A Compost "Bin" in a Bottle

 Rotbots allow children to watch and photograph the composting process over several weeks and then observe and record the growth of a plant in the compost they have created. 

The effect of Worms   Adding worms to a rotbot can provide additional interest and will  enable students to observe the their contribution to the composting process. If worms are to be used two Rotbots should be prepared using the same mix of materials with worms being added to the second bottle only.  I do not recommend adding worms to the Rotbot unless it is certain  that they will be harvested safely at the end of the procedure and not left to die in a neglected bottle.

  In schools Rotbots can be made by students working in twos or in small groups.

 Equipment  required

1 or 2  two litre plastic ‘pop’ bottles, Scissors,( Optional Junior hacksaw and 3 foot length of drain pipe), wooden stirring stick, mesh/muslin, rubber band, label and pen, tablespoon, saucer.


As with all composting a mixture of "greens" (nitrogen rich) and "browns" (source of carbon) are required for the composting process to work effectively. where young children are making the Rotbot it is recommended that only "clean" vegatable material is used. Try to get a range of colours to make the photos more interesting.


Straw or twigs, small pieces of paper or cardboard. (We find that shreded computer paper is easy to add to the  bottle) and makes an interesting contrast in coour within the bottle.


 Grass, vegetable leaves, potato and apple peelings, chopped up  banana skins. Comfrey leaves are very good but can feel prickly so gloves should be worn. It is good to have a  mix of colours at the start of the process so that the layers can be clearly seen in the bottle.


One or two  layers of compost taken from an active compost bin may also  be added.

Growing the Seed

A layer of soil will be need at the time the seed is to be planted.

1 or 2  Drawf Bean  or marigold seeds (I recommend beans as they should provide some pods to be harvested, cooked and eaten.


5-10 composting worms  if these are to be added




  • Rince the bottles and remove labels. (Take a photo at this stage)
  • Adult  Cut the top off the bottle(s) using scissors or a sharp knife. (If using scissors you may find it helpful to  make a small starter cut using a junior hacksaw. Once the top has been removed trim the cut  to remove any sharp pieces of plastic. Some people cover the cut edge with tape as an additional safety precuation.
  • Adult  Drill one or more drainage holes in the bottom of the bottle(s). If a large number of Rotbots are being made I   put the bottles up-sidedown over a short length of plastic down drain pipe  on a wooden frame. When the drill goes through the plastic base it is contained inside the pipe  reducing the risk of drilling my hand or leg. 
  • Prepare the ingredients by tearing or cutting the organic materials (straw, twigs, leaves, grass, soil, vegetable and fruit scraps) into small pieces. The material can be moistened by the addition of water at this stage or if the Rotbots are to be moved later  water can be added once they are in their final position.  If water is added at this stage the mixture should be damp but not soakin wet. The correct moisture level will result in the materials having a water content   like that of a wrung out sponge. 
  • Build up the layers in the Rotbot by putting in straw and twigs first, then the fresh compost (or soil), followed by alternate layers of  grass, shredded paper,  vegetable food scraps and leaves. If the Rotbot is to be kept indoors choose plants with leaves that will not produce an unpleasant smell while composting. Herbs are good, cabbage and similar leaves might be best avoided.
  • Fill to within an inch of the top. The contents may be mixed  together with the wooden stick but I prefer to keep the distinct separate layers of browns and greens as this looks better in the initial photographs.
  • Add three to six tablespoons of water if the materials were not moistened earlier.  If the Rotbots are to be taken home it is best to leave adding the water until they are at home and standing in a saucer.
  • Cover the Rotbot with garden fleece, mesh or muslin and secure with an elastic band.
  • Stand the Rotbot  in the saucer.
  • If using two bottles to see what effect added worms will have  add them  to one of the two bottles once decomposting has started. 
  •  Place in a warm place but not in the direct sunlight (Worms darkness so any worm Rotbots should be covered.


  •   Take weekly photographs to record the changes in the bottle and the rate at which the different Rotbots made by the class decompose .  


  •  As the composting process takes place the material may sink in the bottle. Top up with additional material so that when the final layer of soil is added and the seed is planted will be near the top of the bottle.
  • Add more water it if it starts to dry out

 The compost Rotbot is ready for sowing after four to six weeks. The compost Rotbot is ready when the ingredients look like dark, rich soil.

  •   At this stage a layer of soil should be added

Growing the Seed

  •  Plant a dwarf french bean or marigold seed in the layer of soil on top of the compost.
  •  Place the Rotbot in a well lit place and water regularly.
  • If a  bean seed has been sown you should be able to harvest a few pods to cook and eat


Photos and more information are on the schools page  Schools Composting.



  • Rotbot with seedling growing

  • Seedling growing in a Rotbot