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tapla wrote: Some plants have toxic reactions to various elements at lower levels than most other plants. Spider plants (and many others in the families Liliaceae [and Marantaceae]) just happen to tolerate compounds of fluorine poorly and react negatively to even what we would consider relatively low concentrations. E.g., drinking water commonly has 1 ppm fluoride added, and 1 ppm in irrigation water is eventually enough to cause marginal and tip necrosis. Add to your tap water another heavy source of fluorine like single superphosphate, diammonium phosphate, triple superphosphate, or even resin coated slow release fertilizers, and you can almost be assured of appearance problems.
I should clear something up on the perlite thing. You CAN leach perlite of a good part of the fluorine-containing compounds if you rinse it thoroughly several times before you incorporate it into soils. Negative reactions from perlite are most often seen soon after potting or repotting - before your irrigating has had a chance to leach the fluorides from the soil.
Highly aerated soils are best. For all my houseplants, I use a mix of:
by volume
1 part screened (1/8-1/4"), uncomposted pine or fir bark
1 part screened Turface or NAPA floor-dry
1 part crushed granite (grower grit or #2 cherrystone)
gypsum
See following picture. Don't be alarmed that it looks like gravel. It works wonderfully. You get a highly aerated soil that lasts practically forever. The price you pay is the effort to find the ingredients to make it yourself and the effort to water more frequently.
I think a fertilizer with a 3:1:2 RATIO (24-8-16, 12-4-8, and 9-3-6 are examples of 3:1:2 ratio fertilizers) is best (for spiders and 95% of your other houseplants) because it comes closest (almost exactly) to supplying nutrients at the same ratio in which plants use them. This allows you to keep soluble salt levels at their lowest possible concentration while guarding against deficiencies.
Since Fl availability increases as pH decreases, we would probably want to stay away from urea-based fertilizers like MG. Dyna-Gro makes a fertilizer called 'Foliage-Pro' 9-3-6. It derives most of it's N from nitrate sources rather than urea, and it contains ALL the essential elements (most soluble fertilizers lack both Ca and Mg, and often other essential elements). I highly recommend it as your all-purpose fertilizer.
So again:
* Get your watering procedure down pat - don't over-water - you'd have to work very hard at over-watering plants in the soil I described above.
* Always fertilize and water with an eye toward maintaining the soluble salts level in the soil at the lowest level that will prevent deficiencies. Spiders don't need high fertility levels, so low doses of the fertilizer I suggested will go a looonng way toward keeping salts low
