I first wrote about biochar when it was all the rage — the panacea that would bring soils across the globe back to life. There have been a multitude of peer-reviewed studies since then, academic parlance for a lot of water under the bridge. What these studies tell us is that for some chars, some soils, some crops and some measured variables, biochar is pretty terrific. However, for the broad range of materials produced, soils where they might be used, crops that might be grown and range of variables to consider, char is not a solution. It is just a tool to consider. Bring the char, but also bring the no till, compost, cover crops, limestone and a range of other options to restore function and health to your soils. In other words, showing up just with the char is like coming with just a Phillips head and finding you should have brought a regular screwdriver or even the whole toolbox.

The combustion (pyrolysis) can be carried out for different amounts of time and at different temperatures and pressures.  Long slow cooking at lower temperatures will generally produce more biochar than quick fast cooking at higher temperatures. Other critical characteristics of the biochar including surface area, pH, and nutrient content, will vary based on both feedstocks and combustion temperatures.
Let’s start with a review. Char is the general term used to describe burning organic matter under conditions with minimal oxygen. Biochar is not a new compound or a new concept. We’ve used it for generations of weekend barbecues, long before the gas grill became commonplace. In that case we refer to it as charcoal. What you put in the fire and how you cook it can have a big impact on the resulting product. There are different ways to produce biochar including torrefaction (burning at 200-320°C for several hours), fast pyrolysis (burning at 350-700°C for several hours) and gasification (burning at greater than 800°C for seconds to minutes). The slower and cooler processes will leave more material to add to the soil. In a nutshell, char and biochar are interchangeable terms. Burning is the process.
The initial interest in char was related to its persistence in soil. Think of leaving a good steak on a flaming grill for hours. What is left is literally burnt to a crisp, and not at all palatable. You put a raw or even well-done steak in a soil and the soil microbes will turn that into dinner in hours. Those that run composting operations know how fast things can disappear. However, when you burn that steak to a crisp not even the soil microbes want to eat it. A piece of charcoal in a compost pile will still be there long past the thermophilic phase and even the curing phase.
Same deal with putting that charcoal in the soil. Biochar provides a persistent source of carbon for soils. There is consensus in the literature on that. However, persistence is just one of the things that we hope carbon can do in soil. Carbon is the backbone of soil organic matter (SOM) and I have been singing the praises of SOM for years. SOM has a wide variety of benefits (see “Soil Health Indicators,” June 2018). These include carbon storage, nutrient supply, increased water infiltration rates and water holding capacity, mitigation of salty soils, reduced erosion, improved soil structure and improved plant yield. The question is does char provide the same benefits as active SOM or, phrased a bit differently, what exactly are the benefits when you add char to soils?

Product Variations

One of the big things about char is its structure. And that depends on the combustion process. A benefit associated with certain chars is the very high surface area of the material. The microbes won’t eat it, but they may be able to live in it. For those microbes (or soils) that are hoarders, there is also plenty of room to store stuff in char. Stuff can include heavy metals, water, and different nitrogen molecules. Depending on the feedstock and combustion method, the char can also have value for soil fertility. Burning the carbon can also produce a product with high value as an alternative to liming. Those are some potential benefits. Let’s see what the literature says.
Spokas et al. (2012) looked at the literature to see what the impacts of biochar actually are. They noted that the term “biochar” covers a wide range of products with widely different properties. For example, the authors report that the nitrogen in different chars ranges from 2 to more than 50 kg for each ton of material. Phosphorus goes from 0.3 to close to 60 kg per ton. In other words, without labeling requirements, it is not clear if you are getting fertility with the char.
One factor that is more consistent is liming value. This same study reported the pH of most chars to be well above 7 with a few outliers in the mid 5’s or acidic range. This review reported that while some studies showed big yield gains with char, others reported large decreases in yield. A second review focusing solely on plant yield found an overall increase of about 10 percent (Jeffrey et al., 2011). That mean included studies showing a 28 percent decrease in yield and up to a 39 percent increase in yield.
That uncertainty is less of a gamble if your soils are acidic and sandy or at least neutral and medium in texture. Yield increases also varied based on the feedstocks for the char with an increase of 28 percent for chars made from poultry litter and a decrease of 28 percent for chars made from biosolids. The study also reported that the best responses were when char was added to soil at 100 tons/hectare (about 40 tons/acre). Spokas et al also noted that the costs to produce the material were unlikely to be recovered by sales of the product for general agronomic use.
This does not mean that biochar has no place. It means that specific biochars are likely very good for specific things under specific circumstances. Some examples might be binding metals on contaminated sites or reducing nitrous oxide emissions from corn growing on fine soils. They might also be important for reducing odors during composting. Finding the char that is right for you will likely be the next focus of scientific studies. A critical part of this will be defining the char equivalent of the US Composting Council’s Seal of Testing Assurance (STA). No one wants to pay a high price for the wrong panacea.
Sally Brown is a Research Associate Professor at the University of Washington in Seattle (slb@uw.edu) and a member of BioCycle’s Editorial Board.