What Plant Fertilizers Do I Need?

A brief explanation of the numbers on a bag of fertilizer

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Shovel and Rake

Table of Contents

When you walk through the plant fertilizers, there are a million options and it feels like you can never choose the correct one. How do you know what plant fertilizer to buy? Fortunately, there are only a few major plant nutrient requirements for most plants and knowing this make choosing a fertilizer a lot easier. If you haven’t yet, check out the Nutrient Knowledge page from Mosaic.  Applying fertilizer is vital to good production, especially in raised beds.  I would suggest putting fertilizer on at least once if not twice a year.

Fertilizers come in both organic and synthetic formulas.  The sources listed here are all organic sources since most people prefer organic gardening and it is easier for the average homeowner to get them.  Some nutrients do require that you help them to distribute throughout the soil.  You can do this a couple of different ways.  


You can dig down and till the nutrient into the soil, like I did with my tomato plants when I put ground eggshells around them.  You can also water them in, which is what I do with the majority of my garden.  Be sure to have a good irrigation system set up.  I prefer drip irrigation systems myself and I walk you through setting one up in this post.

Most bags of fertilizer will have 3 numbers on the front to indicate nitrogen, phosphorous, and potassium. The ratio of these three major nutrients is what is important when choosing a fertilizer to give you the desired effect. Again, that means that it isn’t the overall number but how they relate to each other that is important. A fertilizer where all the numbers are equal, for example 16-16-16, is a good overall fertilizer since it won’t push your plants to one specific direction. However, a plant fertilizer with a balance like a 20-5-5 will be better for a plant that needs high nitrogen.

Major Plant Fertilizers


Nitrogen Fertilizer Uses and Sources

Nitrogen is one of the most important plant nutrients for plants to grow properly. Any plant that is basically just leaves, corn, lettuce, and grass, requires high levels of nitrogen. Nitrogen helps plants grow a lot of healthy green foliage. The good news is that one of the best sources of nitrogen is coffee grounds and most coffee shops will give them to you for free. Eggshells and fruit pits are also great sources of nitrogen. A generally yellow appearance, especially in the mature leaves, indicates a need for more nitrogen in a plant.

Nitrogen has been overused in the last few decades. Unfortunately, this has caused huge environmental concerns. Nitrogen is very mobile within the soil and can easily be washed away which creates nitrates and nitrites downstream. This is something that can easily be dialed back by only using the amount of nitrogen that you need and not washing it downstream.


Phosphorous is essential for plants to convert energy at growing points, at its roots and blooms. The easiest way to add phosphorous is by adding bone meal and banana peels to your compost. Plants lacking phosphorous is have weak stems and stunted growth. Any plant that produces blooms or flowers needs phosphorous to grow, from roses to peppers.

Phosphorous Uses and Sources


Potassium Uses and Sources

Potassium is vital for giving your plants an all-around resistance to bad situations. It helps plants to reduce their water loss, resist insect invasion, and prevents diseases. Plants that are low on their potassium will develop yellow around the leaf edges and have weak stalks with poorly developed roots. Add potassium to your compost with kelp meal, alfalfa meal, or ashes from a fireplace. All plants can benefit from added potassium but perennial plants benefit more than others.

Secondary Plant Fertilizers


Magnesium helps move phosphorous around the plant to produce chlorophyll and proteins. Signs of deficiency are similar except that the leaves tend to cup upward slightly. Similar to phosphorous again, anything that blooms needs higher levels of magnesium, easily obtained from Epsom salts or Dolomitic lime.

Magnesium Uses and Sources


Sulfur Uses and Sources

Sulfur was originally used quite effectively as a fungicide and insecticide however, it has been found to be more important. It is necessary for proper amino acid production and winter hardiness as well. Since sulfur can easily be washed away by irrigation, consider adding Epsom salts or grass clippings to your compost to boost the content. Most plants in the Brassica family, broccoli and cabbage, require a high sulfur content to grow well.


Since it is vital for new cell walls, calcium is important for all young plant and fruit growth. It also helps the absorption of other nutrients. You can easily add calcium to your compost by adding gypsum, egg shells, and bone meal. A lack of calcium leads to weak roots and yellow younger leaves. One of the most obvious indicators of low calcium is blossom end rot in tomatoes. Blossom end rot is when the end of the tomato turns mushy, brown, and rotten.

Calcium Uses and Sources


Plant fertilizers are required in order for a garden to thrive to its highest ability. When you know your plants need some nutrient, give them some good balanced fertilizer with a little bit of everything or some good compost. If you have a soil test done, that’s the best way to tell what your plants may be short on. It’s also a good way to test pH to make sure it’s neutral and the nutrients that are in the soil can be taken into the plant.

Leaves are important indicators of what a plant needs as well. Which leaves are affected, the young or the mature? What is the pattern of colors and damage on the leaves? The overall nature of the plant helps as well. If the growth is stunted or the roots are weak, that could indicate other deficiencies as well. Don’t be afraid to take plants to a local county extension center to see if they can help you determine what you need as well.

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Head Homeowner

Hey everybody! I'm Sydney, the head homeowner here. Let me know if you have any questions you didn't find the answer to. Tell me what projects you're working on. I love to hear from all my readers.