Organic vs. Inorganic Fertilizer: Which Is Really Better?

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Sydney has over 15 years of experience in lawn maintenance, landscape design, and organic gardening. She has an A.A.S. in Landscape Design/Organic Produce Production from the Nebraska College of Technical Agriculture.

Why You Should Fertilize

There are two main reasons why fertilization is a good idea:

To improve the nutrient content of your soil. Unless you are blessed with soil that already contains abundant nutrients in just the right proportions, you will probably need to fertilize at some point. Even if you rely mostly on preformulated potting soil, you may need to fertilize your plants to give them the specific minerals they need.
To improve the structure and quality of your soil. The right fertilizer components, applied properly, can actually improve your soil’s moisture control, aeration, and population of beneficial microbes.

It is important to understand, however, that fertilizers are not miracle drugs for your soil. They can’t compensate for soil that is too salty, seeds that aren’t healthy enough to germinate, or many soil-borne diseases (source).

Before you plant, do two things:

Have your soil tested. This will tell you which nutrients are already present and which are missing. A simple DIY soil test (link to Amazon) can also help determine if there are any problems with the structure of your soil, which will affect water drainage and root growth.
Investigate the needs of the plants you intend to grow. Plants don’t necessarily require an exact balance of nutrients, and different plants have different nutritional needs.

What Organic and Inorganic Fertilizers Have In Common

The labels “organic” and “inorganic” seem worlds apart, but when it comes to fertilizers, the two are not as different as they seem.

Organic fertilizers come only from plant or animal sources–nothing produced in a lab.

Inorganic fertilizers are synthetically produced, but contain the very same chemicals and nutrients that organic fertilizers provide.

With a few exceptions, organic and inorganic fertilizers have the following things in common:

Nutrient content
Quick-release and slow-release varieties
Availability for purchase

Nutrient Content

The “big three” nutrients that all plants need are nitrogen (N), phosphorus (P), and potassium (K). Other minerals, including sulfur, magnesium, and calcium, are important, but all plants will suffer if they lack nitrogen, phosphorus, or potassium.

For that reason, to be considered complete, fertilizers available to buy will be labeled with N-P-K numbers. For example, a 10-10-10 fertilizer will have balanced proportions of nitrogen, phosphorus, and potassium (10% each), whereas an 18-6-12 fertilizer will be 18% nitrogen, 6% phosphorus, and 12% potassium.

The remaining contents are harmless fillers like perlite or sand (source).

These formula variations are part of the reason it’s important to test your soil. For example, your pre-fertilized soil may already be high in nitrogen, but low in potassium, which means the best fertilizer for you will be low in nitrogen.

The formulas also vary for grass, vegetables, and flowers. Fertilizers are not “one size fits all;” the best one for you will be the one that best supplements what your soil provides to meet the needs of your plants (source).

All fertilizers–both organic and inorganic–use N-P-K labels or have measurable ratios of nitrogen, phosphorus, and potassium. Furthermore, while the sources of these minerals may vary, the actual chemical compounds are the same.

For example, regardless of the nutrient source, plants can only absorb nitrogen in the form of nitrate ions (NO3^–) or ammonium ions (NH4⁺). These ions can come from organic or synthetic fertilizers; the plants reap the benefits either way (source).

Quick-Release and Slow-Release

As their names suggest, slow-release fertilizers deliver nutrients to your plants much more slowly than quick-release fertilizers. Both slow and quick-release fertilizers are available in organic and inorganic forms.

Slow-release fertilizers (also called controlled-release fertilizers) allow gardeners to fertilize less often, and nutrients are less likely to drain away.

Quick-release fertilizers provide necessary nutrients as soon as plants need them, but those nutrients are less likely to linger in the soil like slow-release varieties.

Slow-release organic fertilizers include compost, rock phosphate, and corn gluten. Quick-release organic fertilizers include blood, bone, and fish meal (source).

Some recommend starting your planting season by adding both slow and quick-release fertilizers to the soil before you transplant any seedlings. That way, your plants have both immediate and continued access to nutrients.

Again, the choice between slow-release and quick-release formulas is a matter of responding to what your plants need and what your soil already provides.

Availability for Purchase

Compost and manure are not the only organic fertilizer options. Many garden centers and online retailers carry organic fertilizers right alongside inorganic ones.

AgroThrive All Purpose Organic Liquid Fertilizer (link to Amazon) is a popular option that is available online.

If you choose to buy an organic fertilizer, look for a label indicating that it’s been certified by the U.S. Department of Agriculture or the Organic Materials Review Institute.

Differences Between Organic and Inorganic Fertilizers

While your plants will benefit from either organic or inorganic fertilizers, they do have several important differences.

Nutrient levels
Efficiency
Effect on the soil

Nutrient Levels

Inorganic fertilizers’ N-P-K ratios are determined by the manufacturer. There are no surprises in store for the gardener.

Organic fertilizers, on the other hand, have N-P-K ratios that are comparatively low. Compost, for example, the most widely used organic fertilizer, has a 1-1-1 N-P-K ratio. Fish emulsion has a ratio of 4-2-0 (source).

This does not mean that organic fertilizers are worse for your plants or soil, and highly balanced inorganic ones are superior. Unless your soil test shows that your soil is infertile, you may not need much–or any–additional nutrients.

The comparatively low nutrient ratios simply mean that one single source of organic fertilizer may not provide enough nutrients in a single application.

Organic gardeners can use different fertilizer sources to supplement nutrients that are missing, or they can purchase organic fertilizers that already contain a blend of nutrients from different organic sources. However, expect these blends to still have somewhat lower N-P-K ratios.

Efficiency

Part of the problem with the low nutrient ratios of organic fertilizers is that they are less efficient than inorganic fertilizers. For example, ten pounds of compost with a 1-1-1 ratio can accomplish as much as only one pound of inorganic fertilizer that has a 10-10-10 ratio.

This means that organic gardeners will have to buy or produce higher amounts of fertilizer over the course of the growing season, if they need to add any to their soil at all.

Furthermore, while there are some quick-release organic fertilizers, many of them, including compost, need to be broken down even further by microorganisms in the soil before they nourish the plant. This makes them less efficient at delivering nutrients, as well (source).

Effect on the Soil

Perhaps the most important difference between organic and inorganic fertilizers is their effect on the soil.

When applied properly, inorganic fertilizers will not damage your soil, but they also will not provide any long-term benefits. When inorganic fertilizers are overused, they can create a crust on the soil, making it tougher for you to work.

On the other hand, the most powerful advantage of organic fertilizers is their ability to improve soil structure and quality. Compost is a particularly superior soil conditioner, for those who have the patience to incorporate it into their soil season after season.

The following are some of the most notable positive effects of organic fertilizers, especially compost:

They can improve the drainage in tight, clayey soils.
They improve the airflow in the soil, which promotes healthy roots.
They can enhance water retention in sandy soils.
They make the soil easier to cultivate and till (source).
They feed the beneficial microorganisms that live in the soil and break down nutrients for plants (source).

All of that soil conditioning is in addition to the provision of nitrogen, phosphorus, potassium, and other minerals that plants need to thrive.

So Which One Is Better?

The unsatisfying answer to this question is, “It depends.” Organic and inorganic fertilizers have their own respective strengths and weaknesses, and they simply don’t meet all of the same needs. Consider the following factors to decide which is right for you:

The outcome of your soil test. You may need to make only small adjustments to your nutrient levels, in which case an organic fertilizer can keep you from overfertilizing and harming your plants. If you need to add a significant amount of nutrients, inorganic fertilizers will be more cost-effective.
The short-term needs of your plants. If you have a crop in dire straits, a quick-release, water-soluble inorganic fertilizer will be the fastest and cheapest solution.
The long-term goals you have for your soil. If you want to improve the general structure and fertility of your soil, you can’t do better than compost.

You might also consider a blend of organic and inorganic fertilizers as the season advances and your needs change. As long as you avoid over-fertilizing, blending the two won’t cause any harm.

Conclusion

Commercially available organic and inorganic fertilizers are capable of delivering the same nutrients and can be found in quick and slow-release formulas. They are not equally efficient, and only one–organic–can raise the quality of your soil over time. When choosing a fertilizer, consider your soil content, crop needs, and your long-term goals for your soil.