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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…

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