What Do the Numbers 10, 12 & 8 Mean on a Bag of Fertilizer?

Plants need proper nutrition for health and to produce the foliage, flowers or produce you are hoping for. If a soil test shows nutrients are needed for the plants you grow, then you will need to supplement through either organic matter, such as compost, or a commercial organic or synthetic fertilizer. The numbers displayed on fertilizer packages indicate the percentage by weight of the three nutrients plants need in the greatest amounts for healthy growth. Called macronutrients, meaning primary nutrients, they are nitrogen (N), phosphorus (P) and potassium (K), commonly referred to as NPK. The numbers on the fertilizer bag always indicate these nutrients in the same order. Fertilizers also sometimes contain micronutrients and trace elements.

Ratio of NPK

If the NPK on a bag of fertilizer is 10-12-8, it means the bag contains 10 percent nitrogen, 12 percent phosphorus and 8 percent potassium. Expressed as a ratio, describing the relative proportions of each nutrient, the package contains 5 parts nitrogen to 6 parts phosphorus to 4 parts potassium. Because plants need these nutrients in different proportions, the numbers tell gardeners the kinds of plants the fertilizer will support. For example, a strong dose of phosphorous can help a flowering plant produce buds and blooms, so its fertilizer would be likely to have a higher middle number, such as 5-10-5 or 4-8-6.

Nitrogen for Green

Nitrogen promotes vigorous, leafy growth and dark green color. Plants that aren’t getting enough nitrogen often languish, and their leaves turn pale green or yellow. They’re also more susceptible to diseases. Too much nitrogen, however, promotes quick, weak growth that stresses the plant’s root system. Too much nitrogen also discourages flowering, and in runoff, nitrogen pollutes the local water shed.

Phosphorous for a Good Start

Phosphorus is essential for healthy root development and for getting plants off to a good start. It plays a vital role in photosynthesis and the production of flowers, fruit and seeds. It aids respiration, storage and transfer of energy, and cell division. Phosphorus deficiency results in poor plant growth and small fruit, sometimes accompanied by purple stems and leaves, especially in young plants.

Potassium for Flavor

A factor in stress and disease resistance, potassium also contributes to flavor and color, so it is especially important in vegetable and fruit growing. It helps regulate plant systems such as photosynthesis, growth speed and respiration. It also contributes to drought resistance and winter hardiness. Symptoms of potassium deficiency are yellowing leaves, often with burnt edges, and shorter stalk length.