What do the Numbers Mean on a Bag of Fertilizer?

What do the Numbers Mean on a Bag of Fertilizer?

Bags of garden fertilizer have three numbers on the label in bold, referred to as the fertilizer analysis or “grade”. The first number refers to the percentage of nitrogen (N) in the mix; the second number is the percentage of Phosphate (P2O5); the third number is the amount of Potassium (sometimes referred to as potash) (K2O). These three numbers, expressed as N-P-K, are the primary nutrients necessary for plant health. A bag of 10-10-10 fertilizer contains 10% nitrogen, 10% phosphate and 10% potassium. A bag of 5-10-5 is 5% nitrogen, 10% phosphate and 5% potassium. The remaining ingredients are inert substances like sand, sawdust, sterile dirt, peat moss, ground corn cobs, perlite, rice hulls, limestone or other ingredients. High percentages generally indicate synthetic substances were used to manufacture the fertilizer. Organic fertilizers tend to have lower percentages and slower release of nutrients.

Inert ingredients – those that do not react with the fertilizer or feed the plants – are intended to dilute or reduce the concentration of the fertilizer’s active ingredients, which can burn delicate roots and stems; keep the fertilizer from drying out or hardening and clumping, which makes it unusable; or make the distribution of the fertilizer easier.

What does NPK do for plants?

  • NPK stands for Nitrogen, Phosphorous, and Potassium. They are the 3 most important nutrients for plant growth, sometimes referred to as macronutrients.
  • Nitrogen is vital for a plant’s growth, development, and defense against diseases and stress.  Every part of a plant needs nitrogen – it helps plants photosynthesize, gives them their green color, and make the proteins they need to produce new tissues. But if too much nitrogen is added, the plant may grow lots of foliage and stems but few fruits, vegetables, roots or flowers. In extreme cases, too much nitrogen can actually kill a plant. A lack of nitrogen causes the lower leaves to turn yellow and the whole plant to turn pale green. Plants like legumes, however (beans, peas, and others), do not take up nitrogen from the soil, but from the air. They return nitrogen to the soil when the legume dies and decays.
  • Phosphorus is a vital component for plants. It stimulates root development, improves flower and seed formation, increases stalk and stem strength, helps the plant resist disease, and is critical in photosynthesis. Phosphorus deficiency causes stunted growth and poor flowering and fruiting. Too much phosphorous – applying when it is not needed – can increase chlorosis, a yellowing of the leaves caused by insufficient chlorophyll.
  • Potassium is important for many plant processes, but most importantly for gardeners, it supports the yield and quality of the edible parts of the plant. Signs of potassium deficiency are stunted growth and yellowish lower leaves.

Nitrogen is the one element that frequently needs replenishment, as it’s easily lost to rain and irrigation. Plants need this primarily just before growing and during the early stages of growth when they produce as many new cells as possible. Adding Nitrogen late in the season does little for the plant as it’s difficult for the plant to utilize. When added late, it will remain in the soil until next season, move below the root zone, or be leached away by rain and weather. Ultimately, plants will only use what they need of any element, regardless of how much you add.

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