Making Compost in the Nursery

Making Compost in the Nursery

What is composting?

Composting is the breakdown of any organic material over time through the action of micro-organisms into a crumbly, dark, soil-like product in which none of the original material can be easily identified. Various organic waste materials produced by farming such as husk, effluent, vegetable waste, stubble and so on can be used to produce compost.

There are many benefits of using compost including:

  • Increasing organic carbon protection of soil from erosion
  • Increasing soil structural stability
  • Improving soil moisture holding capacity
  • Increasing water infiltration and reducing water runoff addition of nutrients (as slow release)
  • Enhancement of growth of a wide range of soil organisms.

There are three distinct phases in composting. These include:

  • Hot phase
  • Cool down phase
  • Maturation phase

During the hot phase the digestive processes of the micro-organisms cause the temperature of the compost pile to increase. Internal temperatures of 130°-140°F are routinely reached. These temperatures are sufficient to kill some disease organisms and weed seeds.

During the cool down phase fungi become more active in the compost processes. This is when stems, stalks and other tough fibrous materials are broken down.

The maturation phase begins after the compost has cooled down. In some instances worms move into the compost breaking down some of the materials and mixing the compost. The compost is ready to use as soon as most of the original material is no longer recognizable.

Nutrient supply and types of organic material

Organic materials are substrates for bacteria, fungi and other micro-organisms involved in composting. Compost is the end product of these digestive processes.

Compost that is suitable for growing seedlings requires a mixture of dry dead materials with living green materials. Dry dead materials are consumed slowly and contain high levels of carbon but low levels of nitrogen. Living green materials are consumed quickly and contain low levels of carbon but high levels of nitrogen. If there is too much nitrogen in the compost, some of it will turn to ammonia that will be lost.

The balance of carbon-rich dead materials and nitrogen-rich green materials is referred to as the carbon/nitrogen ratio. The optimum carbon to nitrogen ratio is about 30 to 1. Grass, animal manure and fresh green plants are high in nitrogen. Leaves, brush, sawdust and wood chips are all good sources of carbon. Blending these carbon sources with nitrogenous materials can provide a satisfactory carbon to nitrogen ratio.

Size of compost pile

A desirable size of a compost pile for a nonmechanized operation is 2.5m wide and 1.5m high. If the pile is much larger, then air circulation will not be sufficient to ensure the speedy decomposition of the organic materials. On the other hand, if they are too small, compost piles will not develop the heat required for speedy decomposition. Compost piles should not be less than 1m by 1m.

Particle size

Reducing the size of the organic material prior to composting increases the surface area available to micro-organisms. Reducing particle size also improves the internal structure of the compost pile which makes controlling aeration and moisture easier.

Size reduction can be mechanized with power choppers, shredders or hammer mills. A small gasoline-powered shredder can process up to 7m/hour of organic materials. This could process enough compost for a nursery that produces one million seedlings a year.

Available oxygen

Most micro-organisms active in composting require oxygen to live. These micro-organisms are considered aerobic. The carbon dioxide produced during decomposition needs to be removed. If enough oxygen is not available, anaerobic micro-organisms will thrive and slow the decomposition as well as produce a bad smell.

Oxygen will move into the pile if it is loose and there is plenty of space between particles, like when straw is mixed in the pile. These factors have been quantified and are considered the basic requirements for accelerated compost production.

Compost making

Typically, compost should be made in pits – a pit that is 2m long by 1m wide and 1m deep will produce enough compost for up to 8000 small polythene tubes.

The first layer in the bottom of the pit is 10cm of forest soil. The second is 10cm of leaves or grass which should be compacted by walking on top of it. The third is 10cm of manure. Except for the first layer, each layer should be watered with three cans of water before adding the next layer. These layers are then repeated in the same order until the pit is full. Normally, there will be three layers of each material in a 1 metre deep pit. The compost pit should be completed with a final 10cm layer of soil which is compacted by walking on it, and the finished compost heap should be the same level as the surrounding ground.

Good quality compost should take about 8 weeks; macadamia husk can take up to 12 weeks. It is very important not to use compost before it is ready as beneficial organisms will not have established, and nitrogen will have been temporarily taken by the decayed organisms and be unavailable to plants.

When the compost is ready it has the following distinct characteristics:

  • Smell: nice and earthy, with no bad (sour or rotten) smells
  • Feel: moist and earthy, not wet and sloppy or dry and powdery
  • Appearance: original organic materials are not distinguishable. Pile contains dark soil-sized particles
  • Temperature: pile stops getting hot.
  • C:N ratio: between 15:1 and 30:1

The composting process takes longer if there is:

  • Insufficient water, or
  • Too much carbon-rich material

Testing compost maturity

Immature (unfinished) compost may stunt or kill plants. There are several simple tests to determine compost maturity before using compost as a growing media or incorporating compost into a potting mix. Two tests will be examined:

  • Plant germination in compost
  • Plant germination in compost extract

Plant germination in compost

This simple test consists of germinating spinach directly in the compost.

  • Fill two pots with the compost to be tested.
  • Moisten each pot thoroughly.
  • Plant 50 spinach seeds in rows in each pot.
  • After 7-10 days count the number of seeds that have germinated.
  • If 75 per cent or more of the seed sprout and grow, then your compost is mature and ready to use.

Plant germination in compost extract

This test consists of germinating legume seeds on paper towels that have been moistened with a liquid extracted from the compost to be tested and with water for a control.

Making the compost extract

  • Measure ½ cup of compost.
  • Put this compost into a 1-litre jar with a lid.
  • Measure 1½ cup of clean water, and add this to the compost in the jar.
  • Secure the lid onto the jar and shake it. Allow this mixture to soak for 2 hours.
  • After 2 hours, strain the contents of the jar through a cheese cloth then filter into a clean jar.
  • Divide the test seeds into six groups of ten. Two groups will be treated with water and will serve as controls. Four groups will be treated with the compost extract.
  • Make six stacks of paper towels or filter papers (if available). Moisten four stacks of the towels or filter paper completely with the compost extract. Moisten two stacks of the towels or filter paper with water.
  • Drain off any excess moisture.
  • After moistening the paper towels or filter paper, place 10 seeds on each of the stacks.
  • Fold the paper towels over the seeds or cover with filter paper.
  • Place the folded paper towels into a re-sealable plastic bag.
  • Place the bags with the seed in a dark space at room temperature.
  • After 24 hours remove the paper towels from storage and count the number of seeds that have germinated. Record this number and repeat this process each 24 hours for a total count (72 hours).

Home composting methods

Home composting depends on traditional or small scale simplified composting technologies at low cost. (E.g., simple pit method, heap method, bins, rotating drums). But each method uses the same scientific principle though it differs in procedures and equipment. Suitable home composting systems may differ from place to place depending on environmental, economic and social conditions of the people. In East Africa, space limitation has been a critical issue for many local authorities and this has crippled the implementation of home composting programmes.

Common home composting systems

  • Heap method
  • Pit method
  • Traditional basket methods
  • Rotating drums
  • Composting bin systems
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