Session Two Oral Presentation a Charcoal as Soil Conditioner and Nutrient Retainer – Studies in the Humid Tropics, Amazônia, Brazil

Agriculture in the humid tropics is mostly limited by low soil fertility and invasion of weeds. The nutrient retention capacity of most soils is low and applied mineral fertilizers are rapidly leached into subsoil. Small farmers can not afford continuous mineral fertilizer input to compensate for the losses. Hence the dominating agricultural praxis is shifting cultivation accompanied with slash and burn agriculture. Slash and burn agriculture is practiced by about 300 to 500 million people, affecting almost one third of the planet’s 1500 million ha of arable land. The most commonly observed change in soil following slash-andburn clearing of tropical forest is a short-term increase in nutrient availability due to ash deposition, rise in pH and soil heating. But a large proportion of nutrients is lost during combustion of biomass. After a few cropping cycles the land is abandoned due to nutrient depletion and weed invasion. The contribution of tropical rainforest burning to global warming is significant and the subsequent loss of soil organic matter causes nutrient depletion and further CO2 emissions. In the Brazilian Legal Amazon a hint to sustainable permanent agriculture might exist. This is the socalled Terra Preta de Indio or Amazonian Dark Earth (ADE). The sustained fertility in charcoal-containing ADE and the frequent use of charcoal as soil conditioner in Brazil and other parts of the world (mainly Japan) provided the incentive to study the effects of charcoal application to a highly weathered soil. For many small land owners in the vicinity of Manaus charcoal production provides a significant proportion of the family income. To carbonize the woody biomass (slash and char) instead of burning it (slash and burn) would sequester CO2 if regrowing resources are used and maintain high SOM contents. On that score the authors described slash and char as a reasonable alternative to the widely practiced slash and burn agriculture. Soil charcoal additions were described to stimulate microbial activity and reduce leaching of nitrogen fertilizer. Microbial immobilization is an important nutrient retention mechanism in those soils highly affected by leaching. We suppose that microbial immobilization of easily leached nutrients (such as nitrogen) and solubilizing of mineralbound phosphate could be further mechanism besides an increase in CEC and nutrient sorption how charcoal amendments improve nutrition of crop plants. In a series of experiments, we are studying the use of charcoal in agricultural management of a highly weathered Xanthic Ferralsol on terra firme north of Manaus. In a randomized complete block design with five replicates, 15 organic amendment combinations (compost, chicken manure, litter and charcoal) were established and plant growth of sorghum (Sorghum bicolor) was observed for 4 cropping cycles. Plots fertilized with mineral fertilizer (NPK and lime) and additional charcoal application showed a 40% increase in plant growth and 50% increase in yield as plots mineral fertilized alone. A maximum yield increase of more than 800% to due charcoal application was obtained in the second growing season without new fertilization. Charcoal amendments alone had no effect on plant growth. These results are evidence of charcoal’s nutrient retention and/or sorption capacity and its positive effect on crop productivity. Additionally we measured soil nutrient content, pH, CEC, soil physical parameters and microbial biomass and activity. The microbial population growth rate after addition of an easily degradable substrate (glucose) reflected the soil fertility and plant biomass production very well and the difference in charcoal amended and fertilized plots and fertilized plots without charcoal amendments was significant. The influence of charcoal to the microbial community and nutrient dynamics was studied in further experiments. Soil charcoal amendments increased the microbial activity and decreased losses of nutrients.