The most suitable clay to use is red terracotta clay, collected from a site free of weed seed. Often, digging deep will ensure that no unwanted seeds become part of the mix! The clay needs to be dried and ground finely to ensure a lump free mixture. Two bricks can serve as an effective grinder. Other clays, the blues and whites are not suitable for this process, as they contain minerals, which interfere with the growth of the seedlings.

http://en.wikipedia.org/wiki/Terra_cotta

terracotta原先在英文是磚紅色之意,字源是意大利文,指baked earth(烤過的土),就是陶製品。

赤陶 terra-cotta
一般指由粗而多孔的黏土製成的塑像和器皿等。燒成後的黏土顏色從淡赭變為紅色,通常不上釉。是最有實用價值的一種陶器,價格低廉,用途廣泛,經久耐用。