Understanding different nutrients your plants need and how to care for them properly

We don’t feed plants. We provide nutrients and they produce their own food. Here is a houseplant nutrient lesson in less than 500 words.

Plant nutrients fall into one of two categories, macronutrients (large amounts) and micronutrients (small amounts).

Macronutrients

Elements recognized as macronutrients for plants are Carbon, Hydrogen, Oxygen, Nitrogen, Phosphorus, Potassium, Calcium, Magnesium, and Sulfur. The carbon, hydrogen, oxygen, and sometimes nitrogen, are supplied by the air and water (we’ll ignore C, H and O). The rest come from the soil, entering the plant in a liquid solution through the roots (although some can be absorbed through the leaves).

While all these macro-nutrients are necessary for good plant growth, the range of requirements may change with the life cycle stage, or the type of plant it is. They are all involved in more than one plant process, but here is a simplified list of their major functions:

• Nitrogen—promotes green, vegetative growth
• Phosphorus—promotes flowering
• Potassium—stem integrity and protein synthesis
• Calcium—cell wall and membrane integrity
• Magnesium—component of chlorophyll and enzyme activator
• Sulfur—important component of plant proteins

Micronutrients

Be very careful with micronutrients as deficiency and toxicity can easily result by misapplication, and individual species often have significantly different requirements.

• Boron
• Chlorine
• Copper
• Iron
• Manganese
• Molybdenum
• Nickel
• Zinc

There is not enough room in this post to cover all the deficiency or toxicity symptoms, but they are all involved in many plant processes.

Many potting soils have some form of fertilizer mixed in with it. That may be fine for the first few months, but you will eventually need to supply nutrients for your houseplants. When purchasing fertilizer for your house plants the first thing to notice is the guaranteed analysis. This is shown as three (sometimes four) numbers, often referred to as NPK (nitrogen, phosphorus and potassium, in that order).
If your houseplants are not flowering plants, just for greenery, then the first number (nitrogen) should be a little larger than the other two. For flowering plants, the second number (phosphorus) should be a little higher than the other two. If your plants go from flowering to vegetative, year after year, then the three numbers should be even.

Houseplant fertilizer usually comes in one of three different forms—slow-release pellets, plant stakes, water-soluble. Slow-release pellets are usually mixed in with the potting soil, so when it is used up, you will need to either repot the plant in fresh soil, or use a different form of fertilizer. Plant stakes are also slow-release and are pushed down into the potting soil for the roots to access the released nutrients. Water-soluble fertilizer will be added to the water.

Most houseplants do best if they are fertilized during the active growing season (summer) and no fertilizer (or very little) during the winter months.

Here is a great certified crop advisors module on Plant Nutrient Functions and Deficiency and Toxicity Symptoms from Montana State University.