Definition of NPK ratioCategorized under "General"
Definition as written by paulgrow:
This term refers to the ratio of nitrogen (N), phosphorus (P), and potassium (K) in fertilizers.
It is always expressed as N-P-K. For example, a fertilizer labeled as 24-6-6 has 24% nitrogen, 6% phosphorus, and 6% potassium
In this example, the remaining 64% of the fertilizer is comprised of inert matter
Definition as written by Terry:
Remembering which nutrient does what is easy, if you'll remember this simple phrase: "up, down, all around"
up: Nitrogen promotes top growth
down: Phosphorus promotes good roots
all around: Potassium benefits the whole plant
Definition as written by tapla:
A fertilizer's NPK ratio is established by its NPK %s, and a fertilizer's RATIO tells us something quite different than its NPK %s. 24-8-16, 12-4-8, and 9-3-6 are all popular soluble synthetic fertilizers. Their NPK %s are all very different, but they are ALL 3:1:2 RATIO fertilizers. Divide all the numbers in 24-8-16 by 8, and you come up with 3:1:2. Divide 12-4-8 by 4, and 9-3-6 by 3, and you come up with 3:1:2 RATIOs - sort of like reducing a fraction.
The fertilizer example used above with NPK %s of 24-6-6 is actually a 4:1:1 ratio fertilizer.
Plants don't use nutrients at the same rate, so why supply them at the same rate, which ASSURES excesses and/or deficiencies? Plants use roughly 6X as much N as P, and about 3/5 as much K as N. When all the calculations are done for the fact that P is actually reported (on fertilizer packaging) as P2O5 and K as K2O, we discover that the 3:1:2 ratio fertilizers supply nutrients in almost the exact ratio at which plants use them, which is no accident, and is a distinct advantage. The amount of nutrients plants use varies widely, but the ratio varies little from plant to plant. Sequoias, sunflowers, snapdragons, and sedum, all use nutrients in very close to a 10:1.5:6 ratio, which is very close to what all 3:1:2 ratio fertilizers provide (after factoring for how P and K are reported on the label).
A fertilizer's Ratio tells us much more about how appropriate a fertilizer is than its NPK %. 20-20-20 and 10-10-10 are both 1:1:1 ratio fertilizers, but the 20-20-20 has twice the amt of nutrients per given weight as 10-10-10. Similarly, 24-8-16 is twice as concentrated as 12-4-8, so you would use half as much 24-8-16 as 12-4-8 to make a solution of the same strength.
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