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I am looking for some good vegetables which will produce enough biomass that, once the main crop is harvested, the plants can be composted and returned to the soil, and will leave the soil better than they found it, in terms of organic matter content. are there such plants? If so, what are the most effective ones?

Note: I know that the soil will be left with fewer nutrients after cropping. This question solely asks about building humus levels in the soil through composting leftover plant material.

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    Isn't that a contradiction? What you are looking for is something that leaves more nutrients in the soil than before, even though you extract nutrients via the vegetable that you are presumably going to eat. So where would those nutrients come from? – drat Jun 18 '14 at 10:14
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    legumes (Beans, lentils) fix nitrogen in the soil, when you leave parts of the plant to rot you also add carbon to the soil. But I'm no gardener. Not sure wether anything can be done about other nutrients. – mart Jun 18 '14 at 11:10
  • @drat Note. I didn't say a plant which leaves more nutrients than it takes out. I said a plant which improves the level of organic matter (humus) in the soil it is grown in, once the plant is composted and added back to the soil. – J. Musser Jun 18 '14 at 19:48
  • @mart I like the idea of legumes. see updated version of the question for better topic clarification. – J. Musser Jun 18 '14 at 19:52
  • No, improving the soil organic matter (OM) content is not a contradiction as OM is not directly taken up by crops. OM rather decomposes (the speed will mainly depend on soil moisture, temperature and OM input). If the input during the crop growth period (non harvested plant parts left in the field, ploughed under or so) exceeds the amount of OM broken down you can improve soil quality in that aspect. – thijs van den bergh Jun 29 '14 at 12:23
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The best option, as you probably know, would be to grow a cover crop right after your main crop. The cover crop should have a mix of plants that produce nutrients and biomass. Generally, this means a mix of legumes and grains. I like to use fava, oats or wheat and an annual vetch. The cover crop should be left on for the winter, then dug in to the soil in the spring. This helps to prevent erosion during the season when you aren't planting. I live in an area with a very short growing season, so I make do with planting a cover crop over only 1/4 of the garden in any given year. When I turn the cover crop over in the spring, I don't plant spring veggies in that 1/4 of the garden. This way the cover crop can break down a bit before I start planting. One important thing to note is that you should not let the legumes bear fruit - I cut them back when they start flowering. When the beans develop, the nitrogen is being pulled from the roots into the legumes. You don't want that.

Now, if you don't think a cover crop will work for you, the suggestion of legumes is a good one. You won't get all of the nitrogen benefits if you are allowing them to set fruit, but you will get some biomass. French sorrel is another good one. It is a perennial vegetable that you can use first thing in the spring. After that, I cut mine back to the ground once or twice during the growing season, and let the leaves lay on the soil. You probably already do this, but other plant parts like onion and garlic tops and squash plants can be added back, as well. The only ones I'd avoid adding back to the soil would be brassicas, tomatoes and potatoes.

  • I actually do use cover crops and green manures when the veggies are out. My question is supposed to be asking 'which vegetable plants I can grow are the most efficient at building biomass in the soil'. Not necessarily adding nitrogen either, although that would be a nice side. – J. Musser Jun 20 '14 at 1:03
  • In that case, legumes, corn, sunflower and grains would all add mass. Whether you'd want to include sunflower and grains as veggies, I do not know. – michelle Jun 22 '14 at 4:58
  • I was looking for a more scientific answer, in regard to a list of which are best, by how much they add to the soil. Perhaps in lbs per acre, or similar. – J. Musser Jun 24 '14 at 22:37
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    The answer will strongly depend on the region where you are located. A way forward would be to lookup productivity values for different crops in your region and then estimate what fraction would be harvested. If the harvested fraction is low (as in beans for example) you have a good candidate crop for improving the organic matter content of the soil. – thijs van den bergh Jul 3 '14 at 17:53

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