On soil health

On soil health

07 May 2008

Got questions about the environment? Ask Umbra.

Dear Umbra,

I asked about fava beans about two weeks ago, and have not had a response back. I have had no luck researching it myself, and would really appreciate a response. I asked at what point in the plant's life did it produce nitro for the soil? For fullest nitro replenishment, should I let the plant seed? Die off? If I pull the plant out, will the nitro stay in the soil? I thank you for your time.

KLK
Aptos, Calif.

Dearest KLK,

You'll notice that I answer two questions per week -- that's 5 percent of all of the questions that are asked. Grist has still not stated its policy on cloning employees, so although I love you all, a personal reply is often impossible at this time. That's why, over New Year's, I put together a little clip-and-save resources list for my dearest readers. You have a gardening question, so your local extension service or Master Gardeners is a first internet or phone stop, even before asking me.

Smile and nodule.

However, I love favas too. And because I am tackling soil health this week, I am going to answer this seemingly fava-specific question, which actually has broader implications.

First, a bit of background: favas are legumes, members of the Fabaceae family, which includes a huge number of edible and landscape plants. Legumes have various identifying characteristics (they're dicotyledonous, with stipules, etc.); the two most obvious to the eye are the seedpod and the flower, which resemble the seedpod and flower of beans. Alfalfa, lentils, soy beans, green beans, peas, vetch ... these are all legumes, as you'll notice when they fatten their furry pods. Legumes are a key to eating less meat, as we know; they also are vital to reducing our use of fertilizers at a home and farm level.

Proper use of legumes as "green manure" or "cover" crops can help us all reduce our fertilizer inputs and build a self-sustaining garden. Nitrogen is a vital plant nutrient, but the plentiful nitrogen in the air is in a chemical form plants cannot use. Synthetic nitrogen is the original chemical fertilizer, and still today the underpinning of our unsustainable food system. It is petroleum-based and designed for quick action in the soil. It does wonders to produce green, leafy growth in plants; it does little to build the long-term soil health necessary for a sustainable, self-sustaining system.

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Here is where legumes come in: their symbiotic root bacteria, Rhizobium, transforms nitrogen gas from the air into ammonia and ammonium, the forms that can be used by plants. We plant beans; they sprout and grow roots; if Rhizobium is present (or if we purchased and added inoculant when we got the seeds) it will start building little homes on the root hairs, taking nutrients from the plant roots for its own health and feeding the plant with nitrogen in return.

The little Rhizobia nodules (visible on the roots of the plant) are the key to nitrogen transformation, but most of that nitrogen is then used by the legume, stored in its leaves and stems. KLK, then, is asking how exactly to take the next step as a gardener: how do we get the nitrogen provided by these nodules to slough off into the soil and feed the rest of our crops, so that we can avoid buying fertilizer? Do not let the plant go to seed or die. The best way is to cut down the entire legume as it begins to flower. Either chop and mix it right into the soil to let it decompose, or compost it and then return it to the garden.

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I hope this explains why gardening books and advisers are always harping on cover crops as part of the garden rotation. You can't just grow beans to eat and hope they return nitrogen to the soil. They won't -- they've spent all their nitrogen making seeds. To build soil nitrogen with few store-bought inputs, we must dedicate part of our garden to growing legumes and then composting them.

Here lies the fava-lover's dilemma. If you grow edible favas as a nitrogenous cover crop, you must kill them just as they start to flower, and never get to eat the beans. I say don't do it. Either grow inedible favas for cover crop, or use a different legume. It's just too painful to cut down delicious future dinner.

Leguminously,
Umbra

Yours is to wonder why, hers is to answer (or try). Please send Umbra any nagging question pertaining to the environment -- but first check out her FAQs!

The claims made in this column may not reflect the views of this magazine. Neither the magazine nor the author guarantees that any advice contained in this column is wise or safe. Please use this column at your own risk.

Umbra Fisk is Grist Research Associate II, Hardcover and Periodicals Unit, floors 2B-4B.

Comments: (6 comments)

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Thanks for the info

Thanks for the very interesting article. Does the same thing go for all green covers? Do we really have to turn them under during flowering? I thought clover was a soil builder that did not require any intervention...

by timdiller at 7:51 AM on 07 May 2008

saving gas

Hello there

I am a diehard environmentalist waiting patiently for gasoline to hit the $5 mark.

In the mean time, before I make the following suggestions to our California govenor on how to save gas, I would like your opinion on how much we would save if we Californians do the following:

Bring back the 55 mile signs that were taken down a few years back. I am sure they are being stored somewhere.
Start a movement to stop gasoline use for transportation and save gasoline for the organic chemists to play with. I believe that the chemists did go to congress at one time to make that very point.
To expedite $5.00 gasoline how would you go about convincing people that we need higher taxes on gasoline?

by concerned at 10:23 AM on 07 May 2008

Petroleum or natural gas

A question on semantics:

"[Synthetic fertilizer]is petroleum-based and designed for quick action in the soil."

Synthetic fertilizer is made with natural gas (as I have learned from Umbra's blog). I have heard petroleum used interchangeably for nat gas and oil, is that a misnomer or is it an all-encompassing word?

Focus the Nation on January 31st 2008

by tboggia at 11:46 AM on 07 May 2008

cover crop

I grew up on a ranch in South Texas. My grandfather also had some fields, mostly maize and corn. He rotated his crops and every so many years (I think it was three but it could have been more), he would plant what he called "field peas" in a field and then plow them under. He said it not only built up the soil but it got rid of the weeds. I would think you could roto-till the favas or whatever and that would work. People also planted winter rye for the nitrogen too.

My grandfather had beautiful fields. He was an old country farmer (from Norway) and put down the farmers who spent a lot of money on "store bought" fertilizer and pesticides. He used to take me out in the fields at night to "hear the corn grow." And I could! He was my nature mystic mentor.

Anyway, I'm here to tell you - field peas work and Umbra's right - plow them under or compost them. You'll get a good return.

http://goodwordswan.wildflowerstew.com

by swan at 11:53 AM on 07 May 2008

eco-farming tipw

There are probably several such groups but in my area there is a group called Ohio Ecological Food and Farm Association that is all about practicing this kind of sustainable agriculture at all levels right on down to the urban households. www.oeffa.org is their website.

I am sure the occasional Grist contributor Tom Philpott would have some content to add.

My contribution is to suggest google searching the term "green manure". The concept of plowing in the Nitrogen fixing plants to enrich the soil in this manner is associated with the use of that term in practice here in the US.

Keep on Bean there, Umbra!

Moving toward sustainability with hopefulness, one revolution at a time.

by 2wheeler at 2:33 PM on 07 May 2008

Re:

Petroleum includes natural gas and oil:

A naturally occurring complex mixture composed predominantly of hydrocarbons in a gaseous, liquid or solid phase.

A generic name for hydrocarbons, including crude oil, natural gas liquids, natural gas and their products.

A mixture of liquid, gaseous, and solid hydrocarbon compounds found naturally underground.

by yamisamre at 1:54 AM on 08 May 2008
[ Parent ]

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