Bioavailability and mechanism of modified rock phosphate in calcareous soil

Phosphorus is among the most plant essential nutrient elements. High yields and good qualities of crops are efficiently promoted by the application of plenty soluble phosphorus fertilizers. Utilization efficiency of soluble phosphorus fertilizers, however, remains generally low because of its unique chemical properties. This has posed a significant challenge for natural resources of phosphorus and rock phosphate. Furthermore, the use of rock phosphate resources as high concentration phosphorus and compound fertilizers was limited in China. It was therefore important to develop the most efficient utilization technique for the sustainable usage of rock phosphate resource with meaningful economic benefits of phosphorus fertilizer incorporation. Rock phosphate activation to release efficient, non-acid-driven phosphorus and utilizable byproducts in industries and agriculture was one such technique. Although few current studies conducted in acid soils indicated high bioavailability of new phosphorus fertilizers derived from activated rock phosphorus (the so-called modified rock phosphate), less studied was conducted in calcareous soils. This paper explored the availability of modified rock phosphate in calcareous soils of North China and the possible mechanisms of activation effects. The findings of the study could enhance industrial production and application of modified rock phosphate in different soil conditions. In this paper, rock phosphates from different producing areas Kunming City of Yunnan Province (RPⅠ), Deyang City of Sichuan Province (RPⅡ), Mianzhu City of Sichuan Province (RPⅢ), Kaiyang City of Guizhou Province (RPⅣ) were activated by both organic and inorganic activators to get modified rock phosphate. The P bioavailability of the modified rock phosphates in calcareous soils were also tested for spring wheat in pot experiments. Compared with the rock phosphates, the results showed improvements in available and soluble P in modified rock phosphates. Also while the use of inorganic activator improved available P, organic activator largely improved soluble P. Modified rock phosphate improved dry matter content, and P uptake and utilization efficiency of spring wheat in calcareous soils. Modifying rock phosphates with organic activator had more significant effect on spring wheat growth. Measured structures using the Infrared Spectrogram (IS) technique showed significant changes in rock phosphate structures after activation. Compared with normal rock phosphates, increased characteristic H2PO4- and HPO42- spectrograms were noted. The increasing effects varied among rock phosphates producing areas and activator types. This indicated that activation promoted release of available P in rock phosphates. Rock phosphates from different areas were different in chemical components such as phosphorus, calcium, iron, aluminum and magnesium. Chemical measurements showed that activation effects were related with rock phosphate oxide content. In the four rock phosphates, RPIII showed the best activation effect. MgO content of RPIII was the highest and total oxide content was the lowest. This was the possible reason behind the best effect produced by RPIII — i.e., the higher MgO content induced a higher solubility of rock phosphates. Different activators exhibited different activation effects even for the same rock phosphate. Although IS structure measurement suggested that inorganic activator was better, soil plant experiments showed that organic activator was better. Due to these contrasting results, further studies were recommended in this direction.