Phosphorus buffering capacity of substrate clays and its significance for plant cultivation

Abstract

Abstract Peat is commonly used as the main component of horticultural substrates, but it has a very low buffering capacity for the anionic macronutrient phosphorus (P), which can be increased by the addition of clays. The aim of this study was to characterize the P adsorption capacity of different substrate clays and to evaluate its significance for plant P uptake. Substrate clays were characterized with a single‐point batch experiment and adsorption and desorption isotherms. The data were fitted to the Langmuir equation for a calculation of the maximum adsorption capacity. Additionally, the contents of oxalate extractable Fe and Al (Σ Fe ox + Al ox ) were determined. The influence of a varying P adsorption capacity of the clays on the P availability to plants in the respective peat–clay substrates and pure peat was investigated in a growth experiment with Impatiens walleriana fertigated with 0, 17, and 35 mg P L −1 solution, respectively. The observed and calculated (Langmuir) P adsorption capacity of the clays could be well‐characterized by both the batch experiment and the adsorption isotherms and was highly correlated with the Σ Fe ox +Al ox . A higher P adsorption capacity of the clay amendment in mixed substrates resulted in a lower P concentration in the substrate solution, while the CAT extractable P concentration ( P CAT ) was the same. Plant growth and shoot P concentrations were enhanced in the substrates, showing a higher P adsorption capacity, since plants were able to take up the whole amount of P CAT , and also part of the non‐CAT extractable P. However, the release rate was too low to ensure optimal plant growth, which was in accordance with the result of the desorption experiment. The absolute extent of P release was increased with the increasing P adsorption capacity of the clays and higher degree of P saturation ( DPS ).