas.
Beans are an important source of protein for humans and animals. There are many types of beans you can plant, including soybeans, garbanzo beans (chickpeas), broad beans (fava beans), runner beans, lima beans, and common beans (black, kidney, pinto, and green).
For more information, check out this article on beans from Wikipedia.
Clover (also known as trefoil for its three-section leaves) is a legume that grows well in the temperate zones of the Northern Hemisphere. Clover prefers a soil pH between 6.0 and 7.0. Clover can grow as an annual, biennial, or perennial.
Clover is often used as feed for livestock, since it will regrow repeatedly after harvesting. Clover is a primary source of nectar for honeybees.
As with all legumes, the roots of bean plants contain bacteria that fix nitrogen from the air into the soil. You can also till bean plants into the soil to help restore nitrogen and other nutrients to the soil.
For more information, check out this article on clover from Wikipedia.
Alfalfa, also known as lucerne, is a perennial legume. Alfalfa looks somewhat similar to clover, and is similarly used as feed for livestock.
Thanks to its deep root system, alfalfa is highly resistant to drought. Alfalfa can also help to prevent soil erosion, in addition to restoring nitrogen to the soil.
If you have trouble with soil erosion, check out my article on how to prevent soil erosion.
Alfalfa prefers a soil pH of 6.8 to 7.5 (close to neutral). Alfalfa can be harvested between 3 and 12 times per year, depending on where it is grown.
Most varieties of alfalfa become dormant in the fall, in response to lower temperatures and shorter days. This makes them an ideal summer crop.
However, there are varieties of alfalfa that are nondormant, often grown in Mexico, Arizona, and Southern California.
For more information, check out this article on alfalfa from Wikipedia.
Alternatives To Nitrogen-Fixing Plants: How To Add Nitrogen To Soil
There are other ways to add nitrogen to soil, apart from planting nitrogen-fixing plants such as legumes. One of the most common ways is to use manure in your garden, so let’s start there.
Manure is simply animal waste and bedding. Manure from chickens, cows, or horses is often used in gardens to provide additional nitrogen, along with other nutrients.
One important thing to remember is that manure should be completely decomposed before applying it to your garden. Otherwise, you can harm your plants with salt in the animal waste or by raising the soil’s nitrogen level too quickly.
If you have livestock of your own, you can add the manure to your compost pile to decompose it before adding it to your garden. If you do not have any animals, you can ask a local livestock owner, who will often be happy to have someone haul away a large pile of manure.
There are certain types of manure that you should not use in your garden. For example, dog or cat waste should not be used as manure.
For more information, check out my article on where to get manure.
Compost is another good option for restoring nitrogen and other nutrients to your soil. The best part about compost is that you can make it from yard waste and kitchen scraps that you may already have available.
Common yard waste used in compost includes grass clippings, raked or mulched leaves, and dead plant matter from the year’s garden.
Common kitchen waste used in compost includes fruit and vegetable scraps, coffee grounds, and eggshells.
As with manure, you will want to wait until compost is completely decomposed before adding it to your garden soil. You can keep your compost in a pile or in a compost bin and mix it regularly to help along the process of decomposition.
Remember that “greens”, such as grass clippings, will contain more nitrogen, and thus will make the best compost for replacing nitrogen in your soil. However, you do want to include some “browns”, such as dried leaves, in your compost pile.
For more information, check out my article on how to make your own compost.
Buying fertilizer from the store is one option if you need a quick boost to the nitrogen level in your garden soil. Most fertilizers will contain a good amount of each of the “big three” nutrients: NPK, or nitrogen, phosphorus, and potassium.
The three numbers you see on a package of fertilizer tell you the percentages (by weight) of these three nutrients. The first number will represent the percentage of nitrogen in the package of fertilizer.
For more information, check out my article on NPK ratios.
Remember that it is possible to over-fertilize your plants, which can cause fertilizer burn due to high levels of the salts that are present in many fertilizers.
For more information, check out my article on over fertilizing your plants.
It is also possible to give your plants too much nitrogen, which can cause them to grow lots of leaves and shoots (green growth), while neglecting to produce flowers and fruit.
For more information, check out my article on low-nitrogen fertilizers.
Other Options For Adding Nitrogen To Your Soil
There are some other options for adding nitrogen to small areas of your garden, such as blood meal and fish emulsion. These are ideal for supplementing nitrogen to individual plants.
As you might guess, blood meal is a powder made from blood waste from slaughterhouses. It contains high amounts of nitrogen, and a small amount of phosphorus and potassium.
Fish emulsion is a fertilizer made from fish, and contains nitrogen as well as lesser amounts of phosphorus and potassium. Fish emulsion is expensive, so it might be best to dilute it and use it as part of a fertilizer mixture that contains compost and other less expensive options.
Now you have a much better idea of what plants add nitrogen to soil. You also know some alternative methods to add nitrogen to your soil.
For more ideas, check out my article with a list of nitrogen fixing plants.
I hope you found this article helpful – if so, please share it with someone else who can use the information.
Want to read more about legumes and other cover crops that restore nutrients to soil? Check out my article on green manure (cover crops).