08/03/2026
Nice write up explaining what these numbers mean!
All About Fertilizer
Three of the most important nutrients for plant growth are nitrogen 👎, phosphorus (P) and potassium (K).
The NPK numbers listed on all commercial fertilizers represent the percentages of these macro-nutrients.
For example, fertilizer labeled 5-10-5, would contain 5% nitrogen, 10% phosphorus, and 5% potassium by weight. The remaining 80% are filler to facilitate the spread of the fertilizer.
Nitrogen encourages healthy leaf growth. Phosphorus promotes good seed germination, healthy seedlings, flowers, fruits and root growth. Potassium (aka Potash) contributes to the overall health, and makes the plants more stress and disease resistance.
Organic fertilizers feed the soil, chemical fertilizers feed the plant. Organic fertilizers must be incorporated into the soil where micro-organisms will release their nutrients as they decompose. The chemical fertilizer (except slow-release fertilizer) is best dissolved in the water before feeding to the plant.
Organic sources of nitrogen include alfalfa meal (4-1-1), blood meal (11-3-0), fish meal (10-6-2), soy bean meal (7-2-1), cottonseed meal (6-0-1), and fish scrap (5-3-0). Phosphorus is high in bone meal (2-5-0) and fish meal (10-6-0). Potassium is high in kelp meal (1-0.8-4) and wood ashes (0-2-6).
Many vegetables benefit from some complete fertilizer (5-10-5, 5-10-10, and 12-12-12). Soil tests can help you decide what nutrients are needed but nitrogen is commonly needed replenishing. Consider light but frequent applications every 3 to 4 weeks. Avoid letting the fertilizer come into contact with the plant stems to avoid burning.
Thank you to Master Gardener Donna Lee for this info.
https://ucanr.edu/blog/real-dirt/article/understanding-fertilizers-and-amendments
Photo: Fertilizer bag sample label, Dennis R. Pittenger, California Master Gardener Handbook