How to make best use of your compost

In this blog, we guide you on how best to use your mature well-rotten compost in:

• Vegetable patches and raised beds
• Seed trays
• Hanging baskets  
• Flower beds
• Lawns

Whether you make your own compost or buy it from a garden or DIY centre, it can vary enormously.  Using our understanding of what compost contains and how each component affects plant growth, we suggest how to make best use of your compost. 

After the lists of tips, we look in more detail at the different types of compost, what they contain and how this affects how it is used.  Finally, we end the post with a section on how to improve your next batch of compost.

Preparing your vegetable patch and raised beds

-    Dig in compost rather than adding a layer on top of the soil
-    Only add to beds you intend planting heavy cropping plants that enjoy a rich soil (follow plant guides on crop rotation etc)
-    Don’t add too much. 10% is good, 20% max. Too much compost can be bad.

Growing media for seed trays and potting on seedlings

-    Use the right product! (read more below)
-    If making your own media, follow known recipe – e.g. John Innes 1,2 3
-    SoilFixer is working on our own recipe using biochar, colloidal humus and compost 

Making up your hanging basket and container mix

-    Sieve compost (remove all bits >6mm)
-    Blend with soil and sand
-    Add colloidal humus
-    Don’t forget it will need feeding (NPKs) after 3 months

Greenhouse - tomato or cucumber beds

-    Use deep beds (>20 cm deep)
-    Add colloidal humus (for water retention)
-    Ensure compost was hot composted to kill off pathogenic types of fungi and seeds. (Hot greenhouse will exasperate issues with dangerous molds and fungi). 

Flower beds

-    Dig it in to 10 cm depth. (Ok – we know spreading around on top of soil is easier but colloidal humus is most beneficial in the root zone).
-    Ensure compost is exceptionally well matured. Either hot compost at 60C or leave your cold heap for a minimum of 18 months to reduce number of viable weed seeds. Otherwise all the weeds will grow back into your flower bed. 

Lawns

-    Add a thin (1cm) layer of sieved compost in spring to kick start the grass season. It will help reduce fertiliser needs, control moss and reduce the need for watering.

Defining compost - what is compost?

Below we 'dig in' to the detail to help define what is compost, PAS100 compost, humus, colloidal humus, fertilisers, growing media and soil improvers.

Compost is a brown crumbly mass in which no original waste plant or animal material is distinguishable. That is, you cannot tell which bit was the banana skin. You will note this is a very broad definition that can cover a vast range of things. Adding to the definition, compost has:

• An earthy smell -  it should not smell putrid, acrid or drain-like
• Is a dark brown almost black material
• Is very wet and sticky and pliable 
• Is mostly fine particles (<3mm) with no sign of original waste or big bits. (Woody waste breaks down slower - it is almost always the case that home compost has large bits of un-composted material. If you have time, it is better to sieve these out and add them back into next compost heap).

PAS100 compost is merely a compost that meets a QA standard. (The PAS standard is the early stage of BSI kite mark). Notably, each batch of compost is checked to ensure it is stable and mature. It is now common that all PAS100 compost passes through a 6mm mesh sieve. The standard makes no claim on the ratios of different components  in the compost - this can be problematic – especially when the green waste contains a lot of wood waste. 

Homemade compost and PAS100 compost can vary enormously in texture. This is due to the ratio and structure of the two components – colloidal humus (anywhere from 1-10%) and particulate organic matter (POM); i.e. all the partially degraded matter which ranges in size from small from 0.5mm to large 6mm pieces. 

Humus is usually reserved to describe ‘very well-matured’ compost, but can be used to describe any compost, and specific types of dark peat.  We have chosen to use the term ‘colloidal humus’ as this accurately describes the valuable component of compost we believe gardeners should focus on. 

Colloidal humus a wonderful substance. By definition all colloidal particles are ultra-fine (<50 microns) so dried-out colloidal humus would easily pass through a kitchen (flour) sieve. The second property of colloids is absorption of water to create a suspension - think jelly or wallpaper paste. Colloidal humus when separated from the rest of the compost is a thick-gooey-slimy-mush that won’t go through a sieve without being pushed.   

In our view, colloidal humus is THE important ingredient in compost because it holds and releases both nutrients and water. The plants interact with the colloidal humus via the root zone so it is logical that colloidal humus needs digging in.

A fertiliser is a mix of plants nutrients (normally the NPK elements) added to soil specifically to enhance growth. Commercially a fertiliser must meet a defined specification – hence you will always see numbers such as 5:10:10 indicating the % ratio of NPK. Although compost has a fertiliser value, is not labelled for sale (classed) as a fertiliser because the total value of NPKs is low (<5%) and because they are not readily available - they are released slowly over time as the compost degrades.

A soil improver is any substance added to soil to improve its properties related to plant growth – but it excludes items classed as an NPK fertilisers and lime. All composts come under the broad classification of soil improvers. 

A growing media is a defined as substance for growing plants in. There is a huge range , soil obviously, compost often is used (!), even vermiculite and rock wool are used as growing media in specialist areas. Everyone would like a 'single' growing media that works for all planting. This is unrealistic as plants are adapted for a huge array of environments. One easy example to look at is acid loving plants – you will see ericaceous growing media and compost for acids loving plants.  

Compost is a soil improver and it is often sold and used as a growing media. Compost makes a poor growing media. You may have read one of the many blogs from gardeners bemoaning the results from attempting to grow seeds in various commercial composts. Complaints range from rotting stems, deformed growth and no germination. Seeds and seedlings very often do not grow well in "pure" compost. Compost can be too rich (high levels of NPK), immature (e.g. acidic). It can be too friable, drying out far too fast. It can  the opposite too heavy for root growth.  Many seedlings grow best in growing media designed for the purpose of germination and early growth. This usually means a mixture of compost plus sand and clay (or soil) to provide good drainage and physical structure to hold the roots.

The most widely known formulation for potting mixes are John Innes 1 & 2. There are other proprietary seedling growth mixes – many may have compost in the title. So look for the key words “seed and potting mix growing media”. Things are getting better as labelling is improving. But we still suggest reading the detail.

How do I improve my next batch of compost?

You need more colloidal humus. The best way to achieve this is to add a ‘humification agent’ into your compost heap. (Follow this link to our compost humification page)
We have a number of gardeners experts and bloggers testing CHA -  come back to read about their results.