Compost is great stuff. It is comprised of decayed and decaying organic matter (plant residues), soil, humus and vast amounts of microscopic bacterial, fungal and arthropodal life.
The properties of compost are both physical and biological. Physically, the addition of compost to your soil improves soil structure, making soil easier to work and gardening more pleasurable. Compost will improve drainage in clay soils by creating spaces for the water to flow through, but will also improve water retention in most soils as the millions of tiny pieces of organic matter hold onto water.
The biological properties of compost flow from the activities of soil biota. Soil biota are present in much of the world’s soils, and populations will expand rapidly given the right conditions. The making of good compost optimises conditions for soil biota, resulting in a life-rich addition to your garden.
Soil biota, like all life on this planet, need certain conditions to thrive. A well built compost heap, with lots of vegetable matter from the garden, will provide the carbon, nitrogen, oxygen, nutrients, ph (acidity level) and moisture necessary to create compost over time.
In the compost heap, the soil biota are busy. They eat, grow and die at a great rate, proliferating as they consume and decay the matter of the heap, their own dying bodies re-releasing more available nutrients for the next generation.
By the time the compost has matured and is finished, it has broken down the coarse organic matter of it’s ingredients into a biota-rich, friable organic matter composite. It is concentrated soil life, ready to invigorate your beds.
So when you compost your beds, you are improving the physical properties of your soil, as well as inoculating them with a broad range of biota that help to make nutrients in the soil available to plants both symbiotically (an exchange between living entities), and in readily useable forms as the biota die and decompose.
Over time, the soil biota population will decline as they consume the available organic matter, and as the nutrients in the soil are absorbed by plants and therefore unavailable to the biota. This is why it's important to dress your beds with compost regularly, restoring both biota and the food they need to thrive.
Hot composting is more labour intensive and finicky (gathering materials, layering and turning the heap regularly) than cold composting (dumping all organic scraps in a heap). Hot composting is also faster if done correctly, and will kill pathogens and weed seeds if the heap gets hot enough. However, hot composting results in more CO2 emissions as well as burning off significant amounts of the nutrients stored in the composting materials.
Here at Golden Valley Farm I cold compost because I am lazy.
I try to start a heap with more fibrous plant material such as bean or pea plants, tomato plants or broccoli stems, allowing (initially at least) a little more air circulation around the base. The heap then naturally grows in dense and sparse layers as different crops are finished and their residues are taken to the heap. In the photo below, a half formed heap is shown with the most recently added layers of weeds and dirt, then leek tops, then old tomato vines. (You can see the finished heap I am using in the background.)
At any given time I will have 2 or 3 heaps on the go: one new heap in the process of being built, one maturing heap, and one finished heap which I am using in the garden. My heaps are given a year or more to decompose, and contain more than 4 tonnes of compost when finished.
Depending on which crop I am preparing a bed for, I will use the compost coarse from the heap (broad beans, potatoes), raked to a medium tilth (hand-sown and transplanted crops) or fine sieved (if sowing with the six-row seeder). You can see the sieve in the photo above.
Regularly composting your garden builds soil organic matter content, makes the earth easier to use, and ensures healthy populations of beneficial soil biota, meaning you will have happy plants!