Why Homemade Compost is Best

Peat Free Organic Compost

Peat Based Compost

Environmental

Positive – No waste collection or transport involved.

OK - Waste is recycled and used, but lots of transport and processing costs.

Negative – Unsustainable and destroys habitats that take thousands of years to make.

Composition

You know what goes in and hence what comes out.

PAS100 badge is a good quality indicator, but always check label for contents.

Always read the label. A wide variety including reduced, low, green &          eco-friendly.

Cost

Initial outlay for equipment, plus cost of bulking agent if unable to make own.

Average of £3-6 per bag.
£0.05-£0.20p per litre.

Time

It takes time to make good compost, this differs between hot and cold composting.

It takes time to make good compost, this differs between hot and cold composting.

Growing Medium versus Soil Improver

Would require sieving before using for seed germination.

Option to match product to specific growing need – eg seed germination, planting tubs. The term “multi-purpose compost” is used to describe growing media, but may contain a number of compounds.

Water

Compost is a giant sponge that holds water until the plant roots need it. 

Water Holding Capacity (WHC)

Good but depends
on humus levels.

Good – often artificial additions are added
– eg SAP gels.

Peat holds
1-2 times its own
weight in water.

Water Retention (How quick the sponge dries out)

Rewetting
(How easy to re-wet once dry)

It all depends on the % colloidal humus in your compost and peat.

Organic matter such as peat and compost become hydrophobic when fully dried.
They are very hard to re-wet and if fully dried rarely recover  full water
holding capacity.

Peat has poor water retention and re-wetting capacity.

Aeration

Roots need oxygen. Oxygen moves through soil via pore spaces and gaps around particles. Anything (like clay and humus) that holds water and swells to block pores limits aeration and impacts plant development. The crumb structure is a big issue. Much of the work to develop 'compost growing media' is around the balance between the need to hold water and the dramatic impact water has in reducing aeration.

Aggregation
(<1mm)

Humus has such a strong WHC that the colloidal structure swells and blocks airflow.

Multipurpose compost is designed for this purpose. John Innes and many other mixes have sand and
grit added!

Peat has the ability to hold water without too
much impact
on aggregation.

Aggregation
(10mm)

In our trial we found that small amount of 1-10mm were good, but large amounts were very bad – too many pockets of air allow the mix to dry out and seedlings wilt.

Interesting – few mixes add larger particles. Although we can in our trails, 1-10mm worked as well if not better than small <1mm.

Nutrients - NKP

Nitrogen
(Foliage Strength)
Potassium
(Overall Health) and Phosphorous
(Root and Flower Growth)

Has nutrients held from decay. (Note if too high can be a problem – especially in germination). Contains both ‘ready now’ and slow
release nutrients.

Normally mixed for the intended use of the compost.
The chemicals added are unlikely to be listed on the label – but likely to be blood/bone meal type sources based on availability and cost.

Virgin peat is low in
‘ready now’ nutrients.

Cation Exchange Capacity (CEC)

Many gardeners will not be familiar with the CEC. An analogy would be to consider a sponge that holds all plant nutrients until the roots need them. A bigger and better sponge is better for your plants. CEC is a measure of the size and strength of the sponge.

Humus
(a specific part of compost) is one giant nutrient sponge.

Few compounds come close to matching humus.

Values do vary with % humus. Humus in composts varies widely from as little as 1%-to 30%! It has a profound impact!

Depends if clay or loam is added. All composts tend to be c100.

Few bags list a value as it is harder to measure.

Peat (Sg moss) as used most growing media has low CEC it does not hold nutrient well. (This in part explains why peat based growing media get exhausted after 4-4 months growing.

(However they can be refreshed by adding liquid or solid fertilisers).