This is a question I get all the time and although many gardeners use these terms interchangeably, there is a difference. Let’s see if we can clarify the two words and decide when to use compost and when mulch would be better. There are definitions for different types of composting systems and mulches and some FAQs answered about problems with composting.
So what’s the difference between compost and mulch?
Compost is decomposed organic matter used as a soil amendment to add nutrients and improve soil structure, while mulch is a layer of organic or inorganic material placed on top of the soil to suppress weeds, retain moisture, regulate soil temperature, and protect plant roots.
What is mulch?
Mulch is a layer placed on top of the soil. It can be a physical membrane (such as plastic, cardboard, horticultural fleece or even stones) used to suppress weeds, to deter insects or pests, or an extra layer placed on the soil to retain moisture in summer. Mulch, like leaf mould, can be layered on to boost soil fertility, as it rots down eventually. In winter, a layer of mulch can keep underground parts of plants cosy and warm, helping to avoid frost damage.
What is compost?
Compost comes in different types, but essentially compost is a method of re-utilising green or brown kitchen and garden waste that is decomposed in the heap by bacteria and insects, to enrich the soil that can be used to grow other vegetables, flowers and trees. Compost is best dug into the soil in small amounts so that its goodness can be utilized by the roots of the plants. Composting annually enriches your soil and fills it with essential nutrients for plant growth like nitrogen, phosphorus, and potassium.
Open-air composting systems are known as aerobic, that is they use oxygen to decompose waste.
Anaerobic composting systems exclude oxygen to convert waste into liquid and these are usually used indoors to recycle kitchen waste, dairy and meat products.
Let’s look at all of these in more detail and answer the common questions about composting and mulches.
Types of composting systems
Aerobic composting systems use oxygen to decompose green waste into a soil improver. Think of a circular plastic bin composter, with a lid, where you add different layers like grass clippings, food waste like potato peelings, shredded paper and cardboard or uncooked vegetable waste.
Any outdoor compost heap, placed directly on soil utilizes oxygen so these methods are all known as aerobic. Making compost involves a chemical reaction where bacteria and fungi get to work on the food waste and break it down into different elements and nutrients over time.
4 different types include:
A typical circular purchased plastic bin with a lid and a slot to remove finished compost at the bottom. These are widely available commercially, from garden centres or your local council.
- A homemade compost heap laid on soil, lined with twigs or cardboard can be made from wood pallets, chicken wire, stakes etc.
- Rolling compost makers turn the waste inside using a handle and if you do this regularly, it adds extra oxygen to decrease the time it takes to produce finished compost. These are more expensive but they make compost much quicker due to the extra oxygen provided by the turning. You get quite fit using these too!
- Wormeries are containers filled with waste and also a handful of earthworms who digest the waste on offer, and then excrete their waste. The worms make tunnels in the waste container while they move, giving it plenty of oxygen and their poop adds to the richness of the finished compost. Placed in a container on a balcony, they will provide you with fabulous compost after a few months. Using worms to compost waste is known as vermiculture.
- Aerobic systems normally take several months to complete their cycle.
Anaerobic composting systems
Anaerobic composting systems are used to treat waste that cannot be composted such as cooked meat bones, poultry leftovers, fish bones, and dairy waste products like yoghurt, cheese or cream as well as green kitchen waste, like fruit peelings or orange skins.
Typically, a specialist bin is provided with starter kits that use activators to reduce the food waste to liquids, which also squeeze the waste into a compound section. After 10 days or so, this releases a liquid rich in nutrients derived from the waste collected. Air is completely excluded so this is anaerobic composting.
The end product is not soil but usually a liquid with high nutrient content, which can be diluted to use on indoor plants. The actual waste does not completely transform as it does in your compost heap either. Food content becomes almost pickled, after the fermentation process, and this waste will still need to be disposed of.
Anaerobic systems exclude oxygen and the bacteria prefer conditions to be damp or even wet. So burying the remaining Bokashi bin waste in a trench outdoors and re-covering this with soil or sealing waste in a bin or a polythene bag to exclude air and allow time to work its magic on it.
Bokashi is a Japanese system where a waste bin is used with added bran to activate the waste. It works by the addition of an innoculated bran, that contains bacteria and microbes that can help to convert the waste into liquid. The waste is added to the bran and then another layer goes on top, so the waste is sandwiched between them and then air is excluded by closing the bin tightly. The whole process takes from 10 to 14 days. Air is excluded by pressing down the waste excluding air, which then converts it into a liquid feed often known as Bokashi tea, but it also leaves the fermented solid waste which you can add to an (aerobic) normal compost heap. Or you can bury it.
Some bin composters use coffee grounds or sawdust instead of this bran, as activators to convert dairy products, cooked meat and fish in a closed-air system by reducing the oxygen content. However, the treated bran helps to control pathogens as cold composting never reaches the sterilisation of pathogens like hot composting does.
Compost trenches can be dug a foot deep with the size of your garden determining the size of the trench. Dig and lift the grass initially in a row, and place it aside to re-use for covering later, and then fill the excavated trench with the fermented waste. The liquid can also be sprinkled in this trench if you like. Then cover it up with the excavated grass clods, turned upside down, to rot into the trench and some extra soil and allow it to settle. Wait for a month or two and then you can plant seeds in the top layer of the trench.
Another popular method of using the fermented waste leftovers from Bokashi bins is to bury them in bags or bins underground, where it will eventually rot down and become useful for your soil. You just need to have some soil to dig a hole large enough for it and to be patient until the waste decomposes and becomes a fertile area for plants. Dig it back up after several months and then spread it on the soil where the air will help to break it down completely, and you can use it as a new patch of garden to plant up.
Now for different types of mulches…
Organic mulches are made from natural materials whereas inorganic mulches are often made of plastic or horticultural fabric. Corrugated iron sheets can be used to mulch an area so be creative!
If you want to suppress weeds then a layer of plastic or cardboard can do the trick. Cut up old manure or fertiliser bags or any large packing that arrives at your house. If you use cardboard, make sure there is no printing on the card (as these can leach chemicals into the soil), and then weigh it down with stones. Old blankets are very useful if you want to warm up an area or suppress weeds to make a patch easier to dig.
Make round cardboard collars with a small opening for the stem to put around the stems of brassica plants to deter cabbage root fly.
Cardboard is a good way to recycle waste card to suppress weeds in any area that has been wild for a while. Place the card over a section of weedy ground and secure it down with a few large stones or cover it with grass clippings. This removes light from the area so plants underneath will turn pale or yellow. Leave it for at least a month or over the winter and by spring, you normally have a fine collection of slugs and snails which you can easily relocate to your compost heap when you lift the card. Once the weeds have died back remove them, and dry them before adding them to compost then you can dig the area thoroughly.
Stones are useful to surround certain plants because in summer they prevent moisture loss by providing a physical barrier whereas in winter, they retain heat so they keep the roots of overwintering plants warmer. You can buy decorative stones if you like or just use whatever stones you can find in your garden space. If you grow bulbs like dahlias, daffodils or spring bulbs, you can mulch around them after flowering to keep them warm and protected. Stones can also provide a decorative border to remind you that there is something interesting in the ground there. Rake the stones off just before they are meant to flower.
Made from the bark of trees wood chips are often used as mulch for paths, placed over a layer of horticultural fabric. It makes the environment look organic and works well to make paths but it does make wheelchair access impossible so think about your users before you add a bark path.
Sawdust is another alternative and this is often added to flower beds both to suppress weeds and also to add some organic matter to the soil. It is particularly useful to gardeners whose soil is clay-based as it helps to break up the stiff soil and give it a bit more air.
Use chicken wire and stakes to make a circular shape and allow the leaves to decompose. This allows air to circulate freely, aiding them to rot down over the winter. The odd rain shower will speed up the whole process too.
Leaves can be collected in autumn and allowed to rot down in a plastic bag, or a chicken-wire collector (pictured). When they rot down several months later, you can crumble the mixture into place over delicate plants. You can add a layer 2-3 inches (5-7 cm) thick. It will not only add to the organic content of the soil but it keeps the soil underneath warmer too in the colder winter months. Worms love leaves and usually overwinter in them, as they provide tasty snacks and a layer of heat. While leaf mould does not count as a fertiliser, it certainly helps with water retention in hot weather and also acts as a soil improver of the structure of the soil overall. If you find any worms as you move it, then add the worms too; their poo will also add to soil fertility too.
Allotments offer gardeners supplies of leaf mould, made from local green waste collections. At my allotment, the supplies are very generous and you can lay it out like this before you decide where you actually want it.
To offer some frost protection, place a thick layer of leaf mould around plants that will remain in the soil over the winter, like spring bulbs or dahlias. Use finished leaf mould as a protector for bulbs. Flowers will often appear earlier with leaf mould because their roots feel warmer.
If you are lucky enough to have lots of leaves, then collect some into bags, and you can add a handful to pots whenever you plant seeds or when adding new soil to larger pots. You can crumble it into flower beds too to condition it.
When do I use compost and when should I use mulch?
Maybe you can use both!
The time to use compost is when you want to boost growth in hungry plants like tomatoes, beans, fruit and vegetables. Compost adds nitrogen to the soil along with a healthy does of microbes that are essential in your soil. Dig in compost well, before you plant your seeds but ensure that you are not planting directly into the compost. It is better to mix it with existing soil to add fresh nutrients each year.
Use mulch in the spring if you want the plant to have lots of water in summer because it stops (or slows down) water evaporation. Use mulch to suppress weeds at any stage or season too because the thick layer will stop many of them from reaching the soil surface. Use mulch in the autumn for frost protection and to add some nutrients to the soil as the mulch decomposes.
Why does my compost heap smell?
The main reason for bad smells is that not enough oxygen is making its way into the container. Make sure you cut large pieces of waste into smaller pieces which are easy to decompose. Try to move the compost around when you add new waste.
Some people insert tubes or pipes into the heap mid-way down to enable them to move the waste.
If the texture seems very wet, you can remove the bottom compost and re-load it in the top and try to move it around manually. Make sure you add some brown waste to help it dry out like torn-up recycled paper, cardboard or dry leaves.
If it appears too dry, then add some grass clippings or plenty of vegetable waste. Have a look to see if there are lots of worms at the end and sheltering in the hot lid.
If you have a large heap, try to use a stick to mix it well and add waste in layers so that you have a wet (green) layer with potato peels apple cores, fruit cores or grass clippings etc, followed by a dry (brown) layer, which can consist of shredded paper, dried leaves, crushed eggshells or torn up egg boxes and newspapers. Then add another layer of green waste on top. This mix usually avoids smells.
The last culprit could be a member of your family or a visitor who has added some cooked food or leftover meat to the pile. This causes a different smell to normal compost and it also attracts vermin like rats to the area so do not add cooked food to your compost heap. See more below on what to add and what to exclude.
The question arises what does a good compost heap smell like? It should not smell sour, rank or disgusting. There should be an earthy smell like when you dig soil but placing a heap a bit away from your back door is a good idea if you really do not like the smell. Gardeners like me love to open it up in winter just to get a good sniff of this “black gold”, as it is often known.
Why can’t I add meat to my compost bin?
This rule is only for exterior aerobic compost bins. The reason to avoid cooked food is that it attracts rats and mice by the smell. Once they arrive, it is difficult to get rid of them so this rule is for prevention.
By now, you know that you can add meat, fish, poultry and bones to anaerobic compost indoors as long as you chop it into small portions so it will ferment easily.
By now do you know the difference between compost and mulch? Compost is dug into the ground and mulch usually sits on top is an easy way to remember it. Compost improves the health and fertility of the soil while mulch protects individual plants from moisture loss, weed competition or frost damage. You are aware of the chemical miracle taking place under the lid of your compost heap and in the Bokashi bin too. Recycling your waste at home is so environmentally friendly and important for the planet so I hope this article encourages you to start a compost heap and use it to boost your plants this summer. If you have any more questions about composting, comment and let me know and I will happily try to answer any issues that may occur.