How To Recycle Compost For Seed Sowing

Can We Recycle Compost For Seed Sowing? Here’s How I Do It And How I Feed The Seedlings If The Compost Is Nutrient Depleted.

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There’s no need to buy new compost every time we want to sow seeds or grow more plants. We can recycle compost many times. As a retired commercial of several decades I’m sharing my secret to keeping the cost of growing down that many people don’t know or understand.

Can Compost Be Recycled?

Yes, that’s the simple answer. But it pays to understand a few things before we try it. Thats what this article about.

Does Compost Get Nutrient Deficit?

Yes, to an extent it does. But when sowing seeds that needn’t be an issue. Here’s why.

Freshly sterilised recycled compost ready for growing seeds .. Recycle Compost

When we grow seeds or plants in compost or soil the growing plant will use some of the available nutrients as it grows past the cotyledon stage. The cotyledons are the seed leaves that emerge out of the seed. There are either one or two of them. For example grass is a monocotyledon and has a single leaf emerge from the seed whilst many vegetables, such as carrots, brassicas and tomatoes, have two seed leaves and are called dicotyledons.

In the initial stage of growth the seed depends on the energy stored in the seed and requires no additional help. But once it gets to the next stage, at the first true leaf stage (that’s the first leaf after the cotyledon(s) emerge), it is going to start to take some nutrients from the growing medium .. that’s the soil or compost to you and me.

So if we grow a plant for long enough it takes a lot of the available nutrients from the compost. The key word in that sentence is “available”. Because many nutrients aren’t available initially. The simple explanation for this is that they are tied up doing other things! Usually that is in another form or chemically bound to soil particles, and will become available when the temperature, moisture level or other conditions changes.

It isn’t necessary to go into the science or all the details of this process here. Just take my word on it for now and I’ll write an article on the processes another day.

So, though the plant may have taken some nutrients from the compost, it doesn’t mean that more will not be released later. In that sense many composts aren’t actually exhausted and will have enough nutrients when we use them again.

However care needs to be taken with this and, if in doubt we can boost the nutrient levels when we reuse compost. One way is to water the plants with a nutrient rich solution as they grow. I don’t advocate using teas for this purpose as there is no way to know their strength. I’ve written about compost teas elsewhere so will not go into detail on that here.

The second way is to add some nutrients to the compost before we sow our seeds. You could use a synthetic “artificial” fertiliser for this but they tend to be harsh and release too much nutrient at once. This is why some purchased composts have those little fertiliser “eggs” in them. They are slow release pellets of fertiliser and release their goodies when the temperature is high enough for plant growth (isn’t science clever).

Or better still in my mind is to use an organic fertiliser. I prefer Vitax Q4 but you could also consider hoof and horn or bone meal. Hoof and horn has a high nitrogen analysis and little else is available whilst bonemeal is going to supply nitrogen and potassium. Q4 is also a mix of nutrients and contains NP&K plus many trace elements. I prefer it to any other potential additives and have been using it since the 1980s.

The amount of fertiliser to add is best determined by reading the instructions on the container, but my best rule of thumb is a fist full per large bucket of compost. I also used a fist full per square yard for growing hundreds of thousands of lettuce each year in my greenhouses

How To Prevent Weed Growth In Recycled Composts

If we grow plants in soil or compost and want to recycle it for seed sowing we have to accept that a few weed seeds may well have blown in to it. Clearly delicate seedlings aren’t going to appreciate us using selective weedkillers to control weeds .. and I never use or advocate weedkillers anyway. I’ve written about herbicides and chemical weed control elsewhere and many methods use obnoxious chemicals.

Coriander seed in freshly sterilised recycled compost in module trays.

So, if we can’t use chemicals how can we control or prevent weeds? Prevention is the key here. I prevent my weed seeds by sterilising the soil or compost. Here’s how I do it.

Back To Basic Principles

I follow the basic principles used by amateur and commercial growers for many years. I use heat to sterilise the soil or compost. For centuries steam has been used to sterilise soil in greenhouses and conservatories. Permanent underground steam pipes were often laid in growing areas and were fed by boiler produced steam. The steam heated the soil and sterilised it.

In commercial greenhouses they would forgo the permanent pipes and use a steam plough instead. This device was used like a subsoiler and pulled slowly through the soil at depth. The difference being it was connected to a large boiler by a flexible steam hose and injected pressurised steam in to the soil. The heat killed off all the pests and diseases that might otherwise affect later crops.

After the steam injection age came the methyl bromide age. A plastic sheet was placed over well rotated soil in the greenhouse and methyl bromide was pumped in under the plastic. It sank into the soil and killed everything. It was very efficient but had a downside. Methyl bromide is now known to be much worse than carbon dioxide when it comes to the climatic greenhouse effect. It’s an example of where accepted practices are later seen to be so very wrong! It’s now banned, thank goodness.

Compost Heating!

So my method is based on heat.

I take the material to be recycled and put it in the microwave. It needs to be moist as the moisture turns to steam and sterilises the compost. It’s a modern take on what our forebears did in greenhouses and conservatories.

This method kills weed, pests and diseases. To be sure it works use a cooking thermometer and check the temperature in the depths of your container. It needs to reach around 70C and stay hot for as long as possible.

This method works for me. The only weed seed that ever seems to survive is one I’ll write about another day. But they are easy to remove as they germinate and a bit more heat kills them if need be.

Using Heat Treated Recycled Composts

Use of the heat treated recycled composts is straight forward. Use as you would any other compost.

Brassicas germinating in recycled compost fours days after sowing in module trays .. recycle compost.

In my case my material is a 50:50 mix of top soil and well rotted farmyard manure. And some of it has been recycled 5-6 times. Provided I follow the above nutrient levels advice it goes on forever.

The cost of running the microwave for a few minutes is small compared with buying new compost. The only downside is the volume needed. I can only teat a few kilos to a time so if I want to heat treat a lot of compost it’s a bit of a faff. But I’ve a work around.

If the reason for treating the compost is weed, and not pest or disease control, there is no need to treat all of it. Just heat treat the top few inches of a container, replacing “weed” containing compost with clean heat treated material. Simples.