Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Composting process

  1. Dec 5, 2015 #1
    Dear PF Forum,
    My church concerns about climate change/global warming condition now. And we try our best to cope (altough in a very small scale) to lessen the effect.
    Now, I'd like to know about composting. But I have no idea what to do.
    What stages are there in composting
    What kind of bacterias are there in bioactive compound involved in composting process.
    How to grow/harvesting the bacteria in bioactive compound.
    What is the end product and the usefulness of composting end product, etc...

    Perhaps I should begin from the end and work my way up in subsequent postings

    The end product of composting is fertilizer, and I'd like to know what elements are in a plant.

    I try to find links that explains the elements composition in a plant, but it's difficult, instead I found this link
    The elements composition human body are (in mass)
    Oxygen 65%
    Carbon 18%
    Hydrogen 9%
    Nitrogen 3.2%, and also phosporus, calcium, potassium, sulfur etc..
    1. Is it safe to assume that the elements compositions in plants roughly resembles those in human body?

    Some 30 years ago when I was in junior high, I once read that someone conducted an experiment.
    He/She grew a plant from a pot bucket filled with soil. All he/she did was only watering the pot.
    And when the plants grow bigs, the soil was measured. The mass of the soil decreased very little.
    IF the CHON composition (Carbon, Hydrogen, Oxygen, Nitrogen) in living organism is roughly 95% (since I don't know the answer of question number 1)...
    Was the experiment possible? It's been a long time ago that I read the article.
    2. Is it true that plant gets their
    Oxygen from air
    Carbon from CO2
    Hydrogen from water
    Nitrogen from air? 80% in atmosphere


    So here are the gist of my questions.
    1. Is the element composition in plants roughly resembles that in human body?
    2. Is it true that plants gets their
    Oxygen from air
    Carbon from CO2/air
    Hydrogen from water
    Nitrogen from air?

    Thanks for any replies.
  2. jcsd
  3. Dec 5, 2015 #2


    User Avatar

    Staff: Mentor

    Warning: I'm not a biologist. Many on PF are more knowledgeable than me on this, but I think that my answers below are generally correct.

    Roughly. You'll find some number for corn here: http://nutrients.ifas.ufl.edu/nutrient_pages/BSFpages/EssentialSub2.htm
    (I guess it also depends how you calculate the percentage. In the link above, it is % per weight.)

    Mostly from water. The reaction for photosynthesis is
    CO2 + H2O + light → C6H12O6 + O2


    The vast majority of plants get their nitrogen from the soil. Hence the need for fertilizer.

    However, I don't know what all this has to do with composting. You'll find many hits by googling "composting primer," with lots of practical advice.
  4. Dec 6, 2015 #3
    That's all right. Your answers are very good. You have been helping me very much. I suspected that much about element composition in living organism. Since I can't find the link on other organism other than human, so I post this thread.
    Your link above is the one that I need. :smile:
    There are differences for oxygen and carbon in plant and human (other animal?) being.

    Forgot about H2O for oxygen. :smile:


    First, I'd like to know how useful is this composting thing before I study composting. What compost gives to plant or soil. What elements that plants need from soil.
    So, for CHO the plants get them from watering and air.
    There are Nitrogen, Sulphur, Potassium, Calcium as well, that I think the plants get from soil. Which I think what the plants need from fertilizer as well.

    Based on the link that you gave me. CHO holds 95.3% the weight of the plant, which the plant gets from air and water.
    So the soil and fertilizer are only responsible for 4.7%, right?
    In the what I really want to know is this after studying the element composition in plants and where the plants get them. (95% from water and air)
    What I want to know is this.
    1. How many percent does the fertilizer give to the plant?
    2. How many percent does the soil give to the plant?
    3. How many percent does water and air give to the plant?

    Just suppose that fertilizer responsible for 1% weight of the plant, then...
    So I know, that once I composted several kg organic waste, and this waste turns into CO2 and H2O (this can make the composting process very efficient, before the bin will be full. CO2 will be exhausted from the pipe in the composter and the water can be expelled from a house at the bottom) and left some compost behind, and when I scoop just 1 spoonful of fertilizer and mixed it with water, this 1 spoonful (1 gram) fertilizer can grow 100 gram plants. And I can find how effective this composting process is.

    Thanks for any replies.
  5. Dec 6, 2015 #4
    Oops, sorry buddy.
    The photosynthesis reaction as I recall is.
    6CO2 + 6H2O -> C6H12O6 + 6O2
    6CO2 -> 12 moles oxygen
    6H2O -> 6 moles oxygen, I think it's mostly from air. But I do forgot about H2O
  6. Dec 6, 2015 #5
    Yes, it does take several CO2 and H2O molecules to produce a single carbohydrate molecule, but the general principal is correct.
    If you want to be completely precise about it then 6CO2 + 6H2O -> C6H12O6 + 6O2 is not correct either.
    Photosynthesis is a highly complex set of reactions involving intermediate compounds, especially phosphates, and the above formula is just a summary of the net result.
    The process also depends on light absorbing pigments such as chlorophyll.
    The wiki article seems to be reasonable enough.
    Last edited: Dec 6, 2015
  7. Dec 6, 2015 #6

    jim mcnamara

    User Avatar

    Staff: Mentor

    There are autotrophic and heterotrophic organisms. Autotrophs make their own 'fixed' carbon from energy derived from sunlight (or metals/sulfur for some bacteria -- example undersea high temperature vent ecosystem). Fixed carbon is reduced carbon like sugar, a 'carbon hydrate', carbohydrate. Heterotrophs use fixed carbon previously created by some other organism.

    Humans and animals are heterotrophs, for example. Green plants are autotrophs.Both types of organisms use fixed carbon to run their living processes -- in the process we call respiration (not the normal meaning of this word):


    Composting is the breakdown of fixed carbon, largely cellulose and lignin, into simpler compounds or into carbon dioxide. This is important because herbivorous animals have gut bacteria to do the 'composting', if you will. Cows derive nutrition from grass. Grass as it is found growing is made of complex indigestible carbohydrates, that bacteria turn into digestible carbohydrates. If you ate grass you would derive very little nutrition, and lots of 'fiber'. Fiber is the term we use for these indigestible fixed carbon molecules in plants. Wood, for example. Some bakeries add microcrystalline cellulose to bread to increase fiber:

    https://en.wikipedia.org/wiki/Microcrystalline_cellulose -> sort of double speak for processed wood pulp. Mmmm woody bread.

    Without the action of 'composting' bacteria, the carbon cycle would slow down, and most heterotrophic life on earth would be in trouble. You should consider reading about the carbon cycle:
    Last edited: Dec 6, 2015
  8. Dec 6, 2015 #7
    @rootone, thanks. But it's the "composting" process that I want to learn. I'll read about photosynthesis someday later. It's an interesting topic, photosynthesis.
    @Jim Mc Namara, I'll contemplate your answer. Thanks.
  9. Dec 6, 2015 #8
    Composting can be thought of simply as the reverse reaction compared to photosynthesis.
    Bacteria are using carbohydrates as an energy source and producing Water+CO2+ while releasing energy (as heat).
    Composts heaps can get surprisingly hot.
  10. Dec 6, 2015 #9
    Reverse action, is that the main concept of composting? Reverse reaction of photosynthesis?

    Before I study composting further, because I don't know about composting (and ehm chemistry :smile:). I want to know this...
    Does composting really expell water and CO2?
    says that CHO make up 95% of plants.

    So if composting expell water and CO2, if we put organic waste in composter bin and composter expells water and Co2, so the mass of the waste will be left about 5% to 10%?

    Thanks for any replies.
  11. Dec 6, 2015 #10
    I'm not sure if the reduced mass gets down to as low as 5%, but yes it does reduce significantly.
    There is always going to be some amount of residue that bacteria don't use, (the bits of phospher, calcium, and nitrate stuff that end up getting incorporated into plant material).
  12. Dec 7, 2015 #11


    User Avatar
    Gold Member

    No problem. Really not a problem, as during photsynthesis excess oxygen is expelled trhough the leaf stomata. During times when photosynthesis is impossible ( darkness, low light levels, etc ) the plant still has to respire. In this case, oxygen is supplied through the air. The plants have no way to store the excess oxygen production for later times.

    Roots also need oxygen for respiration. No photosynthesis happening there with an excess of oxygen production, so oxygen uptake for respiration is from the air ( ie air pockets in the soil ) or from dissolved oxygen in soil water.
    You can "drown" a plant, by having the soil too wet for the roots.

    Respiration and photosynthesis both occur at the same time during sunlight hours - it is not just one or the other.

    Other means of obtaining oxygen for respiration is through lenticels.
    Might as well read up on it through wiki,
  13. Dec 7, 2015 #12


    User Avatar
    Gold Member

    Are you putting the compost product in a kiln to dry it out?

    Compost would have anywheres around 40, 50, 60 % moisture.
    Too dry and the bacterial action is curtailed during composting.
    Too wet and the compost becomes oxygen deprived and you get a stinky - anaeorobic bacteria take over from the aerobic ( aerobic is what you want )
  14. Dec 7, 2015 #13
    @256bits Thanks for you advice. Tough some members before have posted links. I still study them.
  15. Dec 14, 2015 #14
    Plant growth and composting are not really the same process, you can forget all the stuff about fertilisers. Composting is a method of encouraging the biological processes that break down organic matter, this is usually down to the action of fungi and bacteria, in doing this they alter the structure of the material and release many of the nutrients stored in plants. What you end up with is a material that is a good soil conditioner to improve soil structure which is rich in plant nutrients, added to the soil it improves the growing conditions for other plants. You can buy things that you can add that speeds up the process or you can make a special effort to add egg shells and the like to avoid to much acidity, the bacteria need moisture but not to much and turning the pile one or twice, if there is a lot, adds a bit of oxygen. It can take a while for the process to finish particularly if there's a lot of hard matter like wood but Its certainly useful, gardeners like it, it encourages plant growth and it reduces the need for any artificial fertilizers. In general terms plants over their lives remove more CO2 from the atmosphere than they add and reducing the amount of waste that requires processing also reduces the amount of CO2 generated by this processing.
  16. Dec 16, 2015 #15
    My composting experience is limited to the North Western Europe Climate but i would expect that there are great similarities.

    Theoretical considerations aside. When you start to compost you might want to consider practical stuff as well. Such as :
    1) Where do you want the use the end products - compost (& perhaps compost leachate = nutrient rich water leaking out of your compost)
    2) Is it more practical to compost where you want to use the compost or where you produce your organic waste.
    3) Do you have a balanced supply of organic waste ? For optimum results you should strive for a balance between 'brown', fibrous, woodlike materials and 'green' nitrogen rich (mostly food) residue. You might have to bring in dry leaves or some similar material. In urban areas you
    4) What kind of stuff do you want to grow in the compost you are using? Flowers, herbs, veggies, rice..... Do you want to use the compost as a mulch, anti-erosion, etc.....
    5) Who does the tending of the mulch heap(s) ? What tools are available ?
    6) What kind of pest problems might you have if you make a mistake ?

    Most problems will be minor if you take care and you should be able to produce good mulch shortly.
    There are a lot of good threads on the internet. Use 'composting in the tropics' for starters. You will find video etc....

    You might also want to look at permaculture sites from Queensland, Australia. http://permaculturenews.org/ Permaculture pays a lot of attention to composting and there are a lot of forums where you can get practical advice.
  17. Dec 16, 2015 #16
    @Geology Erwin
    As a matter of fact, I just joined this forum a week ago
    There is a "composting" forum in there. Helpful Gardener is very good. But PF is much better. I can have "the science" of composting in PF, while in gardener, "How to compost..."
    Should combine both of them.

    While we are here...
    Perhaps you can answer me.
    1. What is the end product of aerobic composting?
    2. What is the end product of anaerobic composting?
    3. I read that there are 4 stags of composting
    Cooling down

    Is mesophilic an aerobic or anaerobic process?
    Is thermophilic an aerobic or anaerobic process?
    Is Cooling down an aerobic or anaerobic process?
    Is Maturization an aerobic or anaerobic process?

    Thank you very much.
  18. Dec 16, 2015 #17
    I do composting by using this composter bin.
    They say that it's for anaerobic composting.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook