Plant Legumes to Help Add Nitrogen to Your Garden Soil

Crop rotations in which you plant legumes will lead to better garden soil and bigger garden harvests.

Fava Bean Growing

Broad bean, also known as fava bean, is a hardy plant grown all over the world.

Photo by Fotolia/Witold Krasowski

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Crop rotation is a part of both traditional and organic agriculture. Its roots extend back to around 6,000 B.C., not too long after the dawn of agriculture in the Fertile Crescent. The basic idea is that the farmer grows something different on a given plot of land every year. Crop rotation has many benefits, including reducing the damage from pests and disease, and retaining the nutrient balance in soil. An increase in yield of 10 to 25 percent has been found in studies of farms that practice crop rotation, and — if you include legumes in your rotation — you can avoid depleting the soil of nitrogen by doing so. With a little planning and record keeping, you can bring the benefits of crop rotation to your home garden.

Benefits

Crop rotation results in increased yields, although scientists are not sure exactly why. There are several factors that may contribute.

First of all, if you grow the same crop on the same plot of land year after year, you are enriching the soil and environment with pathogens and pests of that crop. For example, if you grow squash and your plants attract squash vine borers, the borer will consume the vine and then lay eggs that will develop into grubs in the soil. The next season, the grubs will emerge. If squash is planted in the same patch, the insects do not have to search for food. The same basic idea applies to soil bacteria, fungi and viruses. Growing the same crop on the same plot leads to a cycle of disease.

Secondly, different crops have different nutrient needs. All plants fix carbon dioxide from the air to meet their needs for carbon. Plants also need nitrogen, phosphorus and potassium as major nutrients. (Nitrogen, phosphorus and potassium are the “N,” “P” and “K” listed on bags of fertilizer.)

In addition, plants require smaller amounts of micronutrients, and different crops have different requirements. For example, in many fruits — including tomatoes — calcium is required at levels almost as high as nitrogen, phosphorus and potassium. Planting the same crop over and over on the same plot of land runs the risk of depleting key micronutrients. In addition, plants differ in the amount of nitrogen, phosphorus and potassium they require, and macronutrient deficiencies can arise if a “heavy feeder” is planted in the same location year after year.

Including both deep-rooting and shallow-rooting plants in a rotation may also help improve the soil. Deep-rooting plants help break up the deeper, compacted layers and soil, while more shallow-rooted plants help aerate the soil near the surface.

So, planting different crops in different plots every year helps move them away from potential diseases and pests, and moves them away from plots where they have depleted some of the micronutrients they require. In addition, different root structures may contribute to better soil structure. And, if you add one more element to your garden crop rotation, you can reap even bigger benefits: By adding legumes to your crop rotation, you can actually maintain higher nitrogen levels in your soil.

Legumes and nitrogen fixation

Plants get the carbon they need right out of thin air. During photosynthesis, plants fix carbon dioxide (CO2) gas from the atmosphere and, using energy harnessed from sunlight, combine the CO2 with water (absorbed through from the roots) to form sugar.

Plants need nitrogen, too — and our atmosphere is full of it. About 78 percent of the air we breathe is nitrogen gas (N2), compared to the 0.04 percent carbon dioxide used in photosynthesis. Unfortunately, from a plant’s perspective, they can’t utilize gaseous nitrogen. Most other organisms, including animals, protists or fungi, can’t either. However, various bacterial species can take nitrogen gas from the air and convert it to a source of nitrogen usable by other organisms. Legumes form symbiotic relationships with some of these nitrogen-fixing bacteria, and exchange sugar for nitrogen. Legumes are plants in the family Fabaceae (formerly Leguminosae). Legumes that could play a part in a garden crop rotation include common beans (Phaseolus vulgaris), lima beans (Phaseolus lunatus), peas (Pisum sativum), lentils (Lens culinaris), soybeans (Glycine max), fava beans (Vicia faba), peanuts (Arachis hypogaea), chickpeas (Cicer arietinum), cowpeas (Vigna unguiculata), alfalfa (Medicago sativa), clover (Trifolium), and vetch (Vicia).

The bacteria responsible for nitrogen fixation in association with legumes are gram-negative soil bacteria from the genera Rhizobium and Bradyrhizobium. These are collectively called rhizobia. These rhizobia enter the roots of the legume and form a nodule, a portion of the root that is swollen. Sugar from the plant feeds the nodule-dwelling bacteria, and they in turn fix nitrogen for the plant, turning it into ammonia (NH4+). Cells in the plant convert the ammonium into usable urea derivatives and amino acids.

When rhizobia are living in the soil, they don’t fix nitrogen, although some other types of free-living soil bacteria do. Some cyanobacteria also fix nitrogen on their own. While the nodule is active, it will turn pink or reddish-purple. As an annual legume grows, nodules will form, turn colors, then recede while new nodules are filled. As the annual reaches maturity, the plant redirects its sugar away from the nodules and to the beans in the pods, and the nodules lose their color and become nonproductive. In a perennial legume, a longer, finger-shaped nodule will form and be almost constantly active.

Contrary to some explanations of the benefits of legumes, the nodules do not leak significant amounts of nitrogen directly into the soil. Nitrogen fixation is energetically expensive, and plants pay for it by supplying sugars to the nodules. The plant, in turn, uses nearly all of the resulting nitrogen. In addition, even with the nodules, legume growth may still result in a net loss of nitrogen in the soil. Common beans, for example, can add up to 50 pounds per acre of nitrogen to the soil, but they require more than this to yield well.

Commercially, usually 30 to 50 pounds of fertilizer per acre is added to grow them. Other crops, such as soybeans, fava beans, cowpeas and peanuts can produce 250 pounds of nitrogen per acre. These crops are typically not fertilized with nitrogen. Legume cover crops such as clovers, alfalfa and vetch can yield up to 500 pounds of nitrogen per acre.

It’s common for garden sources to say that legumes add nitrogen to the soil. It’s more accurate to say that they — in conjunction with the rhizobia — fix enough nitrogen to greatly reduce the demands for outside nitrogen, compared to other plants. There are other nitrogen inputs in soil — including free-living nitrogen fixers, animal manure and, of course, man-made fertilizers. Growing legumes in one part of your cycle greatly diminishes the amount of outside nitrogen required. There are other plants that form symbiotic relationships with bacteria to fix nitrogen — such as alder trees, which form a symbiotic relationship with Frankia bacteria — but legumes are the only food crop to do so.

Growing legumes

Legumes are not difficult to grow, and do well in most garden soils. The bacteria required to form nodules is present in most soils, but you can boost the levels of rhizobia by adding a soil inoculum. These are available from most companies that sell garden seed. Different species of bacteria inhabit different legumes. For example, the inoculum for soybeans will not help if you plant common beans. Once you’ve planted a species of legume in a garden plot, the bacteria will remain in the soil for years, and you shouldn’t need to re-inoculate the next time you plant that crop.

Although legume nodules fix nitrogen, it takes a while for the nodules to form and the bacteria to multiply to the point that significant amounts of nitrogen is being produced. So, adding a small amount of nitrogen-containing fertilizer to your soil when legumes are planted can help the young plants grow more quickly. Also, nodules supply the “N,” but not the “PK,” so adding a little phosphorus and potassium around the time the plants bloom can help your crop.

Finally, remember that to enjoy the benefits of the nitrogen produced in the nodules, stir the plants back into the soil after harvest. Alternately, compost the plants and return their humus to the soil.

Crop rotations in time ... and space!

There are a variety of crop rotations recommended by various authors or gardening groups, and, of course, many gardeners practice a haphazard form of crop rotation — simply avoiding planting the same thing twice in a row. Some crop rotations include leaving one or more of your garden plots fallow some years. The idea behind leaving a plot fallow is to deprive plant pathogens of their hosts, and to allow organic material from the previous season’s rotation to break down in the soil, releasing its nutrients. In addition, a small amount — up to 5 pounds per acre — of nitrogen will return via free-living nitrogen fixers in the soil.

When letting a plot go fallow, planting a cover crop can help prevent erosion and provide organic matter to disc into the soil. If the cover crop is a “green manure” crop, the soil can also be enriched with nitrogen.

In most cases, gardeners keep all of their garden plots in production every year. In these cases, it’s highly advantageous to work edible legumes — beans or peas — into your rotation. Here are some common crop rotations and their advantages and disadvantages.

The simplest rotation

Let’s start with the simplest practical crop rotation plan. This would only be of interest to someone wanting to grow a single crop, such as tomatoes. In this rotation, the gardener would divide his garden into two plots. The first year, she would plant the first plot with tomatoes and leave the second plot fallow. The next year, she would reverse this, and the pattern  repeats.

If the gardener were interested in a second type of vegetable, not in the same family — i.e., not peppers, eggplant or potatoes — she could alternate plots and keep both plots productive instead of leaving one fallow each year. In commercial farming, the high prices commanded for corn and soybeans mean that a corn to soy alternating rotation is fairly common. In this simple rotation, if both plots were kept in production each year, you would have to apply more fertilizer compared to a situation in which one of the plots was left fallow. On the home scale, this would not present much of a problem. The gardener could simply apply granular fertilizer, compost, or a mixture of the two.

Although this alternating rotation would work for the dedicated tomato gardener, there are a couple of potential problems. First, the gardener might develop micronutrient problems in the plots. Tomatoes need calcium, magnesium and sulfur to almost the degree they need nitrogen, potassium and phosphorus. They also require a whole slew of other micronutrients in smaller quantities. In a two-year tomato-to-fallow rotation, one or more of these nutrients may gradually be depleted. More importantly, this rotation would eventually enrich any tomato pathogens in the plots. This could lead to an outbreak of disease that would require the gardener to quit planting tomatoes, or any plants in the Solanaceae family, in those plots for at least a couple of years.

More practical rotations

In practice, the home gardener will have a list of plants she is interested in growing, and a fixed amount of garden space. The garden can be divided into any number of plots, and each plot could have its own rotation pattern, with three-year, four-year, or longer rotations, if desired. In many cases, mine included, rotation is not extensively planned — the gardener simply avoids planting the same thing in the same patch of ground. Also, let’s face it, sometimes what you most want to grow conflicts with the rules of crop rotation. In an otherwise healthy garden, planting, say tomatoes, two years in a row in the same plot will probably be fine. This is especially true if there was no sign of disease the previous season.

You can assemble a workable garden rotation simply by dividing your garden into a reasonable number of plots and following these rules of thumb:

1. Keep a record of what you plant each year and where it was planted. Consult this when planning the next season’s garden. You can plan crop rotations if you like, or treat each season like a puzzle.

2. If you enjoy edible legumes, include them in the rotation in every plot. If you don’t, work clover, alfalfa, vetch or other “green manure” cover crops into the rotation every three or four years.

3. Rotate crops by biological family. Some older garden guides group vegetables into functional categories, such as leafy vegetables, fruits, roots, etc. However, this can cause problems as a biological family can have members that yield more than one type of vegetable. For example, the Solanaceae family includes tomatoes, peppers, eggplants (which are fruits, botanically), and potatoes (tubers).

4. Follow the legume season with a plant that requires a lot of nitrogen. These so-called “heavy feeders” include corn, tomatoes and most greens. Follow the “heavy” season with one or more seasons of “light feeders” before rotating back to legumes.

5. To break the cycle of disease, use at least three-year cycles within each plot. Longer cycles are better. And if possible, try to plant members of the same family as far away as possible from the previous season’s patch. For instance, if you planted squash in one patch, try not to plant pumpkins in the adjacent patch the next season. This can sometimes be hard to do and is something that you can usually ignore if last season’s garden was healthy.

6. If a serious outbreak of disease occurs in a small garden, retire all members of that plant family from the rotations in every plot for at least two years.

7. Even with legumes in your crop rotation, expect that you will need to add some fertilizer to your garden plots each year. Although crop rotation, and the benefits of planting legumes, helps, it can’t completely counter the effect of growing vegetables in the same plot year after year.

8. Gardening is a hobby, and it should be fun. Don’t let the idea of crop rotation force you to plant anything that you don’t enjoy growing or discourage you from planting the same thing in the same plot for two years in a row. Likewise, some plots in your garden may not be suitable to grow every type of vegetable. This may be due to partial shading, soil pH, drainage, or other reasons. Don’t let a crop rotation plan compel you into planting something where you know it won’t thrive.   

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Chris Colby is an avid gardener who lives in Bastrop, Texas, with his wife and their cats. His academic background is in biology — a Ph.D. from Boston University — but his main interest is in brewing beer.