Biochar lasts hundreds to thousands of years once it is in the ground. That spread is the mean residence time measured in soil, and it is wide for a good reason. Biochar soil lifespan is set at the kiln and not in your garden, so two bags that look the same can behave nothing alike. One number on the product sheet does most of the deciding. Ask for it before you pay.
The Amazon shows what a long residence time looks like. Terra preta soils there were built up over thousands of years. The source was charcoal waste from pre-Columbian hearths, and that charcoal is still in the soil today. Growing seasons came and went by the thousand. The char stayed. Farmers have worked char into fields for more than 2,500 years, so this is not a new idea getting its first test.
The check buyers are pointed to is the hydrogen to organic carbon ratio. You want an H:Corg ratio below 0.7. Char that meets that mark lasts hundreds of years or more in soil. Char that misses it does not earn the same claim. The figure comes from a lab test on the finished product, so it is something the seller reports and not something you can judge by eye.
That ratio also predicts how much carbon survives the first century. Weng and Cowie report BC100, the share still present at the 100 year mark, at 60% to 90% for char under the 0.7 mark. Heat matters too. Biochars made above 1,020°F (550°C) are dominated by inertinite. Getting there costs you material, though. Only 10% to 50% of the carbon in the raw feedstock ends up in the finished char, and the figure depends on the system.
BC100 is shorthand and nothing more. It is the share of carbon left once a century has passed, and it works as a plain way to compare two bags. A char that holds 60% at a century and one that holds 90% are not the same product, even when the price per pound is equal. Ask for the figure. If the seller has never heard of it, that answer tells you plenty on its own.
Permanent is not quite the right word for biochar soil lifespan either. A BC100 of 60% to 90% means some share does leave during that first century, even in the stable grades. What stays keeps going, and that is why the top of the range reaches into the thousands of years. Treat it as a range and not a promise. Any batch lands somewhere inside it based on how it was made, and no seller can hand you an exact year.
For a home garden that changes the planning. Compost breaks down and gets reapplied every year or two. Biochar does not run on that schedule, so the work of spreading it lands one time rather than every spring. The bed you amend now still holds that carbon when someone else is growing in it.
Request the H:Corg ratio and the pyrolysis temperature in writing. A supplier who tests the product has both ready, and those two numbers decide how long the carbon stays put.
So ask two questions before you pay. What is the ratio, and what heat was it made at? Price tells you nothing about biochar carbon permanence, and a cheap bag with a low ratio beats a costly one cooked too cool. Get both numbers and you have a real read on biochar soil lifespan instead of a guess.
Read the full article: Biochar: What It Does and When It Fails
