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MAKE THE MOST OF LEGUMES TO FIX AND KEEP NITROGEN IN YOUR SOIL

by Janet Wallace

 

"Legumes are an integral part of crop rotations, providing nitrogen ..." We’ve all heard this and we all grow legumes to add nitrogen, but can we rely upon legumes to always add nitrogen?

It’s true that legumes can add relatively large amounts of nitrogen to the soil, but simply growing a legume does not ensure nitrogen will be added. Sometimes legumes don’t nodulate and the nitrogen is not fixed. Other times, the plants fix nitrogen but the nitrogen is removed at harvest. For example, if peas are grown and the plants pulled up when they are harvested, there is probably no net gain of nitrogen to the soil. At other times, some of the nitrogen can be leached away, unless captured by catch crops or carbon-rich material. Fortunately, there are many steps farmers and gardeners can take to help add and retain nitrogen.

 

Nitrogen Fixation

Nitrogen fixation occurs when specialized strains of bacteria (rhizobia) infect the roots of legumes (beans, peas, clover, vetch, alfalfa, lupins, sweet clover, etc.). Nodulation occurs when the legume roots form a growth, called a nodule, around the bacteria. Within the nodules, the bacteria convert atmospheric nitrogen into usable plant compounds (such as ammonia and nitrate). After the plant flowers, the nitrogen moves from the roots into the seeds. Eventually the legume dies and releases the stored nitrogen as it decomposes. About half of the nitrogen will be released within a year after incorporation into the soil. The rest of the nitrogen will be released over a period of a few years.

 

Inoculation

Legume seed should be inoculated to ensure contact between rhizobia and the seed. Farm and garden supply stores often sell a few types of inoculant, each of which inoculates several related legume species. These inoculants are readily available, but not as effective as inoculants specific to only one species. Inoculant should be kept refrigerated until used and replaced each year. To inoculate seed, moisten the seed with water (or with a very dilute sugar and water solution) and mix in the inoculant. Plant the seed immediately after mixing as the inoculant will die if it dries out.

Rhizobia can survive in the soil for a couple of years, so if you are planting a legume in the same place it grew two to three years ago, assuming it nodulated successfully, rhizobia are probably still in the soil. Also, if you are planting a legume that is closely related to a common weed on your farm, there are likely rhizobia present in the soil. In these cases, inoculation with a commercial inoculant is not necessary for N-fixation, but it does provide a cheap and easy way to possibly increase the amount of nitrogen fixed.

You can also acquire their own inoculant by collecting soil from around the roots of nodulating legumes. Store the soil in a cool place over winter and next spring, mix the soil with seed of the same legume species. This is not as convenient as using commercial inoculant, but it seems to work.

 

Seeding & Establishment

Most legumes need well-prepared seed beds that are firm, level and weed-free. After seeding, the seed should be harrowed (or raked) and tamped in. Legumes need weed-free beds since they are relatively slow to become established and are vulnerable to weed competition.

Frost-seeding and overseeding are effective ways to help legumes become established. To frost seed, broadcast seed onto frozen ground in the spring, just before the soil starts to thaw. The freeze-thaw movement of the ground will pull the seed into the soil. Frost-seeding gives the seeds an early start; they can be growing while the rest of the ground is still too wet to be worked.

To overseed, broadcast seed into an existing stand of other crops. For example, red clover is often overseeded into grain fields in the spring after the grain has become established. After the grain is harvested, the clover can continue growing for another season. Legumes can also be overseeded a few weeks before dieback of the main crop. This way, the canopy begins to open just as the legumes need more light.

Legumes combined with grains or grasses provide excellent green manures. The legumes can be overseeded or broadcast with the grasses. The grasses or grains control weeds, add organic matter and can provide structure for viny legumes (e.g. vetch) to climb on; the legumes provide nitrogen. After the green manure is turned under, the carbon in the grains or grasses will tie up some of the fixed nitrogen, preventing it from being leached. After trying a number of green manure combinations, I was most impressed by crimson clover mixed with annual ryegrass, and oats mixed with common vetch. (To combine green manures, cut the grain/grass seeding rate in half and mix with the legume at 2/3 the regular legume seeding rate.)

Ideally, the legume seedbed is kept moist for several weeks after planting. Many legume seeds are close to the surface and take a couple of weeks to emerge. Consequently, the seeds are quite susceptible to drying. Also, the bacteria in the inoculant will die if the inoculant dries out. For these reasons, legumes are often planted in the spring or overseeded or planted just before rain is expected.

 

Checking for nodulation

So, now the legume is growing, but is it fixing nitrogen? If you are curious, carefully dig up the roots of a few legumes that are about to flower (4-8 weeks old). Look for round or oblong growths on the roots: these are nodules, the site of nitrogen fixation. Their presence indicates that the legume was successfully inoculated and probably is (or was) fixing nitrogen. Ideally there should be a few large nodules or many small nodules. For example, a hundred small nodules on a clover plant or two dozen large faba bean nodules is excellent. To see if the plant is actively fixing nitrogen, break a few nodules in half. Active nodules are red or pink inside; inactive ones are green or black. You might want to pull up some leguminous weeds to see what their nodules look like; weeds often provide excellent examples of nodulation.

If a legume isn’t nodulating, there are a number of possible explanations. The legume may be too young or too old – nodulation peaks just before flowering. The legume may have been stressed – legumes slough off the nodules when stressed by drought, mowing or other adverse conditions. The soil may also be too rich – legumes are lazy. If there is a considerable amount of nitrogen in the soil, the legumes will simply use the soil nitrogen rather than fix nitrogen. Legumes fix the most nitrogen when no nitrogen source has been added to the soil for the last two years.

 

Incorporating green manures

There’s a common assumption among gardeners that you can plant peas or beans to add nitrogen to the soil and harvest a crop from them as well. Unfortunately, there’s a trade-off involved. Nitrogen fixation in legumes peaks at flowering. After that, the nitrogen moves to the seeds. In the seeds, the nitrogen is converted into amino acids, the building blocks of protein. (This explains why the dried seeds of legumes such as beans are high in protein.) When the beans or peas are removed, most of the fixed nitrogen is harvested as well. If the rest of the plant is removed, there is likely no net gain of nitrogen. Removing the plants and the roots could result in a loss of nitrogen.

To add the most nitrogen to your soil, green manures should be incorporated when they start to flower. Legumes break down quickly, and the nitrogen is released into the soil as nitrates. Nitrates are readily available for plants, but are also susceptible to leaching. So, even if a green manure is successfully established, nodulated and incorporated, the nitrogen may still be lost. If a green manure is turned under in late summer or early fall, and the soil stays bare throughout from the fall rainy season through the next spring rain, much of the nitrogen may be leached.

There are many ways to retain the nitrogen in the soil. The nitrogen can be taken up by living plants such as catch crops. Catch crops can be planted after the legume is turned under, and will take up the nitrogen and hold it in their tissues until they decompose. The catch crops are incorporated the following season. Excellent catch crops include fall rye, ryegrass, oats, white mustard, oilseed radish, buckwheat and phacelia.

The other way to hold nitrogen in the soil is by adding carbonaceous material to bind up the nitrogen. (Carbonaceous material is what composters call ‘browns".) For example, straw can be tilled into the soil. Another way to incorporate carbon-rich material is to grow the legume with a grass or grain. When the combined green manures are incorporated in the soil, the nitrogen will be tied up by the grain or grass, and there should be no problem with leaching.

In the farm or garden, legumes can be used as green manures by incorporating a plowdown crop into the crop rotation every three to four years. Legumes can also be used for intercropping or companion planting. For example, soybeans can grow between corn rows, clover can be a living mulch around brassicas or nightshades, and clover can be overseeded into grain stands.

If you are interested in more information on using legumes, catch crops and other cover crops, the Nova Scotia Organic Growers Association (NSOGA) Cover Crop Research Project has produced a 50-page guide entitled Under Cover: a guide to using cover crops in the Maritimes. The book costs $10 plus $2 shipping & handling. For a copy, contact Janet Wallace, RR #1, Margaretsville NS B0S 1N0, e-mail <jwallace@istar.ca>.

 

Janet Wallace is a researcher with the Nova Scotia Organic Growers Association’s Cover Crop Research Project. The project is currently funded by Environment Canada’s Action 21 Program, and Janet’s position is subsidized by the On-Site program.

 

Copyright 1997. Janet Wallace.

Reprinted with permission. All rights reserved.


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