The Effect of Rock Phosphate Acidification and Vermicompost on Phosphorus Release Kinetics in a Calcareous Soil

Phosphorus (P) is one of the most important elements necessary for plant growth and production of agricultural products. In calcareous soils, phosphorus deficiency is a general issue due to high pH, high soil calcium carbonate content, lack of organic matter and moisture. Phosphorus absorption capacity depends on different soil reactions such as: adsorption, sedimentation, stabilization and release. The speed and amount of plant available P depends on the soil reactions. Studying the kinetics of P release from soil is a good indicator to check the status of P uptake by plant. The kinetics of P release in soils is a subject of importance in soil and environmental sciences. The aim of this research was to investigate the kinetics of P release and derive the most suitable equation to describe the release of P from a calcareous soil when subjected to the acidification of rock phosphate and the addition of vermicompost.

In order to investigate the ability of acidified rock phosphate and vermicompost in P release, an experiment was conducted with 2 replications on a light-textured soil with low OC and Olsen-P (1.2 mg/kg). One hundred grams air dried calcareous soil was transferred into special containers and 5 treatments including: 1- control (soil), 2- rock phosphate, 3- acidified rock phosphate (20 CC nitric acid 0.1 N and 5 g rock phosphate), 4- vermicompost, and 5- acidified vermicompost (20 CC nitric acid 0.1 N and 5 g vermicompost) were applied. The treatments incubated two weeks in 20±2℃ temperature. The Kinetics of P release was studied by adding 20 mL of 0.5N NaHCO3 to, one gram of air dried treatments. Extraction times were considered to be 0.25 h to 256 h (in 11 times) based on the time of adding the NaHCO3 extractant until filtering. After adding the extractant, the samples were shaken and centrifuged. After filtering, the concentration of released P in samples were determined by spectrophotometer (Model: CE 292 Series2, ultraviolet). For higher accuracy in the measurements, acid-washed containers were adjusted based on the amount of soil moisture which was dried in the oven (105℃). Finally, the P release data were fitted to different kinetic equations. The effect of different fertilizer treatments on P release in specified times and then kinetics parameters were investigated and compared with the control.

Addition of acidified and non acidified rock phosphate and vermicompost increased the amount and speed of P release in the calcareous soil. Six kinetic equations were fitted to describe the release of P in the period of 0.25 h to 256 h from the soil to evaluate the effect of the treatments. The highest release of P was in vermicompost and acidified rock phosphate treatment, which were an organic fertilizer and a source for preparing phosphate fertilizers. To describe the release rate, kinetic equations were used. The best equations were chosen by highest coefficient of determination (R2) and the least of standard error (SE). The zero, first, second order equations could not describe the release of P in the studied calcareous soil. The R2 value decreased from the zero to second order equation. The simplified Elovich equation described well the release of P from the soil with the average R2 of 0.79 and with the average SE of 0.4. Comparison of the average effect of the studied treatments with the control showed that the acidifed vermicompost and rock phosphate treatments increased the capacity and speed of P release compared to the control. On the other hand, acid addition has increased the capacity and speed of P release in the calcareous soil.

The findings indicated an initial rapid release of P, which then decreased over time. Notably, the application of vermicompost and the acidification of the soil with rock phosphate resulted in a pronounced and accelerated release of P. Generally, organic fertilizer treatments exhibited a higher release of P compared to chemical fertilizer treatments. This observation is in accordnce with the findings of the data presented by Ghorbanzadeh et al. (2009), who explored the P release potential of bone meal. Their data demonstrated that the acidification of bone meal accelerated and enhanced P release. To further enhance the practical relevance of these results, it is recommended to conduct this research in the presence of plants.