Fertilizer is any natural or synthetic material added to your soil or another growing medium to increase its growth rate, quality, and yield. There are two main goals of fertilizers. They either change the soil’s aeration and water retention, or they add or improve the nutrients in the ground.
People have been using fertilizer for ages and were primarily added to increase crop yields to feed an ever-growing population. For instance, it’s believed that both ancient Babylonians and Egyptians used animal manure and minerals on their barley and other crops to improve their growth rate. And in 1842, the entrepreneur John Bennet Lawes patented a new form of manure (who knew you could patent manure?) treated with sulfuric acid and phosphates. Most consider this to be the beginning of the artificial fertilizer industry.
As a home gardener or urban gardener in the 21st century, you’re probably less concerned about feeding your community and more focused on either supplementing your grocery budget or just experiencing the joys of working in the soil. But with such a rich history of fertilizers across the world, we can use this knowledge to improve the growth, beauty, and productivity of our plants.
Most commercial fertilizers typically provide one or more of three main macronutrients:
Many will also contain secondary macronutrients or soil amendments:
The following micronutrients are often included, as well:
Understanding NPK Values
When deciding between different types of fertilizers, you’ll likely notice a set of three numbers on the fertilizer bag or bottle, such as 10-8-10. These are the NPK numbers, which stand for (N) Nitrogen, (P) Phosphorus, and (K) Potassium. These specific macronutrients are needed by all plants. Knowing the particular needs of your plants will help you decide what your NPK ratio should be.
Together, the numbers on the fertilizer bag refer to the level of nutrient concentration in the fertilizer. If, for instance, you see a fertilizer with the numbers 10-5-5, then that fertilizer has two times more nitrogen than it has phosphorus or potassium.
But what does this actually mean for you? Depending on what your plant needs, you should purchase a fertilizer with the appropriate macronutrients. Nitrogen, for instance, helps with leaf development on the plant. At the same time, phosphorus is mainly responsible for flowering, fruiting, and plant root development. And potassium is associated with cellular strength and how nutrients, carbohydrates, and water move throughout the plant tissue.
So what NPK ratio do your plants need? If you’re gardening with soil, an excellent way to check your ratio starts with a soil test. You can purchase one from your local state extension office. This will tell you the current condition of your NPK and other nutrients.
When submitting the soil test, you can even list the specific plants you plan to grow, and the extension office can give you the NPK ratio needed to help them thrive. From there, if you’re lacking in any or all of the three main macronutrients, you can add nitrogen fertilizers, phosphate fertilizers, and potassium fertilizers.
If you’re using a soilless mix, it may already have the needed macronutrients to feed your plants for the first few months. And for most plants, an equal analysis fertilizer – or one that has an equal number of NPK (such as 10-10-10) – will usually meet your needs. From there, it can be a bit of trial and error. If the leaves on your kale are starting to yellow, you may need to ramp up the amount of nitrogen fertilizer being applied.
The soil’s pH level can also affect how certain macronutrients are absorbed by the plant. In some cases you may need to consider changing the pH with additives like lime or sulfur. Before doing this, though, speak with your local extension agents for advice.
Organic Fertilizer vs. Inorganic Fertilizer
When deciding between fertilizers, people often ask if they should use organic or inorganic fertilizers. Both options provide great benefits, and both come with some potential pitfalls.
Organic fertilizers are only made up of plant or animal-based materials – compost, leaves, manure, feather meal, blood meal, bone meal, fish emulsion, etc. Inorganic fertilizers are manufactured artificially and can contain minerals – many of which are mined from deep inside the earth and synthetic fertilizers made up of chemicals.
Inorganic fertilizers are usually more affordable than organic fertilizers. Their nutrients are (in most cases) available for immediate use by your plants. As far as cons go, inorganic fertilizers can cause chemical imbalances if misapplied. Inorganic fertilizers have also been known to cause leaching.
Inorganic fertilizers include chemically processed compounds, such as ammonium nitrate, potash fertilizer, and micronutrient fertilizer, as well as others.
Organic fertilizers will only release nutrients in warm and moist soil, which usually is the appropriate time for a plant to need additional nutrients. That said, they also rely on small organisms to break down the organic material, meaning it can take even longer for the fertilizer to successfully release its nutrients. The cost of organic fertilizers is typically more than inorganic fertilizers. But organic fertilizers continue to improve your soil, even long after you treated it. So this extra cost up front could end up saving you in the long run.
Types of Fertilizer
There’s a wide variety of fertilizers that gardeners can use to cause plant growth, root growth, yields, and hardiness. Often a combination of these fertilizers and techniques is needed for your specific plants.
A granular fertilizer is packaged in a dry form and made up of tiny pellets spread over the soil. Depending on the specific type and brand, granular fertilizers will last between one and nine months. Popular granular fertilizers include the Miracle-Gro Shake ‘n Feed All Purpose plant food, which will support root system strength and development and feed your plants for up to three months.
Granular fertilizers come in two primary types: slow-release and quick release.
Slow or quick-release fertilizers refers to the speed at which nutrients are released into the soil. With a slow-release fertilizer, the nutrients are released steadily as the soil’s conditions and small organisms break them down.
While slow-release fertilizers release their nutrients over time, quick release fertilizers rapidly release their micronutrients and macronutrients into the soil. These quick-release fertilizers are often water-soluble, meaning they will break down quickly and get to work on your garden.
Organic Granular Fertilizer
An organic granular fertilizer is a dry form of fertilizer made up of small pellets. It’s sourced from natural materials, such as cottonseed meal, blood meal, feather meal, bone meal, manure, and others. It is not made up of any synthetic sources.
Jobe’s Organics 09525 Purpose Granular Fertilizer is a good option when looking for organic granular fertilizer.
Liquid Fertilizer (or Water Soluble Fertilizer)
A liquid fertilizer or a water-soluble fertilizer can be applied to the root system or the plant itself, which is called a foliar application. When added to the roots, there is a slower release, but the effects can last longer. With a foliar application, the plant absorbs the nutrients quickly for near-immediate use. That said, these benefits on a foliar application are not long-lived, though they can be a good way to correct nutrient deficiencies.
Advanced Lawn Food Natural Liquid Fertilizer is a great liquid fertilizer option that you can use in your urban garden today.
Fertilizer spikes are compressed spikes made up of macronutrients, micronutrients, and other ingredients. They can either be made up of organic or inorganic materials. To apply them, you’ll need to hammer the spikes into the soil around the plant or tree. From there, the spikes are broken down by activity from either bacteria or fungi.
The idea behind fertilizer spikes is that they are a slow-release fertilizer that can meet your plants’ needs over a long period. However, it’s difficult to know precisely how much fertilizer you need. Due to the way the fertilizer stakes release nutrients, there is also concern that it may not reach the plants’ deeper roots. Not only this, but they may not distribute nutrients across soil evenly either.
In most cases, it likely makes more sense to use one of the other types of fertilizer, which are typically more cost-effective and easier to distribute evenly. If you would like to try using fertilizer spikes, start by talking to your state extension office, which can give specific recommendations for your situation.
Fertilizing Your Garden
Many types of fertilizers can and should be used. While the goal of fertilizers is often the same, they each have different methods of releasing and activating the nutrients that your garden needs. Start with a soil test in your garden, and then make the necessary changes to keep your plants thriving all throughout their growing season.