All plants require certain elements to keep them healthy. Those elements they need in relatively large amounts, including nitrogen and magnesium, are known as macronutrients. Those they need in very small amounts, such as copper and zinc, are referred to as micronutrients, or trace elements. Plants that are stunted, have discolored leaves or that fail to flower may be showing signs of nutrient deficiencies. Each nutrient supports plants in specific ways, and knowing what each element does can help you to determine what your plants are missing.
Plants require nine macronutrients: nitrogen, magnesium, calcium, potassium, oxygen, sulfur, phosphorous, hydrogen and carbon. Sometimes hydrogen, oxygen and carbon are not included as macronutrients, since plants get these from air and water, but they are just as essential as the other elements on the list. Plants that are missing any of the macronutrients typically show obvious signs of poor health and fail to thrive, and often by the time such a deficiency is discovered it has already done serious harm to the plants.
The micronutrients plants need are zinc, molybdenum, boron, cobalt, iron, manganese, chlorine and copper. While not required in large amounts, these elements are essential for supporting plant growth and function, and without them plants are not able to make use of the macronutrients. Many chemical fertilizers don’t contain trace elements, but natural fertilizers such as manure and compost normally have most or all of these. Soils known to be deficient in any of these elements should be enriched with compost or another type of complete fertilizer.
Nitrogen is present in virtually all commercial fertilizers and is identified on the label by the NPK number, which shows the amount of nitrogen (N), phosphorous (P) and potassium (K) in the mix. The number on the label indicates how many pounds of each element is present per 100 pounds of fertilizer, so 18-18-21 indicates the presence of 18 pounds of nitrogen, 18 pounds of phosphorous and 21 pounds of potassium in every 100 pounds of fertilizer. Nitrogen is essential for the growth of leaves and stems, and it helps to give plants a rich, green color. If it is lacking, plants will be pale and sickly-looking, and the oldest leaves will look worse than the newer ones. Too much nitrogen can cause plants to put all of their energy into developing greenery, and they will fail to bloom or produce fruit.
Magnesium supports plants by facilitating the distribution of phosphorous throughout the plant. Since phosphorous is essential for developing flowers, fruit and roots, a lack of magnesium results in plants that fail to produce fruit or that are unable to take in enough other nutrients due to an underdeveloped root system. The oldest leaves of a plant with a magnesium deficiency will have edges that appear yellowish, and in the center of each will be a light green area shaped like a lance.
- University of California Marin Master Gardeners: Feed Your Garden Well
- Learn2Grow: Pick up the Fertilizer Essentials
- The Mother Earth News: Hydroponic Nutrients -- Fertilizer for Your Hydroponic Garden
- North Carolina State University Extension Service: A Gardener's Guide to Fertilizing Trees and Shrubs
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