Fertilizing A Garden

Why You Should Fertilize Your Garden

You may be just starting a garden for the first time, or a seasoned gardener. Either way you should make sure your soil can provide the needed nutrients to your crops. To make sure this can happen, start with your soil preparations. A vegetable garden does best when the soil allows for good water drainage, can deliver nutrients, and has enough pore space for root development. Adding organic matter will help all of these conditions to improve.

A couple of days to a couple of weeks before you start planting your garden you should add a slow release fertilizer to your garden area. Organic fertilizers like manure are slow-releasing by nature, but an additional slow-releasing organic fertilizer could also be added.

The big reason fertilizer should be added to your garden is because of the continual withdrawal your plants make all season long. During the growing season your plants are transforming the raw nutrients in the soil to tasty, nutrient rich foods you and I eat.

This pathway leads directly to what you give your plants to start with. Providing healthy soil will lead to healthy plants for a healthy person. We have provided this page as a guide to help you make educated actions to grow your best garden yet.

What Is Fertilizer?

Fertilizer is made up of the elements needed for plant growth. These elements become the building blocks for amino acids, proteins, juices, and fibers that plants become. It comes in forms that deliver that nutrients quickly and over a period of time.

A balance of this quick-release and delayed fertilizer is needed to sustain your crops over their growing season. They are available in naturally occurring forms as well as synthetic or processed options. What you use is really a matter of preference while both routes have their benefits.

Most fertilizers will contain the basic components of Nitrogen, Phosphorous, and Potassium. These components are always listed in that order as numbers on the packaging. The numbers represent how much of each element, or a compound containing the element, can be found in the fertilizer mix.

To know how much you should add to your lawn or garden we recommend having your soil tested. Getting your soil tested is easy. Just contact your local agricultural extension office for details on how to test your soil and where to send it.

NPK Ratio - The Numbers on Fertilizer Bags

Nitrogen (N)

- Nitrogen is largely responsible for foliar growth. When you apply Nitrogen you should notice more leaves appearing within a couple of weeks. When growing a vegetable garden you shouldn't apply much Nitrogen until flowering starts. Even then, once you start getting into the main fruit production part of the season you should stop all fertilizer applications. This method will encourage fruiting while providing the necessary foliar support during key nutrient uptake times. Be careful not to apply too much Nitrogen. Too much Nitrogen will lead to lots of foliage with little fruiting. Too little will cause leaves to yellow, stunted growth, and small fruit development.

To calculate how much nitrogen is in a bag of fertilizer multiply the total weight of the bag by the concentration expressed as a decimal. If you have a 100lb bag that is an N value of 10 multiply 100lbs by 0.10 N to get a result of 10lbs of Nitrogen within a 100lbs bag of fertilizer.

Phosphorus (P)

- Phosphorus helps promote healthy stem, root, and fruit development because of its role in cell growth. Having a good concentration of Phosphorus will encourage better energy transfer in photosynthesis. Signs of not having enough Phosphorus in your soil include early leaf drop, weak flower stems, and buds remaining closed. Most of the Phosphorus used in fertilizer comes from Phosphate rock and is available in the compound P2O5. Elemental Phosphate does not naturally occur. For this reason the value you see expressed on a bag of fertilizer actually represents the concentration of the compound P2O5.

If your fertilizer recommendations are given per how much P2O5 you will need no additional conversions are required. However, if your recommendations are given per how much Phosphate you need, calculate how much is in a bag of fertilizer by figuring how much of the bag is P2O5 by multiplying the total bag weight by the concentration as a decimal. If your fertilizer bag is 100lbs with a ratio of 10-5-8 this calculation would be 100x0.05=5 lbs P2O5. Then multiply the 5lbs by a conversion factor of 0.437 to get a result of 0.2185 lbs of elemental P in a 100lb bag of 10-5-8 fertilizer.

Equation: __lbs bag of fertilizer x Concentration of P2O5 = __lbs of P2O5

__lbs of P2O5 x 0.437 = __lbs of P in the bag of fertilizer

Potassium (K)

- Potassium is actually the second highest component of a plant's dry material matter. It is used to encourage the “immune system” of plants by encouraging recovery from insect, disease, and weather damage. Not having enough will result in yellow leaf margins, weak flower stems, and poor fruit development. The most common form of Potassium in fertilizers is potash (pronounced pot-ash). Potash is really a combination of Potassium Carbonate and Potassium Chloride (salt) which is found in naturally occurring evaporated sea beds. The deposits are collected from underground or solution mining.

The amount of Potassium in a given fertilizer is not actually a representation of its elemental form. Like Phosphorus a conversion factor is needed to calculate the amount of elemental K in a bag of fertilizer. For calculations Potash is represented as K2O. To calculate the elemental content of K use the following equation.

Equation: __lbs bag of fertilizer x Concentration of K2O=___lbs of K2O

__lbs of K2O x 0.830 = __lbs of K in the bag of fertilizer

Types of Fertilizers

  • Controlled / Slow Release

    Provide nutrients over time ultimately requiring less applications through the growing season. Slow-release fertilizers should be applied to home vegetable gardens at the beginning of the season to provide nutrients during the peak production weeks and months.

  • Inorganic

    Inorganic fertilizers are often mistaken as unnatural materials because they come from non-living material. This is true, however they are usually sourced from rocks and minerals. Potash for example would be considered inorganic because it doesn’t originate from living plant or animal materials. The benefits of these types of fertilizers may be immediate or slow releasing.

  • Dry

    Dry fertilizers can be spread a few different ways including scattering, by row, around individual plants, or broadcast. Most dry fertilizers should be applied at a rate of 1 pound per 100 square feet. With dry fertilizers you want to be sure not to get it on any of the plant foliage. To protect your plant it is a good practice to spray them off after an application. Watering will also help to start working to fertilizer into your soil.

  • Water Soluble

    Water soluble fertilizers are usually for fast action. This route is great for spring applications to your vegetable garden. By using both slow-release and quick acting fertilizers you can ensure your crops have the nutrients they need throughout the season. To apply a water soluble fertilizer mix with water and apply using a watering can or hose. Check the fertilizer you are using to know if it can be applied to foliage or not.
    • Organic

      Materials from living or once living plants or animals. Organic matter should be added to your garden before planting to help improve your soil quality. Because organic fertilizers are usually slow releasing you will want to add more organic material to your garden over time. If you have poor soils it may take several seasons to achieve the soil health you want. There are several types of organic fertilizers including the following:


    - Compost is the result of plant material decomposing into an organic humus material. When composting, it is important that your materials experience enough time to decompose properly. Using compost material too early can result in nutrients actually being robbed from your growing plants due to the ongoing decomposing processes. An immature compost can also lead to more pests and disease in your garden.


    - Manure is made of the undigested material excreted by livestock. This material includes most of the nitrogen, phosphorus, and potassium in the food these animals eat. Because they don’t take it up, the nutrients are available for your plants. Because of the bacteria present in their excreted material, manure needs to go through a composting process to benefit your crops. Because the amount of nutrients in manure is so much lower than inorganic sources you will need to provide many more applications to truly benefit from it.

    Apply according to the recommendations given on your soil test report. 1 Ton of manure from cattle, hogs, or horses = an NPK ratio of 10-5-10. While the nutrient content of most organic fertilizers is much lower than inorganic sources, it does provide many benefits to your soil such as increased moisture retention, drainage, and reduced compaction.

    Bone Meal

    - A powder or granule made from animal bone. This organic option is a slow releasing fertilizer making it a great application for the early spring and fall. It will also add other micronutrients that are important to plant growth. If you live in an area with acidic soils this is the option for you. If your soils are more alkaline, bone meal won’t add many benefits to your soil as the phosphorus is tied up due to soil pH. To apply bone meal add 3 cups to every 100 square feet of your garden space. Mix into your soil well to avoid problems with attracting other animals to your garden.

    Cottonseed Meal

    - To amend new vegetable gardens with cottonseed meal apply at a rate of 4-6 lbs to every 100 square feet. This application should be made in the spring and fall as it is a slow-releasing organic fertilizer option. Cottonseed meal is also great for areas with alkaline soil that needs to be lowered. Consider using Cotton seed meal to areas with plants that especially love acidic soils, such as blueberries or hydrangeas, for greater success. Its average NPK ratio is 6-2-2. With low concentrations like this it would be a good idea to supplement with another fertilizing method that also has a faster release rate.

    Kelp Meal / Seaweed Extract

    - Seaweed extracts are extremely effective organic fertilizers. Because of its renewable sourcing this method is one of the most sustainable fertilizing options available. Like a vitamin is to us, seaweed extracts can help boost a plant's immune system. In addition to these benefits the molecules that contribute to a seaweed's flexibility aid in improving soil structure and water drainage. When applying this fertilizer you are not only getting better nutrients, you are getting a better environment for your plants to thrive in.

    Fish Emulsion

    - Fish Emulsion fertilizers are high in Nitrogen content making it a good option for use on leafy greens. The benefits of Fish Emulsion does reach the plants quickly making it necessary to reapply frequently. Because it is so plant friendly you can spray your mixture directly on the foliage of your plants without worrying about your plants burning. To use, just dilute about 2 Tbs of the emulsion with 1 Gal of water and spray on, or around your plants.


    - There are a couple of ways to use Alfalfa as a fertilizer. The first is to use it as a cover crop. Because Alfalfa develops deep roots it is perfect for breaking up clay soils to improve soil structure. It also grows quickly outcompeting weeds. And it's full of nitrogen and trace minerals. To use it as a cover crop you will need to plan ahead. If you want to use it more immediately you will need to use the pellet form. When using alfalfa as a fertilizer you should be aware that it does heat up the soil. Because of its heat properties it can also be added to compost piles as long as it is turned to prevent too much heat build up.

    Recommended Fertilizing Times

    Garden Preparation:

    • Apply a general fertilizer and a compost, or manure if you would like
    • 3-4-4 mix for Herbs and Vegetables
    • 3-4-6 mix for Tomatoes
    • Go light on the Nitrogen applications until flowering/fruiting starts
    • Plan to apply fertilizer about every 2 weeks for the first 8-10 weeks

    In Season:

    • 3 after transplanting - Cauliflower, Cabbage, Broccoli
    • After blossoming - Peas, Beans, Cucumbers, Muskmelon
    • After fruit sets - Peppers, Eggplant, Tomatoes
    • For Tomatoes, fertilize again after the first tomato is picked and another time a month after that.
    • When Sweet Corn is 8-10 inches tall, and 1 week after tassels develop
    • When grown to ⅓ of the mature size - Spinach, Kale, Mustard
    • Do not add excess Nitrogen to crops of Sweet Potatoes, Watermelon, Carrots, Beets, Turnips, Parsnips, or Lettuce
    • Stop all fertilizing when plants are in full fruit production

    Hydroponic Fertilizer

    When growing hydroponically it is important to add nutrients to your cycled water. Traditionally plants draw nutrients from the soil through their root systems. With a hydroponic system there is no source of nutrients unless it is added independently. One of the perks of growing with a hydroponic system is that you can control what nutrient sources are used. By controlling the amount of nitrogen, phosphorus, potassium, and other nutrients, you can initiate specific growth stages convenient for your personal or commercial production.

    If you are growing microgreens or other leafy vegetables we recommend using FloraGro, a hydroponic fertilizer with an NPK ratio of 2-1-6. Instructions for how much should be added are right on the bottle making it easy to grow as little or much as you want. One of the benefits of using FloraGro is that the solution is neutral so you don’t have to re-adjust your water pH after every application. And because this is a water soluble fertilizer it can be used on any media making it a perfect go-to nutrient source indoors and out.