Too Much of A Good Thing?

We get questions about biochar often, and recently we were asked: Can you apply too much biochar?  The simple answer is yes, but let’s go into why…

If you have ever proudly dumped a nutrient rich load of compost into your field in one crop year just to circle back the next season to feel like your soil is back to square one, you are familiar with how quickly a garden or field ecosystem consumes organic matter. It does wonders for your tomatoes for one season but you have to add more ($) every year. However, if you add biochar to that compost before applying, some of the nutrients help foster populations of healthy bacteria and fungi that feed the plant over time with their own nutritious outputs. Gradually over a period of years, the biochar should be increasing the percentage of organic matter in the soil so that you can apply less and less compost/fertilizers annually.

The long-term advantages of biochar applications include slower nutrient release from added organic matter, greater stabilization of organic matter, higher nutrient use efficiency, and enhanced cationic retention as a result of improved Cation Exchange Capacity. Many of these benefits are a result of the increased surface area of the biochar, providing space for soil microbes to “store” nutrients, keeping them in your field rather than being leached away during heavy rain events. The beneficial soil microbes thrive with biochar as their shelter and help increase access to nutrients through their own biological processes, divvying it out to the roots over time.

However, living things need shelter, water and FOOD to survive! If we apply biochar to soil that does not contain enough nutrients for the planted crops, there is potential that the microbes will “lock up” some of what the plant needs in their poverty stricken “biochar city” that just got an influx of cheap housing units with no work to do in the surrounding area. The nutrients that don’t get tied up in the biochar might not provide enough for that crop to reach its full potential. The solution developed for this in the agricultural industry is to inoculate or “charge” the biochar with compost, compost tea, or manure, in order to balance nutrients and nutrient interactions, stabilize pH, and improve amendment moisture content to aid in application (fine particles can easily blow away!) This is what makes biochar and compost go hand in hand: the biochar is a seed for humus, that dark beautiful soil, because in combination with rich nutrient dense organic matter, it invites a civilization of microbes to thrive that will provide all of the advantages listed above, helping reduce nutrient applications in year 2 and subsequent years. You can see several studies with these results on our biochar resource page, but there is also so much still to learn about what happens in the rich ecosystem underground!

When applying this information to your own operation, sending off a soil test is the best way to determine what your soil may be lacking, and step one of our project’s biochar application process.  Regional university extension offices have “ideal soil condition” numbers for most Missouri grown commodities, like tomatoes, squash or elderberries, accessible through a google search. Compare the soil analysis to these recommendations, and include minerals or nutrients that are not at the necessary amounts, like Sea-90, basalt flour, gypsum dust etc. along with the compost.

On all four of our Phase 1 farms, we applied biochar and compost in a 50:50 ratio, combining it in large totes, fully saturating the combination, and allowing it to sit for two weeks behind a shed or corner of a barn. Then when the weather was right, we top dressed elderberry rows, soybeans, Sheep pasture and cover cropped poultry pasture. For a 400 sq meter plot, we applied 250lbs compost and 250lbs biochar. This looked like a nice sprinkling of sesame seeds on an artisan bun, not too dense a covering. This process could also look like adding a layer of biochar to cover your compost pile each time you clean out the barn and shove more manure in, or like one of many layers in a raised “lasagna” style urban ag bed, alongside rock dusts, sea minerals and other mineral sources.

There are many other factors to consider when deciding to use biochar for soil health, including minimizing or eliminating the use of chemical applications that can kill the microbes in the soil or throw off the balance of their ecosystem (fungicides, herbicides, pesticides, overuse of synthetic fertilizers). The recently released (May 2024) NRCS guidelines for Soil Carbon Amendment Conservation Practice #336 really gets “in the weeds” of it all better than I can do in a conversation email, and includes suggestions for compost applications as well.

Yes, too much biochar is a thing – while it is 70% or higher concentrations of carbon and a plant could live in a 100% Biochar growing medium in a hydroponic greenhouse, it wouldn’t get very far without adding compost or fertilizers. And depending on the feedstock used to make the biochar (sugarcane bagasse, rice hulls, oak/timber chips) the amount you add could alter your crops preferred pH.

– Written by Jackie Casteel