Beginner Gardening: Should I repot my Desert Rose?, 1 by tapla

tapla wrote:
The numbers on the fertilizer packages represent the percentage by weight, of NPK in that particular product (almost - I'll come back to that in a sec), but the %s are actually much less important than the ratio. 24-8-16, 12-4-8, and 9-3-6 are all examples of fertilizers that have the same ratio, 3:1:2. Notice I write the ratio using colons and the %s using dashes. 24-8-16 has 24% nitrogen and 12-4-8 has half that, as well as half the P and K of 24-8-16, yet these two fertilizers are exactly interchangeable by doubling the dose of 12-4-8 or halving the dose of 24-8-16.

The reason ratio is an important consideration is, all plants use the same nutrients in almost the same ratio. There is very little variance in the ratio in which plants use nutrients from species to species or genus to genus; some plants just use MORE nutrients than others.

I said that the NPK #s listed on the package almost show the % of NPK because they're quite far from accurate. That's only important when I connect it to what comes next. P is reported as the % of P2O5 (phosphorous pentoxide) and K is reported as the % of K2O (potassium oxide). To get the ACTUAL amount of P in the fertilizer, you have to multiply the % of P by .43, and for K, the factor is .83. The connection occurs when we consider that the average usage of NPK for all plants is actually about 10:1.5:7. After all the factoring and dividing is done, we come up with the fact that 3:1:2 ratio fertilizers supply NPK in almost exactly the same ratio as the average that all plants use.

Bloombooster fertilizers supply far, far more P than containerized plants could EVER hope to use in relation to N. Even 1:1:1 ratio fertilizers like 20-20-20 are high P fertilizers because based on N usage (and almost ALL fertilizer regimens are decided as a function of N) a fertilizer only needs .17 part of P for every part of N. 1:1:1 ratio fertilizers actually supply .43 parts, or about 2.5 times more P than the plant will/can use for normal growth. You can't MAKE a plant take up more P than it wants/needs, and excess P in the soil is not a benefit; rather, the excess is a limiting factor and plays all sorts of unwanted tricks on soil chemistry.

Anyway - If I suggest a 3:1:2 RATIO fertilizer, you have lots of options, but if I suggest 24-8-16, you're limited. If I suggest 9-3-6, you're more limited still, because I think Dyna-Gro is the only entity that manufactures fertilizer in those %s. I've posted hundreds of pictures of my plants here & @ other forum sites, and you would be hard pressed to find anything but perfect foliage/or/blooms on any of them. I use the 9-3-6 on everything I grow, from the most valuable bonsai to the tiny little stuff I just have fun with (see pic).

Sorry to go so deep into the answer. I know all the numbers make it look technical, but it's actually a pretty easy concept to understand if you think about it a little. Thanks for asking. ;o)

Al



This message was edited Mar 15, 2011 4:33 PM