When you purchase garden fertilizer, the ratio of nitrogen (N), phosphorus (P) and potassium (K) will be listed on the container in the form of three numbers. But what do those fertilizer numbers mean, and how do you use them?

“Nitrogen, Phosphorus, Potassium. ‘Up, Down, All Around’. Come on, Jess, it’s so easy!”

This was my introduction to the basics of horticulture from my dependable schoolmate, Ray. Having re-entered college after an extended absence, Ray was my own personal coach helping me understand the science behind the art of horticulture.

For the new or intuitive gardener (like myself), the chemistry behind the action of fertilizing can be intimidating. I have often witnessed gardeners in the fertilizer aisle going from one product to the next trying to decipher the label and the numbers, confused by symbols they barely recollect from their high school science lessons. Up, down all around. That’s when Ray’s little pearl falls from my lips.

This simple phrase is an easy way to remember the primary nutrients your plants require to achieve successful growth:

  • Nitrogen for vegetative growth like plant leaves or grass (Up).
  • Phosphorus for root development (Down).
  • Potassium for everything (All Around).

fertilizer bag

Understanding fertilizer numbers

When you purchase a fertilizer from your local garden center, the ratio of nitrogen (N), phosphorus (P) and potassium (K) will be listed on the container in the form of three numbers. These numbers tell you the percentage of N, P and K in the fertilizer blend they have combined as their special product. For example, if you wanted to purchase an all purpose fertilizer with equal parts N P K, then you might choose a product that had a ratio of 4-4-4. If you were looking for something more appropriate for transplanting, when you want to promote root development, you would want a higher middle number such as a 2-8-4.

The three elements we are discussing above are just the basics–the primary macro-nutrients. Plants also need other elements, known as micro-nutrients, to achieve successful development. Magnesium, Calcium and Iron are also key factors in garden health–the lack of which can cause weakness in your plants.

The only accurate way to determine what your soil has or what it does not have is through a soil test. However, if you take a closer look at your plants, you can sleuth out some deficiencies on your own.

Common soil deficiencies

This chart lists the basic garden macro- and micro-nutrients, along with the “symptom” that indicates a lack of that particular nutrient.

Nutrient LocationSymptom
NitrogenLeaves, stemsNewer leaves are pale green. Older leaves starting at the bottom turn yellow. Stems slender and fibrous.
PhosphorusRoots, flowersPoor root development. Flower drop and delayed fruit set. The undersides of leaves often have purplish cast to them.
PotassiumLeavesLowest and older leaves look wilted or scorched and will often cup downwards from margin inward. Spotting along leaf stem may also be apparent.
MagnesiumLeavesYellowing of leaf tissue between the veins of older leaves (chlorosis) but veins will remain green. Leaves are brittle and will cup upwards. Often deficient in acidic soils.
roots, fruit
New growth is deformed and irregular. Die off in roots begins with tips sloughing off and the remaining area becoming enlarged or bulb like. This is also the source of blossom end rot.
IronLeavesNew leaves will yellow between the veins. Older leaves will be completely yellow. Generally more of a problem in alkaline soils.

Natural vs. synthetic fertilizers

If you have discovered through a soil test or visual inspection that your plants are deficient in any of these minerals, you have two choices as the delivery method to compensate what is lacking.

The most common choice is a salt based or synthetic fertilizer. Due to the production methods, negative environmental consequences and the unknown long-term effects, these products have received much criticism over the years. The spectrum of nutrition they provide is often narrow and without the trace elements (micro-nutrients) that plants need. The benefit of synthetic fertilizers is that they provide large dosages of the macro-nutrients in a way that is readily accessible to plants. This has allowed for agriculture to flourish in nutrient poor soils and thus feed our rapidly growing population.

In my own garden, I prefer to use sources that are naturally derived to support the over all soil complex. The analysis of these substances is often much lower than the synthetic varieties but more compatible to my labor force of worms and other microorganisms like bacteria and mycorrhizae. Natural fertilizers build soil structure because they are of an organic base. They improve aeration, hold moisture and nutrition better and complement the growth of the beneficial organisms. Using natural amendments helps mimic what we would find in a healthy natural system. To compare the scenario to our own bodies, it would be like choosing whole foods versus processed food to provide us with energy and nutrition.

The fertilizer numbers below are examples. Typically these numbers vary somewhat by brand or with custom mixes. Some other useful natural amendments that you can explore are:

Bone meal 4-21-0

Is rich in Phosphorus, a mineral that plants need for healthy root development and flower growth. Bone meal also contains calcium and a little bit of nitrogen, both of which are beneficial to plants. Bone meal is taken up slowly over time, so there is little risk of burning plants with too much of this fertilizer. Bone meal is available at Eartheasy online.

Alfalfa 3-1-2

An all purpose natural fertilizer which aids microorganisms by helping them convert soil nutrients into soluble forms more available for plants. Alfalfa boosts growth rate of seedlings and is an excellent mulch for roses and other perennials. Low risk of burning vegetation.

Kelp 2-1-3

Another all purpose natural fertilizer, kelp meal adds Nitrogen, Potassium and Phosphorus to the soil. Kelp has a large amount of trace minerals and alginic acid that improves soil structure. Be careful because it can burn if over-applied. Kelp meal is available at Eartheasy online.

Green Sand 0-0-5

Is high in trace minerals and potassium. Good for plant strength and healthy leaf development. Green sand is really good for root crops such as beets, potatoes and carrots and has a low risk of plant burning.

The multiple benefits that amendments offer would seem to me to be a much better choice when addressing plant nutrition. I am not looking for instant results in my garden but long-term success. This requires an infrastructure based on healthy soil. Soil is the heart of the garden. Great soil equals great plants!

Check out other fertilizers we offer in our Garden Supplies category.

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