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