Potassium adsorption and release properties of clays in peat-based horticultural substrates for increasing the cultivation safety of plants

Clay amendments are used widely in the horticultural industry in peat-based substrates, as they have the capability to improve the potassium (K) buffering capacity. The K adsorption and desorption behavior of different clays was characterized here to develop criteria for identification of clays suitable for substrate amendment and to evaluate their significance for plant K uptake. K adsorption of saprolitic, bentonitic and relocated clays from the Westerwald area, Germany, was characterized with adsorption isotherms for calculating the maximum adsorption capacity for K. The K desorption was quantified in a percolation experiment using 0.01 M CaCl2 as extractant. The influence of a varying K adsorption capacity of the clays on the K availability to plants was investigated in a growth experiment. The availability of K in substrates was determined by 0.01 M CaCl2 (CAT), Ca-acetate-lactate and NH4 acetate. Different cultivation practices were simulated by varying the start of K topdressing: "immediate," "delayed" and "no" K topdressing. A higher K adsorption capacity of the clays amended resulted in a lower K concentration in the pore solution of clay-amended peat-based substrates, while the CAT extractable K concentration was the same. It also resulted in an enhanced plant growth and higher K concentrations in the shoot without K topdressing. The uptake of K by plants exceeded the amount of exchangeable K in some peat-clay blends where no K was fertilized. The amount of K released from the clays in the plant experiments was in the same magnitude as the maximal desorbable K determined in the percolation experiment and was highly dependent on the mineralogical composition in the order: smectitic > illitic, kaolinitic clay. The K released from the clays contributed significantly to the supply of K to plants.