If you want the easy answer to “what fertilizer can I use for all my houseplants?” – it's 3-1-2. That means any fertilizer with multiples of 3-1-2 (or close to that) will be suitable. Any of these will work just fine for ALL your houseplants.

Most gardeners should use a complete fertilizer with twice as much phosphorus as nitrogen or potassium. An example would be 10-20-10 or 12-24-12. These fertilizers usually are easy to find. Some soils contain enough potassium for good plant growth and don't need more.

But here's the thing, the 3-1-2 NPK ratio is just that, a ratio. In other words, with the ratio of nitrogen 3 times higher than some of the other nutrients, this indicates that the fertilizer supports leaf growth in plants as opposed a fertilizer that encourages blooms and so on.

Some growers prefer to use a high-phosphorus fertilizer, indicated by a larger middle number. You can also keep things simple with a fertilizer especially formulated for tomatoes – usually with a ratio like 3-4-6 or 4-7-10. Most importantly, don't over-fertilize.