A 12-4-8 fertilizer ratio is a good nutritional balance for perennial plants with lots of foliage, like trees, shrubs, and grass. It can be applied to support early seasonal lawn growth in soil that’s deficient in all three macronutrients (nitrogen, phosphorus, and potassium).
As grass grows, it absorbs these nutrients from the ground, and they need to be replenished seasonally, either naturally or by amending the soil at different times throughout the year.
What Is The NPK Ratio?
The NPK ratio is the three numbers on the label of fertilizer bags, referring to the main soil macronutrients: nitrogen, phosphorus, and potassium. The ratio refers to the percentage of each nutrient in the fertilizer.
A 12-4-8 fertilizer contains 12% nitrogen, 4% phosphorus, and 8% potassium. These three essential nutrients play important parts in your lawn’s good development and ongoing health:
- Nitrogen – Nitrogen is an essential component of chlorophyll, the green compound in the leaves of plants that absorbs light to make glucose, which feeds the plant. This nutrient supports growth, especially of the foliage.
- Phosphorus – Phosphorus is important for healthy root growth in grass, also supporting the plant’s overall metabolism. The nutrient is most useful to plants at the beginning of the growing season when roots are growing or reviving from dormancy.
- Potassium – Potassium supports a plant’s overall cellular health: good cell reproduction, strong cell walls, and moisture retention. This all helps a plant stand up firmly and be able to withstand some stress associated with pests, disease, and drought.
Before choosing a fertilizer, it’s important to do a soil test to be sure you aren’t over-providing any nutrients since too many can be as damaging as too few.
What Fertilizer Number Is Best For Growing Plants?
The fertilizer number that’s best for your plants is going to depend on three factors:
- The soil profile – When you do a soil nutrient and pH test before choosing a fertilizer to put down, you’ll know exactly what your yard needs and doesn’t need.
- The plant type – Different plants have different nutritional needs. Grass, for example, doesn’t need phosphorus once it has grown in, but flowering and fruiting plants need phosphorus throughout their growing season to put out blooms and fruits.
- The stage of growth – Grass and plants that are growing from seed or coming out of dormancy will appreciate some phosphorus at the beginning of the season for strong root growth, but later in the season, they won’t need any (unless they’re producing fruit and flowers).
There are as many products available in a range of ratio combinations as there are potential nutritional needs that your lawn may have.
Types Of Fertilizer
The NPK ratios are important to deliver the right proportions of nutrients to your lawn, and the way they get delivered to your grass roots is affected by the form of the fertilizer.
- Synthetic – Nutrients can be isolated and delivered in high concentrations to be most effectively absorbed by plant roots. These come in both solid and liquid forms, and overapplication can damage roots since most formulas are salt-based.
- Organic – Organic fertilizers deliver a slow release of nutrients in their natural forms, like compost, worm castings, and grass clippings. While natural fertilizers take longer to release than synthetic fertilizers, they remain in the ground longer and deliver a consistent supply as the material gets broken down by the topsoil ecosystem.
- Liquid – Liquid fertilizers, both organic and synthetic, deliver nutrients that are available immediately and in high quantity. While plants will quickly absorb as much as they can, these fertilizers will drain from the soil faster than solid forms.
- Solid – Organic material and granular synthetic fertilizers are solid materials that stay in the ground for an extended period of time, breaking down slowly to release small amounts of nutrients over multiple weeks or months.
What Numbers Are Best For Lawn Fertilizer?
All-purpose fertilizer comes in a balanced ratio of 1 to 1 to 1, like 5-5-5- or 10-10-10. If your soil is deficient in only one or another of them, using an all-purpose fertilizer may be over applying a nutrient that might have negative effects on the grass.
For lawn fertilizer, the numbers will be dependent on the time of the year, the stage of growth of the grass, as well as the soil profile. In general, however, the optimal ratio for lawn fertilizer is 4-1-2. The concentration might be multiplied by 4 for a more powerful application in a 16-4-8 fertilizer ratio. This is a little stronger than 12-4-8 because it contains more parts nitrogen.
If your soil test shows that you have enough phosphorus in the lawn, a 12-4-8 ratio might not be ideal, since too much phosphorus can interfere with iron absorption, and by extension, chlorophyll production. Phosphorus makes iron bond to particles in the soil and becomes unavailable for roots to absorb. In this case, a ratio with zero phosphorus content might be best.
In What Season Should I Apply 12-4-8 Fertilizer?
Fertilizers with a 12-4-8 or 16-4-8 ratio are best for an early spring, early summer, or early fall application (based on your soil profile test results). The phosphorus content will help welcome the season’s grass roots as they revive from dormancy and grow new roots for the season, with a healthy supply of potassium to support cell health. The nitrogen will support general growth by encouraging grass blades and chlorophyll production.
Fertilizer high in nitrogen and phosphorus should be avoided in the middle of the summer when temperatures are high and grass may be stressed from heat and dryness. The nitrogen will push growth and can add more stress to the lawn, and any extra phosphorus may affect the availability of iron. To green up your lawn in mid-summer, apply an iron product that has some potassium, like Ironite, whose NPK ratio is 1-0-4.
Similarly, a high nitrogen and phosphorus ratio of fertilizer shouldn’t be applied in mid-fall, when grass is preparing for winter dormancy. If you want to help your grass prepare, an application of a high potassium fertilizer in mid-fall won’t push growth, but help the roots store energy.