What the numbers mean
Every fertilizer bag should have 3 bold numbers. The first is the amount of nitrogen (N), the second the amount of phosphorus (P) and the third the amount of potassium (K). Nitrogen is in the form of ammonium (NH4) or nitrate (NO3). Phosphorus is in the form of phosphate (P2O5) and potassium is in the form of potash (K2O).
These numbers represent the percentage of each nutrient in a bag of fertilizer and are always in the same order - (N)(P)(K). A 20lb bag of 10-10-10 fertilizer contains 2lb (10%) nitrogen 2lb (10%) phosphorus and 2lb (10%) potassium. A 50lb bag of 18-24-6 starter fertilizer contains 9lb (18%) nitrogen 12lb (24%) phosphorus and 3lb (6%) potassium. Note that a fertilizer does not have to contain all three nutrients - urea (46-0-0) is an example.
What the nutrients do
Nitrogen - absorbed by plants in larger quantities than other nutrients
- helps provide green color
- somewhat mobile in soil & should be applied 2 - 4 times a
year depending on conditions
- speeds up (top)growth of plant
- is a component of chlorophyll and essential to
photosynthesis - process by which plants make food
- can be over applied - too much nitrogen and grass will grow
quickly at the expense of root systems
Phosphorus - essential to photosynthesis
- supports root growth and flowering - why there is generally a
larger (P) number in starter fertilizer
- less mobile in soil in Midwest as it tends to bond tightly with
clay soils
- part of every cell in plant
- assists in storage of food
Potassium - second only to nitrogen in plant uptake
- essential to disease resistance and tolerance of
temperature extremes
- helps with root development
- not a fixed part of plant tissue
- helps transport nitrogen and sugars in plant to where they
need to go
- is recycled from clippings - bagging of grass can (over time)
result in potassium deficiency
