Grow What You Can - Potting Compost Trials

 

Potting Compost Trials

Like many gardeners we tend to use a large amount of seed and potting compost. Concerned about the environmental impact of using many commercial composts, as well as worried about the financial costs, we have started to investigate other options and conduct some simple trials.

Compots and seeds used

We thought a good test would be to sow trays of salad mix. This would quickly show how the composts would compare with germination, growing and how long the nutrients last.

The freshly bought seed used was Halcyon Spring mix from Wallis Seeds, containing: Greek cress, leaf radish, pak choi, mizuna, tatsai, mustard suehlihung. As the mix didn't quite fill the trays a single row Spinach Campania, an F1 also from Wallis, was added in each tray.

On 20th March one A4-sized seed tray each in of the following were sown: coir compost, sieved garden compost, sieved garden soil and peat based multi-purpose compost (MPC).

Compost trays
Seed trays, clockwise from the top left: Garden compost, coir, garden soil and peat based MPC

Germination

The first tray of seedlings to really show up was the peat based MPC but the garden soil was hot on its heels.

By 31st March all trays were up and there was not an obvious difference in quality - the spinach is just starting to appear, much more slowly than the salad mix. By mid April the seedlings were growing very well in the coir, garden compost and peat based MPC but noticeably slower in the garden soil. These pictures were taken on 12th April.

Germination
Germination results, clockwise from the top left: Garden compost, coir, garden soil, peat-based MPC
Germination
Coir results, click picture to see larger version
Germination
Garden Compost results, click picture to see larger version
Germination
Multi-purpose compost results, click picture to see larger version
Germination
Garden soil results, click picture to see larger version

Final results

The final results were noted at the start of May with photos taken on the 7th. The pictures show some of the differences quite clearly (click on any picture to see a larger, more detailed version).

As expected the peat-based MPC performed the best with good lush growth of salad throughout the trial. The leaves remained green, if a little pale, and the plants only just started to show signs of bolting.

Almost as good was the tray grown in the garden compost. The leaves seemed a bit less lush but a nicer, darker green. The plants started to bolt a bit sooner. The main drawback was the number of weed seeds which also germinated although this could be overcome with a hotter or more selectve compost heap or treatment of the compost before sowing.

The coir compost performed reasonably but the salad was less lush than the MPC or garden compost. There was noticable yellowing of the leaves towards the end of the trial and the plants also bolted earlier. The main problem seems to be the lack of nutrients after the initial growth, something that could easily be rectified with feeding after the initial germination. It was also the most prone to drying out of all the composts.

Unsurprisingly the tray of garden soil performed the worst, with the least healthy looking plants which also suffered early bolting. There were also problems with weed seeds.

Overall we were pleased with the results as it shows home made garden compost can produce good results and, with some practice and more experiments, coir compost seems a viable alternative to peat-based MPC although it will need a little more looking after.

Final
Final results, clockwise from the top left: Garden compost, coir, garden soil, peat-based MPC
Final
Coir
Final
Garden Compost
Final
MPC
Final
Garden Soil

Further details about the composts

Coir compost

Coir is often sold as an environmentally sound alternative to peat. Before investigating whether these claims are correct we wanted to see how it compared. We also find it appealing that it comes in tightly compacted blocks that are easy to transport and store, very useful when you use a large amount of compost.

The coir we tested was from B&Q and consisted of a block about the size of a house brick that is placed into a bucket containing a few litres of water. The block absorbs the water and expands and after a bit of fluffing up turns into a very light and free draining compost

Unfortunately since conducting this trial we've found it hard to track down coir compost blocks from local shops so will have to try and track down a good online supplier.

Coir block
Coir compost block being soaked
Coir compost
Coir compost, soaked and ready to use

Sieved garden compost

We produce as much garden compost as possible and have three plastic bins in constant use. A mix of vegetable waste, chicken litter and garden weeds and prunings is composted. The bins produce a large amount of bulky organic matter which is easily used in our garden. However, as the bins do not heat up consistantly and stay at a high temperature the compost doesn't kill weed seeds. For a seed compost the garden compost was sieved to remove bulky items

Sieved garden compost
Sieved garden compost

Sieved garden soil

The easiest soil to find is plain garden soil. We sieved ours to take out some of the larger stones. It wasn't treated in any wasy so it contained weed seedlings and wasn't sterile. As our garden soil is light and free draining it also doesn't contain many nutrients.

Sieved garden soil
Sieved garden soil

Peat based multi-purpose compost

As a control for this trial we used a cheap peat based multi-purpose compost bought from a local DIY store. The cost works out at about 50p a litre

Peat MPC
Peat based multi-purpose compost

Back to the garden trials