Horticulture Magazine

Loam-Based Compost

fertile loam

What Is Loam-Based Compost?

A loam based compost is a compost that contains a certain proportion of loam soil. Most potting composts do not contain any soil at all. But there are some – most notably the John Innes composts, that do have some ‘soil’ in their composition.

John Innes composts are popular here in the UK, and in other countries. These originally were developed in the 1930s in the John Innes Horticultural Institute, which was, at the time, situated in Merton, in Surrey. As the name indicates, the crucial ingredients in loam-based composts is loam. But loam is in short supply.

It is worth noting that the common John Innes loam-based composts contain soil ingredients that are not necessarily loam in their modern formulations.

Loam is the name given to soils that are balanced mixtures of mineral components of different sizes: clay, sand and silt. They also have an organic matter component. These soils are the best soils for gardeners. They combine the best qualities of different soil types and are great for plant growth.

The loam in loam-based composts is sterilized to get rid of weeds, pests and diseases. This means that the gardener should encounter fewer problems than they would with a non-sterilized soil-based growing medium.

There are several different common John Innes composts, or potting media that are on the market today. These are:

  • John Innes Seed Compost
  • John Innes Potting Composts (Soil Mix and Fertilizer Base)
  • John Innes Compost No. 1
  • John Innes Compost No. 2
  • John Innes Compost No. 3
  • John Innes Ericaceous Compost (for acid-loving plants)

There are also peat-based and peat-free composts/ growing media with ‘added John Innes’

Read on to learn what exactly goes into each of these different soil based growing media mixes.

What Is It Made Of?

The different types of loam-based John Innes compost have different ingredients, and contain materials in differing quantities. Here are some details to help you understand what is in John Innes loam-based composts, and how to make these products yourself  following their recipes:

John Innes Seed Compost is used for sowing seeds, and for cuttings, and seedlings can be grown on in it until they are ready for pricking out. It comprises:

  • 2 parts loose bulk medium sterilized loam.
  • 1 part peat or peat substitute. (Peat is often replaced with other materials and this is a more eco-friendly choice.)
  • 1 part coarse horticultural sand.

For each 2 gallon bucket of the mix, 10g of superphosphate and 5g of ground chalk are added.

John Innes soil mix is:

  • 7 parts sterilized loam.
  • 3 parts peat or peat substitute.
  • 2 parts loose sand.

And John Innes fertilizer mix (also known as John Innes base) is:

  • 2 parts by weight of hoof and horn meal.
  • 2 parts by weight of superphosphate.
  • 1 part by weight of sulphate of potash.

John Innes Compost No. 1 is suitable for young plants, potted up seedlings, and rooted cuttings. And it is used for short-term potting such as with bedding plants or transplants which will subsequently be placed out in the garden. For each 2 gallon bucket of soil mix (as described above), add 28g of John Innes fertilizer mix and 5g ground chalk.

John Innes Compost No 2 is used to grow many houseplants, and vegetable plants in containers. For each 2 gallon bucket of soil mix, add 56g of fertilizer mix, and 10g ground chalk.

John Innes Compost number three has even more nutrients. It is used for established trees, shrubs and other mature plants, and mature indoors plants that will remain in their containers for a long time. This mix is also good for particularly ‘hungry’ plants like tomatoes, for example. Add 84g of fertilizer mix and 15g of ground chalk.

What Are Its Benefits?

Loam based composts can be beneficial because the loam helps to avoid fluctuations in water and nutrient content in the growing medium. Since the soil part of the media is very stable, this can be especially useful in situations where plants are placed for the long term.

How To Make Loam-Based Compost

It is important to note that you can make your own loam-based compost mixes at home. And you do not necessarily have to stick to, or remain close to, the formulations described above.

One option if you have a good quality loam soil in your garden is simply to use some of that. You can also make a good loam by stacking turfs of grass upside down until it breaks down.

As mentioned above, all commercial loam-based composts use sterilized loam. The sterilization process is undertaken because soil or loam that has not been sterilized can contain weeds, pests and diseases.

It is not especially easy to sterilize loam at home, but it is possible to do so in your oven. Sieve loam soil, or loam from turf stacks, through a 5mm sieve. Then place it in an oven proof tray up to 8cm thick. Heat it for 10 minutes at 160-175 F (71-79 C). You can then mix it with other ingredients to make your compost or potting mix.

Personally, I use a mix of 1/3 loam (from the garden), 1/3 homemade compost (with well rotted manure and bedding from our chickens) and 1/3 leaf mould (from autumn leaves) to make a mix that works well for mulching around vegetables and placing in containers. I do not bother with sterilization and while there will be the odd weed here and there, and the odd problem, these are not too much of an issue in my organic garden.

What Plants Is Loam-Based Compost Good For?

A loam based compost can be good for a wide range of different plants. The right loam-based formulation can be used at all stages of plant growth. But it is important to remember that there are certain plants that need a different formulation, that is more free draining or which contains a specific mix of nutrients.

Loam based composts are generally excellent for the vegetable garden, and for mature plants that will remain in place for longer periods of time. They can be used for indoors or outdoors container gardens. And can often be the best choices when plants are not grown in contact with the natural soil.

When To Avoid Using It

Commercial composts of this type, either John Innes composts, or composts with ‘added John Innes’ should have been specially formulated to allow for good drainage, aeration, fertility and optimal plant health and plant growth. But is is important to note that as with other composts, the quality of these commercial composts can vary considerably. Some are much better than others.

The texture of bought compost with loam will vary considerably depending on the exact composition of the loam, and on where the material came from. There are no legally binding standards for John Innes potting media ingredients.

Many compost makers belong to the John Innes Manufacturer’s Association, and their aim is to provide reliable loam-based potting media that will support good plant growth even when not all ingredients exactly match the original formulations. Look out for the mark on packs to see whether the compost you are considering purchasing comes from a member of this association. Those that are not will not necessarily provide the same quality level and you may not achieve the same results.

Those who are trying to avoid harmful practices and want to garden as sustainably as possible should aim to avoid choosing products that contain peat, non-sustainably sourced loam, and any non-organic fertilizers or other problematic ingredients. If possible, in order to make your gardening efforts as sustainable as possible, you should try to take a DIY approach, using materials sourced from your own home and garden, or as locally as possible.

Frequently Asked Questions

Can You Mix Loam And Peat-Based Compost?

As you have already read above, both loam and peat are included in many compost or potting media formulations. So it is not either or. There are environmental reasons why opting for a peat based compost is not a good idea.

But if you have a peat based compost, you might be wondering whether it will be a good idea to add loam, or a John Innes mix, to this to improve it. John Innes potting media is sometimes added to a commercial soilless peat-based compost. They add it to improve buffering, improve the trace element content of the formulation, or improve the weight of the medium.

However, the manufacturers of these media will have carefully adjusted their formulations to allow for adequate drainage and aeration. John Innes materials have very fine particles, and the risk of adding these to a peat-based compost yourself is that the small particles may create compaction. Issues with drainage and aeration may arise. So generally, it is best either to buy and rely on existing formulations, or to take an entirely DIY approach, as mentioned above.

Does Multi-Purpose Compost Contain Loam?

Most multi-purpose composts are soil-less growing media, which do not contain loam. However, those that have ‘added John Innes’ do contain loam (or loam-like, soil based materials).

What Is ‘Mixed-Loam’ Soil?

A mixed loam soil is a topsoil that contains more or less equal mixtures of clay, sand and silt particles. The mix of particle sizes means that it is easy to work, fertile and moist yet free draining. In your garden, a good quality loam will also contain good amounts of organic matter as well as these mineral particles, and play host to a wide variety of soil micro-organisms – the life below the soil that is essential to keep the ecosystem running smoothly.

A loam-based compost attempts to replicate (more or less) the growing conditions in a natural mixed loam soil for container plants. And is used as a mulch to improve the soil below.

What pH Is Loam-Based Compost?

The pH of a loam-based compost will vary depending on which ingredients in addition to the loam itself are used in the recipe. A loam based compost that contains peat will have a higher pH than non peat based compost mixes.

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