Magnesium balance in four permanent manurial experiments under rainfed agro-ecosystems of India

Magnesium (Mg) plays a vital role in photosynthesis, dry matter production and carbon partitioning in sink organs. Hence, four permanent manurial experiments (20-27 years of duration) under the auspices of All India Coordinated Research Project for Dryland Agriculture (AICRPDA) network centres across diverse agro-ecological regions were carried out to examine the soil exchangeable Mg (ex-Mg), crop uptake and overall Mg balance. Groundnut (peanut), finger millet, rice-lentil sequence and post rainy sorghum were the major crops or cropping systems followed in four permanent manure experiments at Anantapuram, Bengaluru, Varanasi and Solapur, respectively. Nutrient management in all experiments involved control (no addition of nutrients), 100% organic, 100% chemical, and integration of organic and chemical. Except in the finger millet-based system, mean ex-Mg status in the entire profile was higher than the sufficiency level (1.0 cmol(+) kg(-1) as a critical limit). Status of ex-Mg (cmol(+) kg(-1) soil) in soil profiles was in the order: Solapur (3.80) > Varanasi (2.07) > Anantapuram (1.06) > Bengaluru (0.44). A uniform distribution of ex-Mg was observed in plots that received integrated application of organic and chemical fertilisers. In general, improved status of profile ex-Mg (cmol(+) kg-1) over the control was observed in soils under groundnut (0.19-0.78), finger millet (1.90-3.20), and post rainy sorghum (6.50-7.60, except 4.20 in 100% NPK) cropping. Overall, ex-Mg status and balance of different soil types under diverse crop production systems was influenced by several factors, some of which include soil type with varying mineralogy, particle size distribution, nutrient management strategies and rainfall. Significant positive relationships were observed between ex-Mg status and clay content (R-2 = 0.94), soil pH (R-2 = 0.92), cation exchange capacity (R-2 = 0.98) and mean air temperature (R-2 = 0.22), whereas a weak relationship was observed with rainfall (R-2 = 0.01). The study gives an account of Mg balance in major Indian soil types and recommends further attention on Mg nutrition in current intensive agriculture.