One of the most confusing topics for most gardeners is the topic of fertilizers. Just what do those numbers mean? Why do the numbers matter?
The numbers on a fertilizer package represent the % of different nutrients that particular mix has. The three nutrients measure are:
- N = Nitrogen
- P = Phosphorous
- K = Potassium (sometimes called Potash)
Generally, the value to the plant of these three nutrients is unique.
- Nitrogen encourages and feeds the production of green plant material – top growth if you will. You’ll notice that many lawn fertilizers are heavy on nitrogen. An example of a heavy “top growth” or “green growth” mix would be 21-3-3, indicating 21% nitrogen, and 3% each of phosphorous and potassium.
- Phosphorous encourages both root growth and development of flowers, fruits and seeds. It’s an expensive ingredient in the blend, and too much of it is bad for the environment. But it’s critical in the perennial garden.
- Potassium is a critical component of a plant’s ability to develop vigor to resist stress and withstand changing weather conditions.
A well-established lawn needs a fertilizer heavy in nitrogen to feed that heavy and lush top growth. However, pure nitrogen without sufficient P and K will leave the turf susceptible to stress and disease. In our heavy soils down in Parker, I always apply less N and more P and K, because I care more about the grass plant’s ability to build and maintain a root structure that will penetrate and survive the tough soil.
For perennial gardens, you want a mix that is high on P and K. There are different opinions about how much N you want, but certainly you don’t want the N number to be higher than the other two – the most N you would want would be the same as the P and K – say something like 5-5-5.
You’ll notice also that fertilizers often advertise that they’re “organic”. While we do use synthetic blends depending on the situation, my preference is to use organic blends. An organic blend will always have lower numbers, but for the perennial garden you’re not looking for high numbers anyway. The greatest value to the synthetic blends are to the container plants and lawns (assuming you want a high N lawn fertilizer.)
