Terra Preta – Biochar in the Garden

Terra preta is a dark earth in the Amazon basin that people created themselves centuries ago. It formed from ash, bones and faeces, mixed with charcoal from cooking and refuse fires. On the upper Xingu the Kuikuro still build up such earth on refuse heaps today. Biochar is charcoal made specifically for use in soil. It is produced by heating plant biomass without air to 350 to 1000 °C. The nutrients came from the refuse, and the charcoal held on to them. Anyone recreating terra preta in the garden therefore has to charge the biochar with compost or manure first.

What makes terra preta fertile?

Terra preta is less acidic than the soil next to its sites and holds considerably more nutrients. The neighbouring soil sits at about pH 4, terra preta at pH 5.2 to 6.4. Plant-available phosphorus reaches over 200 mg/kg there, compared with about 5 mg/kg in the neighbouring soil.

Aged charcoal carries acid groups on its surface that nutrients cling to. Cation exchange capacity describes how many positively charged nutrients such as potassium or calcium a soil can hold and release again. In terra preta it was about three times as high per unit of humus as in the neighbouring soil. What it means for soil and substrate in the pot is covered in the article on cation exchange capacity.

How do you charge biochar before it goes into the soil?

The biochar goes into the heap from the start and composts with it, a method called co-composting. While composting, the biochar takes up nitrate from the compost. Co-composted biochar carried over 5000 mg of nitrate nitrogen per kg. Nettle slurry (nettle tea) contains only 0.005 % nitrogen and is therefore no use as the sole charge.

  • Mix the crushed biochar in when setting up the heap, one fifth of the volume.
  • Let the heap compost hot for six weeks at 60 to 70 °C.
  • Without a hot heap, mix biochar 1:1 by volume with finished compost or manure and keep it moist for two weeks.

Which charcoal is suitable, and does barbecue charcoal work?

Barbecue charcoal works, because it is ordinary wood charcoal. It comes in coarse lumps, but a particle size of 0.5 to 1 mm works best. Post-processed biochar, for example ground or sieved, gave 14 % more growth on average than untreated. Washing alone brought no benefit on average.

  • Moisten the charcoal and crush it so that no dust rises.
  • Shake it first through a 1 mm sieve, then through a 0.5 mm one, and use the fraction in between.

How much biochar goes into a bed, and what difference does it make?

In 144 Swiss gardens, 1 kg of biochar per m² raised the mean yield by 7.5 % over compost alone. By volume that amount is roughly 5 l. In 45 % of the beds the gain exceeded 10 %; in 31 % the yield fell by more than 10 %. Brassicas, cucurbits and umbellifers responded positively; nightshades responded negatively.

Co-composted biochar yielded 40 % more than compost without biochar across many trials. Together with fertiliser, biochar brought 15 % more than the fertiliser alone. Below an annual mean temperature of 10 °C there was no reliable gain. That covers much of central and northern Europe, where co-composted biochar still showed measurable benefit. In sandy soils plant-available water rose by 47 %, but only at 0.27 to 10 % biochar by weight.

The stable fraction of 97 % stays in the soil for 556 years on average. The remaining 3 % breaks down in the first year. Field reports from South Asia give 100 g per m² for planting holes; the biochar there was charged with cattle urine.

  • With little biochar, put about 25 g per plant into the planting hole at a depth of 15 to 25 cm.

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