Fire Up the Garden – Volcanic Ash, Terra Preta, Biochar

I had the idea of writing about the benefits of charcoal and wood ash for the garden, but in light of the recent goings on up at Gunung Agung I couldn’t go further without first mentioning the benefit of andisols, otherwise known as volcanic soils which are created from lava weathering and volcanic ash.

Volcanoes are natures own industrial scale fertiliser factory, producing beneficial gardening materials such as perlite and pumice as well as a whole host of other nutrients which are contained within the lava and ash.


The recent eruption of Agung brings with it the obvious dangers and inconveniences, and could even harm agriculture. One need only take a trip to the eastern side of the mountain north of Tulamben to see the moonscape left in the wake of the 1963-64 eruption. Huge ‘car-sized’ boulders (sometimes larger) can occasionally be seen lining the road that winds along the coast. I can only assume that they were hurled kilometres forth from the imposing Gunung Agung, that dominates the entire eastern landscape. Nothing compares to the power of nature – I am in awe.


Despite the dangers, the entire Indonesian archipelago is a direct result of tectonic processes and volcanic activity, so you can’t have one without the other – take the bad with the good. But is it really that bad anyway? The materials deposited from volcanic eruptions are in large part responsible for the surrounding fertile lands that produce the diverse forests, whilst also supporting local agriculture. Over the long-term volcanic ash and weathered lava rock produces the world’s most fertile soils primarily due to the release of minerals such as sulphur, potassium, phosphorus, magnesium and calcium.


Over time, chemical and physical weathering allows the lava rock and ash to release nutrients and expand its surface area, enabling soils to hold both more water and nutrients whilst also increasing porosity at the same time. These are things that plants love. It may be hard to convince the locals right now, but this eruption could be a blessing in disguise, so long as farmlands aren’t buried by lava-flows. Volcanic soils are typically highly porous and moisture retentive – there’s a very good reason why perlite (also known as volcanic glass) is used in seed raising and potting mix!


Volcanic ash is a great addition to your garden in more ways than one and I know of people already planning to go up there and collect it by the truckload when things calm down. Volcanic eruptions are central to the natural processes that produce Indonesia’s fertile soils – so if I am the garden doctor, then Agung must be the Garden God!


But if you can’t get your hands on bucket loads of volcanic ash you can just as easily enhance your soil by incorporating wood ash and charcoal, but definitely not charcoal from BBQ briquettes.


There are many reasons to add charcoal to your garden, including raising the soil pH, improving porosity and increasing water and nutrient retention. Charcoal is a highly absorbent form of carbon. It does not decompose in the soil and will absorb nutrients and hold them, rather than letting them wash away with the rain (that’s why orchids are grown on it). Once added to the soil, the benefits of charcoal continue on for years, producing colourful flowers, lush vegetation and nutrient dense fruits and vegetables.


Many ancient cultures have used wood ash and charcoal as part of their agricultural practices to amend and improve the soil. The ancient Amazonians created a thick fertile soil in places of settlement that have been extensively studied and come to be known as ‘Terra Preta’ literally – dark soil.


Scientists studying the remnants of these ancient settlements have found the charcoal amended soil much more fertile than the surrounding forest soils still to this day – despite that the settlements haven’t been inhabited for hundreds of years. The indigenous people of northern Australia also used fire to burn grasslands, allowing the combusted material to absorb into the soil. The resulting lush new growth had the effect of attracting game such as kangaroos for hunting. Importantly one of the best ways to deliver phosphorous back into the soil in absence of animal manure is through wood ash.


These days the common term is biochar. The idea is simple – incorporate charcoal into the soil where it will absorb and retain nutrients leading to more productive crops and gardens. There are many techniques to make your own biochar at home, some involving steel barrels and 24hr slow burns. I know you’d prefer the easier way. Just burn some logs, you may as well have a BBQ at the same time. Once the logs have burnt down into smokeless coals smother them with water so that the fire is extinguished immediately otherwise they will further reduce to ash. Whilst ash can be used in the garden and has its place we are after charcoal here.


Spread the charcoal over your soil, and then work it in completely, add it to the compost too. You can do it all at once or work in sections. A soil amended with biochar will retain a higher level of moisture and nutrients than previously. The charcoal essentially locks up the nutrients, preventing them being washed away with rain or watering, but rendering them their bio-available to the plants at the same time.

Wood ash can also be a good addition to the garden if used in moderation. It consists of about 10% potassium or potash which is a vital nutrient for plants and crops.


In fact, the name derives from early Dutch ‘potasschen’ literally meaning ‘pot ashes’. This refers to wood ashes soaked in a pot of water (the preferred means of production prior to the industrial era of mining and manufacture). The word was later adopted into English as potash from which the word potassium is similarly derived. With plants just as in humans, potassium regulates fluid retention and has a role in transporting food within the plant regulating sugars, starches and hormones – particularly good to use around flowering plants, fruits and vegetables.


Ash has a high of calcium content, sometimes as much as 45%. This high calcium content will raise the pH of the soil, and can be used on acidic soils in place of lime. However home-made ash isn’t a homogenous product, meaning its content will vary. Hardwoods, for example, generally produce more ash and contain more nutrients than softwood. Wood ash obviously isn’t an ideal addition if your soil already has a high pH. There’s also no point in spreading it around acid-loving plants such as rhododendrons, roses, azaleas or strawberries, but you can sprinkle it lightly over the lawn.


Wood ash is best used as a light layer sprinkled sparingly over the soil or compost heap. If you have a lot, don’t add it all at once as the alkalinity could have a disastrous effect on plants – it could easily all end in tears. It is also caustic and will affect the beneficial bacteria and worms if overdone. It’s better to store the ash and sprinkle it on as a thin layer occasionally, incorporating it into the soil at the same time to prevent it blowing away in the wind.


Now get out there and fire up the garden!


Dr. Kris

Garden Doctor


Copyright © 2017 Dr. Kris

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