I have two fertilisers: Chempak bonsai fertiliser (10-30-30) and potassium chloride (0-0-60). If I mix two parts of the chempak to one part of the KCl, does that give me a 6.7-20-40 (1:3:6) fertiliser?

Better still, are there any ready-made fertilisers that are low in N but have P and K in a 1:2 ratio? I'm growing cacti in British gloom, and witholding N while providing all other nutrients seems to be the best way of growing something that's tough as boots and nicely-spined.

This might also be useful for a number of the 'touchy' species. Some cacti can be grown as lime beachballs, but others will simply roll over and die if they're allowed to luxuriate (some of the desert rats do this even when grown in their native climate. No-one's figured that one out yet). I'm sure some growers have tried the everything-but-N approach, but I've found very little information on the composition of each grower's fertiliser. Pots and mixes, those topics can run to several pages, but nobody really talks about what they feed the plants once they're homed. Perhaps it's because the dominant mix online is based on John Innes and grit, and therefore comes with its own supply of fertiliser.

There's an interesting hexalogue linked at the bottom of the post that gives the kind of information I'm interested in. Not only does it extol the virtues of a coarse, mineral mix in which all ingredients have their use, it suggests the use of mineral ingredients that by themselves sustain the plant for much of the growing year.

So... thoughts? Sorry if this is a bit rambly, I'm better when I have a topic to aim at.

Here is a link that might be useful: Article by Dag Panco de Grid