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I'm going into my second year starting seeds in soil blocks, and this year I'm switching from a prepackaged fertilizer mix to making my own. I'm going largely from Elliot Coleman's original soil block recipe, BUT I learned when ordering the fertilizer that the colloidal soft rock phosphate that is available now has a higher concentration of "available" (some places refer to this as "soluble", which makes sense) phosphate than what was common in the past—it's labeled 0-7-0 instead of 0-3-0. But the total phosphate is the same, around 20%. (I got mine online—the company made a blog post about the transition. Fedco Seeds also says, "This product is a better value than Calphos, which we carried for years: the price per ton is just slightly higher but Fertoz products offer twice the level of soluble Phosphorus". So that kinda implies that they think you'll use half as much.) I have no real knowledge or experience but what I've read suggests the remaining phosphorus does matter, at least some of it does/can become available on a meaningful timescale (i.e. months not years), however probably not all.

So the question is, do I use the same amount of rock phosphate, because the total amount of phosphorus is the same? Or does this risk damaging my seedlings from excess (available) phosphorus, and I should use about half as much? Or somewhere in between? I know some people substitute bone meal, which is generally 10+% P (e.g. 3-15-0, 2-12-0, etc), and I assume that's also available phosphorus (shouldn't those NPK numbers be an apples-to-apples comparison to at least some degree?). I don't know what if any other/insoluble phosphorus it generally contains. So that suggests that maybe more available phosphorus is fine. FWIW too I've had issues in past years that seemed to be a matter of not enough phosphorus after a few weeks of growth—I've needed to add e.g. fish emulsion at that point to prevent the seedlings from stalling out. So again, that suggests the full amount is probably fine. I just really don't want to make up a big batch of soil blocks and then find that my seedlings are experiencing phosphorus toxicity.

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    Aren't you overthinking? Seeds and seedlings on first weeks don't need any fertilizers, and later you want to give them just soil (not much fertilizers). Or do you have a special cultivation? Note: seedlings should growth roots so let's them to do the work. Consider also the volume of the a seedling and a plant. Commented Mar 26 at 5:54
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    You're right that seeds don't need fertilizer to germinate, but as soon as the first true leaves start to appear they begin to need nutrients. I apply fertilizer by adding it to the water, but I often don't need to water until after this point, and I don't know how well the fertilizer diffuses all the way through the block when bottom-watering—only an issue at this early stage. Seedlings stay in the blocks until transplant into the ground—this is standard practice for all types of indoor seed starting. Not sure I understand the rest of your comment. Commented Mar 26 at 16:15
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    The seed will provide nutrients for the cotyledon and usually also the first leaves, and in any case for the first phase soil should have enough nutrients. Fertilizers on seedlings often causes more damages then good (remember that too much fertilizers are toxic to plants, or they can block other nutrients to be absorbed). Let's the seedlings try to find nutrients (they need for next phase), and so to build strong roots which you need when you transplant them. Commented Mar 26 at 16:25
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    Do no trust fertilizer industry (which are just selling waste of chemical industry processes), and now they are also selling small quantities at unacceptable prices (compared the price you get for real fertilizers on huge bags). Apply fertilizer: if problems are visible or with soil analysis (so not on seedlings). Again: over-fertilizing have often same symptoms as lack of fertilizers, but much more difficult to correct (and to interpret) Commented Mar 26 at 16:28
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    @GiacomoCatenazzi I know where you're coming from as far as avoiding over-fertilizing. My use of fertilizer with seedlings in the past has been based on problems being visible as you say—stalled growth and reddening leaves. In my and many others' experience the tiny amount of soil present in a block or cell tray does not contain enough nutrients even if planting the seed in a bed of similar soil might. I ask this question precisely to avoid over-fertilizing and the resultant toxicity you refer to. The point about encouraging the roots to stretch for nutrients is a good one though. Commented Mar 26 at 22:51

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Before we can answer, we may need some terminology and how fertilizers are used.

Let's start with fields (agriculture), which was the initial push for chemical fertilizers (note: it works also with natural fertilizers, like straws, manure, ash, etc., just note that in such case it is more difficult to get the NPK numbers, some labels requires to measure it for each deliverable).

We start with the required nutrients per year for a specific crops (according norms), and removing what already is available in the soil (there are some formula which take into account reserve and soluble soil values, but also acidity and other factor which may slow accumulation of nutrients). So we have a number, e.g. Kg/ha (Kilogram per 10_000 m2) per nutrient.

Then, according the fertilizer numbers (as percentage value) and the size of the package, we can estimate how much fertilizer we must buy.

So: on your question: a 0-7-0 has more the double of P as a 0-3-0, so you need less than an half of weight or volume. Just note that it seems some fertilizers on shops are not very accurate (or better they are creative on the numbers or just invented: the small text may give some clues), or the numbers are to be taken if after dilution.

But the main question is about available vs total.

Available or soluble is the amount roots can get quickly, if there is enough water in the soil. For seedlings and usually for annual plants, it is the only data you care.

The total is the available content on long term. Usually the organisms in the soil (bacteria, fungi, worms, etc.) will slowly transform it into soluble nutrients, but some (especially the fertilizers in pots, may just use humidity to decompose, and they decompose also quicker then usual fertilizers, so no good to store).

But this part is also the most tricky part. One should check the chemical composition (which should be described in the packing) and looking e.g. online (there are not many common molecules per nutrient): some may be quicker to be released, and some slower (and also it depends on acidity and temperature). But such research is also needed to check about acidity of the fertilizer (not to worsen the soil). For crops, one know (by asking shops and other people) what it the better compound for the soil of the region. In this case we are speaking of 2 to 5 or 10 years (and some may release nearly nothing on first months or the first year).

So we uses the fertilizers with small available part to improve soil on long term (and by fertilizing one every 2 to 5 years, without risking to over-fertilize).

Because it is tricky, I do not recommend to use such long term fertilizers, but for crops and possibly before starting a new lawn or garden, to improve soil before doing any work. Soluble fertilizers are much easier to handle, but with the caution: it is also much easier to over-fertilize: a quadruple dose on monthly basis is not the same as a normal weekly dose, e.g. for pots. Note for vegetables and flowers on beds and gardens you do not need to fertilize so frequent (but do not over-dose): surface is large, and root will expand so they do not get the entire fertilizer in one shot. Note: manure, straws, etc. are on such category, so you may notice that the effect is lower on first (and maybe second) year.

Then there is a sort of third category: liquid fertilizers (for leaves): these fertilizers are also quicker then soluble fertilizers, so good to use in case of emergency or in case of sandy soil (so also on some pots).

So for seedlings: use just soluble fertilizers (liquid fertilizers for leaves may be difficult to dose). But remember that usually seeds includes fertilizer for early phases (but orchids and other seeds), and soil should provide enough for early phases. On seedling often we should push for roots, not for the green parts until transplanting). Non soluble part is not used: it will take months to be released, and probably your soil is not (much) active (common on soil for seedling).

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