The peas are up despite our April snowstorms. These cold-hardy legumes usually treat us to gallons of delectable peas. But that’s not the only reason I plant them: soon they’ll help fertilize my soil.
Peas and other legumes remind me of the fairy tale about the woman who is ordered to spin straw into gold. She secretly gets help from the magical Rumpelstiltskin, who does the work in exchange for her first-born child. Legumes take advantage of an almost-as-magical conversion: unusable nitrogen (N2, which is almost 80% of the atmosphere by volume) is ‘fixed’ into a plant’s version of gold (ammonia, NH3). But legumes can’t do this themselves. They depend on bacteria that infect their roots and ‘fix’ the nitrogen for them. And the bacteria only require food and lodging in return (not a firstborn child).
Nitrogen really is as valuable as gold for your garden. It is an essential ingredient in the DNA and proteins of all living things, and if your plants don’t find enough, their growth will be stunted. Gardeners and farmers need to add nitrogen back to the soil periodically to keep plants healthy. They might opt to buy synthetic fertilizers, but these are environmentally costly to produce and pollute ground and surface water if over-applied (Abdul-Baki 1996). Compost is a great source of nitrogen (my favorite), although it can be labor-intensive to add to a large garden and the nitrogen doesn’t become available immediately. So using legume-bacteria partnerships to fix nitrogen right in your soil can save time, money and water quality.
It’s easy to see if a legume’s bacteria are fixing nitrogen. Just pull up a pea, bean, clover, peanut, lentil, vetch or other legume, and take a look at the roots. If the right kind of bacteria are present, you’ll be able to see whitish bumps, or nodules, when the plant is a few weeks old. When the nodules turn pink or red, nitrogen is being fixed by the bacteria inside (Flynn, 2015). These symbiotic partnerships are productive: an acre of legumes and bacteria can fix hundreds of pounds of nitrogen each year. If you don’t see nodules, your legumes may not be fixing nitrogen (see below for solutions).
There are lots of possibilities for using legumes in your garden. Read on for three experimentally-supported techniques and additional suggestions for improving your nitrogen content with legumes.
1. Growing ‘hairy vetch’, a cold-hardy legume, before tomatoes may increase your harvest. Hairy vetch is grown to improve and protect the soil (not to be eaten), and its bacteria fix lots of nitrogen. In one experiment in Maryland, vetch was planted in the fall and allowed to grow all winter and spring (Abdul-Baki 1996). It was mowed and left as a mulch on top of the soil before the tomato plants were transplanted.
Compared to the conventional method of mulching with black polyethylene, the tomatoes mulched with vetch grew larger tomatoes and their leaves contained more nitrogen even though they only received half the nitrogen fertilizer. While the polyethylene-mulched plants produced tomatoes earlier, their productive season also ended earlier, resulting in lower overall yields (at least in the hot climate of this experiment). The hairy vetch mulch also saved time and money because the soil didn’t need to be tilled prior to tomato planting, half the amount of fertilizer was applied, and vetch seeds were cheaper than polyethylene mulch. For more on tomatoes, see my post here.
2. Another option is to plant rows of legumes between rows of other plants to boost productivity. One study in Pakistan grew mung beans or soybeans between rows of corn (Ullah 2007). Grown together, the combined yield of corn and beans was always higher than when either was grown alone, especially when the corn was planted with 90 cm between rows. And less herbicide and pesticide was required with this technique than when corn was grown alone.
You might try growing sweet corn or another vegetable alongside a soy bean variety, maybe for edamame. Or try growing ornamental sweet peas between some of your vegetables to beautify your garden and attract pollinators.
3. Legumes also help the plants that grow after them when included in a regular crop rotation. In a review of several studies conducted in the northern Great Plains (Miller 2002), wheat harvests were higher when grown after peas if nitrogen was a limiting nutrient.
While wheat is not typically grown in home gardens, the same principle applies, especially to plants that require lots of nitrogen. Growing peas in a different place each year will benefit the plants that grow after them.
If you’re looking for ways to add nitrogen ‘gold’ to your garden soil, make sure to plant legumes. The following suggestions will help you get the most from your legumes.
- Add a soil inoculant. If legumes haven’t been grown in your soil for 5 years or more, inoculate your soil with nitrogen-fixing bacteria (Miller, 2002). You can buy inoculants online or at many gardening stores. Look for an inoculant that is specific to the legume you plan to grow. They come in various forms; probably the easiest to use is a powder inoculant. Simply coat the legume seeds with the inoculant before planting (check the instructions on the packaging) to ensure that the right bacteria will be present in the soil to infect your plants’ roots.
- Do not fertilize legumes with nitrogen. If there is lots of nitrogen in the soil, legumes won’t fix their own (Karkanis 2016). The exceptions to this are common beans like green beans, which aren’t great at fixing nitrogen and may not even make enough for themselves. Legumes need phosphorous and other nutrients, so be on the lookout for nutrient deficiencies. For example, peas will grow purplish leaves if there is not enough phosphorous in the soil. Compost is a great source of phosphorous.
- Overwinter some legumes (Abdul-Baki 1996). If you have a small garden or wish to maximize your garden space, consider growing some legumes in the fall, winter, or spring. Cover crops like vetch can be planted in the fall and overwintered so that they don’t interfere with your regular harvests. Peas are very cold-tolerant and can be planted early in the spring and then followed by a warm-weather crop like squash in the same year.
- Leave as much of the plant in the soil as possible. Removing roots, stems, and leaves after the plant has died removes nitrogen that could be added to the soil (Flynn 2015).
- Rotate where you grow legumes to reduce diseases and non-nitrogen nutrient deficiencies (Karkanis 2016) (for more information, see my post on crop rotation). Growing legumes before a nitrogen-demanding crop like corn or tomatoes will help boost these plants’ production.
- No tilling is required (and you don’t need to turn over your garden soil) for peas (Karkanis 2016). They don’t like compacted soil, however, so try loosening your soil with a garden fork prior to planting.
Abdul-Baki, A. A., Teasdale, J. R., Korcak, R., Chitwood, D. J., & Huettel, R. N. (1996). Fresh-market tomato production in a low-input alternative system using cover-crop mulch. HortScience, 31(1), 65-69.
Flynn, R. (2015, June). “Nitrogen Fixation by Legumes”. Retrieved from http://aces.nmsu.edu/pubs/_a/A129/.
Karkanis, A., Ntatsi, G., Kontopoulou, C. K., Pristeri, A., Bilalis, D., & Savvas, D. (2016). Field Pea in European Cropping Systems: Adaptability, Biological Nitrogen Fixation and Cultivation Practices. Notulae Botanicae Horti Agrobotanici Cluj-Napoca, 44(2).
Miller, P. R., McConkey, B. G., Clayton, G. W., Brandt, S. A., Staricka, J. A., Johnston, A. M., … & Neill, K. E. (2002). Pulse crop adaptation in the northern Great Plains. Agronomy Journal, 94(2), 261-272.