To grow healthy food we must create soil conditions that enable plants to get the best possible nutrition in their growing cycle. Compost is one of the most important tools organic food gardeners can use to improve soil fertility and plant nutrition. Good compost reduces weeds, improves soil humus levels, structure and fertility, improves soil drainage, helps retain soil moisture levels, increase air and water flow through the soil and stimulates necessary soil microbial activity.
All of this helps create optimal plant nutrition. Composting is also an excellent way to recycle kitchen and household waste so that it becomes a valuable resource instead of pollution causing landfill.
There are two basic methods of making compost. These are hot composting and cold composting. The method you use should depend on how much time and effort you want to spend on compost making, what materials you have available, the quantity of materials at hand, and the result you want to achieve.
Hot composting relies on heat generated by composting microbes to speed up the natural rate of decomposition. It’s a great method to use for quick, good quality, weed free compost, if you have a lot of material to compost, or if you are worried about disease causing microbes breeding in your compost. The heat sterilizes any dangerous microbes and kills unwanted weed seeds so is safe to use on salad, vegetable and herb gardens. This method is also good if you are concerned compost might bring weeds, pests or diseases.
Hot composting requires more work than cold composting though. A hot compost heap requires regular monitoring to ensure it heats up properly (to an internal temperature of at least 450C) and doesn’t dry out or get too wet. Too dry or too wet heaps won’t heat up properly. Hot compost heaps also require regular turning to allow air to circulate and keep the temperature high, and enough material to make a large enough heap (about 1 metre high and a metre wide at minimum). A too small heap won’t generate enough heat to sterilize weed seeds or disease carrying microbes.
If your heap is large enough and contains a balanced mix of compost materials, including sufficient nitrogen rich materials to fuel enough heat generating microbes, the temperature in the heap should begin to rise after three or four days. Monitor the temperature every day. When the temperature begins to drop, turn the heap to re-stimulate microbial activity and cause a new temperature rise. All hot composts require at least 3 or 4 turnings. Turning aerates the heap, mixes the organic matter and re-stimulates the necessary intense microbial activity that heats up the heap.
‘Cool’ composting involves the slow breakdown of organic matter at normal outdoor temperatures. Because cool compost heaps don’t heat up to the high temperatures of hot compost heaps, weed seeds or disease causing microbes might not be eliminated. On the other hand, cool composting enables one to add small amounts of waste to the heap over time, and the heap doesn’t need turning or monitoring.
Cool composting is ideal for conditions where the amount of organic material, labour, time and space for building large compost heaps are in short supply. You can use cool method compost on food gardens, as long as you avoid adding materials that might carry diseases or pests, such as manures or animal products, weed seeds, or diseased plants.
‘Ingredients’ for Good Compost
A good composting mix contains sufficient Nitrogen, Carbon and Calcium bearing organic material, and enough (but not too much) air and water.
Nitrogen rich materials generate high levels of microbe activity and heat, and speed up decomposition. They include animal manures such as fresh chicken litter, cow and horse manure, human and animal urine, and green annual weeds, coffee grounds, fresh lawn clippings, fresh vegetable matter, soft green garden waste, organic fabrics such as cotton, wool and silk, feathers, nettles, old cut flowers, general organic kitchen waste, and leaves, roots or shoots from leguminous plants ( e.g. beans, peas, acacia’s, clover). In its converted form (nitrates and ammoniums) nitrogen is essential for healthy plant growth.
Carbon rich materials take longer to decompose than Nitrogen rich materials, but are a valuable source of humus and fungus food. Carbon rich materials include woody prunings, hedge trimmings, bark, wood chips, dried materials such as dried straw and dried garden trimmings, dried leaves, cardboard and paper, corn cobs, nutshells, sawdust, and wood ash.
While carbon rich materials are slow to decompose, they create a firm structure that allows air and water to penetrate. If a compost heap doesn’t have enough carbon materials it can turn smelly and toxic. On the other hand, a heap that contains too much carbon can dry out very quickly and will decompose very slowly.
Fungi are the main decomposers of carbon, so high carbon materials are ideal for making compost that contains a high proportion of beneficial fungi – ideal for fruit and nut trees. If possible, chip large branches and pieces of vegetation into smaller bits. If this isn’t possible, lay the largest bits on the ground as a ‘bed’ and build your compost heap over these. It might take some years before large branches decompose completely, but these can provide a base for other compost heaps in the future.
Calcium lowers the compost PH and prevents compost from becoming too acidic. Calcium rich materials include green garden weeds, brassica leaves and stalks, kitchen vegetable waste, seaweed, rose leaves, eggshells, lime and wood ash. Acid build up is especially likely if the compost heap is not well aerated, contains a large quantity of acidic material such as conifer clippings or oak leaves, or too much nitrogen rich materials. Most bacteria enjoy a slightly alkaline environment, so calcium also helps stimulate growth of bacteria that are beneficial for many annual vegetables.
Both hot and cold compost are ready when most of the materials in the compost have broken down into a dark, rich, crumbly substance that looks like loamy soil and has a pleasant, earthy smell. It’s not always necessary to wait until compost is completely ready before using it though. You can apply half-decomposed compost as mulch onto your beds, but this may bring weeds if there are seeds in the compost.
By Valerie Payn
Valerie is the author of e-book An Ecological Gardeners Handbook – How to create a garden with a healthy ecosystem and garden sustainably. (Available at Amazon). Valerie also blogs regularly about sustainable landscape design and gardening.