Adding Nitrogen To Soil Organic Gardening

The legume is commonly recommended as a companion plant. It is believed that the excess nitrogen produced by the legume will help feed the companion growing next to it. In the Three Sisters agriculture system the bean provides nitrogen for the corn to grow and since corn needs a lot of nitrogen, it grows better.

Legumes are also an important ingredient in crop rotation. Grow corn one year and follow that up with beans or peas the following year to restore the level of nitrogen in the soil.

There is no doubt that legumes are able to capture atmospheric nitrogen (using bacteria) and convert it to plant usable nitrogen, but how much of this actually benefits other plants? Are legumes a good source of nitrogen for the garden?

Do Legumes Add Nitrogen to the Soil?

Legumes and Nitrification

Legumes, including beans and peas, are able to have a symbiotic relationship with a specific family of bacteria called rhizobia. The plant roots form nodules (little bumps), which house the bacteria. The nodules provide protection for the bacteria and the root provides them with sugars as a food source. In return, the bacteria take atmospheric nitrogen, which plants can't use, and through a process called nitrogen fixation they convert it to ammonia. The ammonia is converted to nitrate as it is absorbed by the plant.

Nitrification root nodules on Wisteria roots (hazelnut for scale)

The plant is then able to use the nitrogen to make proteins and other molecules.

This process is well understood, and is not up for debate. However, what is much less clear is how does this fixed nitrogen become available to other plants, in what quantities and when?

Note: not all legumes make nodules and some scientists believe not all legumes are able to fix nitrogen. Others believe there is a non-nodulating way for some plants to fix nitrogen.

Garden Myths book by Robert Pavlis

Most garden writers just assume that a lot of this nitrogen flows to other plants for them to use.

Nitrogen from Live Legumes

Does nitrogen move from a legume plant to a neighbor plant while the legume is still alive? This must be true if the nitrogen is a benefit to a companion plant.

The answer to this question has NOT been fully answered by science. Some research suggests that the nitrogen does move from one plant to others growing nearby. In one study broad beans were injected with radioactive urea to see where it goes. The garlic growing nearby absorbed some of the nitrogen from the bean, clearly showing it moved while both plants were alive.

Other research has shown no movement of nitrogen between plants. This movement may depend on environmental conditions, type of plants, type of soil, nutrient levels in soil or other unknown conditions.

Soil Science for Gardeners book by Robert Pavlis

We don't have a definitive answer, but if nitrogen does move from legumes to other plants, it is almost certain that the amount is small because most studies can't find any movement at all.

Building Natural Ponds book, by Robert Pavlis

Legumes keep most of the nitrogen so they can use it themselves.

Nitrogen in Living Legumes

Where does a legume store nitrogen? Some insights into this can be found inDesigning and Maintaining Your Edible Landscape Naturally, Robert Kourick, which shows a bean plant and its relative nitrogen levels.

nitrogen in legumes

In a green plant before flowering, 60% of the fixed nitrogen is found above ground in leaves and stem and 40% below ground. The same plant with mature pods has 80% of the plants fixed nitrogen in the seed, 9% in leaves and stem, and remainder in the roots.

Are Your Legumes Nodulating?

In order for legumes to form nodules and host the bacteria, the bacteria needs to be present in the soil. There are different species of rhizobia for different types of legumes. If your soil does not contain the right strain, no nodules will be formed.

Gardeners solve this problem by inoculating seed with the right bacteria at the time of planting. Little packs of bacteria can be purchased from seed companies and you can also buy seed that is already coated with the right bacteria.

Once the bacteria is in the soil, it will survive there for several years, so even a 4 year crop rotation does not need to be inoculated each time.

How do you know if you have the right bacteria in the soil? Grow the legume and have a look at the roots halfway through the summer or in fall. You can easily see the pea-size nodules if they are there and they are most visible as the plant blooms.

If the plant did not make nodules you either do not have the right bacteria in the soil, or you have too much nitrogen. Excess fertilizer will prevent the formation of the nodules since the plant simply does not need the bacteria.

Nitrogen from Dead Legumes

As discussed above, living legumes provide very little nitrogen to the soil or other plants. Once the legume dies, the nitrogen in the plant is returned to the soil, where decomposers (bacteria and fungi) convert the organic matter into free nitrogen ions, like nitrate, which can be used by other plants.

This all sounds great, but the story is a bit more complex.

The legume uses most of the nitrogen it obtained to produce seeds – the beans and peas. If you harvest the seeds or any other part of the plant, you are removing most of the nitrogen before it gets to the soil. In fact, "the residue from a corn crop (a non-legume) contains more nitrogen than the residue from a bean crop, simply because the corn crop has more residue".

Forage crops that are harvested and removed from the land provide almost no nitrogen for future plants because "roots and crowns add little soil nitrogen, compared to the above ground biomass". About 80% of a plants nitrogen is found above ground.

The other issue is one of time. Organic matter takes time to decompose. A rule of thumb is that organic matter decomposes slowly over a 5 year period with some nutrients constantly being released. This is OK for the long term gardener, but if you think that legume will provide a lot of nitrogen for next years crop, you are wrong.

There is some shorter term nitrogen released from legumes. As the plant grows it might shed the nodules as it grows new roots or matures. These discarded nodules are a quicker source of nitrogen for the soil.

Legumes and Companion Planting

Legumes are highly touted as a great companion plant because they add nitrogen to the soil for the partner plants. This is simply not true.

They may provide a reasonable amount of nitrogen in future years, provided you don't harvest a crop from them, but as a companion plant they fail to meet expectations.

Legumes and Crop Rotation

Legumes can provide a good source of nitrogen in future years provided you don't harvest a crop from them. So a legume cover crop makes a lot of sense since all of the nitrogen is returned to the soil.

Keep in mind this is a good long term strategy for building up nutrients in soil – not a short term strategy.

Clover in Lawns

Clover is a legume and many gardeners suggest growing it in a lawn because it 'adds nitrogen' for the grass. The idea that clover is synonymous with lots of nitrogen is a myth. Cutting the clover will add some green plant material to the lawn, but the same happens when grass is cut. Both plants add some future nitrogen as the material decomposes in the lawn.

Clover does not add significantly more nitrogen than grass, however, some of that nitrogen did came from the air as fixed nitrogen, whereas all the nitrogen in grass is from the soil. Leaving the clover in a lawn is a good idea, just don't expect it to add a lot of nitrogen.