Sunday, February 13, 2011

It's alive!

What could be more dead than dirt? We all know the difference between soft, squishy mud between our toes and beach sand. It looks and feels different. Your nose will tell you the difference between a rich, fruity loam and a sterile, powdery clay. But beyond the structure of the soil, what makes plants grow better in one soil over another? I wish that I knew the complete answer, but for now I have some ideas. Here is one difference...

Nitrogen fixing root nodules
After taking a round of antibiotics, your doctor may recommend eating yogurt for the acidopholus bacteria to help replace the naturally occurring bacteria in your body to help you digest food. Did you know that dirt needs bacteria to process and store the nitrogen necessary for plants to grow? Even legumes which are known for the nitrogen producing nodules on their roots require the same bacteria for them to make their internal nitrogen available in the soil. This bacteria, with enough carbon (organic material) mixed into the soil, can process the nitrogen from rotting organic material and store it in the earthy loam to make it available for plants. Your nose will also tell you if things are out of balance. That nitrogen is instead released as ammonia gas, which contributes to greenhouse gases rather than benefiting those plants, you and your family.

I learned about this stuff recently so I wasn't surprised when I noticed that the seed catalogs which seemed more concerned about natural and organic growing techniques were also selling inoculants to mix with nitrogen producing plant seeds. This is like acidopholus supplements for sick garden soil! Of course it isn't always necessary to use this supplement if your soil is already full of this naturally occurring bacteria.  This would occur over several years of adding steer manure. But if you are not sure that your soil has the right bacteria, it wouldn't hurt to give it a little more to be sure.

This year one of the things that I plan to do in my garden is to test this by planting "green manure" of legumes mixed with an inoculant, and tilling that in before we plant the rest of the garden. I have found that Johnny's Seeds has a great selection of these types of seeds. Of course, having not done this type of thing before, I'm going to need to do some more research to make sure how long I need to let this grow but still have enough time to grow the rest of the garden. (It may be that I can only do this year with those areas in the garden that can be planted later in the season.)


  1. Hi, I grew beans for the first time last year and used an innoculant since I didn't know if my soil had that bacteria. What I need to find out is what to do with the bean plant at the end of the season. Do you leave the roots in the ground and compost the rest of the plant? Lots to learn!

  2. I think that the easiest thing for you to do, assuming that the dead plants are still there, would be to cut off the tops and compost them and till in the roots. The majority of the nitrogen benefit comes from the root nodules when the roots are decomposed.

    From what I understand, the nitrogen-fixating bacteria in the nitrogen nodules of the legumes are not beneficial to nearby plants (growing at the same time) or next year's plants until the legume plant dies and releases the nitrogen in its roots into the soil. If you let a bean plant run its course of life, it will use a large portion of that nitrogen to sustain that plant for the season, although you will still get some benefit once the previous year's roots have decayed.

    I believe that the biggest benefit to the soil and other plants is to treat the legume as a cover crop or green manure. The general rule for timing this is to mow or cut them down just as the plants begin flowering, or a bit earlier if you need to start planting the rest of your garden. Wait a day or two for them to dry out a bit and then till them into the soil to decompose. The decomposing process can tie up the nitrogen, so it is important to plan on waiting two to three weeks after tilling before you plant the next set of crops.

    This year I plan on planting a cover crop in my garden as soon as the soil has dried out enough to till. By the time that weather is warm enough to plant the rest of the garden, the cover crop should be cut down and tilled in to benefit the next set of plants.

    Just a caveat... I haven't actually tried this in my current garden. There is a danger of waiting too long and a cover crop going to seed and then coming up again with the next set of plants. They are weeds at that point, since you don't want them any more. I'll keep you posted on my blog with my progress on this topic!