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Compost additives: what do they do?

5 Minute Read
Compost additives help to improve your soil condition in the garden. Use them to increase water drainage, add organic matter, or change the pH of the soil. In this article, we take a look at the compost additives available at The Garden Superstore and how to use them!

Vermiculite is a natural mineral that is great at absorbing liquids, is lightweight and provides the perfect environment for plants to grow when added to compost. Vermiculite is ideal for seed germination and is typically used in potting compost. Vermiculite helps to stimulate root growth, giving plants quicker anchorage and improved uptake of nutrients.

  • High water holding capacity
  • Good air holding capacity
  • Non-abrasive and non-irritant
  • Attracts excess nutrients
  • Facilitates re-wetting
  • High cation-exchange capacity
  • Neutral alkalinity
  • Does not deteriorate in storage (if kept dry)
  • Free from disease, weeds and insects
  • Insulates and minimises soil temperature fluctuations Inorganic and sterile
  • No known toxicity or fire hazard

Sinclair Fine Vermiculite

Fine vermiculite for mixing into compost to increase nutrient and water retention. Sinclair Vermiculite Fine is a natural, non-toxic addition to compost mix that improves aeration and drainage. Fine vermiculite can also provide insulation to seedlings in the nursery.


Sinclair Horticultural Grade Vermiculite

Sinclair Horticultural Grade Vermiculite is porous so it helps retain water & nutrients in the soil for release to plants. Lightweight and has thermal insulation properties which makes it ideal for use as a pot topper.


Perlite is the name for a naturally occurring siliceous rock. When heated, it has the capacity to expand from four to twenty times its original volume. This is due to the rapid expansion of water within perlite, which creates the extremely light perlite that we use for gardening.

The main function of perlite for the gardener is to aid water retention and aeration as well as improve compost mixtures.

  • Prevents compaction 
  • Improves aeration and drainage better than vermiculite 
  • Stimulates root initiation and vigorous growth
  • Holds moisture but does not become soggy 
  • Almost neutral pH 
  • Does not decompose 
  • Free from disease, weeds and insects
  • Insulates and minimises temperature fluctuations 
  • Inorganic, inert and sterile 
  • No known toxicity or fire hazard 

Sinclair Perlite Standard

Sinclair Perlite Standard is horticultural medium-grade perlite for compost mixes to improve drainage and increase insulation. Tropical plants and bonsai thrive in loose free drainage soils. Perlite has a neutral pH and is sterile making it disease resistant.


Sinclair Perlite

Sinclair Perlite Supercoarse is the perfect potting mix for plants that like soil-less tropical environments (e.g. orchids and bromeliads). Perlite Supercoarse allows lots of air around plant roots and has good insulating properties.



Use Vitax Perlite Compost Additive for potting compost mixes for seeds, cuttings potting & planting. Perlite is a porous volcanic rock that aerates soils and retains water & nutrients when added to potting compost. Use on its own for seedlings or as part of a mix in containers.


Garden Lime
Garden lime is used to neutralise soil acidity and is ground from limestone rocks and provides calcium carbonate and magnesium carbonate to the soil. Calcium helps plants to grow healthy roots and stems. Magnesium aids with photosynthesis in lawns. 

  • Neutralised soil acidity
  • Contains calcium and magnesium carbonates
  • Enabled healthy roots and stems
  • Apply in winter before growing crops in spring
  • Increases pH

Garden Lime Powder 25kg

Garden Lime powder for application to vegetable patches and garden beds to neutralise acidity. Use lime in winter before growing your new crops in the spring. Suitable for `a multitude of garden projects and usage. 


Dolomite Microdol H100 Lime 

Dolomite Lime fertiliser includes magnesium carbonate & calcium carbonate. Dolomitic lime can be used to correct magnesium deficiencies in soils and increases pH. 25kg bag of powdered dolomitic lime. Dolomite Lime Microdol H100 is suitable for fruit trees like apples & pears. 

Soil Improvers and conditioners
Soil improvers and conditioners are used to improver the overall condition and fertility of the soil structure. The substrate is added to the soil and worked in to enable a healthy uptake of nutrients which in turn benefit plants and the overall quality of the soil. 

  • Contains organic material to improve soil structure
  • Aids soil structure by providing nutrients, improving aeration and water holding capacity
  • Helps to prevent compaction
  • Depending on the ingredients, soil improvers can raise or lower the soils pH level

To find out your soil type, take a look at our soil health guide.

Westland Soil Conditioner

Westland Soil Conditioner is peat-free soil improver that blends organic manure and composted bark to aerate heavy clay or feed and enrich loose sandy soils. The Gardener’s choice for use in beds & borders to improve the soil nutrients


Calcified Seaweed Soil Conditioner

Thomas Elliott Fertilisers Calcified Seaweed is an organic soil conditioner with calcium & magnesium trace elements. Encourages healthy root development and growth. Improves heavy clay soils and benefits soil bacteria. Not suitable for acid-loving ericaceous plants.

Melcourt Soil Improver

 Increase water & nutrient retention by digging Melcourt soil improver into garden soil to at least 15cm. Use as a multi-purpose compost to improve soil conditions in a wide range of garden landscapes. 
Contains naturally composted green materials.


DCM Vivisol Soil Improver

Organic soil improver with a vegetable base and added bacterium for soil improvement after groundworks or when conditions are poor. Increases the uptake of nutrients through roots. 

DCM Vivisol Agri Soil Improver

Organic soil improver from DCM. The Vivisol®Agri Soil Improver has vegetable raw materials and added bacteria species to improve consistency of root growth in crops. 

Topsoil contains some of the highest concentration of organisms and nutrients and gives your garden a helping hand by enriching your existing soil. It enhances the quality of poor soil types, to help with plant and vegetable growth. Topsoil can contain more nutrients and organic matter than regular soil, meaning it works with you to provide balanced nutrients to nurture your plants. Using Topsoil means overtime your will improve the quality of your soil in the long run, which is great for the future of your growing.

  • Can be used in raised beds and planters as well as a topdressing for lawns
  • Contains optimum levels of phosphorus, magnesium and potassium as well as beneficial pH levels
  • The natural fertility of topsoil promotes healthy growth
  • Enhances existing soil types

Melcourt Topsoil Blended Loam

Melcourt Topsoil Blended Loam has a loam base & organic matter blended to make rich topsoil for lawn dressing or potting compost for container gardening. Peat and chemical-free and 
RHS Endorsed. 

Enriched Topsoil 20mm

Binder’s Enriched Topsoil is a mix of loam and compost. Use blended loam topsoil in garden beds, when planting new trees or shrubs, or in preparation for a new lawn, giving the new grass a great start. Topsoil can also be used to fill in uneven paths and borders.

Mulch is typically an organic material which is added to the soil surface, mulch helps to suppress weeds and improve water retention as well as benefitting the soil's overall health and fertility. It also makes the garden look neat and tidy!

  • Helps to suppress weeds
  • Improves soil moisture retention
  • Boosts soil health and fertility
  • Improves the appearance of the garden

Strulch Mulch - Straw Mulch

Strulch mulch is great for organic gardeners. It is a locally made and sourced product, made in the UK. Strulch has patented the process they use of producing straw mulch. Straw mulch can be used to protect plants from winter frosts when conditions are cold. It can be used around vegetables and fruits in the kitchen garden. 

Composted Fine Bark

Melcourt Composted Fine Bark™ is an organic soil conditioner using sustainable conifer bark sourced in Britain. Melcourt’s unique fine bark mulch is used in domestic & landscape gardens for improving soil structure and quality. Melcourt Composted Fine Bark is 100% peat free and FSC certified.

To shop the full range of mulch we have available at The Garden Superstore, click the button below.


Related Products

Sylvagrow Farmyard Manure

 Farmyard manure is an organic peat-free soil improver which suppresses weeds, adds nutrients and improves water retention. Mulch into beds & borders and around trees and shrubs. RHS endorsed.


Sinclair Hydroleca Pebbles

These clay pebbles are lightweight and their frost resistant properties make them ideal for weather-exposed areas in the garden, on rooftops or for water features. Hydroleca hydroponic pebbles are non-toxic, sterile and have a neutral pH.

 Horticultural Gravel

Melcourt Horticultural gravel increases drainage when added to a compost mix, making it ideal for cactus gardens and succulent plants that like dry conditions. Use as decorative stone on beds & borders. 
4mm – 10mm particle size.

Sinclair Hydroleca Pot Topper

Sinclair Hydroleca pot topper or potting aggregate for your container gardening. Non-toxic. Sterile. Frost resistant with insulating properties, making it ideal for exposed containers on patios, in window boxes and on roof terrace gardens.

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Tags: compost, Soil health, Soil improvers, vermiculite

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