1 23 Biology and Fertility of Soils Cooperating Journal of International Society of Soil Science ISSN 0178-2762 Volume 48 Number 1 Biol Fertil Soils (2012) 48:109-116 DOI 10.1007/s00374-011-0576-y Phosphorus threshold for arbuscular mycorrhizal colonization of crops and tree seedlings Ashok Shukla, Anil Kumar, Anuradha Jha, Ajit & D. V. K. Nageswara Rao