Humic or Fulvic Acid: What Kind are Your Plants On?

By Harley Smith
Published: July 17, 2017 | Last updated: July 17, 2017 10:30:34
Key Takeaways

Discover the differences between humic and fulvic acids, and the best ways to apply them to your plants.

Source: Kateleigh /

Do you know the difference between humic and fulvic acids? If you don’t, you’re not alone. Even the regulators at the USDA haven’t been able to agree on a definition, and it has been causing a lot of confusion in product labeling.


Nevertheless, humic and fulvic acids are some of the best biostimulant products in nature, improving the uptake of nutrients in both soil and hydroponics, and once you understand the difference between the two, you will soon discover many applications for your garden.

Humic and fulvic acids are intermediate chelators. Chela means claw, so chelates are organic molecules that attach to mineral ions like a claw, holding them tightly enough so they don’t get locked up in the soil, but loosely enough so they are available to the plant on demand.


Humic acid molecules are larger than fulvic acid molecules and make great soil conditioners. Fulvic acids are smaller, more biologically active molecules that are faster-acting and make excellent foliar sprays. Both improve the uptake of minerals, stimulate plant growth, and improve the plant’s natural resistance to environmental stresses.

Humic and Fulvic Acid Combinations

Most humic acid products on the market are actually a combination of humic and fulvic acids. The humic acid fraction consists of larger molecules with lots of positive and negative charges on the surface of the molecules. It isn’t actually taken up through the roots, but it lightly holds onto minerals in the root zone, making them much more available to plants. It is especially helpful in soils with high clay content.

Clay particles and organic matter have a strong negative charge, and they tend to hold onto positively charged minerals too tightly for root hairs to easily take them up, meaning important plant nutrients such as potassium, calcium, magnesium and essential trace elements can be locked up by clay particles. Humic acid forms a bridge between the clay particles and the mineral cations.


Fulvic acid has a much smaller molecular weight, and is more biologically active. Fulvic acid not only surrounds mineral ions, it can also help transport them through the cell membrane and release them inside the cell.

This means fulvic acid makes a great foliar spray, allowing trace elements such as copper, iron, manganese and zinc to be better absorbed through the leaves. Fulvic acid also stimulates the metabolisms of plants, which makes fulvic acid treatments a great way to quickly correct trace metal deficiencies while stimulating plant growth.


Improving Iron Uptake

In soil, humic and fulvic acids help with the uptake of iron. Iron is one of the limiting factors in soil since it is easily oxidized and turned into rust. Once oxidized, iron becomes unavailable to plants, but humic and fulvic acids not only keep iron soluble, they also stimulate cell membranes to take up iron more efficiently.

Iron is a catalyst for chlorophyll production, so as plants take up more iron, they make more of the green pigment that harvests light energy to make sugars. Some of the sugars are used for energy to grow and reproduce, some are stored in the flowers and fruit, and some are leached from the roots to feed plant growth-promoting microbes in the root zone. Humic acid in the root zone also stimulates the uptake of nitrates and other essential elements.

Adding Seaweed Extracts

Humic and fulvic acids work even better when combined with seaweed extracts. Seaweed extracts are loaded with natural plant-growth hormones such as auxins and cytokinins. Humic and fulvic acids hold onto the auxins in an exchangeable form to amplify their effects on plants.

In fact, Virginia Tech’s 10-year study on organic biostimulants found that five parts of humic acid combined with two parts of seaweed extracts worked 50% better than either product alone. If you want more lateral root growth and more root mass, you can easily make your own humic acid and kelp root stimulant.

Fulvic Acid Foliar Sprays are Perfect for Clones

Fulvic acid foliar sprays make a great tonic for sick clones, especially when combined with kelp. If your cuttings are slow to root, or if the leaves start to turn yellow and fall off, try using a fine mist of fulvic acid and seaweed at the recommended 5:2 ratio.

Rooting hormones are actually made by plants in the leaves and transported to the roots to initiate new growth, so a foliar spray with fulvic acid and kelp is fast-acting when absorbed by the leaves.

Research shows that plants start to respond to fulvic acids in as little as four hours, and vigorous root growth and stress recovery is often evident in as little as two days. Before you give up on your precious seedlings or clones, try a little fulvic acid tonic first. (Also see: The Benefits of Adding CO2 During the Cloning Stage)

Humic and Fulvic Acids Provide Stress Protection

Humic and fulvic acids also help protect your plants against stress. In the Virginia Tech study, it was found that the 5:2 ratio of humic acid to kelp stimulated plants to make 50% more of a powerful plant protection agent called superoxide dismutase (SOD), an enzyme that sponges free radicals and protects the chloroplasts and membranes of the cell.

To be effective, SOD must be turned on by either an iron/manganese complex or a copper/zinc complex. If the enzyme is activated, a single SOD molecule can perform more than 1,000 chemical reactions per second in the cell, so if you condition your plants against stress with humic acids and kelp, plants will stay green longer and recover faster.

Improved levels of SOD can help protect plants against heat stress, drought stress, UV stress and salt stress. Just remember to condition plants against stress before the stress happens.

Humic and Fulvic Acids for Hydroponic Use

In hydroponics, both humic and fulvic acids work well, but there are minor differences. If you use RO (reverse osmosis) water, humic acid may be a slightly better choice. Humic acid contains both humic and fulvic acid fractions and it has a buffering effect on pH.

RO water alone has practically no buffer for pH. When using RO water, the pH can spike upwards during rapid vegetative growth, or crash during heavy fruiting and flowering—sometimes overnight. But humic and fulvic acids help buffer pH.

In nature, humic and fulvic acids raise the pH of acidic soils and lower the pH of alkaline soils, neutralizing both conditions, while having a moderating effect on your plants. Humic acid also adds more than 62 beneficial trace elements to the water. Since RO water is stripped of nearly all of its minerals, humic and fulvic acids make a great water treatment for hydroponics.

If you use hard water, shift the balance towards fulvic acid. Hard water is usually high in calcium and magnesium ions. Since fulvic acid molecules are smaller than humic fractions, they are better at surrounding the calcium ions to help keep them soluble and available to plants.

Fulvic acid is also more acidic than most humic acid products. Since hard water is generally higher in pH, fulvic acid may be a better choice when using well water in hydroponics. (See: The Top 4 Reasons Your pH is Out of Whack)

What’s On the Shelf?

It’s not always easy to determine which products on the store shelf contain humic or fulvic acid. That’s because humic and fulvic acids are complex molecules, and regulators have a hard time defining exactly what they are.

In most states, manufacturers aren’t even allowed to put the words fulvic acid on the labels. Ultimately, the Association of American Plant Food Control Officials (AAPFCO) is the group that decides on formal definitions and testing procedures for humic and fulvic acids.

Afterwards, the departments of agriculture in various states set labeling standards. In the meantime, look for the words humic acid on the label and check out what color the product is.

Products rich in humic acids are usually dark brown to black, and fulvic acids are usually yellow, orange, or amber.

Experiment with humic or fulvic acids to make your own custom tonics and before long, you will find the perfect recipes to grow healthier, faster-growing, more stress-resistant plants.


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Written by Harley Smith

Profile Picture of Harley Smith
Harley Smith is the director of research for NPK Industries. A veteran in the hydroponics industry, Harley has more than 18 years of consulting and educating experience. He is regarded as an expert on plant nutrition and organic bio-stimulants, performing research and new product development in the US and Europe.

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