What good aeration means and why the pile fails
Aerating is not just turning the heap now and then. It means oxygen reaching the inside of the pile, where the microorganisms that break down the matter live. If the compost becomes compacted, oxygen runs short, acidic or ammonia smells appear and the process slows down. On a terrace in Sotogrande, with sea air and high humidity, a small, covered pile can keep its centre damp and compact for weeks if it is not turned.
The practical test is simple: when you open the heap it should smell of woodland earth, not rot. If a handful of material clumps like a wet sponge, it lacks structure and has too much water. If it is dry and crunches, it lacks moisture. Aeration comes from combining coarse material, thin layers and scheduled turning, not just adding more waste.
- Do the fist test: squeeze a sample; it should feel damp without dripping.
- Watch the colour: uniform dark brown means a good process; black, shiny patches mean a lack of air.
Open bin versus closed bin: how to choose
An open bin, such as a mesh ring or three-bay system, lets air circulate and allows turning with a fork. It is the most tolerant option in a hot climate if you have space and some shade. Its weak point is that in summer the outer layer dries quickly and it attracts wildlife. A closed bin, with lid and base, controls moisture and odours better on terraces, but it needs more handling: if it is filled with fine waste and no structure, the inside compacts and air cannot get in.
The decision depends on space and the type of waste. Anyone who prunes shrubs and has a garden benefits from an open bin with chopped woody material. Anyone who only produces kitchen waste needs a closed bin with a layer of dry material and more frequent turning. Inland in the province, with drier summers than on the coast, a closed bin helps conserve moisture; on the coast, an open bin in good shade reduces the risk of excess water.
- Choose a closed bin if you live in a flat or on a small terrace; an open one if you have a garden and pruning waste.
- Place the bin in afternoon shade, both on the coast and inland.
Material proportions: structure comes first
For air to circulate, the mix needs volume. Green, damp, nitrogen-rich waste such as fresh leaves, grass or fruit scraps tends to compact. Dry, woody waste such as fallen leaves, unprinted cardboard, shredded twigs or straw holds open the gaps through which oxygen enters. A rough guide is more dry material than green, added in thin alternating layers rather than big blocks.
A common mistake is adding freshly cut grass in thick layers. Within a few hours it forms an impermeable mass that smells bad. The solution is not to remove it, but to mix it with dry leaves or twigs and spread it in a thin layer. Another mistake is over-shredding: material that is too fine compacts. Leaving some twigs a finger thick improves structure.
- Alternate thin layers: one of green waste, two of dry, coarse waste.
- Chop large waste, but keep some woody material to create air pockets.
Turning, moisture and season: the real calendar
On the Costa del Sol, from spring to autumn the compost can heat up quickly and dry on the outside. Weekly or fortnightly turning in that period helps redistribute moisture and air. In winter, with mild temperatures, the process continues, but it is best to turn less and protect the pile from heavy rain. Inland, with cold nights, the heap's activity drops and too much turning cools the centre further.
Correct moisture by watering if it is dry or adding dry material if it is waterlogged. Covering the bin with a breathable sheet or lid prevents heavy rain from saturating it. A good indicator is the inside: it should be warm in spring and autumn, and damp without dripping. If the heap does not heat up, check the mix and the size: a very small pile loses heat and a very large one without air rots.
- Turn more in spring and autumn, less in winter; adjust to the climate of your area.
- Cover the pile during heavy rain to prevent waterlogging.
Common mistakes and how to correct them
One of the most common mistakes is burying kitchen waste without mixing it. It compacts, attracts flies and produces bad smells. The correction is to open a hollow in the centre, put the waste in and cover it with dry material. Another mistake is adding cooked waste with oil or meat in large quantities: it slows the process and attracts animals. In small amounts and well covered it can be integrated, but it is not the basis of compost.
The idea that compost should always be covered and left still is also wrong. Without air there is no compost, only putrefaction. If you notice a rotten smell, turn the pile, add dry material and leave it more open for a few days. If it smells of ammonia, there is too much green waste: correct it with dry leaves and stir. Gardening in Sotogrande with hedge and pruning waste offers plenty of woody material, ideal for correcting mixes that are too wet.
- Bury kitchen waste in the centre and always cover it with dry material.
- If a smell appears, act within 48 hours: turn and add structure before it compacts.
How to tell the compost is ready and how to use it
Mature compost is dark, smells of earth and the original waste can no longer be recognised. If twigs remain when you sieve it, return them to the pile as structure. It is best not to use immature compost in pots or seed trays: it can burn roots and compete for nitrogen. In the garden, spread it in a thin layer on the soil or work it in shallowly to improve structure without excess.
On the Costa del Sol, the best time to apply compost is autumn, before the rains, or early spring. In sandy soils it helps retain moisture; in clay soils it improves drainage. A useful approach is to try it in a small area and observe how the soil and plants respond before spreading it over the whole garden.
- Sieve compost before using it in pots and return coarse remains to the pile.
- Apply in autumn or early spring, in a thin layer and without burying it deeply.