A patch of poor soil on a former cassava field in Amazonas turned into a small test of an old Amazonian secret. Researchers in Brazil found that tiny amounts of Amazonian dark earth, a human-made soil built up over centuries by pre-Columbian peoples, gave two native tree species a clear early advantage in degraded land. In the first 180 days after planting, pink trumpet tree seedlings grew up to 55% taller and developed stems 88% thicker than untreated plants. Brazilian firetree seedlings also grew faster, gaining 20% in height and 15% in stem diameter. That kind of jump matters in places where forests have been cleared, soils compacted, and recovery slowed by years of agricultural use. It also hints that the power of Amazonian dark earth, often called ADE or terra preta, may lie in more than fertility alone. “The key factor was not the amount of nutrients per se, which doesn’t vary much, but rather the microorganisms, which were quite different, especially the fungi. In plants treated with dark earth, the microbiota around the …