Crops treated with phosphorous (P) fertilizer raise crop yields and return-on-investment. But phosphorous, a crucial plant nutrient, is sometimes the limiting factor in crop growth. Phosphorous management in P-limited, highly weathered acidic soils or soils containing calcium carbonate (i.e., calcareous) is challenging due to rapid P sorption reactions known as “P-fixation.” Added P fertilizers are not always effective in calcareous soils, as the P precipitates out as calcium phosphate or adsorbs onto calcium carbonate. So, applying the right amount of P in the right form is challenging. The source of the phosphorous has been thought to be irrelevant, leaving the growers’ choice to price and application method preference. Often excess phosphorous is applied, resulting in environmental degradation. A better understanding of the behavior of P in soils could lead growers to select the right form of fertilizer or researchers to develop new fertilizers that could increase profits and be better for the environment. This work, carried out in part at the U.S. Department of Energy’s Advanced Photon Source (APS) and published in the Soil Science Society of America Journal, studied P partitioning in various liquid fertilizers, with different dilutions, and the presence or absence of a gelling agent in two mildly calcareous soils from Kansas. Results from controlled laboratory incubation studies in combination with synchrotron-based x-ray techniques, will help researchers and industries to revisit the use of soil-test P methods and develop new fertilizers.
To avoid P deficiency, growers may apply more fertilizer than plants take up. Surplus P washes into water bodies, where it can fertilize a massive algae bloom. Decomposition produces CO2, lowering the water’s pH. High acidity inhibits shell formation in bivalves and slows the growth of fish and other marine organisms. Decomposition also depletes the oxygen in the water, leading to eutrophication. Without oxygen, fish and other organisms cannot survive. This is exemplified by the Gulf of Mexico, where Mississippi River runoff may produce a dead zone nearly the size of New Hampshire.
There is no single solution to reducing phosphorous pollution while increasing or maintaining crop yields. The amount of P left behind by plants after fertilizer application depends on its form and the way it is applied.
The three incubation experiments in this study used the same experimental protocols, including phosphoric acid, polyphosphates, and orthophosphates. Soils were incubated for four weeks in Petri dishes and sectioned into concentric rings from the application point for analyses (Fig. 1). Resin-extractable P, sequential extraction and synchrotron-based x-ray absorption near-edge structure spectroscopy were used to probe reaction products and pathways. The spectra for two sets of samples were obtained at the X-ray Science Division Spectroscopy Group’s 9-BM-B x-ray beamline of the APS, an Office of Science user facility at Argonne National Laboratory, and at beamline 6B1-1-SXRMB of the Canadian Light Source.
This research revealed that liquid that is more dilute or contains P as polyphosphate has less chance of Ca-P precipitation. Phosphorus movement is restricted in calcareous soils, most of the P remained in the first soil section. A larger amount of solution allows the P to travel further in soil from the application site thereby reducing chances of Ca-P precipitation. In addition, P suspended in an organic gel may slow the chemical kinetics of calcium-phosphorous interaction, leaving P in the soil. In mildly calcareous soils, P fertilizers behave differently depending on source and formulation. Polyphosphates and orthophosphates, both in liquid form, partition differently. For growers, the correct selection of P source may improve P uptake efficiency allowing a reduction in total P application.
Although this was a laboratory-scale study and field verification is needed, these types of studies are essential for our understanding of the molecular-level mechanism and reaction pathways of different fertilizer sources in challenging soils. Future farmers working in calcareous soils may find that application of polyphosphates, increasing the dilution of P, or co-application of gel to be effective methods for lowering total P applications. Researchers still must determine how P partitioning changes over a growing season and from year-to-year and test the impact on actual plant uptake and P use efficiency in the field.
— Dana Desonie
See: Joseph J. Weeks, Jr., and Ganga M. Hettiarachchi*, “Source and formulation matter: New insights into phosphorus fertilizer fate and transport in mildly calcareous soils,” Soil Sci. Soc. Am. J. 84,731 (2020). DOI: 10.1002/saj2.20054
Author affiliation: Kansas State University
Correspondence: * firstname.lastname@example.org
This work was supported by the Kansas State Research and Extension, The Fluid Fertilizer Foundation, Bio Huma Netics, Inc., and the USDA National Institute of Food and Agriculture NC-1187 Multistate Research Project. The authors thank Advanced Photon Source and the Canadian Light Source Incorporated for providing the opportunity to use their synchrotron facilities; the beamline scientists Tianpin Wu, George Sterbinsky, and Lu Ma at beamline 9 BM-B of Advanced Photon Source; and Yongfeng Hu and Qunfeng Xiao at SXRMB 06B1-1 of the Canadian Light Source. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.
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