On what surfaces is phosphorus absorbed in acidic soils?

Phosphorus is primarily absorbed on the surfaces of aluminum (Al) and iron (Fe) oxides as well as clay minerals in acidic soils. In such environments, the solubility of phosphorus is affected by the presence of these oxides, which have a high affinity for phosphorus ions. The acidic conditions lead to a higher concentration of Al and Fe ions, which bind with phosphorus, making it less available to plants. Additionally, clay minerals also provide surface area for the adsorption of phosphorus, further enhancing its retention in the soil.

Other options do not represent the primary adsorption mechanisms in acidic soils. For example, while organic matter plays a role in nutrient retention overall, it is not the primary surface for phosphorus in acidic conditions. Sand, having very little surface area and poor nutrient retention capabilities, does not effectively absorb phosphorus. Lastly, soil particles and gravel generally do not contribute significantly to phosphorus adsorption compared to Al/Fe oxides and clay minerals, particularly in acidic soils where those oxides are more prevalent. Therefore, the choice reflecting Al/Fe oxides and clay minerals accurately captures the critical interactions that take place in these soil conditions.