Document Type : Research Paper

Author

• Roghayeh Shahriaripour

Department of agriculture, PayameNour university, Tehran, Iran

Abstract

Introduction Phosphorus (P) is the second nutrient and plays a key role in plant growth. The availability of P in the soil depends on the P fractions, which influence the primary productivity of agricultural ecosystems. Having sufficient and accurate information about soil mineral phosphorus is very important for the development of sustainable agriculture in arid regions. Awareness of phosphorus deformation in different soils is also very important for phosphorus fertilizer recommendation. When phosphorus is used in its soluble forms, it is rapidly converted into unusable forms for the plant. Native soil phosphorus is often in unusable form for the plant. Therefore, having enough information about phosphorus deformation in different soils is very important to recommend phosphorus fertilizer. The objective of this study was to find out the effects of long-term use of different sources of phosphorus on available P for pistachio plants in southeast Iran.
Materials and Methods The aim of this study was to find the effects of long-term use of different sources of phosphorus fertilizers that can be used for pistachio plants. For that, 168 different soil samples from 63000 ha of Sirjan pistachio orchards of Kerman province. Jiang and Gu method was used to isolate and determine the mineral forms of phosphorus in the soil. Each P fraction was extracted as follows: 1g (oven-dry weight) of sample soil weighed out into a 50 mL polyethylene centrifuge tube. A volume of 40 mL of the first extractant, NaHCO3 (Table 1) was added and placed in an automated mechanical shaker at 25° C with 110 rpm for 1 h to allow time for the solution top equilibrate. The tubes were then centrifuged at 6000 × g for 15 min and the supernatant was carefully filtered through Whatman No. 42 with minimum loss of soil. Reactive P in the supernatant was determined using the ascorbic acid method at 882 nm. This method separates phosphorus in the forms of di calcium phosphate, octa calcium phosphate, aluminum phosphates, iron phosphates, occluded phosphates and apatite. The data showed that Ca10-P was the most abundant P forms in the soils tested.
Results and Discussion Results showed that the total P was high in these soil samples and the abundance of P fractions was in order to Ca10-P type≥ Al-P >Ca2-P type> Occluded P ≥ Fe-P > Ca8-P type in two depths. These results suggested that continuous P application leads to plant available P convert into unavailable P forms such as Ca10-P. results indicate that NaOH-extractable P has resulted from active Fe oxides. Also, by comparing two depths of soil in the study area, it can be concluded that the occupied phosphorus in the surface layer is more than the deep layer. Finally, it can be concluded that by increasing the stability of various forms of phosphorus, their contribution to other forms of phosphorus increases, so that the concentration of apatite is much higher than phosphate-calcium phosphate. Results showed the amount of available phosphorus (Ca2-P) in the surface layer is more than the deep layer, which can be attributed to surface application of Phosphorus fertilizers. However, there are sections where phosphorus in the surface layer and the bottom is almost equal. Amount of this form of P is less than Al-P and Fe-P. It may be due to increase in Al and Fe contents in in calcareous soils.
Conclusion High amounts of total P clarifies that by optimally managing the use of phosphorus fertilizers and soil conditions, the required P of the plant can be provided without additional use. From the results reported in this study, it appears that in the calcareous soils tested, the abundance of different forms of P was in order Ca10-P type≥ Al-P >Ca2-P type> Occluded P ≥ Fe-P > Ca8-P. furthermore, it observed that Fe-P, Al-P and Oc-P are important forms of P and on average constitute 30% of the sum of inorganic P forms. These findings demonstrate that the ability of NaOH to extract Al-P will be reduced with an increase in Al-P content and that NaOH is unable to remove Al-P completely. Therefore, it seems that more research is needed to find some light on whether NH4F can be omitted from the regular P fractionation schemes in the highly calcareous soils of Iran.

Keywords

• fractionation
• phosphorus
• chemical forms
• strategic crops