مجله آب و خاک (Jan 2017)
Inorganic Phosphorus Fractions and Their Relationships with Soil Characteristics of Selected Calcareous Soils of Fars Province
Abstract
Introduction: Phosphorus (P) is the second limiting nutrient in soils for crop production after nitrogen. Phosphorus is an essential nutrient in crop production. Determination of forms of soil phosphorus is important in the evaluation of soil phosphorus status. Various sequential P fractionation procedures have been used to identify the forms of P and to determine the distribution of P fractions in soils (Chang and Jackson, 1957, Williams et al., 1967; Hedley et al., 1982), but are not particularly sensitive to the various P compounds that may exist in calcareous soils. A Sequential fractionation scheme has been suggested for calcareous soils by which three types of Ca-phosphates i.e. dicalcium phosphate, octacalcium phosphate, and apatite could be identified (Jiang and Gu, 1989). These types of Ca-phosphates were described as Ca2-P (NaHCO3-extractable P), Ca8-P (NH4AC-extractable P) and Ca10-P (apatite type), respectively. In this study, the amount and distribution of soil inorganic phosphorus fractions were examined in 49 soil samples of Fars province according to the method described by Jiang and Gu (1989). Materials and Methods: Based on the previous soil survey maps of Fars province and According to Soil Moisture and Temperature Regime Map of Iran (Banaei, 1998), three regions (abadeh, eghlid and noorabad) with different Soil Moisture and Temperature Regimes were selected. The soils were comprised Aridic, xeric, and ustic moisture regimes along with mesic, and hyperthemic temperature regimes. 49 representative samples were selected. The soil samples were air-dried and were passed through a 2-mm sieve before analysis. Particle size distribution was determined by hydrometer method (Gee and Bauder 1996). Also, Cation exchange capacity (CEC; Sumner and Miller 1996), calcium carbonate equivalent (Loeppert and Suarez 1996), organic matter content (Nelson and Sommers 1996), and pH by saturated paste method (Thomas 1996) were determined . Inorganic phosphorus sequential fractionation scheme was preformed according to the method described by Jiang and Gu (1989). Olsen-P fraction that was extracted by NaHCO3 (Olsen and Sommers 1982) was regarded as P-availability index. Also, Total-P by perchloric acid (HClO4) digestion (Sparks; 1996) and organic P were determined.. All of the extraction procedures were performed in duplicate and the amounts of P were colorimetrically measured in the supernatants by the ascorbic acid method of Murphy and Riley (1962).The relationships between forms of P and some of the soil properties were established using correlation method. Results and Discussion: The chemical data of the soils showed that soils were calcareous with CCE range between 9.94 to 74.27 % ( average 51.10%) and pH range between 7.02 to 8.36 (average 7.85). Also, the amounts of CEC were between 5.35 to 29.39 cmol (+) kg-1(average 16.68 cmol (+) kg-1). The results showed a wide range in content of Phosphorus fractions. The amount of total Phosphate ranged from 301.87 to 1458.68 mg kg-1 with an average of 626.63 mg kg-1 . Calcium Phosphate ranged from 147.83 to 666.90 mg kg-1 with an average of 324.79 mg kg-1, that comprised 85 and 52 percent of inorganic and total Phosphorus, respectively. The amount of Fe-P ranged from 0.38 to 59.18 mg kg-1 with an average of 7.56 mg kg-1 that comprised 13.64 and 8.34 percent of inorganic and total Phosphorus, respectively. Also, the amount of Al-P ranged from 20.49 to 123.09 mg kg-1 with an average of 52.28 mg kg-1that comprised 1.97 and 1.21 percent of inorganic and total Phosphorus, respectively. The results of correlation study showed that available Phosphorus was significantly correlated with Ca2-P, Ca8-P, Al-P, Ca10-P, and Pt (total phosphorus). So, in calcareous soils, awareness of soil properties and phosphorus fractions and their relationships are important for evaluation of phosphorous status in soil and understanding of soil chemistry that influence soil fertility. Conclusion: The relative abundance of inorganic P forms were in order of Ca10 – P > Ca8- P > Al –P> Ca2-P> Fe-P. Among the inorganic P fractions, Ca-P had the highest value and varied from 147.83 to 666.90 mg kg-1, which accounted for 53 percent of the sum of P fractions, occurred in H2SO4 extractable P fraction, which is attributed to primary Ca–P minerals, indicating their weak weathering nature. Also, correlation study showed that available Phosphorus was significantly correlated with Ca2-P, Ca8-P, Al-P, Ca10-P, and Pt. This result indicate that these fractions probably can be used by plant.
Keywords
