In a field experiment, microbiological and chemical amelioration of a highly deteriorated alkaline soil (pH 10.6, exchangeable sodium percentage 95) was undertaken during 1989–92 using two reclamation technologies: namely, growing Karnal grass (Leptochloa fusca) as a first crop in the absence of any amendment (biological reclamation) or applying gypsum as a chemical amendment for different cropping sequences. After ten months average dehydrogenase activity, used as a measure of catabolic activity of micro-organisms under anaerobic conditions, was 49.5 μg triphenylformazan (TPF) per g soil in treatments where the soil was reclaimed by growing Karnal grass, compared with 26.8 μg TPF by applying 50% of the estimated requirement of gypsum. After three years of reclamation, average dehydrogenase activity was 118.7 μg TPF per g soil with Karnal grass and 96.1 μg TPF with gypsum. After three years average microbial biomass carbon was 28% greater in the gypsum treatments (206.5 mg per kg soil) compared with Karnal grass treatments (161.7 mg). However, at this stage the ratio between average dehydrogenase activity and microbial biomass carbon was greater with Karnal grass (0.77) than in gypsum treatments (0.47). The microbiological properties changed more than the chemical properties of alkali soil as the time period advanced. After three years of reclamation, the average pH of the alkali soil dropped from 10.6 to 9.45 and the average exchangeable sodium percentage was reduced from 95 to 47.5. The increase in total organic carbon was 64% and in available nitrogen about 38% compared with the original soil. Higher microbial biomass carbon in gypsum treatments corresponded with lower exchangeable sodium percentage, higher availability of nitrogen and potassium and greater total yield of sensitive winter clovers.