Hi all. I have been experimenting with a soil mixture to suit the challenging conditions of the Tropics. I have been using the standard shop bought Searls cactus mix for years with varying degrees of success. I have found that this mixture tends to be very good in the first few months then rapidly declines in quality there after. One of the most noticeable issues I have with this mix is that after a long dry spell the soil needs to be soaked for 24 hours to re-hydrate it. This is very time consuming and a little tricky to get right. Especially for cacti with large tuberous roots. I.e. Lophs, arios, astros etc. Also, due to the extended soaking times I have found that plant mortality rates increase. This is particularly true around December - January when temps are a steady 38 degrees with 95% humidity all day everyday.
My mix is as follows:
30 litres premium potting mix. Screen out all large organic chunks with a half inch screen
20 litres quincan gravel 10mm screened. This is a fantastic volcanic scoria gravel, mined from the side of Mount Quincan. One of our local scoria volcanoes. It has some very interesting properties. It holds and slowly releases moisture without making the surrounding media wet.
10-20 litres of coarse sand
5-10 litres of brickies loam.
My observations so far is that this mix is exceptionally well drained and dries out quickly. However, the quincan gravel tends to retain moisture and act as small moisture reservoirs in the soil. This results in a media that remains loose and dry but still sustains growth between waterings. The picture below is the mix I use for the Astro's. It has a higher organic percentage as astros grow rapidly and can handle more feeding. I add a heap more sand and gravel to the mix if I am planting ariocarpus or copiapoa as these plants are highly susceptible to root rot. I have re-potted almost 3/4 of my entire collection in varying combinations of these ingredients with only 1 fatality. A small astro seedling that was sick and damaged anyway.