Does dissolved humic acid suppress orthophosphate adsorption to soils? It depends on the type of clay in the soil

Humic substances (HS) are frequently incorporated into soils to improve soil health and nutrient availability. However, existing literature presents discrepancies regarding the soil-HS interactions and their impact on orthophosphate (inorganic phosphorus (IP)) adsorption, specifically, concerning the influence of soil clay mineralogy on these interactions. This study aims to investigate whether the impact of soluble humic acid (HA) on IP adsorption in semi-arid Mediterranean soils is contingent upon their clay mineralogy. This was tested by constructing IP- adsorption isotherms in the presence and absence of HA of four illite/smectite (I/S) or kaolinitic dominated soils. The results revealed that the addition of HA increased IP adsorption to I/S soils by up to 80%, yet IP did not affect HA adsorption to these soils. This suggests that while HA and IP are adsorbed onto different sites on the I/S clay, HA adsorption probably increases the number of adsorption sites accessible for IP. In contrast, IP and HA adsorption onto the kaolinitic soil was fully competitive: IP reduced HA adsorption and vice versa. It is suggested that new adsorption sites for IP resulted from the HA interactions with the I/S clay planar surfaces. In addition, HA moieties containing cationic (ionized) nitrogen, could possibly interact with clays to increase the number of platelets per tactoid. Unlike I/S clays, kaolinite that possesses low degree of isomorphic substitution, low net negative charge, and high degree of platelets stacking, does not tend to such interactions with HA. Instead, HA will be adsorbed mostly onto the clay edges, competing with IP on adsorption sites.