Overall, plants need 16 specific elements, or nutrients, for proper growth. When enough of each nutrient is present in soil, plants grow optimally. If even one element is in short supply, plants can't grow as well. Think of the weakest-link theory, which says that a chain is only as strong as its weakest link. Your soil is only as fertile as its most deficient nutrient.

  • Nutrients for photosynthesis: The nutrients that plants need in the largest quantities are carbon, hydrogen, and oxygen, which plants use for photosynthesis.

  • Mineral nutrients: Plants generally get mineral nutrients from the soil or from applied fertilizers. Mineral nutrients include nitrogen, phosphorus, and potassium (the familiar N-P-K on fertilizer bags), as well as numerous others. When gardeners talk about feeding plants, they're talking about providing them with extra mineral nutrients.

The mineral nutrients needed in the largest quantities are called macronutrients and consist of nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur. In addition, plants need smaller amounts of so-called micronutrients. The eight micronutrients considered essential for plant growth are iron, manganese, boron, copper, zinc, molybdenum, chlorine, and nickel, all of which occur in very small quantities in most soils. These micronutrients, and other substances found in low concentrations in soils, are sometimes called trace elements. Scientists studying plant nutrition may discover additional micronutrients among the many trace elements in soils.

Plants may take up trace elements that they don't need, but that we humans do. The trace elements iodine, fluorine, selenium, cobalt, arsenic, lithium, chromium, silicon, tin, and vanadium, for example, are considered to be essential for animals and humans but not for plants. Brazil nuts usually contain large amounts of selenium, which has no known nutritional value to plants but which is an important antioxidant for human health. The level of selenium in plants varies due to the selenium content in the soil.

Because they are derived from natural sources, many organic fertilizers contain an abundance of trace elements, including important plant micronutrients. Synthetic fertilizers, on the other hand, often contain just nitrogen, phosphorus, and potassium, so they don't replenish or enhance the other nutrients and trace elements.

Researchers still have much to discover about soil and the interplay among minerals, organic matter, soil life, and plant health, so it makes sense to choose fertilizers and other soil additives that supply a broad range of nutrients. What's certain is that plant ¯ and human ¯ health depends on healthy soil.