~ 262 ~ Journal of Pharmacognosy and Phytochemistry 2020; 9(6): 262-266 E-ISSN: 2278-4136 P-ISSN: 2349-8234 www.phytojournal.com JPP 2020; 9(6): 262-266 Received: 05-09-2020 Accepted: 14-10-2020 GK Surya Krishna Department of Soil Science and Agricultural Chemistry, S.V. Agricultural College, Acharya N. G. Ranga Agricultural University, Tirupati, Andhra Pradesh, India T Giridhara Krishna Department of Soil Science and Agricultural Chemistry, S.V. Agricultural College, Acharya N. G. Ranga Agricultural University, Tirupati, Andhra Pradesh, India V Munaswamy Department of Soil Science and Agricultural Chemistry, S.V. Agricultural College, Acharya N. G. Ranga Agricultural University, Tirupati, Andhra Pradesh, India Y Reddi Ramu Department of Agronomy, S.V. Agricultural College, Acharya N. G. Ranga Agricultural University, Tirupati, Andhra Pradesh, India Corresponding Author: GK Surya Krishna Department of Soil Science and Agricultural Chemistry, S.V. Agricultural College, Acharya N. G. Ranga Agricultural University, Tirupati, Andhra Pradesh, India Status of available phosphorus, forms and distribution of phosphorus in soil under major cropping systems of Nellore district, A.P GK Surya Krishna, T Giridhara Krishna, V Munaswamy and Y Reddi Ramu DOI: https://doi.org/10.22271/phyto.2020.v9.i6d.12894 Abstract An investigation was undertaken during 2018-19 in Southern zone of A.P. to study the Status of available phosphorus and forms of Phosphorus in soil under major cropping systems of Nellore district. The Ca-P was observed to be the predominant P fraction under all major cropping systems could be attributed to high CaCO3 content. The highest mean values for saloid-P (16.67 mg kg -1 ), total-P (538.21 mg kg -1 ) and other forms of P (342.07 mg kg -1 ) were recorded in groundnut-paddy cropping system and Fe- P (68.77 mg kg -1 ) and Al-P (61.85 mg kg -1 ) were highest in paddy-paddy cropping system, whereas available P2O5 (166.82 kg ha -1 ) and Ca-P (173.15 mg kg -1 ) were highest in groundnut-paddy and fallow-bengalgram systems, respectively. pH showed positive and significant relation with total-P (r= 0.507**) and other forms of P (r= 0.573**) where as free CaCO3 showed positive and significant correlation with Ca-P (r= 0.432*) in calcareous soils of S.P.S.R. Nellore district. Available P2O5 showed positive and significant correlation with saloid-P (r= 0.456*), Al-P (r= 0.765**) and Fe-P (r= 0.559**), saloid-P showed significant and positive correlation with total-P (r= 0.477*) and other forms of P (r= 0.510**). Al-P showed positive and significant correlation with Fe-P (r= 0.885**). Similarly, Ca-P showed positive and significant relation with total-P (r= 0.418*). Keywords: Calcareous soils, P fraction, cropping systems Introduction Phosphorus is the tenth most abundant element in the earth’s crust and its average content is nearly 0.12 per cent. In lithosphere, it always occurs in the pentavalent oxidation state. Phosphorus is an essential element for plant growth. Therefore, maintainance of an adequate amount of soil phosphorus through application of inorganic and/ or organic phosphorus is critical for the sustainability of cropping systems. Knowledge of amount of each fraction and their relationship with soil characteristics is very useful in assessing phosphorus nutrition of plants. Since various P fractions and their contribution to available phosphorus provide useful information in assessing the available P status of soils and various forms of soil phosphorus have different solubility under varying soil condition. Available P content increases with the application of P. The increase in available P is a function of certain physical and chemical soil properties, such as clay, organic carbon, Fe, Al and calcium carbonate contents. Marked variation in the different forms of inorganic P is a function of genetic differences among soils (Chang and Jackson, 1958) [4] . Materials and methods Five soil samples from each cropping system (Paddy-Paddy, Fallow-Paddy, Fallow- Blackgram, Groundnut-Paddy, and Fallow-Bengalgram) at 0-15cm depth from farmer’s field were collected from calcareous soils of S.P.S.R Nellore district of southern zone of A.P. The soils were mildly to moderately alkaline in reaction, non-saline and below the critical limits, free lime content indicates that these soils were calcareous. These soil samples were analysed for status of available P2O5 (Olsen-P) and different P fractions (Sal-P, Al-P, Fe-P, Ca-P, total-P and other forms of P). The available phosphorus was extracted by using Olsen’s extractant (0.5 M NaHCO3 of pH 8.5) as described by Olsen et al. (1954) [13] and the phosphorus content in the extract was determined by Murphy and Riley (1962) [12] method using ascorbic acid as the reducing agent using spectrophotometer. The inorganic phosphorus fractions viz., saloid-P, Fe-P, Al-P and Ca-P were estimated by following the sequential extraction procedure as given by Kovar and Pierzynski (2009) [8] . Total phosphorus in the soils was determined by perchloric acid digestion method as described