ASIAN JOURNAL OF CHEMISTRY ASIAN JOURNAL OF CHEMISTRY http://dx.doi.org/10.14233/ajchem.2016.19868 INTRODUCTION Evaluation of phosphorus fertility status of soil is nece- ssary to make a sound phosphorus fertilizer recommendation for optimizing crop yield. Phosphorus is one of the major limiting factors of plant growth in many soils. Plant availability of inorganic phosphorus can be limited by formation of inorganic phosphorus such as Al-P, Fe-P, Ca-P, occluded and organic phosphorus The proportion of each of these fractions governs the response of applied phosphorus The transfor- mations of phosphorus in soils involve complex microbiolo- gical, chemical and biological processes. However, adsorption of applied or native phosphorus to Fe- and Al-oxides [1,2], 1:1 type of silicate clays [3-5] and formation of Fe- and Al-P complexes with humic acids [6] in acid soils are considered as the dominant mechanisms of phosphorus fixation affecting concentration of available phosphorus in soil solution at any point of time. The soils of north-east region are the product of slow diagenetic changes of acidic parent materials causing inherent soil acidity and high precipitation further aggravates this problem due to leaching of basic cations [7,8]. Higher phos- phorus fixation due to presence of large amounts of Fe and Phosphorus Fractions in Soils of Transect of Kohora Watershed of Assam, India SEEMA BHAGOWATI and K.N. DAS * Department of Soil Science, Assam Agricultural University, Jorhat-785 013, India *Corresponding author: E-mail: daskn388@gmail.com Received: 28 January 2016; Accepted: 9 April 2016; Published online: 1 June 2016; AJC-17933 The forms of phosphorus in some surface soils representing various land forms viz. hillock top (crest 0-5 % slope), upper slope (30-35 % slope), middle slope (15-30 % slope), lower slope (10-15 % slope), flatter land (moderately sloping, 5-10 % slope), upland (gently sloping, 2-5 % slope), medium land (very gently sloping, 1-3 % slope) and low land (level to nearly level, 0-1 % slope) in a transect of Kohora watershed of Karbi Anglong district of Assam, India were studied. All the soils were acidic in reaction having variation in soil texture from skeletal sandy clay to sandy clay. Total free Fe2O3 + Al2O3, Ald and Fed of these soils were quite high and showed an increasing trend downward the transect along with clay. The study revealed that the sequential occurrence of various inorganic phosphorus fractions followed the order: Fe-P > Al-P > Organic-P > Res-P > Ca-P. Average inorganic phosphorus fractions as a constituent of total phosphorus in these soils was in the following order: Fe-P (29 %) > Al-P (27 %) > Organic-P (23 %) > Res-P (12 %) > Ca-P (3 %). Both Al-P and Fe-P were significantly correlated with Fed, Ald and Fe2O3 + Al2O3 and negatively with pH, Bray’s I-P, Bray’s II-P and Olsen’s- P. Olsen’s reagent extracted the highest amount of available phosphorus (15.38 to 24.21 mg Kg -1 ) followed by Bray’s-II (13.29 to 23.35 mg Kg -1 ) and Bray’s-I (12.39 to 21.35 mg Kg -1 ). Both Al-P and Fe-P were the major contributing phosphorus fractions to the total and available phosphorus pool and phosphate nutrition to crop plants. Distribution of various fractions of phosphorus in soils of the watershed was the functions of local (micro) variations in land forms rather than interrelationship of phosphorus fractions. Keywords: Phosphorus fractions, Watershed transect. Asian Journal of Chemistry; Vol. 28, No. 9 (2016), 2009-2012 Al-oxides and hydroxides in these soils results phosphorus deficiency [9]. Variations in land forms and land use patterns also have profound influence on the chemical nature of soils which in turn governs the dominance of different phosphorus fractions in soils. Information about different fractions of soil phosphorus compound in a watershed context in Assam has not been generated earlier. Hence, an attempt was made to characterize the soils and to quantify different fractions of phosphorus and their contributions to the total and available phosphorus pool relating physical and chemical properties in soils of a transect of Kohora watershed of Assam, India. EXPERIMENTAL The Kohora watershed of Assam (India) lies between 93°20'E to 94°25'E longitude and 26°30'N to 25°45'N latitude covering a part of Karbi Anglong and Golaghat districts of Assam and includes a part of Kaziranga National Park. The study area covers a part of Kohora watershed under Karbi Anglong district with a geographical area of 295 hectares having humid sub-tropical climate with mean annual rainfall of 2246 mm. Based on contour information available in the toposheet following FAO guidelines (2006) and local modi- fications, eight land forms viz. hillock top (crest 0-5 % slope),