How do plants extract nutrients from the ground?

How do plants extract nutrients from the ground?(permaculture.co.uk)

174 points by terse_malvolio 5 years ago | 58 comments

I've been doing quite a lot of gardening in the past 3 years.

There is a plethora of home gardeners and/or permaculture resources on the internet that generally like as much organic matter as possible.

I some of that strategy as it's convenient at home to supply nutrients to plants without much work.

But, I can't help but feel it's all sort of wish washy science.

https://www.youtube.com/watch?v=KHZHy3_7PPE

^ I found this guys videos much more satisfactory to "need the facts" mindset. He uses knowledge from the agricultural industry instead of the "organic" approach.

In short, plant roots actually love sand, silt and clay, and actually don't like organic matter around their roots as it causes rot. He suggests that you should always just top feed your plants. (throw compost around the top of the plants only) When people say don't over water your plants, it's not because of the water, but because of the organic matter in your pots mixing with water and making sewerage. (e.g. this is why hydroponics works)

Anyway, I'd suggest watching some of his videos on perfect soil for an opinion you don't find much of in home gardening videos.

(This was touchy for me because when I started gardened everyone described my soil as sandy, shit and told me to amend it with X, Y and Z. I was too lazy to listen at the time and noticed that my plants were perfectly fine growing in sand as long as I top fed regularly.)

thatcat 5 years ago | |

The plant roots like the right texture. If you have clay, adding organic matter releases glomalin [0] and creates aggregates that let the water drain more easily, preventing root rot and allowing them to penetrate deeper.

Each of those - sand, clay, organic matter have a pore size distribution and different pressure that it releases water at. The amount of water and the pressure that it's released at form what's called a soil matric potential graph. The important thing is balancing those so that the soil moisture is mostly held at a pressure that the roots can access . There is a soil texture pyramid that illustrates the different soil textures and mixtures.

[0] https://www.dpi.nsw.gov.au/content/archive/agriculture-today...

[1] https://topsoilpros.com/what-is-a-soil-texture-chart/

The guy in the video is calling mulch compost. Compost is supposed to be made of green material which contains nitrogen and other nutrients not bark and wood chips. There are different sources of organic matter that contain different nutrients, pore distributions, etc.

thomasfromcdnjs 5 years ago | | |

Thanks for the cool links.

I believe he knows what compost is, he is just being lazy about using it interchangeably with what you find at most garden shops. Most commercial bags of garden soil and compost both contain a lot of broken down wood. He is just suggesting you shouldn't really have any organic matter around your roots, except for at the top level. It's good to promote mycorrhizal fungi but only at the top layer etc

I think he mostly ranting about how commercial gardening places just sell "soil" that is mostly always comes with added "compost" e.g. https://shorturl.at/mvwzG

Where as they should sell more clay/silt/sand mixes and encourage top dressing instead.

Most people just fill their pots to the top with those bags and it leads to adverse effects.

grumpy-de-sre 5 years ago | |

As someone who's done a fair bit of indoor gardening, including hydroponics I have to vouch for the video. Oxygen in the root zone is absolutely critical. Common potting mixes are almost universal terrible. Far too much rotting organic matter.

These days I do all my gardening in a 50/50% volumetric mixture of perlite and coco coir (buffered with calcium and magnesium). With a small amount of pH adjusted hydroponic nutrient and a watering can, the results you can achieve are just incredible, absolutely no comparison. Plus it's completely pest free so you aren't dragging in mite eggs, and fungus gnats.

Never using potting soil again, and anybody still using it should really reconsider. It's insanely counter-intuitive just how well plants can grow in something as inorganic as perlite. It just doesn't look like it should work.

thomasfromcdnjs 5 years ago | | |

Thanks for this. I'm going to do this with all my new indoor plants. I sort of gave up on em because they constantly have issues with the supplied "soil"

goda90 5 years ago | |

>He uses knowledge from the agricultural industry instead of the "organic" approach.

I can't help but wonder if his knowledge applies specifically to cases where chemicals like fungicide are used. There are quite a few species of soil fungus that are beneficial to plants, including root protection, but these can be wiped out by fungicides, leaving a blank slate for harmful fungus and other pests to take hold. So it would make sense that if there were no beneficial fungus present, then organic matter would be a rot threat.

hinkley 5 years ago | | |

This conversation is simply more evidence that we conflate meadow/prairie plant behavior with woodland plant behavior. Since permaculturists are generally growing bushes and trees common to successional forest biomes, to an extent both can be right. The expected ratios of nitrogen in these two biomes and the ratios of fungi to bacteria differ by at least an order of magnitude, so of course strategies should differ as well.

With the exception of orchardists (whom some of us think doing everything wrong that you possibly can), modern agriculture concerns itself with prairie plants. Where natural woodland fauna can and do mix decaying logs into the soil column, in a prairie it will mostly be decaying roots (and especially decaying root hairs). The kind of rot going on is a bit different.

That said, saprophytic fungi are generally held to leave living tissues alone. There are exceptions of course, but having decaying woody matter in the soil is not a death sentence.

markdown 5 years ago | |

> In short, plant roots actually love sand, silt and clay, and actually don't like organic matter around their roots as it causes rot.

IMO it really depends on what you're trying to grow. I live in the tropics and there are a lot of tropical plants that do indeed thrive when the soil is heavy on the organic matter. I suspect it depends on how far/long these tropical plants have been bred away from their rainforest origins.

thomasfromcdnjs 5 years ago | | |

Yeah, I think your absolutely correct.

Gardening is actually quite hard in this regard. Everyone is always sharing tips and tricks, for different crops, different species of said crops, different climates, different soils, different hemispheres, different sides of a mountain etc

I think there will be a cool database where you can filter by those factors one day but not sure we are quite there just yet.

hinkley 5 years ago | | |

Woodland plants also expect organic matter in the soil column. It's one of the complaints about 'modern' forestry practices that leave no mature wood on the ground to decay. Real forests have rotting logs in them.

It's really conventional farmers concentrating on annuals that want something different. They're growing prairie plants.

arbitrage 5 years ago | |

what types of plants do you grow? what is your climate like?

gardeners' advice that is along the lines of "what you do is wrong, what i do is right" is generally not very good, and you can feel fine telling them thanks on moving on with your life.

thorwasdfasdf 5 years ago | |

Charles Dowding, the father of no dig gardening, suggests, you don't need to dig in any compost or ogranic matter, just put it on top, as you've suggested. Apparently, the worms and the microorganisms then get to work and bring all that organic matter into the ground themselves working it into a nice tilth.

sgt101 5 years ago | | |

Worms! If there are many worms the soil is good. If there are no worms, the soil is bad. Worms like the organic matter and convert it into tiny weeny particles that the plants (bacteria) can absorb, they also open up the soil and create structure so it drains nicely.

Worms!

tastyfreeze 5 years ago | |

Organic matter is mostly feeding the soil life. Sand, silt, and clay are where the minerals in food come from. The soil life makes them available to plants.

jspash 5 years ago |

I'm curious what HN thinks about the current trend of root washing when planting trees. From what I've read the science is still out and people tend to be split 50/50.

The root washers say that by removing all media that the sapling was grown in will give the roots more "incentive" to move into the native soil and thus it will establish quicker.

Whereas the anti-washers say you are slowing the establishment by washing away the helpful microbes as described in the article.

Then there are the people who say, DO wash the roots, but save the water. Plant the tree bare-root, but fill the hole with 50% of the saved water, and use the rest to top-water over the next few weeks. The idea is to get the benefits of bare-root planting along with saving the beneficial organisms.

The reason I ask is that my yearly tree just arrived in the post and I've yet to put it in the ground. I'd love to know what the current thinking is.

jedimastert 5 years ago |

This reminds me of my favorite "thought experiment" to poke at intuition: with no other information that what most people learn in elementary/middle school science (or what I would assume is taught, i.e. the water and respiration cycles), where does the majority of a tree's mass come from?

First we might look at dirt, but that doesn't quite pan out; where does the mass come from, and how would it be replenished? We don't see gaps slowly start forming around roots, and dirt doesn't just build up in other places, so that's probably not it.

Next we might look at water, but water is only made of hydrogen and oxygen, and the oxygen is released. The water also brings in nutrients, but there's no way it brings in enough to generate the majority of a trees mass.

So there's only one place left, which is never anyone's first guess and almost never crosses anyone's mind at first glance: air. It's around this point that most people realize that a majority of a tree is carbon, which all comes in from the air as carbon dioxide.

littlestymaar 5 years ago | |

Edit: this is wrong, see comments below.

The correct answer is water though, not air. Air only accounts for the Carbon content of the tree, which is a big part, but still a minority.

First, a live tree contains a lot of water (30-50%), that's why you need to let wood dry a lot before burning it.

And there's something else:

> Next we might look at water, but water is only made of hydrogen and oxygen, and the oxygen is released.

Not all oxygen is released. All oxygen contained in a plants molecule (mostly cellulose and hemicellulose, which are polymers of glucose, and lignin) comes from water. And there's a lot of it: there's as much Oxygen as Carbon in a glucose molecule.

tastyfreeze 5 years ago |

I am so happy to see this on HN. A couple years ago I started learning about rhizosphere environment. It has changed how I look at my gardening. I now see healthy plants as a biproduct of feeding and caring for the soil. I imagine the soil as the external stomach of plants. When the soil is healthy plants thrive. I get excited when I start seeing larger arthropods in a new bed. It is an indication that the soil ecosystem is very healthy.

To head down the rabbit hole of soil health even further, learning about increasing the cation exchange capacity(CEC) affect on plant health has been fascinating.

bjelkeman-again 5 years ago |

In aquaponic systems, which have plenty of bacteria on the roots, we are running much lower levels of nutrients with very good growth, compared to recommended hydroponic nutrient levels. I think we are observing this interaction that is described here.

k_sze 5 years ago |

I wonder how much of this rhizophagy cycle applies to plants that don't really rely on the soil for nutrients. e.g. many of the insectivore plans (venus fly trap, pitcher plant, etc.).

In fact, if you try to plant venus fly trap in rich soil, it'll probably die.

boomboomsubban 5 years ago | |

Most of those plants would still rely on the soil for some nutrients, I assume their history in poor soil has just led to them developing additional ways to get some nutrients. Like vitamin d in humans.

uniqueid 5 years ago |

On a related note, Richard Feynman explains how a tree grows: https://youtu.be/ifk6iuLQk28

ghshephard 5 years ago |

I love how it starts with: Over the past few years it has become clear that plants are able to extract nutrients directly from soil microorganisms in their roots. - something that was like a baseline of plant biology when I took it in high school 40 years ago, and, according to wikipedia, we've know for over 120 years.