39 P hosphorus (P) dynamics in soil and maintenance of its adequate supply are important for sustainability of native and agricultural ecosystem (Song et al., 2007). Phosphorus may accumulate and its bio-availability may increase in soils after continuous application of fertilizers, which can have negative impacts on the environment as accumulated soil P in the topsoil is a major source of soluble and particulate P in the surface runoff (Daniel et al., 1998). The application of P to each crop in a rotation and low recovery of added P (8 to 33 per cent) has been found to result in its signiicant build up in soils (Brar et al., 2000). The residual P left in the soil is comparatively less soluble form of phosphorus (Murthy et al., 2002). A rice-wheat sequence that yields 7 t ha -1 of rice and 5 t ha -1 of wheat removes more than 35 kg P from the soil. Even with the recommended rate of fertilization in this system, a negative balance of P still exists. The productivity of rice-wheat cropping system is declining due to nutrient imbalance. Application of fertilizer phosphorus is essential for raising the available P content in soils in order to meet the crop requirements at different stages of growth. The P requirements for a higher rice and wheat yields vary depending in soil and climatic conditions. Phosphorus, like any other nutrient, is present in soils in major components that are organic and inorganic. Organic P, mainly conined to the surface layer, is mineralized into inorganic forms. But, the plants mainly depend upon inorganic P forms for their P requirements. Phosphorus accumulates in soil with long-term continuous application of P fertilizers (Zhang et al., 2004; Brar et al., 2004) in soil as less soluble products of phosphorus (Dhillon and Dev,1988; Murthy et al., 2002). The availability of soil P to plants depends on the replenishment of labile P from other P fractions. Nwoke et al. (2004) observed that the changes in different inorganic-P fractions in soils under a wide range of management conditions. Measuring speciic inorganic P fractions and Olsen’s P is a potential approach to better characterization of crop response to applied P. Therefore the objectives of this study were to evaluate the effect of long- term use of inorganic and integrated fertilization on soil phosphorus fractions and their affect on P uptake and crop yield under rice-wheat cropping system. MATERIALS AND METHODS Field treatments The soil samples were collected before transplanting of rice in 2013 from ongoing long-term fertilizer experiment on rice-wheat cropping system. The experiment consisted nine treatments including T 1 :100%N, T 2 :100%NP, T 3 :100%NPK, T 4 :100%NPK (P to wheat only), T 5 :150%NPK, T 6 :100%NPK+FYM (farmyard manure), T 7 : 100%NPK+GM (green manure) and T 8 :100%NPK+SI (straw incorporation) and T 9 : control. The rice (Oryza sativa) variety PR 118 and wheat (Triticum aestvium) variety PBW 621 was grown as test crops during the year. The 100%NPK to rice (120 kg N, 60 kg P 2 O 5 and 30 K 2 O per hectare) and wheat (120 kg N, 60 kg P 2 O 5 and 30 K 2 O and lowest) was supplied through urea, diammunium phosphate (DAP) and muriate of potash (MOP). In T 6 , FYM at 10 tha -1 was applied once a year before the rice transplanting. A 45 days old GM (Sesbania aculeate) was incorporated one day before rice transplanting every year in T 7 . In straw incorporation treatment (T 8 ), the rice and wheat were harvested and remaining straw was incorporated in soil before the sowing of wheat and transplanting rice, respectively. The full dose of P and K was applied at the time of transplanting of rice and wheat. In rice, N was applied in Agric Res J 52 (4) : 39-43, December 2015 DOI No.10.5958/2395-146X.2015.00059.9 *Corresponding author: bsbrar1957@pau.edu Date of receipt: 09.01.2014, Date of acceptance: 05.06.2015 EFFECT OF LONG-TERM USE OF INORGANIC AND INTEGRATED FERTILIZATION ON SOIL PHOSPHORUS FRACTIONS IN RICE-WHEAT CROPPING SYSTEM Sukhvir Kaur, BS Brar* and GS Dheri Department of Soil Science Punjab Agricultural University, Ludhiana - 141004 ABSTRACT Soil phosphorus (P) fractions as affected by long-term use of inorganic and integrated fertilization under rice-wheat cropping system in loamy sand soil of Punjab, India were studied. The inorganic P fractions improved with increase in rate of P applied. The Ca-P is the dominating P fractions in these soils. The inorganic P fraction improved with fertilizer and the effect was pronounced under integrated fertilizer application. The pH and EC were negatively and signiicantly correlated with Sa-P, Al-P, Fe-P and Ca-P, however, the soil OC signiicantly and positively correlated with these fractions. The Sa-P and Al-P were major sources of Olsen ’s exchangeable soil P and were in equilibrium with Fe-P and Ca-P. The crop production (grain and straw yield) signiicantly improved with fertilizer. Grain yield and total P uptake by rice was positively and signiicantly correlated with all soil inorganic P fractions. Key words: Farm yard manure, Fertilizer phosphorus, Integrated fertilization, Soil P fractions