Choosing fertilizers for your garden plants can be a daunting process. Fertilizers differ in important ways, such as what they are made of, their mineral content, and whether or not they are organic. Some fertilizers promote leaf growth, while others promote flower and fruit formation. Which fertilizer to use for your garden plants and when to use it, we explain in this article.
Table of Contents
- Why should you fertilize at all?
- NPK fertilizers: what do the NPK numbers mean?
- Which fertilizer to use for which plants?
- And what about pH?
- Liquid or dry fertilizer?
- Organic or synthetic fertilizers?
Why should you fertilize at all?
Plants need to be fertilized because most soils do not provide the nutrients needed for optimal growth. Even if you are lucky enough to start with a good garden soil, your plants will take up nutrients as they grow, making the soil less fertile. Remember those delicious tomatoes and beautiful roses you grew last year? That required a lot of nutrients from the soil. When you fertilize your garden, you replenish the lost nutrients and ensure that this year’s plants have the conditions they need to thrive.
NPK fertilizers: what do the NPK numbers mean?
There are three essential macronutrients, nutrients that are needed in large quantities, that are most commonly mentioned in fertilizers. Understanding how each of these components affects plant growth and knowing your soil’s deficiencies (through a soil test) can make choosing the right fertilizer easy. Most fertilizer packages list three numbers (e.g., 10-5-5). These numbers represent the percentage 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 a “complete fertilizer,” and when all three nutrients are present in equal proportions (e.g., 20-20-20), it is considered “balanced.” The ratio of each macronutrient to each other determines the predominant response in a plant.
Nitrogen (N) promotes vegetative growth (leaves and plant shoots) and is an important component of the amino acids that make proteins. Nitrogen is also necessary for the formation of the pigment chlorophyll, which makes plants green and is essential for photosynthesis. Since nitrogen, unlike the other macronutrients, easily evaporates and drains from the soil, it is the macronutrient that most often needs to be replenished.
Phosphorus (P) promotes the growth of roots and flowers, as well as fruits and/or seeds that follow flowering. Phosphorus is important for all plant processes that use energy, such as photosynthesis, new cells/growth or water regulation.
Potassium (K) activates enzymes that initiate processes that enhance a plant’s ability to cope with stress. Plant stress is caused by transplanting, heat, cold, drought, disease, and pests, among other things. Potassium is also important for overall plant health.
Three additional nutrients that are important, but in much smaller amounts:
- Calcium is needed by plants in cell membranes, at growth sites and to neutralize toxic substances. Calcium also improves soil structure and helps bind organic and inorganic particles together.
- Magnesium is a component of chlorophyll. Without it, plants cannot process sunlight.
- Sulfur is a component of many proteins.
Which fertilizer to use for which plants?
Below are some examples of how you can use fertilizers to promote the growth you want. These examples assume that the nutrients in your soil are “balanced” but deficient, or that you are using a container of potting soil that contains no nutrients.
For growing seedlings indoors.
Have you grown your own seedlings? Once they have developed two true leaves, you can start fertilizing. At this stage, it’s best to give the plants an organic, diluted, balanced liquid fertilizer to provide the plant with overall nutrients. Some commercial fertilizers, especially non-organic ones, are quite strong and should be diluted by half or more to avoid burning the delicate plants. If you want to fertilize every time you water, dilute even more. Once the plants are larger and transplanted, you should begin more targeted fertilization.
Leafy vegetables and turf in the spring
Once seedlings like lettuce and kale look vigorous and have about 4 true leaves, you should give them a fertilizer with higher nitrogen content to encourage healthy, leafy growth. The same goes for your lawn in the spring or, believe it or not, for bulbs!
Fertilizer for flowering plants
Once flowering plants like tomatoes and zinnias are almost large enough to produce flowers and fruit, you should give them a fertilizer with a higher percentage of phosphorus (P). The larger the plant before flowering, the more flowers, fruit and seeds it can form. Larger plants also provide more shade for the fruits, which can be sensitive to sunburn.
Plants are occasionally stressed, for example during a heat wave, when transplanting, or by pests and diseases. Potassium (K) can help plants cope with this stress by efficiently controlling their systems. For example, potassium helps regulate water so that the plant can respond appropriately to drought by storing water more efficiently. Transplanting also causes stress by breaking up the root ball and exposing plants to new environmental conditions (real sun, cool nights, new soil, etc.). Algae are excellent organic sources of potassium.
And what about pH?
Even if the right nutrients are present in the soil, some of them cannot be taken up by plants if the soil pH is too high or too low. For most plants, soil pH should be between 6.0 and 7.0. You can use a soil test to measure the pH of your soil. You can send a sample to a lab or buy a kit for home and do it yourself. Lime or wood ash can be used to raise the pH. Sulfur or aluminum sulfate, on the other hand, can lower the pH. Remember that it is best to raise or lower soil pH slowly over the course of a year or two. Drastic adjustments can be worse. Again, applying compost is a helpful solution. Compost moderates soil pH and is one of the best ways to maintain the ideal level of 6.5.
Liquid or dry fertilizer?
Liquid fertilizer works faster, but should be used more frequently (weekly to monthly) because it doesn’t stay in the soil very long. It also has an almost immediate effect and is probably the best choice for seedlings grown indoors or for plants that have a visible deficiency. Most dry (or granular) fertilizers are released slowly over one to several months, so you spend less time applying it. It is usually incorporated into the beds before planting. In most cases, plants can also be fertilized during the growing season by applying the fertilizer to the soil around the plant and lightly working it in with a hand cultivator. Dry fertilizer is a great way to provide your garden with all the macronutrients that are important for success. You can apply a more targeted dry fertilizer as needed or use a targeted liquid fertilizer on a regular basis. Always follow fertilizer instructions. Too much fertilizer can burn plants and cause irreversible damage and stress.
Organic or synthetic fertilizers?
Does it really matter to plants where they get their nutrients? Yes, because organic and synthetic fertilizers provide nutrients in different ways. Organic fertilizers are made from naturally occurring minerals and organic material such as bone or plant meal or composted manure. Synthetic fertilizers are made by chemically processing raw materials.
Generally, the nutrients in organic fertilizers are not water soluble and are released slowly to the plants over months or even years. For this reason, organic fertilizers are best applied in the fall so that the nutrients are available in the spring. These organic fertilizers stimulate beneficial soil microorganisms and improve soil structure. Soil microbes play an important role in converting organic fertilizers into soluble nutrients that plants can absorb. In most cases, organic fertilizers and compost provide all the secondary and micronutrients your plants need.
Synthetic fertilizers are water soluble and can be absorbed by the plant almost immediately. If you apply too much synthetic fertilizer, the foliage can “burn” and damage your plants. Synthetic fertilizers give plants a quick boost, but do little to improve soil structure, stimulate soil life or improve long-term soil fertility. Synthetic fertilizers have some advantages in early spring. Because they are water soluble, they are available to plants even when the soil is still cold and soil microbes are not yet active. For this reason, some organic fertilizers also contain small amounts of synthetic fertilizers to ensure nutrient availability.
For the long-term health of your garden, it’s best to feed your plants with organic fertilizers and compost. This will give you soil that is rich in organic matter and beneficial bacteria.
Over-fertilizing or under-fertilizing can attract pests, cause pollution, drive up costs and lead to frustration and crop loss. Proper fertilization gives plants the nutrients they need to grow bigger, better and stronger!