This.
Look on the back of whatever amendment or Down To Earth/Dr Earth amendment blend, on the back you'll notice it specifics whether something is water soluble or not.
Anything water soluble, you will lose in runoff. However, anything NOT water soluble will not be lost in runoff.
This can be a blessing or a curse. Similar to mathematics, there is a specific order of operations at play here that non-soluble ingredients follow.
Water insoluble (does not bond to water, and therefore does not
flow with water. even in runoff) ingredients will typically always follow this same "order of operations";
1) Your plant absorbs it.
2) Your soil absorbs it.
3) It becomes "salt" in your substrate and causes issues.
Lets go with an example that demonstrates why I always rant about multiple top dresses of "light" ingredients as opposed to using "hot" ingredients.
You use Seabird Guano (extremely hot @ 0-11-0)
Your plant only needs 2 Phosphorus, so there is still 9 left to deal with. Your soil (via CEC as
@myke pointed out) can only absorb 5-6 of it.
So, the soil absorbs 6, your plant consumes 2, but: 11 - 2 - 6 = 3. There is now a surplus of 3 Phosphorus from your Seabird guano that is still "stuck" in your soil.
It is "stuck" because it is water INsoluble, it will NOT runoff, it will NOT bond to your soil, and the plant will NOT absorb it. The result? It becomes a salt.
You've fed this 0-11-0 ingredient to your soil, however the plant can only process 2 of it, and your soil can only store 6 of it, so the rest is leftover in your soil medium.
This is exactly why I opt in favor of "lighter" ingredients as opposed to "hotter" ingredients, not because I'm anything special, but because it provides great control over things.
Lets take the same example from above, but with something like Dr Earth's 4-6-5 blend.
Your plant only needs 2 Phosphorus, now there is 4 Phosphorus leftoever. However, your soil can absorb 5-6 of the "excess" Phosphorus. So, 6P right? 2 out of 6 goes directly to the plant and the other 4 goes straight to the soil. NOTHING becomes "salts" at that point, because it is all accounted for. Whatever the plant doesn't absorb, the soil does, and there isn't any excess after that. The result is that you NEVER have salt buildup, and if for some reason you don't top dress one week (or the top dress wasn't "big" enough) the soil has plenty stored up in its "pantry" so to speak.
The end result is that your plant gets fed, the soil gets fed, but there is ZERO salt build up.
This is why Coots (and other gurus) have such a hard on for using "light" NPK amendments in favor of "hot" amendments.
By using light amendments, you have significantly more room for error because you can always add more if you need.
You can NEVER subtract, but you can ALWAYS add. Much better to have too little than too much.
Any of you guys drink alcohol? What causes more problems? Too much alcohol, or too little? And how "thin" is the line between too much and too little? Same goes with nutrition, and even hydration. Plenty of instances of people dying from excess water in fact, but that certainly doesn't mean we should stop drinking water. Right?
You know why I love the "salt buildup" example so much? It is exactly like cooking! Salt is immensely useful in processing food ingredients and bringing out/extracting their flavor. However, using too much salt will quite literally result in a "salt buildup" that makes your food taste like shit.
Treat your plant/soil the same way you treat your food! "Salt it" prior to cooking, and "salt it" during cooking, but do so in a way that doesn't result in "salt buildup". Any of you who've added too much salt to a meal will know everything about "salt buildup". Plants experience that too.
With organics, we don't need high NPK ratios because that will lead to salt build ups that will fuck with our pH, among other issues.
Consider a human trying to gain protein. We want to bulk up and gain mass, right? So protein. Again, let us consider two examples.
Example 1) Go to McDonalds and get a McDouble. 22g of protein, 80mg Calcium, 282mg Potassium, but 920mg sodium and 70mg Cholesterol that we do NOT need.
Now, lets analyze a simple glass of milk.
8g protein, 280mg Ca, 300mg Potassium, and minimal sodium/cholesterol content.
A single glass of milk has better nutritional content than that of a McDouble, and yet most of us go for the McDouble over the milk everytime. Especially me, I love my McGangbangs (McChicken placed in between a McDouble. Still very much a guilty pleasure of mine, no shame).
Plants are no different than humans; we need water, shade/shelter, and nourishment. Consistent water supply, and source of nourishment is what makes the difference, whether its plants or people.
As always, forgive the rant and all the best.