There’s a lot of confusion about terms when it comes to fertilizers. Just stop and peruse the shelves at your local nursery or home improvement store. You’ll find a mind-boggling array of things to sprinkle, spread, spray on, pour on, mix in, pound in; there’s food, meal, pellets, encapsulated nutrients, and don’t forget the conditioner, compost, mulches and more. Are they all fertilizers? Is a fertilizer the same thing as a nutrient? An amendment? A mulch? If not, what’s the difference? And what do they do? Are there legal requirements for something to be labeled as a fertilizer? Let’s find out.
Plant nutrients are the chemical elements taken in by plants that are essential for their growth and development. A fertilizer is a material added to the environment around the plant that directly impacts the plant, providing it with specific nutrients. Amendments are any materials mixed into the soil that indirectly aid plant growth by improving the condition of the soil like its structure or texture, water retention or microbial activity. Mulches are organic or inorganic materials placed on the soil surface to help prevent weed growth, conserve moisture and add organic matter to the soil as they break down.
Fertilizers are NOT plant food! Plants make their own food (sugars and carbohydrates) using water, carbon dioxide and sunlight and combine them with plant nutrients to produce the proteins, enzymes and vitamins essential to plant growth. When we fertilize, we are applying plant nutrients to supplement nutrients naturally occurring in the soil.
The law requires that manufacturers guarantee the accuracy of what is claimed on a product label – if it’s on the tag, it’s got to be in the bag. The term fertilizer refers to a material that guarantees the minimum percentages of primary nutrients – nitrogen, phosphate and potash, found on the container label. The product label may also identify other nutrients, like sulfur, iron and zinc if the manufacturer wants to guarantee the amount contained in the product. In some cases, a fertilizer will contain secondary nutrients or micronutrients not listed on the label because the manufacturer does not want to guarantee their exact amounts. Soil amendments make no legal claims about nutrient content or other helpful (or harmful) effects they may have on the soil and plant growth.
Plants require seventeen different chemical elements for healthy growth though most of these elements are already in the soil or the air and don’t need to be added regularly. These seventeen elements are broken down into four general groups – the essential elements, the primary or macronutrients, the secondary nutrients and the micronutrients or trace elements. The elements essential to all forms of life, carbon, hydrogen and oxygen, are derived primarily from air and water. The three primary nutrients are nitrogen, phosphorus and potassium; they are the most common fertilizer ingredients. Next are the three secondary nutrients – calcium, magnesium and sulfur. The remaining eight are micronutrients – boron, chlorine, copper, iron, manganese, molybdenum, nickel and zinc. While the three essential elements are vital for life itself (no plant will grow without all of these), the remaining fourteen play varying roles in plant growth and health. The following table provides some of the key functions of each element as well as the soil pH range in which they are most readily available. Next time we’ll explore the many types, forms and formulations of fertilizers that are available, including organic vs inorganic materials.