You’ve probably been told to plant beans and peas in your garden because “they’re good for your soil.” But, why are beans and peas good for your soil? Beans and peas are part of the the legume family. Legumes, through a symbiotic relationship with bacteria in the soil, perform nitrogen fixation.
Nitrogen fixation is important in the garden because atmospheric nitrogen is converted, via nodule-forming bacteria, into usable nitrogen for your soil. Nitrogen is an essential building block for strong, healthy plants.
Once nitrogen fixation occurs, how can you ensure that you are getting the most nitrogen compounds out of your legumes and back into your soil? To learn more about nitrogen fixation, why it’s important, and how to incorporate it in your garden – please keep reading!
Why is nitrogen important in your garden? What happens if you don’t have enough nitrogen in your garden?
Nitrogen acts as a building block for your plants. Nitrogen is necessary for plant growth and development. All plants need nitrogen, phosphorus, and potassium to grow. Nitrogen can arguably be the most difficult of the three for a plant to absorb. Nitrogen makes up about 78% of our atmosphere but plants cannot absorb it directly from the air. Nitrogen, and nitrogen compounds, are a major component of chlorophyll and the protein building blocks in a plant.
One of the initial signs of nitrogen deficiency in your garden soil include pale, yellowing leaves. The primary reason that the leaves turn yellow and pale is because nitrogen is an essential building block of the plant’s chlorophyll. A plant’s chlorophyll is responsible for performing photosynthesis. On the other hand, excess nitrogen in soil can lead to strong leaf growth but little to no fruit and vegetable growth.
What is biological nitrogen fixation?
Biological nitrogen fixation occurs when atmospheric nitrogen is converted to nitrogen compounds such as ammonia, nitrate, amino acids, nucleotides, and nitrogen dioxide. Nitrogen makes up the majority of our atmosphere but is unable to be directly absorbed by plants. The nitrogen in the air must be converted to nitrogen compounds that a plant can readily use. Biological nitrogen fixation was first discovered by a Dutch microbiologist by the name of Martinus Beijerinck. Nitrogen fixation, he discovered, occurred when bacteria in the soil would form a symbiotic relationship with legumes. The legumes form nodules on their roots and stems to convert nitrogen into an acceptable form used by plants. The nodules are formed by “rhizobia” and are a bacteria that obtain nutrients from the legume plant and produce nitrogen that is then stored in the plant. Rhizobia are bacteria that fix nitrogen by forming nodules on legume plants. Rhizobia require a host plant, such as a legume plant, to fix nitrogen and will not fix nitrogen on their own.
Examples of nitrogen fixing plants
Legumes act as excellent nitrogen fixing plants for any garden or farm. Legumes form a symbiotic relationship with naturally occurring nitrogen-fixing bacteria in the soil. The bacteria, called rhizobia, infects the roots of the legume plant and forms nodules that fix atmospheric nitrogen in to useful nitrogen compounds. A symbiotic relationship is a relationship between two unlike species. There are different types of symbiotic relationships but in the case of legumes and nitrogen fixing bacteria – it is a mutualistic relationship whereby each species benefits.
Good examples of nitrogen fixing legume cover crops include peas, soybeans, and beans. Cover crops are specifically planted to benefit the soil. The goal of a cover crop is to regenerate the soil and suppress weeds. I’ll note that not all legumes are nitrogen fixers. A subfamily named Caesalpinia within the Fabaceae/ Leguminosae family were found to largely not form rhizobia nodules. Legumes are not the only plants that form beneficial, symbiotic relationships with nitrogen-fixing bacteria. Grasses, sugarcane, maize, rice and buckwheat have been found to form similar relationships.
So now that I’ve convinced you of the benefits of incorporating nitrogen fixing plants in your garden, what are some options? I suggest to growing a legume that is multi-purpose – something that you can eat or aesthetically admire while it grows. Nitrogen fixing legumes well-suited for the garden include:
- Bush beans
- Fava beans
- Runner beans
- Soy beans
Suggestions of how to incorporate nitrogen fixers in your garden
An excellent way to incorporate nitrogen fixers in your garden is to interplant legume cover crops such as beans and peas. Once mature, cut the plant where it meets the soil, leaving it in the ground. Save the leaves and either compost or chop them up and turn it back into the soil. It’s important to reincorporate the entire legume plant back into your garden because:
- before the legume flowers, 60% of the fixed nitrogen is found in the leaves and stems and only 40% found below ground. A mature legume plant stores 80% of the fixed nitrogen in their seeds, and only 9% in the leaves and stem, and 11% below ground.
The primary methods to incorporate nitrogen-fixing plants back in to your soil are:
- The chop-and-drop method is a method whereby you chop all the leaves and plant up and drop it on the soil or ground, providing nutrients for your soil.
- The fertilizer tea method is a method whereby you soak the plant in a bucket of water for about a week or so and then you the “fertilizer” liquid on your plants.
Plant beans in spring/ summer and plant peas in fall/winter/spring. Incorporating nitrogen fixing legumes into your garden rotation will help to build healthy soil for your future crops.
Things to consider
There are some things to consider before incorporating nitrogen fixers into your garden. First, you need nitrogen fixing bacteria present in your garden in order for the rhizome nodules to form. The easiest way to tell is when your legume plant begins to flower pea sized nodules should form on the roots. If they are not present, you may need to inoculate with rhizobia bacteria. Another thing to consider is if there are too many nitrogen compounds in your soil already, nitrogen fixation may not occur. This will happen if you fertilize regularly with nitrogen-rich fertilizers.
Bayton, R., and Maughan, S. 2017. Plant Families: A guide for Gardeners and Botanists. The University of Chicago PRres, CHicago.
Goormachtig, S., W. Capoen, and M. Holsters. 2004. Rhizobium infection: lessons from the versatile nodulation behaviour of water-tolerant legumes. Trends in Plant Science 9: 518-522.
Hirsch, A., Lum, M., and Downie, A. American Society of Plant Physiologists. 2001. What Makes the Rhizobia-Legume Symbiosis So Special?.
Sprent, J.I. 2001. Nodulation in Legumes. Royal Botanic Gardens, Kew, UK.