A guide to compost & garden soil improvers

A guide to compost & garden soil improvers

Soil is essential to growing plants that thrive. Though some gardeners may be blessed with perfect soil, for most of us, it’s far from it. But turning things around is not difficult, as long as you understand what makes up healthy soil.

Whether yours has too much clay in it, is too sandy, too stony, or too acidic, read below to learn how to use improvers, what is the difference between soil and compost, and more.

How to use compost? Why do UK gardeners need it?

Compost is organic matter which is decomposing. It could contain a combination of green matter (e.g. food scraps and lawn clippings) and brown matter (dry leaves and branches). With the help of oxygen, water, and a range of bacteria, fungi, and other decomposing organisms (such as worms, sowbugs, and nematodes), they break down into particles. The resulting matter resembles fertile garden soil and is rich in nutrients. Farmers often refer to it as “black gold”. Humus is very similar to compost, but it’s organic matter that has been fully decomposed by microorganisms without oxygen (anaerobically), so it’s often naturally-formed, as opposed to man-made.

Composting refers to the process of recycling organic materials. While it occurs naturally, gardeners can speed up the process by creating favourable conditions.

What can you compost?

What ingredients can we use to make a good compost

To make good compost for garden soil, you need a combination two types of ingredients:

  • Greens – Fresh organic matter adds nitrogen. You can use fresh grass clippings and prunings, as well as food scraps (vegetable peelings, fruit waste, coffee grounds, teabags, nuts & grains, crushed eggshells, and any other non-animal food scraps).
  • Browns – Brown plant material adds carbon. This includes dead leaves, branches, twigs, and paper.

For a good carbon-to-nitrogen ratio, add two to four parts brown materials for every one part green materials. Stay away from diseased plants, dairy products, eggs, meat, fish, bones, fat, coloured or glossy paper, and pet waste.

How to use compost?

You can place compost on top of or mix it into flower and vegetable beds, rake it into tree beds, combine soil with compost to use on indoor plants, or distribute it on your lawn. You can even make “compost tea” to use as liquid fertiliser.

Note that you should only use finished compost in your garden. It must have a pleasant, earthy smell and a crumbly, smooth texture. The colour must be dark and rich. If you rush and use immature compost, this can attract pests or cause harm to your plants.

To check if your compost is fully decomposed, you can conduct the so-called “bag test.” Place a bit of moist compost into a zip-lock

bag and seal it, pressing the air out first. Let it sit like this for three days, then open the bag. A smell of ammonia or a sour odour means your compost needs more time to mature. Redo the test in a week with a different sample.

Advantages and disadvantages of using compost


  • Improves soil health and contributes nitrogen, phosphorus, and potassium, plus traces of calcium, magnesium, iron, and zinc
  • Provides a long, gradual supply of nutrients as opposed to a more potent but shorter-lived burst of nutrition
  • Reduces food waste and subsequently, methane emissions by preventing kitchen scraps from ending up in landfills.
  • Fights erosion and improves moisture retention
  • Reduces the need for chemical fertilisers
  • Home composting is cost-effective


  • Composting is time-consuming as it takes months for the matter to break down completely
  • A compost pile that is not properly balanced may begin to smell bad
  • Requires space to store and process

Compost vs Soil: What’s the difference?

Is compost the same as soil? They may look similar, making it difficult to tell them apart, but they’re quite different in composition. Now that we know what is in compost from the previous section above, let’s explore the difference between soil and compost and their application.

Soil is the surface layer of the earth that plants grow in. It’s a complex mixture of five ingredients. Apart from organic matter, which is also found in compost, soil contains minerals (clay, silt, and sand), liquids, gases, and organisms. The texture of soil is determined by the percentage of sand, silt, and clay it contains. It also determines its properties. For example, sand is coarse and drains well, while silt is finer and holds water better. Clay is very fine grained, which leaves little space between the particles for air or water, making it unsuitable for gardening.

What does work well for growing plants is loam, a combination of sand, silt, and clay. It can hold moisture and nutrients, while also having the ability to drain.

When to use compost and when to use soil?

Compost has more nutrients but can you use compost as topsoil? Compost helps plants grow faster, however, it lacks the texture, composition, and nutrient balance needed for them to thrive in the long run. 

Soil, on the other hand, is better at retaining its structure and holds moisture far longer. Therefore, you can use a combination of soil and compost to benefit from both. A good rule of thumb is to have no more than 20% of compost in your mix, or 4 parts soil to 1 part compost. When adding compost to their garden, most people apply it either in late autumn or in early spring. 

Compost or mulch?

Compost and mulch cannot be used synonymously. As opposed to compost, mulch refers to any material you use to cover the soil surface, either organic or inorganic. It could mean anything from wood chips, bark, hay, and grass clippings, through geotextiles to crushed seashells, gravel, or pebbles. Mulch is usually applied to control weeds, retain moisture, and regulate temperature. Last but not least, it can help improve the visual appearance of your garden.

Still, it is possible to use compost as mulch. It adds nutrients to the soil when it rains and helps stabilise soil pH. However, compost is not as good at keeping weeds down as mulch.

To reap the benefits of both, gardeners can combine compost and mulch. Always place compost directly on the soil, then cover it with mulch. If you’d like to add compost to an already mulched garden, remove the mulch by raking it aside, work the compost into the topsoil, and return the mulch. 

Types of compost

What are the different types of compost? Examples.

There are different types of compost, each with its own benefits and drawbacks. Let us explore some of the options to determine what is the best compost to buy or make yourself.

Manure – animal manure from farm animals such as cows or horses. left to decompose. It can then be used as a nutrient-rich soil amendment for gardens. It no longer smells or contains dangerous pathogens. It may be made solely from manure, but most often it contains other composted materials like straw or sawdust.Contains Nitrogen, Phosphorous, and Potassium. After being composted in a hot compost pile, it no longer smells, contains pathogens, or weed seeds.If not composted, it can be smelly, difficult to handle, and may contain bacteria.
Green manure – fast-growing plants sown on bare soil and then dug into the ground while still green.They suppress weeds and prevent soil erosion, while also providing habitat for pollinators. Turning green manure returns valuable nutrients to the soil and improves soil structure.You need to wait 2 weeks to a month after digging in before planting in new crops. A green manure crop may harbour slugs and snails.
Vermicompost – earthworms eating and digesting organic materials to produce excrement called casting.Great for blending with garden soil, potting soil, or dressing the tops of garden beds.Worms require care and are sensitive to extreme temperatures and direct sunlight.
Peat-based compost – a base of peat blended with other ingredients such as fertiliser, sand and/or grit, vermiculite or perlite, wetting agents and lime.Good at retaining moisture and holding in nutrients from other sources (fertilisers, other organic matter). It has a naturally low pH level which can be helpful for more acid-loving plants.Mining peat damages habitats and releases carbon into the atmosphere.
Peat-free compost – uses base ingredients such as wood fibre, composted bark, coir, and green compost. Great for sowing seeds; rooting cuttings; filling tubs, planters, and hanging baskets for annual flowers or vegetables.It’s a widely available and environmentally-friendly alternative to peat. It tends to be a little more expensive than peat-based compost. The label must explicitly say “peat-free”, otherwise it’s most likely not. 
Loam-based compost / John Innes – soil-based compost made from a mixture of loam, sand or grit and peat (or peat substitutes). Loam is currently in short supply and is often replaced with other soil-based material.It provides a high level of “buffering”, through good absorption and release of water and nutrients.
– John Innes Seed Compost: great for sowing and cuttings.
– John Innes No.1 Compost: for transplanting young seedlings or rooted cuttings, or short-term potting such as for bedding plants or vegetable transplants.
– John Innes No.2 Compost: for potting up small plants, e.g. most houseplants and vegetable plants in containers.
– John Innes No.3 Compost: for established plants, trees, shrubs and climbers, and mature indoor plants.
Purchased compost may vary considerably from the original specification.
Mushroom compost – a substrate left after growing mushrooms which still contains a limited amount of nutrients. It can contain straw, manure, gypsum, peat, etc.It’s eco-friendly, improves soil water retention, contains high Calcium levels, and attracts earthworms.It can cause water logging with some plants, resulting in rot or fungal infections. It’s high in salt which hurts seeds and seedlings, and contains no beneficial organisms as it is sterilised.

What’s the difference between potting mix and multi-purpose compost?

Potting mix, or potting soil, is a lightweight material used for starting seeds off indoors. It’s made up of a mixture of peat moss, tree bark and other ingredients and is steam-heated to kill any bacteria and diseases. This makes it a light, airy and sterile environment with good drainage for young root systems and small planters.

Compost, on the other hand, is of medium density and is packed with lots more nutrients, making it ideal for fully grown plants in open veg plots or flowerbeds. The richness of the soil comes from all the green and brown matter that combine during the composting process, which reduces the need for chemical fertilisers.

What is the best compost to buy?

Luckily, the labels on the bags do a very good job at explaining the contents and intended use of each type of compost. If you’ve decided to purchase compost, refer to the label and follow any advice given on plant care routines as they may differ. Don’t hesitate to ask workers at your local gardening centre for recommendations, or request help online if you’re shopping from a website.

What do soil fertilisers do? How are they different from compost?

What is the difference between fertiliser and compost?

What are fertilizers and how are they different from compost?

Fertilisers can be applied to the soil or directly to different parts of plants to supply nutrients and boost plant growth.

On the other hand, compost is added to soil to improve its condition, fertility and structure. It helps plants feed themselves. ‘Soil improver’ and ‘soil conditioner’ are alternative names for well-rotted organic matter. There are different types of store-bought organic soil improvers available on the market.

Compost acts more slowly and its application is more flexible and general, while fertilisers are more potent and require more careful selection and dosage. For example, each fertiliser has a specific NPK ratio, suited for different types of plants, as well as a feeding chart.

– Adds nutrients and alleviates deficiency in soil
– The rapid effect of fertiliser on soil allows immediate nutrient availability for plants
– Targets specific plant needs
– Creates nutrient-rich soil
– Aides in soil moisture retention
– Assists in disease resistance
– Helps control weeds
– Overload of nutrients can occur
– Can upset microbial balance
– Generally chemically produced
– Chemicals can be released into groundwater
– Takes time to make your own
– Physically laborious
– Large amount needed for success (problematic for large gardens)

Fertilisers and compost are not interchangeable and should not be used in place of each other. Instead, they can be combined. Compost can work well with organic fertilisers, as it stores the nutrients until they are needed by plants. In addition, organic fertilisers (as opposed to synthetic ones) do not disrupt soil balance and support microbiological life. As for compost, soil that has been regularly treated with it becomes rich and crumbly and may need less fertilisation than untreated soil.

Soil is nature’s gift that keeps on giving. Especially if we take good care of it. With these tips on composting, using soil improvers, and fertilising, you’re certain to have soil with good properties, including improved structure, water and nutrient retention, and beneficial microorganisms, resulting in healthier plants.

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