Plant nutrients 101: A brief overview

by Caroline Brown

A lot of gardeners fumble through the process of choosing a fertilizer, unsure if their plants even need one or what product to choose if they do. I included myself in that category until just recently, when I learned about plant nutrients and fertilizers in the URI Master Gardener's program. One of the first things I learned is that plants are just like people. You have to understand a plant's nutritional needs before you feed it.

All plants need six primary nutrients. Because carbon, hydrogen, and oxygen come from the sun, air, and water, these nutrients are usually in plentiful supply. The other three–nitrogen, phosphorus, and potassium–come from the soil. When a plant has a nutrient deficiency, most likely it's one of these three nutrients that's missing.

Nitrogen is important for plant tissue growth. Nitrogen is about green plant parts like leaves and stems. It makes plants grow tall and leaf bountifully; it's what makes your grass green.

Phosphorus is critical for root growth, helps plants set fruit and flower buds, and makes seeds bigger.

Potassium improves the plant's overall strength and disease resistance, and helps regulate the plant's metabolism.

Fertilizer packaging always has three numbers prominently printed on it — for example 5-5-5, 15-10-5, etc. (see photo). These numbers tell the percentage of nitrogen (N), phosphorus (P), and potassium (K) in that order. So a bag labeled 15-5-5 has 15 percent N, 5 percent P, and 5 percent K.

Different plants need nutrients in different amounts. You should buy fertilizer based on what your plant needs. Regulating nitrogen is important: if a plant gets too much, it can have too many leaves but not enough fruit or flowers. For example, you can see that root crops like carrots would need more phosphorus than nitrogen, for example. Vegetables need a different formulation of nutrients than roses. That's why manufacturers make different products for roses, flowering plants, vegetables, grass, etc.

Another major factor in understanding plant nutrients is soil pH. If your soil is too acidic or too basic, you may have to change the pH. Otherwise, certain nutrients are "tied up" –that is, unavailable to your plants–by the excessive pH of your soil. This can be determined by having your soil tested.

Plants need other nutrients, called micronutrients, in lesser amounts. This includes calcium, sulfur, manganese, iron, boron, zinc, copper, and molybdenum. In certain conditions, your plant may have a deficiency of one of these.

If you're interested in this topic, check back because this is really just the tip of the iceberg. In future posts about plant nutrients, I'll provide additional detail on how you can tell if your plant has a nutrient deficiency, how pH affects nutrient availability, and how to choose a fertilizer.

Photo courtesy of Aggie Horticulture Network/Texas A&M

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6 Comments to “Plant nutrients 101: A brief overview”

  1. Thanks Caroline, this was an interesting post and something I wondered about, even with my limited planting experience. Always thought that I would like to be able to feed garden and house plants but the array of products and do’s and don’ts seemed confusing. I know Ive killed more than my share of house plants over the years but I blame that on over-watering. Ive learned to curb that impulse 🙂

    BFN, Geraldine

  2. Thanks for the compliment Geraldine, I have killed my share of houseplants by overwatering too. I’ll write more about types of fertilizers, specifically plants like what formulas, at some point.

  3. Good information, Caroline,

    Also consider the very important soil microorganisms that enable plant food to be utilized.

    Without a rich mixture of bacteria, enzymes, yeasts and other biological life, no amount of plant food will provide necessary nutrition.

    Earth-friendly natural fertilizers add to the biological life – synthetic fertilizers do not.

    Keep up the good work on this blog…I wish I had more time for mine!

  4. Dan, thanks for visiting my site…hope you visit again. And thanks for reminding me about the importance of soil microorganisms. Your website looks very interesting…you have a unique service.

  5. Caroline,

    Excellent post… Very often people kill their houseplants as soon as they get them home. They repot, water and fertilize. This all puts much more undue stress on the plant.

    All the Best,

  6. Thanks for coming by Gary. You’re correct, and most fertilizer manufacturers just make it worse by pushing their product as a cure all and not giving good directions.

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