There are three essential macronutrients-nutrients needed in large amounts-that are the most commonly referenced in fertilizers. By understanding how each of these components influences plant growth and knowing your soil deficiencies (through a soil test), choosing fertilizer can be easy. On most fertilizer packaging, three numbers are listed, separated by dashes (e.g. 10-5-5). These numbers represent the percent weight of nitrogen (N), phosphorus (P), and potassium (K), always in the same order (also known as "N-P-K"). A fertilizer that contains all three macronutrients is called "complete" and if all three nutrients compose equal parts in the ratio (e.g. 20-20-20), it's considered "balanced". The ratio of each of these macronutrients to one another dictates the dominant reaction in a plant.
Nitrogen (N) encourages vegetative growth (leaves and plant shoots) and is an important component of amino acids that create proteins. Nitrogen is also necessary for creating chlorophyll, which makes plants green and is essential for photosynthesis (the process of a plant making its own food). Nitrogen, unlike the other macronutrients, readily evaporates and drains out of soil, so it is the macronutrient that most often needs to be replenished.
Phosphorus (P) encourages the growth of roots and flowers, as well as the fruit and/or seeds that follows blooms. Phosphorus is important to every plant process that uses energy, such as photosynthesis, new cells/growth, or water regulation.
Potassium (K) activates enzymes, which drive processes that improve a plant's ability to handle stress. Plant stresses include those caused by transplanting, heat, cold, drought, disease, and pests. Potassium is essential for overall plant health, too.
How Fertilizer Helps Plants
Here are some examples of how to use fertilizers to encourage the growth you would like. These examples assume your soil nutrients are "balanced" but deficient, or you are using a container with potting soil/media that contains no nutrients.
Seedlings Started Indoors
Let's say we have some seedlings growing inside with a couple of sets of leaves (seedlings aren't able to absorb nutrients until around the time they have a second set of leaves, called "true leaves"). The best choice at this stage is to give plants an organic, diluted, balanced, liquid fertilizer, so that the plant receives overall nutrition. Some commercial fertilizers, especially non-organic, are quite strong and should be diluted by half or more as to not burn tiny plants. If you wish to fertilize with every watering, dilute even more. Once plants are larger and transplanted, start applying a more focused fertilizer (except potassium at transplant; see Plant Stress).
Once seedlings like lettuce and kale are looking strong, about the size that they have 4 true leaves, start giving them a fertilizer that's higher in nitrogen to encourage green, leafy growth. The same applies to your lawn in spring or, believe it or not, onions! We often think of onions as a root crop, but the bulb is actually a modified extension of the leaves.
Once flowering plants like tomatoes and zinnias are almost large enough to produce flowers and fruits, start feeding them a fertilizer with a higher ratio of phosphorus (P) to encourage abundant and larger blooms and fruits or seeds. The larger the plant prior to blooming, the more food it can make for itself, which translates to more blooms, fruits, and seeds it can create. Larger plants also provide more shade for the fruit, which can be sensitive to sunburn.
Plants occasionally get stressed, such as, during a heat wave, at transplanting, or from pest/disease pressure. Potassium (K) can help plants handle that stress by managing their coping systems efficiently. For example, potassium helps water regulation, so the plant can react appropriately to drought by holding onto water better. Transplanting also causes some stress, as we disrupt the root ball and introduce plants to new environmental conditions (real sun, cool nights, new soil, etc.). Seaweed and kelp are great organic sources of potassium.
Liquid or dry fertilizer?
Liquid fertilizer is faster acting, but should be applied more frequently (weekly to monthly) as it does not remain in the soil very long. It will also have a near immediate impact and is likely the best choice for seedlings started indoors or plants showing a visible deficiency. Most dry (or granular) fertilizer is slowly released over a month to several months, so you spend less time applying it. It is generally worked into garden beds prior to planting. In most cases, plants can also be "side/top dressed"-applying fertilizer to the soil around the plant and lightly incorporating with a cultivator-during the growing season. Dry fertilizer is a great way to balance your garden to provide all the macronutrients for success, and you can either side/top dress a more targeted dry fertilizer as needed, or use a targeted liquid fertilizer regularly. Always follow fertilizer instructions. Too much fertilizer can burn plants, causing irreversible damage and stress.
While this information is designed to help you understand macronutrients as they pertain to fertilizer, we don't want to downplay that other macronutrients (calcium (Ca), sulfur (S), and magnesium (Mg)), as well as micronutrients (e.g., zinc (Zn), iron (Fe), boron (B)) are also essential, just in smaller quantities. Your soil test should also give you an indication of what micronutrients need balancing in the soil. Soil tests are often available through your local Cooperative Extension Office. Keep in mind that some nutrients act in concert, meaning one cannot be absorbed without the other, but again, your soil test will give you a clearer direction. Organic fertilizers often offer some micronutrients along with the macronutrients in the formula, but read the analysis to be sure.
The more you know, the more you grow! Over- and under-fertilization can invite pests, cause pollution, increase costs, and cause frustration and reduced harvests. Look for information on fertilizer needs for specific varieties on the inside of our seed packets. Proper fertilization puts your plants on the road to success giving them the nutrients they need to be bigger, better, and stronger!