Effects of Charcoal as Slow Release Nutrient Carrier on N-P-K Dynamics and Soil Microbial Population: Pot Experiments with Ferralsol Substrate

Giardina et al. (2000) reported that 300 million people annually practice shifting agriculture, affecting 400 million hectares of the planet's 1,500 million ha of arable land. The sustainability of shifting cultivation and slash-and-burn continues to be a topic of discussion. Kleinman et al. (1995) characterized sound slash-and-burn agriculture as an ecologically sustainable agroecosystem because crop yields can be maintained without inputs of non-renewable fossil energy resources for fertilizers, pesticides, and irrigation. According to Woods and McCann (1999) shifting cultivation can be an environmentally friendly analogue to the natural processes of disturbance and regenerative succession in tropical forests. They suggest that the Amerindian population made long lasting improvements to notoriously infertile tropical soils by long-term mulching, frequent burning, and the application of charcoal and ash which increased soil pH and thereby suppressed Al activity favourable for specific microorganisms responsible for the darkening of these soils, called terra preta de índio. The theory that a correlation between shortened fallow periods and yield decline in shifting cultivation exists is questioned by (Mertz 2002), but in general most authors describe recent shifting cultivation above a certain population density or frequency of clearance (shortened fallow periods) as disastrous and leading to soil nutrient and soil organic matter (SOM) depletion (Goldammer 1993; Hölscher et al. 1997b; Silva-Forsberg and Fearnside 1997; Zech et al. 1990). The effectiveness of conventional fertilization on highly weathered and acidic Oxisols in the Amazon Basin is limited by high rainfall, low nutrient retention, and rapid water flow. Easily available and mobile nutrients, such as those supplied by mineral N or K fertilizers are rapidly leached into the subsoil (Giardina et al. 2000; Hölscher et al. 1997a; Renck and Lehmann 2004).

P is usually considered the primary limiting nutrient in plant production on highly weathered soils of the humid tropics because it is strongly bound to aluminium and iron oxides and, thus, not easily available for plants (Garcia-Montiel et al. 2000). Heterophobic phosphate solubilizing microorganisms make mineral bound P available by the excretion of chelating organic acids. (Kimura and Nishio 1989) showed that insoluble phosphates which are not crystallized can be solubi-lized by indigenous microorganisms when abundant carbon sources are supplied.