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The Ultimate Guide to Soil Fertilizer

Explore the ultimate guide to soil fertilizers for optimal plant growth, covering types, application methods, and environmental impacts.

A healthy soil ecosystem is important. It supports agricultural practices around the globe. But what makes this possible is the use of fertilizers—soil fertilizers. So what are these soil fertilizers, and are they different from the other fertilizers? Let’s find out.

Read along as we uncover the different types of soil fertilizers, the best methods to use them, and much more. Whether you are a beginner, an experienced gardener, or an agriculturist, this guide will help you make informed decisions.

Importance of Healthy Soil for Plant Growth

Healthy Soil for Plant Growth

We are well aware that healthy soil helps in growing healthy plants. Let’s understand it like this: fertile soil contains a significant number of microorganisms with only a small amount of harmful ones. 

Biological aspect

The leftover plant and animal materials in the soil, such as fresh leaves and twigs, provide food to several microorganisms in the soil. This ensures there’s enough microbial activity in the soil, making it healthy and wise. 

Physical aspect

The physical condition of the soil, which is often a result of the biological activity in the soil, is also one of the determinants of soil health. Good biological activity enhances aeration in the soil, reducing the chances of compaction.

Chemical aspect

Additionally, healthy soil has a good mix of nutrients, neither less nor more. Chemically speaking, for soil to maintain good health, it should have the right pH, nutrients, and salt content. To be precise, it should comprise magnesium, calcium, and potassium to support a plant’s roots.

When soil is able to maintain its biological, physical, and chemical properties, you can deem it to be healthy.

What is a Soil Fertilizer?

What is a Soil Fertilizer

A soil fertilizer is any substance—natural or synthetic that aids in enhancing the health and vitality of the soil. With this, the soil fertility receives a boost as it gets replenished by elements that may have moved out with the last harvest.

Sometimes, even natural calamities like floods take away the essential elements, leaving the soil infertile and incapable of supporting healthy plant growth.

In a soil fertilizer, there are three main ingredients: nitrogen (N), potassium (K), and phosphorus (P). These are written as N-P-K and each value depicts the quantity of that nutrient in the fertilizer bag.

For instance, a fertilizer labeled 25-5-4 has 25% nitrogen, 5% phosphorus, and 4% potassium, plus some other useful ingredients.

Understanding N-P-K ratios and their meaning


The main component required for photosynthesis in a plant, giving them their peculiar green color.


It helps improve the immunity of the plant and builds its resistance to pests and diseases.


Plants can owe their root development, flowering, and fruiting to this component in the fertilizer.

These nutrients, along with other micronutrients and trace elements, are important for a plant’s health. In their absence, the plant shows stunted growth, poor yield, and sometimes even death. 

Types of Soil Fertilizers

Soil fertilizers are mainly of two types:

  • Organic fertilizers
  • Inorganic fertilizers

Organic fertilizers

Organic fertilizers

These come from natural plant or animal matter, either fresh or dried. While they are found to be easygoing and useful for the environment, they are slow-releasing in nature but good overall in the longer run.

Organic fertilizers usually contain only one type of nutrient, which makes them perfect for addressing one type of nutrient deficiency in a plant. When used correctly, these provide plants with sufficient amounts of micro, macro, and trace elements. 

Organic fertilizers are further categorized into:

  • Animal-based fertilizers
  • Plant-based fertilizers
  • Mineral-based fertilizers

Both plant and animal-based fertilizers release nutrients upon decomposition in the soil. The most common example of this is animal manure, and it has been in use for a long time. On the one hand, it adds nutrients to the soil, while on the other, it increases the water-retaining capabilities. 

The added benefit of this fertilizer type is that it provides a natural habitat for the earthworms. As these insects feed on the decaying organic matter, they release their waste into the soil, increasing its nutrients. Also, as they move, they create small tunnels in the soil for the air to circulate and plant roots to grow. 

Inorganic Fertilizers

Inorganic Fertilizers

The opposite of organic fertilizers is inorganic. They are essentially a byproduct of minerals and rocks and are often a result of physical and chemical reactions in them. However, it must be noted that the origin of their raw materials is animal or vegetable. 

Inorganic fertilizers are laden with three essential plant nutrients: nitrogen, phosphorus, and potassium. The elements are available in many different combinations. 

You can choose an inorganic fertilizer with an NPK value as per the plant’s requirements. For instance, if you own an ornamental plant with big leaves that are turning yellow, you might need a high-nitrogen fertilizer such as urea (46% nitrogen). 

Plants absorb inorganic fertilizers faster than organic fertilizers. Also, they dissolve easily in the soil and are readily available by the plant. 

Types of Inorganic Fertilizers

Nitrogen fertilizers: These are used as a source of nitrogen, particularly during the growing phase of a plant. Examples include urea, ammonium nitrates, tetra amines, and sodium chloride or nitrate.

Potassium fertilizers: as the name suggests, these are a rich source of potassium for plants. Muriate of potash is the most commonly used potassium fertilizer. It is 100% water-soluble.

Phosphorus fertilizers: These fertilizers add inorganic phosphorus to the soil. It converts to water-soluble inorganic phosphate as orthophosphate ions before being absorbed by the plant. Some microorganisms can convert rock phosphate into bioavailable forms.

Superphosphate: It is recognized as the first chemical fertilizer and was made by combining sulfuric acid and bone meal in 1840. Later on, bone meal was replaced by rock phosphate. It is useful for both fruiting and blooming in plants. 

Sulfur, calcium, and magnesium fertilizers: Apart from the above, calcium, magnesium, and calcium are also important nutrients. These are also known as secondary nutrients, as they are required in lesser quantities than the above-mentioned primary nutrients. 

Best Practices for Using Soil Fertilizer

When and how to use soil fertilizer for different types of plants

An interesting observation about plants is their dose of nutrition. These sedentary living beings don’t require nutrition every day and as often as humans do. Moreover, the requirement varies from plant to plant and from species to species.

The first example is of perennials. Plants such as bee balm, butterfly weed, coneflowers, etc., do not need any amount of fertilizer except for the initial addition of compost or manure. At the same time, there are plants like phlox, lupines, and daylilies that need fertilizers in early spring and again in summer.

Next is annuals. Annual flowers growing in baskets, pots, and bags need a consistent dose of nutrients for healthier blooms. A fast-release fertilizer application every two weeks works best for them. However, annuals planted in the soil can be sustained on slow-release organic fertilizer with mandatory fertilization during the growing season.

Deciduous trees and shrubs require phosphorus and potassium to develop healthy roots. They need more nutrients in the first 3 years; after that, they can sustain the rich planting mix they were sown into with a slow-release granular soil fertilizer. The fertilizer should be applied during early spring and when the new growth spurs. 

No fertilization for houseplants during the fall or the winter season. So feed them when they are growing, i.e., between March and September. Add an all-purpose soil fertilizer every month during this phase. 

Proper Application Methods 

To spread your soil fertilizer evenly, here’s what you should do:

Broadcasting or top-dressing

Broadcasting top dressing

In this method, the fertilizers are scattered randomly on the surface of the soil either manually or with machines. These soil additives should be mixed with the soil immediately to ensure it is not lost to water runoff. An ideal method for sowing a large number of seeds, this one works for plants where later on thinning is easy.


The fertilizers are placed either above or below the seed. The method is also remarked as a starter application since it assists in starting plant growth. You can choose to use side banding or side row banding method as per the requirement.

Side dressing

When fertilizers are applied to the sides of the plants, we call it side dressing. You can lay down soil fertilizer in a straight line close to the rows of plants on both sides. But before that, it is recommended to learn about the nutritional requirements of the plants. While some plants, such as corn, require a frequent dose of nitrogen, others, like sweet potatoes, work fine without it. 

How to Choose the Right Fertilizer?

Adding the right fertilizer is extremely important for the optimal growth of your plant. That said, here are a few factors you must consider before selecting the soil fertilizer.

Know your plant

Before you pick a fertilizer, you must know the type of plant you are going to use it for. Not all plants require fertilizers. Some can grow even without fertilizers, while others need a frequent or heavy dose of fertilizers. For example, tomatoes require a higher dose of nitrogen. Look for signs before fertilization.

Furthermore, plants don’t have a preference for fertilizers. It does not matter to them where their nutrition is coming from – organic or inorganic. They have the same response to both of them, i.e., the uptake and absorption. 

Analyze the soil

Ideally, the right time to test the soil is just before the fall fertilization or after harvesting the crop. Both these times are considered perfect for knowing about the nutrient deficiencies in the soil. 

The results of these soil tests will help you select the fertilizer. You will also learn about the soil pH, besides the nutrient levels such as nitrogen, calcium, potassium, phosphorus, sulfur, magnesium, and many more. Annual soil tests should be conducted at the same time every year.

Analyze the fertilizer

While organic fertilizers have an estimated value of nutrients, the commercially available fertilizers are labeled with a set value of nitrogen, phosphorus, and potassium, i.e., the NPK value. In addition, some fertilizers contain extra nutrients. Check it first. 

If you are unable to find a fertilizer with the ratio indicated by the soil test, then go for one that is the closest. Note that matching nitrogen value is more vital than the other two nutrients: potassium and phosphorus. 

Impact on the Environmental 

It is highly important to consider the environmental effect of the soil fertilizer you plan to use. For instance, overuse of fertilizers can change the acidity levels of the soil, thus affecting its fertility. This excessive fertilizer runoff can also cause contamination of the water bodies.

Too much and frequent use of chemical fertilizers can also lead to greenhouse gasses, a key factor in climate change. While tackling environmental impact is important, it is wise to use the type of soil fertilizers that cause the least amount of impact.

Some Safety tips for Applying Soil Fertilizer 

Here are a few safety measures that you must undertake when using soil fertilizers:

Store fertilizers safely

Whether you are using organic or synthetic fertilizers, store them in a cold and dry place. Also, these should be kept in a place that is inaccessible to the kids and pets (if any) in the house. This is important as these fertilizers are extremely detrimental to health if consumed.

Wear Protection

Wear Protection

When your skin comes in contact with high nitrogen levels, it can cause a chemical burn. Therefore, keep your body covered. Make sure you have a facial mask and hand gloves, and cover your body by wearing long sleeves and full-legged pants. Also, save your eyes by wearing safety glasses and staying protected.

Parting Thoughts

Soil fertilizers are an easy way to provide nutrition to your plants. When used appropriately after all the necessary analysis, they turn out to be the most potent food for all your plant types. Just make sure you keep yourself and your family safe in the process, and the soil will reward you with healthy plant growth. 

Additionally, when buying a soil fertilizer, go for a certified fertilizer seller, for high-quality products. 

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