Alfisol soils of rainfed semi-arid tropics (SAT) are degrading due to several physical, chemical, and biological constraints. Appropriate soil-nutrient management practices may help to check further soil degradation. A long-term experiment comprising tillage and conjunctive nutrient use treatments under a sorghum (Sorghum bicolor (L.) Moench)-mung bean (Vigna radiata (L.) Wilkzec) system was conducted during 1998-05 on SAT Alfisols (Typic Haplustalf) at the Central Research Institute for Dryland Agriculture, Hyderabad. The study evaluated soil and nutrient management treatments for their long-term influence on soil quality using key indicators and soil quality indices (SQI). Of the 21 soil quality parameters considered for study, easily oxidisable N (KMnO 4 oxidisable-N), DTPA extractable Zn and Cu, microbial biomass carbon (MBC), mean weight diameter (MWD) of soil aggregates, and hydraulic conductivity (HC) played a major role in influencing the soil quality and were designated as the key indicators of 'soil quality' for this system. The SQI obtained by the integration of key indicators varied from 0.66 (unamended control) to 0.83 (4 Mg compost þ 20 kg N as urea) under conventional tillage (CT), and from 0.66 (control) to 0.89 (4 Mg compost þ 2 Mg gliricidia loppings) under reduced tillage (RT). Tillage did not influence the SQI, whereas the conjunctive nutrient-use treatments had a significant effect. On an average, under both CT and RT, the sole organic treatment improved the soil quality by 31.8% over the control. The conjunctive nutrient-use treatments improved soil quality by 24.2-27.2%, and the sole inorganic treatment by 18.2% over the control. Statistically, the treatments improved soil quality in the following order: 4 Mg compost þ 2 Mg gliricidia loppings > 2 Mg Gliricidia loppings þ 20 kg N as urea = 4 Mg compost þ 20 kg N as urea > 40 kg N as urea. The percentage contribution of the key indicators towards the SQI was: MBC (28.5%), available N (28.6%), DTPA-Zn (25.3%), DTPA-Cu (8.6%), HC (6.1%), and MWD (2.9%). The functions predicting the changes in yield and sustainability yield index with a given change in SQI were also determined.Additional keywords: semi-arid tropics, soil quality indicators, sorghum-mung bean, sustainability yield index.

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Crop fertilisation with potassium in rainfed agriculture in India is not practised, merely on the assumption that Indian soils are rich in potassium and crops do not need external K supply. However, under continuous cropping in rainfed regions, huge crop K removals are reported, up to 150–200 kg/ha annually, depending upon amount and distribution of rainfall and biomass production. Thus, most of the crops essentially deplete soil K reserves. The present study evaluates the soil K reserves under diverse rainfed production systems and categorises rainfed soils based on different soil K fractions. Depth-wise sampling was done from 21 locations across different soil types under 8 production systems, and various fractions of soil K were determined. Total K was highest in Inceptisols (1.60–2.28%), followed by Aridisols (1.45–1.84%), Vertisols and Vertic sub-groups (0.24–1.72%), and Alfisols and Oxisols (0.30–1.86%), showing a wide variation within each group. Nonexchangeable K reserves were found in a proportionate manner to total K in most of the soil profile. Unlike nonexchangeable K reserves, Vertisols had higher exchangeable K than Inceptisols and Alfisols/Oxisols. Nonexchageable K showed significant positive correlation with total K in Inceptisols and Vertisols, whereas it was non-significant in Alfisols/Oxisosls. However, significant positive correlations were recorded with exchangeable K and nonexchangeable K in all soil types, indicating the dynamic equilibrium between 2 soil K fractions. Nonexchangeable K reserves were included along with exchangeable K in categorising soils into 9 groups for evolving better strategies to manage soil K fertility in rainfed agriculture in India. Finger millet and groundnut crops at Bangalore and Anantapur regions (category I) need immediate attention on K nutrition, as these soils are low in both exchangeable and nonexchangeable K. Similarly, crops grown on soils of S.K. Nagar, Ballowal-Saunkri, and Rakh-Dhiansar, with low exchangeable K and medium nonexchangeable K, would need K fertilisation as these crops (maize and pearlmillet) are K-exhaustive (category II). Pearl millet and upland rice in category III and cotton in category IV need K additions at critical stages. Upland rice in category V needs a maintenance dose of K. In category VI, cereal crops may not need K additions immediately as they have medium exchangeable K and high nonexchangeable K. Long-term sorghum cropping may need K supply after few years (category VII). Soils in category VIII are adequate in nonexchangeable K and medium exchangeable K and the crops, groundnut, cotton, sorghum, and soybean, may not need external K immediately. For soils in category IX, K fertilisation is not required to the crops (sorghum and soybean) as these soils have high exchangeable and nonexchangeable K.

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