INFLUENCE OF ORGANIC MATTER ON PHOSPHORUS AVAILABILITY IN SOILS OF KANSAS, WITH CONTRASTING LAND USE PATTERNS

Understanding mobility of soil phosphorus (P) is important in agriculture. Soil organic matter (SOM) plays a vital role in controlling P mobility. In this study we characterized inorganic and organic properties of soil samples from two regions in Kansas, Konza Prairie Long Term Ecological Research (Konza LTER) and Hays. Soils were collected from ~20 cm depth from the surface after clearing debris. The soils were dried at 60˚C for 2 hours and crushed to <0.2 mm particle size. The soil pH was measured in 1:1 slurry with deionized water. pH of Konza soils was found to be 7.0±0.6, while that of Hays was 8.1±0.4. Water-leaching experiment was done by mixing soil in deionized water for 24 hours in 1:10 w/w ratio, and presence of ions and water extractable SOM were measured in the extracts for the soils from both the locations. Results indicated that these soils contained 24-hour water-leachable concentrations of PO₄^3- as 0.6±1.1 mg/kg for Konza soils and 2.4±3.1 mg/kg for Hays soils respectively. The major cations to be found from Konza soils were Ca²⁺ K⁺ and Mg²⁺, while the major anions were NO₃^- and Cl^-. The concentrations of total soil P measured by Bray extraction methods, found to be 234±205 and 492±214 mg/kg from Konza and Hays soils respectively. Spectroscopic analyses of the water-extractable soil organic matter revealed presence of highly humic-like, aromatic, and plant-derived SOM, however, the humification in Hays soils was greater than the soils from Konza as indicated by humification index (HIX) of 7.5±0.3 and 8.9±0.1 respectively. The results confirm that the soils from both the locations are calcareous in nature leaching higher amounts of Ca²⁺ and Mg²⁺ upon reaction with water. The dissolution of P-bearing minerals like apatite are the primary source of water-leachable PO₄^3- in these calcareous soils. There are significant differences between the total P and water leachable PO₄^3- ions for the soils from the two locations. The difference in humification of the SOM might be an important factor since presence of humic-like SOM interferes with PO₄^3- adsorption on iron (Fe) and aluminum (Al) oxides via competitive sorption mobilizing PO₄^3- to soil water. Another major factor that may have contributed to the difference could be the difference in land use in these areas where Konza is a natural grassland with no human intervention, but Hays has an agriculturally influenced soil in the area.