Optimization of Clay Amendment for Improving the Re-Wettability of Peat-Based Growing Media

Peat is an important growing media constituent which exposes hydrophobic properties if getting dry. The addition of clay induces a coated surface with hydrophilic properties which improves the re-wettability. The formation of an effective clay coating on peat during processing depends predominantly on the amount of clay applied, clay fineness and mineral parameters. Peat surfaces consist mainly of C. When these are covered with clay minerals rich in Si, the degree of coverage can be described as the C/Si ratio, determined by energy dispersive X-ray spectroscopy. A moderately decomposed sphagnum peat was amended with four different clays from the Rhenish Massif, Germany (10, 20, and 30 kg clay/m(3)). Aggregate size varied from <0.063 to 2.0-4.0 mm and water-uptake characteristics were determined using a capillary-rise method. Water uptake was improved for all samples by amendment of clay, strongly depending on clay parameters. At C/Si ratios <20, where the surfaces of peat are most completely coated with clay minerals, all clay-peat systems showed the highest water uptake rate. Saprolitic and translocated clays, consisting mainly of illite and kaolinite, had the strongest effect on surface coverage degree. Here already 20 kg/m(3) were sufficient to reach lowest C/Si ratios indicating maximum coverage degree. These clays showed fastest water uptake (50 vol. % within 10 min), whereas bentonite showed only minor effects (1.47 to 3.63 (% v/v)/min). A translocated clay was most effective to improve the water uptake rate, admixing small clay amounts. The results show a good correlation of the C/Si ratio and re-wettability. The determination of the C/Si ratio in a relatively short procedure is a method to estimate the water uptake rate of different horticultural growing media. The combination of these methods can be used to identify suitable clays for improving re-wettability, optimum clay fineness and the amount of clay needed.