Hi, I wanted to chime in and mention that it is absolutely true that scoria is different from pumice--and each acts differently in soil mixes. I can't say if scoria is useful in bulb mixes; I have only tried it on a limited basis. But, I sure do like using it in soil mixes for cacti and succulents where I find if very useful in this wet climate (greater Houston area). I attribute the benefit to the fact that scoria has big bubbles and air pockets (1/16 inch or 1/8 inch, and sometimes larger). I find that by using scoria, I can create a lot of underground air pockets, which seem to be as important as fast drainage in this often humid and rainy climate. I've got 2 bulbous plants growing in mixes of 30% scoria, 30% coarse sand, and 30% perlite (10% humus). The 2 species are Zephyranthes chlorosolen (from very sandy/rocky soil in south Texas, and Alophia drummondii from the same location. Both plants do very well, as well as they do in mixes that are 50% humus and 50% coarse sand. It was just an accident these 2 species came to be in the "cactus mix" and I don't think I'll plant more. Humus and sand are much less expensive materials for soil mixes than is scoria. Conroe Joe Clear and Sunny here, high temp was 65 and low is predicted around 50 F (18 C and 10 C). Fall has arrived finally.