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?
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.
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.
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 (link no longer available). This movement may depend on environmental conditions, type of plants, type of soil, nutrient levels in soil or other unknown conditions.
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.
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 in Designing and Maintaining Your Edible Landscape Naturally, Robert Kourick, which shows a bean plant and its relative nitrogen levels.
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 plant’s fixed nitrogen in the seed, 9% in leaves and stem, and the 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.
Does the Three Sisters Agriculture Work?
I discussed this in more detail in Three Sisters Agriculture. The part of the story where the bean provides nitrogen for the corn is a myth.
References:
- Image of root nodules; by Rowan Adams
I checked the reference you linked to with the statement “other research has shown no movement of nitrogen between plants.” Not only is this the same reference you linked to for the urea study showing positive movement, but in the introduction of this paper, there are at least 7 studies referenced that show small amounts of nitrogen movement between plants. This directly contradicts your conclusions.
I am very disappointed in the fraud you are promoting.
If you have a look at that introduction in that link you will see references to studies that found as little as 2% nitrogen transfer and “Sierra and Desfontaines (2009) studied N transfer using 15N
leaf feeding method and concluded that N transfer from root exudates of jackbean (Canavalia ensiformis L.) would be a useful but minor process compared with N release from root turnover in soil”
Numerous studies have demonstrated that nitrogen-fixing plants do transfer nitrogen to nearby plants while they are still alive (references below). Most likely this is due to the continual shedding of root tissue and the natural death and decomposition of symbiotic microbes. Just as we are perpetually shedding dead skin cells, plants are also continually casting off dead root tissue into the soil and leaching carbon, nitrogen, and other molecules. Some of that carbon, nitrogen, and other essential nutrients becomes available for other life. Killing cover crops before they seed and working them into the soil maximizes the benefits, but it’s also true that living nitrogen-fixing plants are adding nitrogen to soil and that some nitrogen is transferred to nearby plants. Of course the details depend on the plants in question and their environment and some of the mechanisms and details remain unclear.
References:
https://link.springer.com/article/10.1007/s13593-016-0396-4
https://link.springer.com/article/10.1007/s11104-011-0882-z
https://www.sciencedirect.com/science/article/abs/pii/003807179190189Q
https://link.springer.com/article/10.1007/BF02856678
https://acsess.onlinelibrary.wiley.com/doi/abs/10.2134/agronj1994.00021962008600020010x
However, unbeknownst to most gardeners, the residue from the corn plants contains more nitrogen than the residue from a bean crop, simply because the corn crop has more residue [biomass]. A perennial or forage legume crop only adds significant nitrogen for the following crop if the entire biomass [stems, leaves and roots] is incorporated into the soil. If a forage plant is cut and removed from the field, most of the nitrogen fixed by the forage is removed. Roots and crowns add little soil nitrogen compared with the above ground biomass.” Nitrogen Fixation by Legumes Guide A-129 W.C. Lindemann, Soil Microbiologist C.R. Glover, Extension Agronomist, New Mexico State University (from my book in progress)
Could you explain this more clearly:
“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.”
Does this mean you don’t allow the plants to flower, or the plant to go to seed? Or you allow them to do so but then till them under?
I have a nitrogen deficiency and looking at cost effective ways to restore my soil (I’m willing to wait until next year if needed)….and already bought some beans. Although now I need to figure out if they do the fixin’ 😉
Thank you!
Going to seed or not is not important. The important point is that if they produce seed, that you don’t harvest the seed.
Robert, I notice in your articles that you constantly are right in what you say but conclusions are wrong. Like here. If people just read this but don’t fully understand N-fixation they will conclude that legumes are to never to be used. Your right that if I were to harvest my cowpeas then I wouldn’t be adding much nitrogen to the soil. But cover crops you don’t let go to seed as this causes weeds and low nitrogen added to the soil. But if I till in those same cowpeas or clover I am adding 80-300lbs of nitrogen to the soil, along with tons (literary) of organic mater and when they decompose even more nitrogen is added. You forgot to say that by killing said legumes when they first start to flower is how to maximize N-fixation and how to properly use cover crops. I grow peas to harvest but also plant clover with them as a green mulch and cover crop and add more nitrogen to the soil than any crop needs. Legumes can reduce to eliminate any need for N. It just takes proper use and timing to get to work correctly. Please in future posts don’t just justify one point while not even explaining the correct way, or even counter arguments about your topics. If you actually farm for a long time you realize that what works for you may not work for her. But what works for her doesn’t work for you. Is she right or you. I say both are right, and wrong. You need to “DIG DEEPER” and realize that under different situations, weather, soil types, that different results will occur.
Discussed under section, Legumes and Crop Rotation.
I’ve read about using legumes as part of a green manure mix. It sounds like this method could add nitrogen since you are not harvesting a crop.
Correct.
The number (in percents) are found on page 265 of my Edible Landscape book. At maturity, 70-90% of the nitrogen is in the seeds.
I have updated the post with this information – Thanks Robert.
Yet another great article challenging conventional notions
I was going to make Lee Reich’s point but he beat me to it.
I do feel you are being a little unfair to the amount of nitrogen returned to the ground in plant debris and its gradual release to other plants especially in nature. I don’t think good gardeners remove as much as you say when they take their crops.
Not quite sure about most of the nitrogen going to the seed – although it is significant
At least fixation makes a great contribution to the actual crop in areas of low fertility. And nitrogen fixers make a significant contribution to natural ecologies in infertile places
A lot of the nitrogen gets concentrated in the fruit and if this is harvested, so is much of the nitrogen. I tried to find numbers to show how much nitrogen each part of a legume has, but never did find the data.
Nitrogen fixation is critical in natural areas.
Clover in the lawn also has another key advantage, it is often green in the summer when dry weather comes and grasses are turning brown. Plus it’s good for bees if you let in flower.
You make a good point about green manure/cover crops, if legumes are allowed to seed most of the nitrogen goes there. Best to cut it as the flowers fade. That way you get the bees and some nitrogen.
Having a rotation with peas or broad beans that are cropped won’t do a lot for the soil, other than a small amount of compost. Which will happily rot on the surface if you’re practicing minimal tillage.
Another excellent post. My only beef is the statement: “The other issue is one of time. Organic matter takes time to decompose. A rule of thumb is 5 years.” This implies that it takes 5 years when, in fact, the N is released over the course of 5 years (or more). You do make this a little more clear in a subsequent statement. Good agriculture should not be based only on short term gains. Part of this involves building up a spectrum nutrient reserves in the form of organic matter in the soil, which obviates the need for emergency dosing of the ground — or leaves! — with concentrated nutrients.
Valid comment – I’ve made a change to make the message clearer. Thanks.
Hi Robert, Great piece. I agree with the Three Sisters conclusion. For a detailed drawing of how Nitrogen moves around see page 265 of my Edible Book. Best, Robert