Deutscher Tropentag, October 9 - 11, 2002 in Kassel-Witzenhausen "Challenges to Organic Farming and Sustainable Land Use in the Tropics and Subtropics"

Soil Fertility Breakdown in Soils of Subtropical South Africa Used as Kitchen Garden

¹University of Kassel, Department of Soil Biology and Plant Nutrition, Germany
²University of Kassel, Institute of Crop Science, Germany

Abstract

Cabbage plants at one plot (L₁) showed chlorosis and necrotic parts on the leaves. At another plot (L₂), the plants grew better and did not reveal any symptoms, but the yield was low as well. Nutrient deficiency was not confirmed by the analysis of plant material. This study we tried to analyse whether these deficiency symptoms were related to specific soil physical, chemical or biological properties. Samples were taken from an ecological homegarden close to Pietermaritzburg, subtropical South Africa. Soil samples were taken at three depths (0-10, 10-20, 20-30 cm) from two plots that are close together.The two gardens did not differ significantly in particle size distribution at any depth. The mean contents of sand, silt and clay were 13, 42 and 45%. The bulk density was significantly larger at site L₁ an average of 1.25 g cm^-3 in comparison to 1.04 g cm^-3 at site L₂. Also, the soil pH was significantly increased at site L₁ with an average of 5.6 in comparison to 4.0 at site L₂. Soil organic C, microbial biomass C, ergosterol, and basal respiration declined all markedly with depth. The basal respiration rates were similar at both sites on a depth-specific level, contrasting the contents of soil organic C, microbial biomass C and ergosterol, which were significantly elevated at site L₂. If the concentrations were converted to the total amounts stored per hectare at 0 to 30 cm depth, soil organic C was only 10% (18.9 versus 17.2 t) larger at site L₂, but microbial biomass C 110% (1680 versus 790 kg) and ergosterol even 220% (5.1 versus 1.6 kg). The ergosterol-to-microbial biomass C was nearly doubled in the new garden soil at 0-10 cm depth in combination with maximum microbial biomass C content. The microbial biomass C-to-soil organic C ratio ranged from 0.25 to 1.10%, declined markedly with depth and was fairly doubled at site L₂. The metabolic quotient qCO₂ revealed strong depth-specific variations of between 18 to 38 mg CO₂-C d^-1 g^-1 microbial biomass C at site L₁ on a two to three times larger level than at site L₂.

Keywords: Basal respiration, biomass N, ergosterol, microbial biomass C

Contact Address: Rainer Georg Jörgensen, University of Kassel, Department of Soil Biology and Plant Nutrition, Nordbahnhofstraße 1a, 37213 Witzenhausen, Germany, e-mail: joergewiz.uni-kassel.de