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It seems reasonable that there ought to be at least a few symbiotic plant varieties that benefit from improving soil condition for other plants.

If not that, are there plants that essentially improve soil conditions if not by improving land fertility?

Improve may be relative as certain plants do have different requirements so I am imagining a sort of 'balancing' plant.

Maybe the answer is something like 'most non-fruiting plants' but I'd think most of those still are about getting what they need and not improving it for others.

For the sake of argument, say you have a near-barren lawn that only has little bits of scattered grasses. Aside from manually adding the fertile soil, can plant-life improve it?

15

Legumes are often used for this purpose. From Wikipedia:

Legumes are notable in that most of them have symbiotic nitrogen-fixing bacteria in structures called root nodules.

And

Within legume nodules, nitrogen gas from the atmosphere is converted into ammonia, which is then assimilated into amino acids (the building blocks of proteins), nucleotides (the building blocks of DNA and RNA as well as the important energy molecule ATP), and other cellular constituents such as vitamins, flavones, and hormones. Their ability to fix gaseous nitrogen makes legumes an ideal agricultural organism as their requirement for nitrogen fertilizer is reduced. Indeed high nitrogen content blocks nodule development as there is no benefit for the plant of forming the symbiosis. The energy for splitting the nitrogen gas in the nodule comes from sugar that is translocated from the leaf (a product of photosynthesis). Malate as a breakdown product of sucrose is the direct carbon source for the bacteroid. Nitrogen fixation in the nodule is very oxygen sensitive. Legume nodules harbor an iron containing protein called leghaemoglobin, closely related to animal myoglobin, to facilitate the conversion of nitrogen gas to ammonia.

But in a lawn, there isn't really a way to improve conditions (naturally) without amending the soil. Adding decomposed organic matter in bulk is the way to go. It takes years, but you can also go from barren land to lawn with the use of intensive green manuring, where nitrogen fixers and mass growers are grown and plowed under for the specific purpose of improving the soil. In good soil, rye (in winter) and buckwheat (in summer) will add biomass fast. Good nitrogen fixers will include vetch (in winter) and red clover or alfalfa (in summer). There are many, many more, but these are the reliable ones I've tried that come to mind right away.

So green manuring with balances of leguminous/non-leguminous plants is the most effective means of plants building soil. In nature, what you see as topsoil is darker than the subsoil because of humus, which is amorphous decomposed organic matter. You will not get much more than a very small fraction of an inch of new humus per year in nature. Topsoil took a very, very long time to form.

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    Although this is true, I think it avoids the question. No plant, not even nitrogen fixers, add nutrients as they grow. Plants produce biomass and organic matter which will recycle back into the soil if left there. Not even nitrogen fixers add to the soil in life, if you take the biomass offsite the nitrogen goes with them. There are deep taprooted plants that mine minerals, but the minerals are only added to the soil in decomposition of the organic matter. Everything adds when its parts decompose. – Alex Oct 3 '14 at 23:19
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    That said, the nitrogen fixers do get an extra boost from the air, so they decompose more nutrients than they took from the soil. This may not be true though if you harvest a part of the crop for other uses, like pole beans. – Alex Oct 3 '14 at 23:20
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    @Alex trees, shrubs and woody vines add biomass when the leaves fall, perennials when they die back, annuals of course when they die. The point is, most plants build soil, but to do it fast, you need to intervene, growing special plants in special conditions, and turning them under prematurely. And FYI, legumes do not produce nitrogen, the bacteria in the root nodules do. They release the ammonium they converted from the nitrogen into the plant, which often releases some into the surrounding soil. That's why grass grows better in a patch of clover than on its own, when under low N conditions. – J. Musser Oct 4 '14 at 0:10
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6

There are many potential problems that could exist in poor soil and there are endless types of plants that can help repair poor soil. In your example of "near-barren lawn", one potentially serious problem you are likely to have is soil compaction. If that is the case, then all solutions you attempt may fail until you directly address that issue.

If you have some time, here is an excellent video that goes over 6 no-till options for building healthy soil (wood chips, straw bales, soil/compost bombing, sheet mulching, Ruth Stout method, living mulch, and hugelkultur): https://www.youtube.com/watch?v=tV6sbGWE0E4 In each other these methods the soil can improve year over year without needing to import fertilizer to do it.

Also, if your interest is more around lawns. Here is an excellent approach called "Organic Lawn Care For the Cheap and Lazy": http://www.richsoil.com/lawn-care.jsp

5

Yes, living plants do provide for bacteria and fungi and other soil organisms while they are alive and thriving. Plants need bacteria, fungi and bacteria, fungi need plants. They've got a symbiotic relationship as well as a biotrophic and necrotrophic relationship.

A few reports I've read give plants credit for a 40% addition of carbon around the roots to attract bacteria/fungi! While they are alive and thriving. Plants need help to take up the nutrients critical to their survival. When we till or disturb the topsoil a lot of this carbon is released into the atmosphere as CO2. Supposedly a huge contributor of CO2!

Regardless, live plants feed their soil organisms. Nitrogen fixing plants as well as nitrogen fixing bacteria help the crop and soil but gets complicated. Planting legumes won't necessarily negate the need for additional nitrogen.

This report about symbiosis of plants and animal/bacteria/fungi is pretty good. Dry, but to the point (pages 191-192).

Using decomposed organic mulch is the best way to improve any soil! In fact, the only way. Just applying it to the top (controls germination of weed seeds) of the soil is all that is necessary and it makes a huge difference in one season. No mixing/digging on your part. In fact, it is better that you 'till' the soil as little as possible.

I double dig my garden beds and planting beds one time only at the very beginning. Keeping plants thriving in the soil, improves the soil. Mulch on top keeps improving the soil (decomposed organic mulch), immediately. During the winter a green cover crop is powerful! Plant in the fall after cleaning up old vegetable plants. In the spring just use a spade to turn the (clover, buckwheat, annual rye...etc.) over before the plants can go to seed, dig out drainage trenches around the bed tossing the soil on top of the turned under cover crop and allowing a good month to decompose.

A bed of soil that has been raised (due to double digging and no traffic to compact) with live plants, good decomposed organic mulch, extended release organic fertilizer and an avenue (trench) for excess water is all one needs for 'improving' the soil and having a complex, healthy ecosystem! And deep, infrequent watering.

4

There is evidence that nitrogen fixing plants such as clover and some beans do improve nitrogen levels if the roots are left in place, but generally speaking, plants don't improve the soil while they're growing - they improve the soil when they're dying, hence a green crop, ploughed in, works in that way. Biomass or humus rich materials added to soil keep the general fertility up rather than just improving nitrogen levels alone, improve bio diversity within the soil, and increase mycorrhizal activity, but this is in the process of breaking down, not while they're actually growing.

  • What about trees in forests? They build up soil overtime, even while alive, and then there are prairie/grasslands, which also build up soil while maintaining a live polyculture. – J. Musser Oct 4 '14 at 17:00
  • @J.Musser: I bet you know the answer to that already, if you think for a minute - forests are allowed to retain the fallen leaves on the ground - they rot down and contribute to the soil. Equally, grasslands are left to their own devices and dead vegetation rots back down. Death does serve a very useful purpose in the world, to the still living, or at least, the decomposition process does. – Bamboo Oct 4 '14 at 17:09
  • But, the plants that built the soil were still very much alive. Of course parts of them must die to build soil. – J. Musser Oct 4 '14 at 17:45
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    Plants DO take care of bacteria/fungi while they are alive. In a stable, long-established ecosystem, nutrients are tied up in the live biomass. When land is 'mined' of plants/trees, the bulk of nutrients are also taken away. We have to replace nutrients when ecosystems have been 'mined'. While an ecosystem thrives, just enough dead material and nutrients are released for necessary new growth. Plants 'donate' a large portion of their food production whilst alive to feed the organisms necessary to uptake nutrients to make food. When they die and they are left in this ecosystem they add more! – stormy Oct 8 '14 at 21:09
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    @stormy - well yea, there's a symbiosis going on - exchange between the bacteria and fungi and the plant itself, but that doesn't really 'improve' soil as such, only enough for the plant to thrive during its life time, aided by dead, decaying parts from the plants - but as you say, the biggest improvement is when they contribute biomass as they rot down after death. I didn't bother to go into that for the Question - I find its usually better to keep it basic purely for clarity. – Bamboo Oct 9 '14 at 9:55
3

If by nutrients, you mean acids, hormones and such, then yes, I'm sure most plants contribute acids and other chemicals to the soil (probably different ones from different plants), which will affect different plants in different ways. Different plants may attract different kinds of microbes, and animals, which may also affect the soil nutrients (different ways for different kinds).

Different acids and other chemicals have different effects on different things. Whether it's an improvement (and what it's an improvement for) depends, but it can be an improvement, I believe. Different acids may make different elements more or less available to different plants.

If by nutrients, you mean elements, that's another matter that other people seem to have attempted to answer, with the discussion on nitrogen-fixing. Legumes tend to have microbes that fix the nitrogen (the plants themselves don't actually do it). Some other plants, I've read, can hinder this process (such as calendula, I believe), and may not be good to plant with legumes.

I suppose a venus fly trap may catch insects, and their left-over skeletons may eventually somehow contribute to the soil.

Trees can grow deep roots to bring nutrients from way down to the surface (the trees drop their leaves, which contain nutrients from down below, which may be able to contribute to the topsoil in ways).

Because many plants have delicious fruit, they influence humans to grow them, and the humans may improve the soil as a result (as well as the plant breeding; so, it's a synergistic effect). It's kind of like a symbiotic relationship, except humans aren't actually attached to the plants.

I suggest learning about cover crops. There's more to good soil than fertility (like soil structure, and probably more stuff). Buckwheat is supposed to be a good one (but not for nitrogen fixing).

2

Comfrey is a chop and drop perennial plant that is very beneficial. The plant leaves can be cut off 2-3 times a season and used as a stinky tea or used as a mulch. There is little cellulose in the leaves so they break down quickly. The roots go deep once it is established and like trees it gets the minerals deep in the soil. It is considered invasive by some. The Hybrid variety like the Russian Comfrey Bocking 14 strain does not produce seed, but does flower. It can be propagated via root. The non-hybrids produce seed. If you consider comfrey just understand that once it is there it will be hard to remove it. Most people opt for the hybrids and put them in areas they do not till or dig in and then harvest the leaves for their gardens or for healing poultices.

To get them established I use my post hole digger and made a 4-5 deep hole and filled with compost and other soil amendments then plant the root (I use Bocking 14). They will get 2 feet wide and 1 1/2 foot tall.

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Applicability will vary with what you consider "lawn" - some people are very grass-centric there.

Clover, particularly "Dutch White Clover" can indeed be added to a lawn in difficulty (such as described) and will, over time, benefit it, being a legume, and a nice short green mowable plant. For people that douse lawns with broadleaf "weed" killers, this does not work, as those "weed" killers will kill clover.

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I have read that hemp will replenish nitrogen and nutrients in soil.

Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted.

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    "I have read..." Please provide some citations. Thanks! – Niall C. Oct 10 '15 at 14:40

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