Fertilizer Research 32: 313-319, 1992. © 1992 Kluwer Academic Publishers. Printed in the Netherlands. 313 Nutrient transformation in soil due to addition of organic manure and growing crops H. Zinc 1 K.L. Sharma 2, J.C. Bajaj, S.K. Das 2, U.M. Bhaskara Rao 2 and R.K. Rattan Division of Soil Science and Agricultural Chemistry, Indian Agricultural Research Institute, New Delhi 110 012, India Received 14 August 1991; accepted in revised form 7 July 1992 Key words: Maize-mustard crop sequence, 'methanised' FYM-bioslurry, inter-relationships, zinc fractions, transformation Abstract Organic matter is the major source of zinc in soil. The availability of this nutrient is dependent on the release from organic matter through mineralization and reaction with soil particles. Addition of bioslurry, containing 70 mg Zn kg -1 will influence availability of Zn through its effect on transformation reaction in soil. The present study was conducted to determine the distribution of major chemical forms of Zn in an alluvial soil, to understand the changes in zinc fractions due to bioslurry application and cropping and to find out the inter-relationships and equilibria between the fractions. Soil solution + exchangeable Zn (Zn-CA), specifically sorbed Zn by inorganic sites (Zn-ACC), specifically sorbed Zn by organic sites (Zn-PYR), Zn occluded by free oxides (Zn-OX), and residual zinc (Zn-RES) constituted 0.3, 4.5, 16.6, 16.3 and 57.3 percent, respectively of the total Zn content (Zn-TOT). Application of 13.32 t ha -1 bioslurry increased Zn content in Zn-CA, Zn-PYR and Zn-RES by 72.7, 93.2 and 36.4 percent, respectively over control. Zn occluded by free oxides (Zn-OX) was found released by the dissolution action of organic compounds present in bioslurry and the amount of Zn so released was transformed to Zn-RES, Zn-CA and Zn-DTPA. Growing crops increased Zn content in Zn-RES fraction only. Linear positive relationships between Zn-CA, Zn-PYR, Zn-RES and DTPA-Zn and bioslurry levels marked the significance of bioslurry in stabilising the status of these fractions. Path coefficient analysis and intercorrelation studies indicated the existence of equilibrium between different Zn fractions in soils. Introduction 'Methanised' farm yard manure, bioslurry con- tains substantial amount of total zinc (Zn), the availability of which to the crops depends on chemical reactions in soil. Studies on Zn have 1 Part of Ph.D. Thesis of the senior author 2 Central Research Institute for Dryland Agriculture, P.O. Saidabad, Hyderabad 500 659, India revealed that the extent and nature of its trans- formation are mainly governed by the reaction time in soil, amount of Zn added, the rate of extraction, type of clay mineral, and extent of organic matter content in soil [13]. Further, Viets [22] suggested the concept of various pools of micronutrients existing in soil and defined five pools as (i) water soluble, (ii) exchangeable, (iii) adsorbed, chelated or complexed, (iv) secondary clay minerals and insoluble oxides, and (v) pri- mary minerals. Subsequently, attempts have