Monday, May 2, 2016


Soil in the rain forest is some of the poorest on earth. Plants absorb the nutrients they need through their roots, relying heavily on the plants being soluble in water. A rain forest, true to its name, rains almost constantly. That rain picks up the nutrients in the soil and washes them away. The various living organisms try to hold on to those nutrients by locking them away in their bodies, but eventually those nutrients are returned to the soil. The soil cannot hold on to them. So when explorers discovered lenses of dark, black, fertile soil in the interior of the Amazon Basin, it came as a big surprise.

The soils came to be called terra preta soils and have been the subject of much study. Due to the high concentration of pottery sherds, bones, charcoal, and other indicators of human life, it was obvious that the soils were made by a previous civilization. But it was initially unclear why the soils retained such a high degree of fertility, with fertility possibly even increasing over time instead of degrading as would be expected. It turned out that the cause was the concentration of charcoal in the soil that was doing it.

The study of this soil led to the discovery of biochar, a form of charcoal produced by pyrolysis, creating the charcoal at high temperatures and in a relatively low oxygen environment. The physical and chemical structure of biochar acts a lot like the carbon commonly used in water and air filters. It is extremely porous, leading to a high surface area, one that is really good at cation exchange. For the lay person, that means it bonds with a wide variety of compounds, holding them in place. In a carbon filter, this means it bonds with soluble lead, arsenic, and chlorine, things you want removed from the water so it is safe to drink. In soil, this capability is more applicable to nitrogen, phosphorus, and potassium. Biochar in soil can hold on to the very nutrients that plants need to survive and thrive.

The benefits don't stop there, though. Because of biochar's porosity, it is also very good at retaining water. Interestingly, the open structure of biochar seems to be an ideal support for microbial life. Beneficial bacteria and fungi thrive in the environment created by biochar. The nutrients bound to the biochar are easily accessible to the microorganisms crawling all over the surface, where they can become a part of the life cycle of the soil, eventually to end up in plants.

So what does it mean for food production? Biochar has a huge potential in agriculture. One of the great frustrations of modern agriculture is that soil fertility is falling. To combat that, soils are heavily treated with synthetic fertilizers. Those fertilizers wash away readily in the rain, meaning that more need to be added. But it also causes a problem downstream. All that fertilizer in the water causes an algae bloom. That algae bloom is followed by the algae dying. As the algae in the water column starts to rot, it steals oxygen from the water, killing fish, crustaceans, and anything else, creating a dead zone. The annual dead zone on the Gulf of Mexico reached 6400 square miles in 2015. All that fertilizer used to make that dead zone was purchased by farmers, each one hoping that that fertilizer would go to their plants.

So what if something could be added to the soil that helped all that fertilizer stay in place? What if that amendment also increased water retention, thereby increasing drought tolerance? What if it also increased beneficial microbial activity, the very activity that supports plant growth? And where does it come from? We really like having trees in our cities, and we like them to be well trimmed. Those trimmings typically head for the landfill. What if we diverted that waste product instead and made our soils better? That biochar could be added to farmland, and just like in the Amazon Basin, that fertility could be realized for hundreds of years. Biochar can take hundreds or even thousands of years to degrade in a natural environment, and it improves the soil that whole time.

But what about more modern, higher tech growing methods? Could biochar be used as media for hydroponics or aquaponics? I have seen a lot of discussion of the possibility online, but very little actual data on whether it works or not. I think that an analysis of what biochar does and how it would apply to hydroponics and aquaponics might be in order.

Again, biochar absorbs nutrients and holds on to them. It will do this with huge amounts of nutrients. Now, biological activity can access those nutrients (remember the "exchange" part of cation exchange) and help feed them to the plants. But that means two things for aquaponics and hydroponics. The first is that the biochar is going to absorb a LOT of nutrients until it is filled up. In land-based agriculture, the biochar is typically "charged" or pre-filled with nutrients before being added to the soil. In hydro- and aquaponics, that doesn't necessarily have to happen, but the grower needs to know that the biochar will take its fill before the plants can get it, and that process can take some time, perhaps weeks or months.

The second thing to recognize is that it is the biological activity that exchanges all those cations. Fungi is particularly active in that process, but bacteria are also important. Without that living system, the biochar will just act as a nutrient sink that will have to be filled before a regular nutrient profile can be maintained.

Biochar in a properly alive media would have a stabilizing force on the nutrient load of the media. Once it is full, the bacteria and fungi can access it if nutrients drop too low and it will absorb when nutrient loads are too high. Adding it while a tank is cycling might help lessen the stress on the fish, but the grower might want to refrain from adding plants until the nitrate level starts to climb, indicating that the biochar filter is full. Also, adding it as a supplement to the media rather than as a media in itself would be a good idea, perhaps 20% or less.

As for me, I do aquaponics with soil. The soil I create is a vibrant, living community that holds its own nutrients pretty well and should have no trouble accessing nutrients held in the biochar. I am working on expanding and creating new aquaponics beds and will be trying biochar as a supplement to the soil in the system, probably at around 20% of total volume. I will report back on how that worked when I have more information.

1 comment:

  1. I am eager to hear about your experiments! I know lots of people excited about using bio-char in fields, but I live in a dry climate where I think that unburned wood is more valuable as worm food than burned wood as a soil structure. But, bio-char as a soil base for aquaponics and other extremely intensive techniques is very interesting!