The lovely bones: Terra preta to save our terra firma?
September 15, 2010 1 Comment
Charred bones set to save the world?
The Amazon river basin is home to the famous Amazonian “dark earth,” or terra preta, which recently made the news as more proof of large civilizations in the tangled Amazon forests. This soil is renowned for its fertile properties, its loose consistency, water-holding abilities, and of course, its dark color. The people who used this earth generations ago—as many as 3000 years ago, according to one researcher—may have had no active interest in “greening” the planet, but they were very interested in getting a good crop yield for their efforts. Terra preta probably gave them just that.
Ancient farming wisdom
Somewhere along your educational path, you may have learned a few tips about farming. Rotate your crops. Let fields take a break. Till the soil. What you may not have understood as clearly were the natural processes that drove this farming wisdom.
When farmers turn over the soil, they loosen it. Earth happens to be the largest sink of carbon on terrestrial Earth, and when we move it around, some of that carbon gets released. When we try to fertilize it using dead and decomposing organic matter—compost, manure—this approach works in the short term to restore some nutrients, but microorgansims make pretty quick work of these organic remnants, returning carbon to the atmosphere again as carbon dioxide.
Thus, standard farming techniques of tilling and fertilizing and applying manure are short-lived efforts to keep the soil nutrient rich enough for planting. If nutrients are low, crop yield will be, too. And then there’s the water consideration; if the soil holds too little water or too much, that will also affect crop yield.
Magical fairy dust for crops
These factors all combine to make the terra preta soil look like magical fairy dust for crops. The soil actually is charcoal—or, in the lingo of the scientists who work with it, biochar. It is made from the rapid, pressurized burning of dead stuff—bones, tree bark—and manure. Pack it all into a metal container with a little hole for some pressure to escape, heat it to about 400 degrees Celsius, and you’ve made yourself some biochar. It apparently looks just like the charcoal you’d use at a cookout, but it has many more uses.
The carbon in the biochar is pretty inaccessible to microorganisms that would break it up, so it lasts a lot longer in the soil than your average, uncharred manure. In fact, it’s so long lasting that it’s still around in the Amazonian river basin long after the ancient farmers who used it disappeared. In addition to being a nutrient-rich and nutrient-tight source of carbon, biochar also is quite grabby with water, holding much more water than your average soil sample. That feature means that less water is required to grow crops in a biochar-laced field than would be needed in a regular, every-day kind of field.
Could soil invented in the Amazon save the Amazon?
Plants growing in the stuff do so faster, more robustly, and in greater numbers, primarily because of the rich nutrient source the biochar provides. Research indicates that the optimum combination is biochar plus fertilizer, which gives the greatest crop yield compared to either alone or neither. Using biochar could dramatically enhance global crop yields while decreasing water use and without adding a single acre of cropland. Using soil invented in the Amazon to save the Amazon rainforest has a nice “the circle is complete” aspect to it.
Although biochar has the drawback of having to be made and transported, its benefits to the planet don’t end with crop yield and water savings. The smoke generated from its preparation, in a process called pyrolysis, can be collected and used to form bio-oil, a form of renewable energy. In addition, biochar has potential as a sponge to soak up phosphates and nitrates from fertilizers before they reach our waterways, a sort of barrier against pollution. Last, this dark, magical fairy dust not only reduces carbon dioxide emissions from cropland but also significantly decreases methane and nitrous oxide emissions, both greenhouse gases that are far more potent than carbon dioxide but get considerably less press.