Florian Wichern, Christian Richter, Rainer Georg Jörgensen:
Soil Fertility Breakdown in Soils of Subtropical South Africa Used as Kitchen Garden

FLORIAN WICHERN¹, CHRISTIAN RICHTER², RAINER GEORG JÖRGENSEN^1
1University of Kassel, Department of Soil Biology and Plant Nutrition, Germany
²University of Kassel, Institute of Crop Science, Germany

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. In 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, 20-30cm) 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 at an average of 1.25gcm^-3 in comparison to 1.04gcm^-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 all declined 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 30cm depth, soil organic C was only [10]% (18.9 versus 17.2t) larger at site L₂, but microbial biomass C was [110]% (1680 versus 790kg) and ergosterol even [220]% (5.1 versus 1.6kg). The ergosterol-to-microbial biomass C was nearly doubled in the new garden soil at 0-10cm 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 almost doubled at site L₂. The metabolic quotient qCO₂ revealed strong depth-specific variations of between 18 to 38mg CO₂-Cd^-1g^-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

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Contact Address: Rainer Georg Jörgensen, University of Kassel, Department of Soil Biology and Plant Nutrition, Nordbahnhofstraße1a, 37213 Witzenhausen, Germany, e-mail: joergewiz.uni-kassel.de