Organic amendment addition distinctly influences phosphorus solubility in tropical volcanic soils with different mineralogical properties

The abundance of short-range order (SRO) iron and aluminum minerals with high phosphorus (P) binding capacity makes this essential nutrient a major limiting factor for agriculture in tropical volcanic soils. These reactive minerals also possess a significant capacity to form organo-mineral associations, thereby contributing to stabilizing soil organic carbon. Adding organic amendments can enhance P use efficiency in volcanic soils due to the competitive effect of organic ligands with P for the adsorption on the surfaces of SRO minerals. Here, using two tropical volcanic soils from Costa Rica with low (LR) and high (HR) mineral reactivity, we assessed the effect of short-term incubations with compost at various rates (0%, 5%, and 20% m/m) on the solubility of different P fractions (measured as P-Olsen and P-CaCl₂). Additionally, we determined P adsorption isotherms to evaluate changes in the adsorption behavior of this nutrient. Furthermore, in a bioassay using Sorghum bicolor as a model plant, we investigated the potential influence of compost addition on the agronomic efficiency of P fertilizers in these volcanic soils.

The results of the incubation experiment showed that the addition of organic matter mostly affected the desorption of the P pool related to the capacity (Q) factor of soils (measured as P-Olsen), whereas the P intensity factor (I) (measured as P-CaCl₂) remained mostly unchanged. The I factor was significantly increased only at the highest rate of compost addition in the LR soil. Description of the adsorption isotherms using the Langmuir equation revealed changes in the adsorption behavior of P due to the addition of compost. The maximum P adsorption capacity (Q_max) of the soils decreased as the amount of added organic amendment increased, particularly in the HR soil. The binding affinity of P (K_L) to the mineral surfaces was also reduced due to the compost addition, and this effect was more pronounced in the HR soil. Lastly, higher agronomic efficiencies of P fertilizers were measured when compost was incubated in the HR soil, whereas in the LR soil the agronomic efficiencies of P fertilizers were unaffected by the compost addition. This study contributes to unraveling how the competitive interaction between organic ligands and P is modulated by the mineralogical properties of volcanic soils, particularly due to the reactivity of SRO minerals. Overall, our results indicate that the addition of organic amendments can be an effective alternative to improve P availability in soils with abundance of SRO minerals. From a broader perspective, adding organic amendments to soils with high P binding capacity would result in a “win-win” situation that contributes to improving the use efficiency of this limited resource while promoting the C storage in soils.