Soil fertility breakdown in a subtropical South African vertisol site used as a home garden

Soil samples were taken at three depths (0–10, 10–20, 20–30 cm) from an old home garden site, subtropical South Africa (KwaZulu Natal Province), exhibiting symptoms of soil fertility breakdown, and from a new garden site, used formerly as extensive grassland. The aims of our study were to investigate (1) whether soil fertility breakdown is reflected by soil biological indices and (2) whether these indices provide information for improving the situation. The bulk density was significantly larger at the old garden site, with an average of 1.25 g cm^-3 in comparison to 1.04 g cm^-3 at the new garden site. Also the soil pH was significantly higher at the old garden site, with an average of 5.6 in comparison to 4.0 at the new garden site. Soil organic C and all soil microbial indices declined markedly with depth. If the concentrations were converted to the total amounts stored per hectare at 0- to 30-cm depth, soil organic C was only 10% greater (18.9 versus 17.2 t) at the new garden site, but microbial biomass C was 110% (1,680 vs 790 kg) and ergosterol even 220% greater (5.1 vs 1.6 kg). The microbial biomass C to soil organic C ratio ranged from 0.25% to 1.10%, declined markedly with depth and was almost twice as high at the new garden site. The metabolic quotient qCO₂ revealed strong depth-specific variations of between 18 and 38 mg CO₂-C day^-1 g^-1 microbial biomass C at the old garden site on a two to three times higher level than at the new garden site. Soil micro-organisms are much more strongly affected by compaction than by acidification at the present two sites. Whether land use practices led to the increase in bulk density at the old garden site or to the decrease in soil pH at the new garden site is still not fully understood. For these reasons, it is very difficult to recommend a feasible melioration plan.