Organic fertilizer N-P-K chart
The analysis on the bag is not what the plant gets this season. This table pairs each amendment's N-P-K with the fraction of its nitrogen that actually mineralises in year one.
Organic amendments: N-P-K and what is actually available
An organic amendment's analysis is not what the plant gets this year. Nitrogen in blood meal is almost immediately available; nitrogen in compost is mostly not. Reading the N-P-K alone is the reason organic beds get systematically under-fed while looking well amended.
| Amendment | N-P-K | First-season N | lb per 1 lb available N | Notes |
|---|---|---|---|---|
| Finished compost | 1-1-1 | 15% | 666.7 | Values swing widely with feedstock. Builds structure more than it feeds. |
| Composted cattle manure | 1-1-1 | 35% | 285.7 | Mild and bulky. Safe to use heavily. |
| Composted poultry manure | 3-2-2 | 55% | 60.6 | Much stronger than cattle. Easy to over-apply phosphate with. |
| Blood meal | 12-0-0 | 90% | 9.3 | Fast nitrogen, no phosphate or potash. Burns at high rates. |
| Feather meal | 12-0-0 | 60% | 13.9 | Same nitrogen as blood meal, released far more slowly. |
| Bone meal | 3-15-0 | 50% | 66.7 | A phosphate source. Only useful where the report shows P low. |
| Alfalfa meal | 3-1-2 | 50% | 66.7 | Gentle and balanced. |
| Cottonseed meal | 6-2-1 | 60% | 27.8 | Slightly acidifying — useful on beds running alkaline. |
| Fish emulsion | 5-1-1 | 90% | 22.2 | Fast and liquid. A side-dressing, not a base dressing. |
| Kelp meal | 1-0-2 | 40% | 250.0 | Bought for micronutrients rather than for N-P-K. |
| Worm castings | 1-0-0 | 30% | 333.3 | A soil conditioner. Weak as a nitrogen source. |
| Greensand | 0-0-3 | 10% | — | Very slow potash. Years, not a season. |
First-season availability is the fraction of the nitrogen that mineralises in year one. The rest is not lost — it releases over following seasons.
Why the fourth column is the one to use
Take blood meal at 12-0-0 and 90% first-season availability: 100 ÷ (12 × 0.90) ≈ 9.3 lb to deliver 1 lb of available nitrogen. Finished compost at 1-1-1 and 15% availability needs 100 ÷ (1 × 0.15) ≈ 667 lb for the same pound — around a third of a cubic yard. Both are legitimate; they are simply not the same tool. Compost is bought for structure and CEC, and it feeds slowly as a side effect.
The phosphate trap
Manures and poultry litter carry phosphate along with the nitrogen, in a ratio plants do not use. Feeding a bed to its nitrogen requirement with composted poultry manure over-delivers phosphorus year after year, and excess phosphorus is both a lock-up problem in the bed and a runoff problem outside it. If the report already rates P as high, take the nitrogen from something that carries none — blood meal, feather meal or fish emulsion.
Fast, slow, and why it matters
Fish emulsion and blood meal act within days to weeks — side-dressings for a crop that is already growing. Feather meal, alfalfa and composted manures release over a season, which suits a base dressing worked in before planting. Greensand is measured in years; buying it for this season's potassium is buying the wrong thing.
FAQ
What is the N-P-K of compost?
Around 1-1-1, and only about 15% of that nitrogen becomes available in the first season — so roughly 667 lb of compost carries one pound of available nitrogen. Compost is worth using for structure, organic matter and CEC; it is a poor way to hit a nitrogen recommendation.
Which organic fertilizer has the most nitrogen?
Blood meal and feather meal both run about 12-0-0. The difference is speed: roughly 90% of blood meal's nitrogen is available in the first season against about 60% for feather meal, so blood meal acts in weeks and feather meal across a season.
Is bone meal a good general fertilizer?
No — it is a phosphate source, around 3-15-0. Use it only when the soil test rates phosphorus low. Most home garden soils that have been amended for years are already high in phosphorus.
How much manure per square foot?
It depends on which manure and on the nitrogen recommendation, but composted cattle manure at 1-1-1 and 35% first-season availability needs roughly 286 lb per pound of available nitrogen. Poultry manure is about three times stronger and carries much more phosphate with it.
Amendments in this table
Which one you want follows from the report — the nitrogen number and the phosphorus rating between them decide it.
- Blood meal or feather meal when the report shows nitrogen low and phosphorus already high — they carry no phosphate at all.
- Bone meal only when phosphorus genuinely rates low.
- Kelp meal is bought for micronutrients, not for its N-P-K.
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Sources
Analyses and availability fractions are typical published values and swing widely with feedstock — composted manures especially. Treat them as planning figures, not as a guarantee.