Every gardener has seen the three big numbers on the front of a fertilizer bag or box.
Most know what the numbers stand for (in order): Nitrogen – Phosphorus – Potassium, also known as N-P-K.
These ingredients are always listed in this same order.
The higher the number, the higher the water-soluble percentage of that ingredient in the fertilizer. For instance, Sulphate of Ammonia, rated at 21-0-0, is 21% soluble nitrogen. It contains no potassium or phosphorus. On the other hand, something like SuperPhosphate, rated at 0-20-0 contains 20% phosphorus but no nitrogen or potassium.
Why should we care specifically about these three elements over all others? What exactly do they do?
- Nitrogen promotes green growth above the soil.
- Phosphorus promotes flowering, blooming, and root growth
- Potassium promotes general vigor with compact, robust growth.
These attributes are sometimes learned by the memory aid, “Up, Down, All-Around.”
DON’T GARDEN BY THE NUMBERS
So do higher numbers equal better fertilizers?
Healthy plants grow in healthy soil.
Soil amendments and fertilizers certainly play a role – sometimes the crucial role – in promoting plant growth. But highly soluble, super-concentrated ferilizers can actually harm plants, the soil, soil microbes, and the environment.
Fast release nitrogen can burn seedlings. You’ve probably seen plants with brown edges on their leaves. That can be a sign of fertilizer burn.
Further, highly soluable fertilizers wash through the soil quickly, leaving little or no nutrients behind for continued growth.
And finally, high concentrations of phosphorus run-off can pollute water and kill fish.
FERTILIZE TO MEET PLANT NEEDS
While N-P-K gets all the glory on the label, other ingredients, such as minerals and trace elements are just as important to the health of your plants. Use the Big Three numbers on the front of the bag as a guide.
Then turn the bag or box over and look at the rest of the ingredients. A long list of nothing but unpronouncable chemicals is unlikely to promote a healthy environment. On the other hand, trace minerals and other insoluble elements will bind with the organic matter in your soil. They are unlikely to wash out as quickly, and so may slowly release nutrients to your plants throughout the growing season.
FEED THE WHOLE GARDEN, NOT JUST THE PLANTS
Soil rich in organic matter will also be rich in life – most clearly illustrated by the presence of earthworms. Burning, quick release fertilizers kill worms, leaving you with dead soil that demands constant re-fertilization. Compost and minerals, in contrast, provide food for the worms, who turn it into one of nature’s richest fertilizers: worm castings.
Organic fertilizers tend to have “lower numbers” than chemical fertilizers, in part because organics are less water soluble. And yet, that very fact gives them an edge in promoting sturdy, compact plants with deep roots – plants more able to resist insects and disease.
Do you have to be 100% organic to have a healthy garden? No, of course not.
But you’ll find that shifting your focus to organics will, over time, build healthier soil. As your soil improves, your plants will improve. You’ll be less tempted to reach for pesticides, and the beneficial insect population will be able to do its part in keeping insect damage down.