Organic Compost Chemistry

Okay…so I was kind of joking about doing a regular weekly gardening column, but having spent rather longer weeding and feeding this week than I intended to, I need to get something written for Sciencebase today that wouldn’t be too demanding. So here’s a quick guide to composting your kitchen and garden waste.

These are the fast-rotting greens that should definitely be in your compost heap. These all provide moisture and the all important organic matter and nitrogen for your compost. They also quickly accumulate bacteria and fungi that start the rotting process – the aerobic decomposition process – and generate necessary heat to get the compost heap going and produce rich humus from the break down of plant cellulose and the other complex molecules
in your kitchen and garden waste.

  • Grass clippings
  • Tea bags
  • Egg shells
  • Raw vegetable peelings
  • Fruit skins
  • Unused salad leaves
  • Dead flowers
  • Nettles
  • Rhubarb leaves
  • Spent bedding plants and annuals

You must also add materials such as cardboard and fallen leaves, sawdust, twigs, bark, and crumpled or shredded paper as much more slow-to-rot materials to build up air spaces in the compost heap and to add fibrous bulk to the compost. A good balance of these so-called green and brown waste products in your compost bin, will produce nice crumbly, but moist compost that earthworms love within 9-12 months. Speaking of earthworms red wigglers (Eisenia foetida or E. andrei) are great composters.

Get the balance badly wrong (too much grass, too many banana skins) or to much leaf waste and you’ll end up with either a dry pile of leaves and twigs or a smelly sticky mass, that is rotting but on the whole anaerobically. 50:50 green to brown is about right and don’t forget to fork it occasionally, but try to keep it nice and warm too. A good compost heap will get up to 77 Celsius through the heat generated by aerobic activity. Cornell University has the figures on carbon and nitrogen content of particular waste products.

The plant remains (including any that have passed through an animal gut) contain organic compounds: sugars, starches, proteins, carbohydrates, lignins, waxes, resins and organic acids. The process of organic matter decay in the soil begins with the decomposition of sugars and starches from carbohydrates, which break down easily, while the remaining cellulose and lignin break down more slowly.

The overall humification process leads to humus, which is a stable organic substance that essentially decays no further but fertilises soil with which it is mixed and provides nutrients for the next generation of plant growth.

There are some things you should not add to a compost heap or bin, partly because they reduce the quality of the compost, lead to the spread of weeds, or attract rats, foxes, and cats:

  • Meat
  • Cooked vegetables
  • Dairy products, particularly lumps of cheese
  • Animal and human waste
  • Perennial weeds and seed heads
  • Sanitary and infant hygiene products

Of course, if you have chickens or goats to feed (we don’t…yet) then most of your kitchen scraps can be used to augment their growth rather than feeding the soil. Chicken guano is great for compost heaps by the way, full of nitrogen and phosphorus and minerals. As too is fairly well-rotted horse manure, but don’t add too a high a proportion.

Finally, a compost heap needs potassium and trace minerals, such as calcium, iron, boron, and copper. These are all essential to microbial metabolism and are usually present in sufficient quantities in the waste materials you add to your heap.

Compost also needs phosphorus and adding a source can help the composting process considerably. It’s a very good idea to make sure there’s a good supply of this essential element being added regularly to your compost bin. There’s no need to rush out to the garden centre to pick up a bottle of “phosphorus”, you release adequate quantities at just the right concentration, together with nitrogen in your urine. I don’t need to spell it out do I? Just make sure the neighbours don’t get a nasty surprise and if you’re a female gardener perhaps invest in a SheWee…

8 thoughts on “Organic Compost Chemistry”

  1. “… we should be composting all of our domestic waste including toilet waste as well as re-using our dishwater etc. I believe this is relatively common in Scandinavia, but there are an infrastructure and cultural barriers to surmount elsewhere.”


    Regarding the cultural barriers you mentioned, the public health sector may need to be re-educated and sanitation regulations may require revision before it becomes commonplace to dispose of organic waste of all sorts on site. Eventually something will need to be done, though, because it is becoming prohibitively expensive to connect to a municipal sewer system or install and maintain a private septic system.

    I just thought I’d offer the information for those of your readers who might want to take further steps to reduce their household contribution to their local landfill. Oh, and good article.

  2. @David – Yes, I realise you *can* compost meat and dairy. Our “green bin” collectors do just that with a huge industrial composting complex, but I was talking about the kind of general garden compost heap or bin that requires no heavy construction.

    But, you’re right, we should be composting all of our domestic waste including toilet waste as well as re-using our dishwater etc. I believe this is relatively common in Scandinavia, but there are an infrastructure and cultural barriers to surmount elsewhere.

  3. Actually, there is a way to compost meat, cooked vegetables, dairy products, etc. These are all organic materials that contain nutrients that plants can use after microbes, bugs, and earthworms have done their work.

    Initial setup requires a galvanized garbage can, some concrete mix, some foam (optional) and a scoop. Neatly cut the top third of the garbage can with a saber saw. Roll the cut edges of both parts of the can with a pliers to prevent injury. Line galvanized lid with plastic and cast a concrete lid using premix concrete. Allow lid to harden for a week. Cut a piece of extruded polystyrene foam to fit loosely inside the top section of the can. Glue to flat side of concrete lid with foam and panel adhesive. Using the top third of the can as a template, dig a hole two to three feet deep. Put the topsoil in the bottom two thirds of the can. Insert upper third of garbage can ring so that it sits flush with the ground or slightly above. Position the concrete lid with the foam down. Place a scoop in the lower part of the can and cover with the galvanized lid to keep the soil dry.

    Now you can put whatever sort of organic material you need to dispose of in this “nutrient reservoir” without worries. The list is endless and includes everything listed above except sanitary and infant hygiene products. Periodically sprinkle dirt (and ashes if available) on top of organic materials that wouldn’t normally be introduced into a regular composting system.

    When the nutrient reservoir is almost full, pull the garbage can ring, cover with six inches of topsoil, and start another one. You can let it sit for a year or you can put a large bottomless pot over the reservoir and plant squash, melons, or some other vining plant. Another possibility is to make a tiered arrangement with the nutrient reservoir at the center. The outer rings can contain flowers or strawberries; the innermost ring a vining plant.

    It seems a shame to flush nutrients down the toilet. It’s a waste of water and fertilizer. Combined with kitchen and yard waste in a nutrient reservoir, human urine and feces are rendered harmless by the biological activity of microorganisms.

    Urine is easy to collect. Diluted with water it also can be used to fertilize lawns, moisten compost, or fertilize garden plants.

    Feces can be collected using a homemade toilet chair. I built one to use on job sites that consists of a box fitted with a toilet seat and a five gallon bucket. A bucket of humus with a scoop completes the ensemble. Covered with humus, the feces does not emit any noticeable odor.

    Finally, in Deuteronomy 23:12-13 one reads, “You shall also have a place outside the camp and go out there, and you shall have a spade among your tools, and it shall be when you sit down outside, you shall dig with it and shall turn to cover up your excrement.”

    Fecal matter is already half composted because it consists of 50 to 60 percent dead gut microbes. Once the composting process is complete, a month’s worth of feces can fit in a shoe box.

Comments are closed.

If you learned something from Sciencebase, enjoyed a song, snap, or the science, please consider leaving a tip to cover costs. The site no longer runs Google ads or similar systems, so your visit is untainted.