Gardeners are often told how important nutrients are to plants. But what is a nutrient, actually?
“Nutrients are the chemical elements that plants need to build their tissues and operate their life processes,” said Sharon Yiesla, plant knowledge specialist at The Morton Arboretum in Lisle. “All their molecules are built from atoms of certain basic elements, and we call those nutrients.”
There are 17 elements that are critical for all plants. The major building blocks of plants’ tissues and their food are carbon, hydrogen, and oxygen — elements that they collect from the air and water. The other 14 important elements must be absorbed from the soil. “When we fertilize plants, we’re supplying a boost of these nutrients for plants to take up through their roots,” Yiesla said.
Nutrients are not food. “Calling fertilizer ‘plant food’ is a misnomer,” she said. “Plants make their own food through photosynthesis.” Nutrients are the elements that they put together through chemical processes to make food, grow, construct seeds and fruit, bloom, and do all the other things they need to do to be plants.
These elements are naturally available in most soils, so fertilizing often isn’t necessary. “Use fertilizer only when you know there is a need for it,” she said.
Plants can use only limited amounts of each nutrient, so unnecessary or excessive fertilizing can actually harm them. “It’s also a waste of money,” Yiesla said. Excessive fertilizing can cause environmental problems if the surplus elements build up in the soil and run off to pollute rivers and lakes. “When it comes to fertilizer, more is not better,” she said. “Just enough is best.”
Sometimes plant symptoms suggest a need for a nutrient supplement. For example, chlorosis, a yellowing between leaf veins, may indicate a need for more nitrogen, iron or manganese. The type of soil in your yard can make a difference; clay soil tends to hold nutrients better than sandy soil. Nutrient availability is also affected by the soil pH — whether it is acid or alkaline. The most definitive way to determine whether your soil is low on important nutrients is to get a soil test (see mortonarb.org/soil-test).
Three of the nutrient elements — nitrogen, phosphorus and potassium — are needed in large amounts, so they are often called primary macronutrients. They are the major ingredients in most fertilizers. Nitrogen promotes leafy green growth such as stems and leaves. Plants need a continuous supply of nitrogen, but not an oversupply. Too much nitrogen can force leafy growth at the expense of healthy root growth. It can also push plants to grow leaves at the expense of flowers and fruit.
Phosphorus supports root growth and fruiting. “Most Illinois soils have plenty of phosphorus, so you probably don’t need to add any unless a soil test says it’s lacking,” Yiesla said. Potassium supports disease resistance and plant hardiness.
Fertilizer labels show the proportions of the macronutrients as a standard ratio, called an NPK ratio after the chemical symbols for the three elements. For example, a synthetic lawn fertilizer’s label might say 32-0-4, meaning that by volume it is 32 percent nitrogen, 0 percent phosphorus, and 4 percent potassium.
Three other elements — calcium, magnesium and sulfur — also are needed in relatively large amounts. The remaining eight plant nutrients, including iron, boron, copper and zinc, are needed only in very small quantities. Since all these elements are usually available from the soil, fertilizer products include them only in trace amounts or not at all.
Both organic and synthetic fertilizer products can supply the elements plants need. “A plant can’t tell whether an atom of nitrogen came from a blue crystal or from compost,” Yiesla said. “It will be used just the same in a chemical reaction.”
For a gardener, though, there are differences. Synthetic fertilizers are created by industrial processes. They contain no carbon and are water soluble. Many deliver a fast jolt of nutrients to plants, often far more than what the plants can use. Some, called slow-release fertilizers, are designed to release nutrients gradually from coated pellets.
Most organic fertilizers are made from the remains of plants and other organisms. They contain carbon and usually a wide range of micronutrients. They release their nutrients slowly so they don’t jolt plants, but they also don’t make an immediate visible difference. Other organic fertilizers are made from finely ground rock minerals.
“Even with organic fertilizers, it’s possible to overdo it,” Yiesla said. Always apply any fertilizer according to the instructions on the label.
You can also increase the nutrients available to your plants by adding organic matter, such as compost. Dig it in at planting or top-dress the soil around established plants.
“Organic matter consists of the remains of dead organisms, mostly plants,” she said. As they decay, all the nutrient elements they contained in life are released into the soil, where the roots of your plants can absorb them. “It’s a cycle of life,” she said.
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or plantclinic@mortonarb.org). Beth Botts is a staff writer at the Arboretum.
