The availability of P is often insufficient and limited by accumulation in soils. This led to the necessity of solutions for the recovery as well as recycling of secondary P resources. Batch experiments were conducted with CaCl2 and citric acid to characterize P release kinetics from vivianite, hydroxyapatite, and bone char. While the P release during the CaCl2 treatment was so low that only vivianite and hydroxyapatite showed a slightly higher release with increasing CaCl2 concentration, the increase of dissolved P was more pronounced for citric acid. The application of citric acid resulted in a 32,190-fold higher P release for bone char. Fourier-transform infrared spectroscopic data suggested higher instability of hydroxyapatite than for bone char. The kinetic data showed that bone char, especially at a lower particle size, had a higher long-term P release than hydroxyapatite or vivianite. The suitability of hydroxyapatite and bone char as a poorly soluble, but sustainable P source is better than that of vivianite. However, the efficiency as a P fertilizer is also dependent on present soil P mobilization processes. The results underline the importance of the accessibility of fertilized or naturally bound P for plant roots to benefit from the excretion of organic acids.

Phosphorus (P) fixation is a global problem for soil fertility and negatively impacts agricultural productivity. This study characterizes P desorption of already fixed P by using KCl, KNO3, histidine, and malic acid as inorganic and organic compounds, which are quite common in soil. Goethite, gibbsite, and ferrihydrite, as well as hydroxide mixtures with varying Fe- and Al-ratio were selected as model substances of crystalline and amorphous Fe- and Al-hydroxides. Especially two- and multi-component hydroxide systems are common in soils, but they have barely been included in desorption studies. Goethite showed the highest desorption in the range from 70.4 to 81.0%, followed by gibbsite with values in the range from 50.7 to 42.6%. Ferrihydrite had distinctive lower desorption in the range from 11.8 to 1.9%. Within the group of the amorphous Fe-Al-hydroxide mixtures, P desorption was lowest at the balanced mixture ratio for 1 Fe: 1 Al, increased either with increasing Fe or Al amount. Precipitation and steric effects were concluded to be important influencing factors. More P was released by crystalline Fe-hydroxides, and Al-hydroxides of varying crystallinity, but desorption using histidine and malic acid did not substantially influence P desorption compared to inorganic constituents.

In acidic soils, phosphorus availability is affected by its strong affinity for mineral surfaces, especially Fe‐ and Al‐hydroxides. Plant roots have developed adaptive strategies to enhance the availability of phosphorus, including producing and exuding low molecular weight organic acids with a high affinity for phosphorus that competes with high molecular weight organic ligands formed during humification and mineralization. The aim of this study was to characterize the kinetics and mechanism of phosphorus desorption from Fe‐ and Al‐hydroxides of variable crystallinity, as well as binary Fe:Al‐hydroxide mixtures. Long‐term desorption experiments (56 days) were conducted with CaCl2, CaSO4, citric acid, and humic acid as competitive sorptives. CaCl2 and CaSO4 were selected as general inorganic sorptives and citric and humic acids were selected as organic ligands produced by organisms in the rhizosphere or following humification. The cumulative phosphorus desorption increased following the order CaCl2 < CaSO4 < humic acid < citric acid. Amorphous ferrihydrite and Fe‐rich Fe:Al‐hydroxides exhibited much less desorption when exposed to inorganic solutions than the crystalline and Al‐rich Fe:Al‐hydroxide mixtures. Models of the desorption data suggest phosphorus desorption with citric acid is diffusion‐controlled for ferrihydrite and Fe‐rich amorphous Fe:Al‐hydroxides. When humic acid was the sorptive, metal‐organic complexes accumulated in the solution. The results suggest organic compounds, especially citric acid, are more important for liberating phosphorus from Fe‐ and Al‐minerals than inorganic ions present in the soil solution.