What is biochar? Biochar is wood chips that go through a process called pyrolysis, basically burning without much oxygen to produce wood charcoal. So biochar is another name for charcoal when not used for food. (Note - commercial charcoal bought for food is not the same as biochar, I'll explain later.)
We are specifically talking about wood derived charcoal prepared for use in soil. The benefits are amazing and this process has been going on for thousands of years.
Biochar, called terra preta, which means dark earth, dates back to 450 BC and 950 AD in the Amazon Basin. The pre-Colombian culture used this technology for agriculture to support a population of 100,000. There are plots that have been found that are 1 acre in size all the way up to 100 acres. They would burn their trash piled in kitchen middens, this would include bones, human waste, manure, and tree and plant residues. Then later they would spread the char and plant.
So why does biochar grow plants better?
Biochar has an enormous amount of pore space and locks in nutrients and organisms, as well as some minerals derived from the parent plant. One gram of biochar can have a total surface area of 1000 square yards. So, because of all the pore space, this makes a perfect home for symbiotic organisms that help with plant growth. Biochar must be activated or charged before use, a process that takes a week or two. Otherwise, char is not as effective and could take longer time to work.
During the curing or charging process, the char is inoculated with compost tea and fertilizers and minerals are added for fertility. Once the char is charged with these materials and time has elapsed to settle in, the char is ready to amend with soil. Once char is in the soil, the plant roots interact with the char and nutrients are shared. Biochar will last an extremely long time in the soil, thousands of years. Some of the original biochar fields, or terra pretta, are still intact in the Amazon where ancient civilizations utilized this resource.
There is a difference between biochar and ash, ash is also good for the soil and has calcium, potassium, phosphorus, and other trace elements. Ash will raise the ph of the soil, so if your soil is acidic you can add this instead of lime or calcium. Ash can be used as a fungicide and will also kill aphids, but be careful if you raise the ph of the soil too much you can negatively affect plant growth. The other main difference between ash and char is ash does not have any pore space to hold in water or nutrients. Use ash sparingly and it's best used in compost pile, just keep a bucket next to the heap and sprinkle in a bit when you add food stock.
Biochar is not the same as commercial charcoal; it is not recommended to use charcoal from commercially bought briquettes. Most of these commercially made charcoals have added binders that might cause problems in the garden. So, make your own char by burning wood instead. You can sift through the ashes of an old burn pile and fish out the chunks of char, or you can make a burner that will slowly burn without oxygen. Any way you do it, the benefits are tremendous and could be a key to sustainability in the future.