Are you mystified by the numbers on the commercial fertilizers available at your local gardening center? What do those numbers stand for? And what should they mean to you?!
Most plants need a combination of the macronutrients oxygen, carbon, hydrogen, nitrogen, phosphorus, and potassium in order to stay healthy. The fertilizers in your local store usually contain a combination of the last three of these elements, also known as N-P-K (the elements’ periodic table abbreviations). These are the three essential macronutrients which are found in soil.
Plants with a deficiency in any of these nutrients are more likely to succumb to fungal diseases, pests, or to simply not thrive. Just like humans and animals need a balanced diet to live their best lives, plants need to “eat” well too.
Let’s talk about what these macronutrients do for our beloved plants. Simply put, nitrogen primarily “feeds” green herbaceous growth in your plants. Phosphorus especially helps plants flower and build strong root systems, and Potassium helps regulate and balance general plant growth rate.
If your soil is lacking in any of these macronutrients, or if it has become imbalanced, then you should consider applying a fertilizer to amend it. You can perform a soil test to see the current balance of nutrients in your own soil on your own (somewhat accurate), by mailing samples to a lab (more accurate), or by hiring your local county extension agent or regional/state agronomist (usually most accurate). The results of the test will give you an idea of what your soil may need.
Before you begin perfectly balancing your soil based on these test results, however, you should also know that different plants require different ratios of Nitrogen, Phosphorus, and Potassium. Roses, for example, need especially high amounts of Potassium to thrive. Evergreens need a higher amount of Nitrogen, and lawns need higher amounts of both N and P.
When your see the large selection of fertilizers at your local store, the wide range of N-P-K numbers may feel overwhelming, but if you remember that these numbers are simply ratios, they should feel less intimidating. A 10-10-10 fertilizer has 10% N, 10% P, and 10% K, with the remaining 70% of its bulk made up of inert fillers like sand, limestone, or sterile soil. These fillers can help “dilute” or spread out the fertilizer so that it isn’t too intense for your plants, and so that it’s harder to overfertilize.
Put a different way, a 50-pound bag of 10-10-10 fertilizer has about 5 lbs of N, 5 lbs of P, 5 lbs of K, and 35 lbs of fillers. Another example: a 100-pound bag of 8-14-4 fertilizer has about 8 lbs of N, 14 lbs of P, 4 lbs of K, and 74 lbs of filler.
Other things to consider when choosing the right fertilizer for your plants are the porosity of your soil (very porous soils will allow nutrients to wash away more quickly, heavier soils hold them in place longer), the time of year (how much growing season is left?), and the age of your plants (established plants often need different nutrition than babies). There are also several micronutrients that plants also need in varying smaller amounts in order to stay healthy.
Soil science and fertilization are fascinating areas of study. I hope this short explanation helps you understand how to read your store-bought fertilizer options, but also whets your appetite to dig deeper and learn more about how to give your plants the best nutrition possible!