What to do with prunings?

We’ve been busily pruning apricot trees this week, making the most of this beautiful autumn weather and enjoying the glorious colours.

If your apricot tree is like ours, with a spot of gummosis, then it’s very important to pick the diseased prunings up and dispose of them properly to help prevent disease next year.

The best way is to return them to the soil somehow. Large animals (sheep, goats, horses) just love to eat prunings (especially if they still have green leaves on them), and will often break them down enough to put the remains straight into a compost pile.

Now that we have Tessa’s in-house micro-dairy here on the farm as part of the Harcourt Organic Farming Co-op, the prunings go straight to a bunch of cows who think fruit tree prunings are a high treat!

Another great technique is to chip the prunings. You can either leave them in a pile to age and then put them back on the trees, or put them into a compost pile. Learning how to make your own compost is one of the “must-have” techniques for all gardeners that are serious about growing their own food. It’s really hard to find good quality compost to buy (not to mention quite expensive, as it’s something you need to apply regularly), plus it’s one of the best ways to capture the nutrients from your garden ‘waste’ and return them to the soil. If compost-making is still a mystery (or keeps going wrong), our Compost That Works online short course will get you on the right path.

Larger prunings also make good kindling or firewood, or can be turned into biochar using one of the techniques we’ve described in other blogs, so there’s really no need to waste anything!

Biochar in a bathtub

We became big fans of biochar years ago when we went to this workshop, and we’ve also written about it here. We even held our own workshop on the farm to learn how to make biochar vessels out of 44 gallon drums.

The bathtub biochar method in action
The bathtub biochar method in action

Despite all of that, we’ve never made it ourselves, because it seems like a lot of effort to produce a small amount of biochar.

That may all have changed! We were lucky enough to go to a small, privately run workshop recently that introduced us to a much simpler system for making biochar in a bathtub.

It was hosted for us by Grow Great Fruit members Win Westerhoff (below left) and Clare Claydon, and the workshop was run by Jim Sansom (below right), who started by talking us through the main steps of the method.

Jim Sansom explaining the bathtub method
Jim Sansom explaining the bathtub method

The bathtub method is based on the ‘Kontiki Cone Kiln’, which is a large (about 1.6 m across) kiln for making biochar, but is a much simplified version.

One of the challenges with this method is that there’s no hard and fast rules about when to stop the burn. “Judge when to stop” was Jim’s advice, “when a stick has lost its identity as a stick”.

Another thing to know before you start is don’t use a cast iron bathtub, or it may crack when you light a fire in it. And obviously don’t use a modern plastic or fibreglass bath either – you need to find an old steel bath at your local tip shop.

At the workshop we went through the actual process of making biochar under the expert tutelage of Jim, Clare and Win — and it wa:s surprisingly fast and easy.

Here’s how it works:

Step 1 – Fill the bottom of the bathtub with light, dry fuel.

Starting the fire in the bottom of the bath with light, dry material (and a bit of metho if needed!)
Starting the fire in the bottom of the bath with light, dry material (and a bit of metho if needed!)

Step 2 – Light it on fire. (Pro tip: use a bit of metho if needed). Aim to get a good strong fire going along the whole length of the bathtub.

Step 3 – Once it’s burning, pile up with bigger stuff, but make sure it’s burning the whole length underneath. (Pro tip: this is much easier on a still day than a windy one.)

Step 4 – Stack the bath until it’s 3/4 full. If the fire is burning well, it shouldn’t be too smoky. It’s fine to use bigger wood in the bathtub as long as it’s all roughly the same size so it burns at about the same rate.

Hugh keeping the fire going evenly along the whole bathtub with a rake
Hugh keeping the fire going evenly along the whole bathtub with a rake

Step 5 – Use a metal rake to pack the wood down hard, and to keep the fire burning evenly and everything burning at the same rate. The gases will be pushed out of the wood and are then burned in the bathtub, which prevents oxygen getting down into the fire. If too much oxygen gets into the fire the wood will just burn away to ash. New research has shown that if the biochar is made at a lower temperature you end up with biochar that has a higher level of organic matter.

Hosing the bath to put the fire out when you judge it's finished
Hosing the bath to put the fire out when you judge it’s finished

Step 6 – When you judge that most of the material is blackened to char, quench the whole thing with a hose.

Hugh and Clare admiring the finished product
Hugh and Clare admiring the finished product

It’s OK for sand, dirt, clay and mud to be on the wood and in the cooking process, and in fact this may be beneficial by creating a more highly mineralised final product.

How do you apply the finished product? We’ve covered this before in other blogs, but here’s the methods that Clare and Win use:

  • dig a trench, tip in a barrow load of biochar, add compost and a scattering of blood and bone;
  • broadacre – spread with a fertiliser spreader and then go over it with harrows.

The point is finding some way to incorporate biochar into your soil where it will work its magic.

Of course it’s not magic, it’s science! Biochar has remarkable effects in soil by providing both a stable form of soil carbon, and by providing amazing (and extensive) habitat for soil microbes. If you still need convincing of just what an awesome impact this can have on your soil, you may want to take our Soil Biology and the Soil Food Web short course.

Gardening with biochar

Biochar is one of the most useful soil additives you can use, and while you can add it any time of year (like compost), autumn is a great time to do it before your trees go to sleep in winter, to boost the organic matter in your soil and provide more habitat to help your soil microbes survive over winter.

In the first biochar workshop we ever went to a few years ago, the presenters started by saying “Let’s clearly appreciate and understand how biochar is truly amazing and differs from fertilisers or other gardening soil additive products.”

Well, that piqued our interest, for sure. They went on to explain that one of the ways biochar is unique is that it acts on four aspects of soil at the same time:

  • physical
  • hydrological
  • chemical
  • biological

What is biochar, we hear you ask, and why should we care? Basically, biochar is just charcoal produced in a special way from plant matter and stored in the soil.

Biochar...just like charcoal
Biochar…just like charcoal

It’s brilliant for the soil and your fruit trees (we’ll tell you why in a minute), but one of the best things about it in these times of climate change is that it’s a stable, long-term way of taking CO2 out of the air, and putting it back in the soil, where it belongs. It’s not much to look, as you can see – it just looks like charcoal.

Making biochar is one of those skills (like making compost tea) that can seem a bit fiddly at the beginning, but once you know what you’re doing and you’ve got a system in place it’s a fantastic way of providing your own top-notch soil amendments, basically for free.

Learning how to make biochar at a workshop at the farm
Learning how to make biochar at a workshop at the farm

After all, it’s been produced for thousands of years (you might have heard of the amazing terra preta highly fertile soils that were created in the Amazon basin through use of biochar), well before modern technology was available, so it can’t be that complicated!

Making biochar in a milo tin
Making biochar in a milo tin

So, why is it good for fruit trees? The main benefit is that it increases soil fertility, primarily by increasing the amount of organic carbon stored in the soil.

But there’s also lots of evidence now that biochar also provides a perfect habitat for soil microbes and stimulates their activity, making your soil much more biologically active. It also improves cation exchange capacity, which is a measure of the availability of the nutrients in your soil.

You can turn ordinary biochar into “superchar” by innoculating it with things like compost tea, worm tea and microbe food, as Lann demonstrated at the workshop. This charges the biochar with both microbes and microbe food before you put it in the soil, reducing the time it takes for it to start working its magic.

But it doesn’t stop there, there’s lots of other benefits, including:

  • it improves water quality,
  • it increases retention of water in soil, and 
  • it helps nutrients stay in the soil rather than leach out.
Healthy soil with lots of organic matter and microbes
Healthy soil with lots of organic matter and microbes

If you want to find out more about the microbes that will move into the biochar “hotel” you provide by adding biochar to your soil, and exactly how vital they are to your soil health, take our Soil Biology and the Soil Food Web short course.

Thanks to John Sanderson and Lann Falconer, both Environmental Engineers for Earth Systems, for their fantastic biochar knowledge and workshops. Read our detailed blog about their workshop here.