Humic Acid vs Biochar: Differences & Uses in Garden

There are more and more products aimed at improving the soil structure of your lawn or garden.

What is the difference between humic acid and biochar? Which is a better choice for plants?

Humic acid is better than biochar for plants. Apart from working as a soil amendment in improving water and nutrient uptake, it also works as a biostimulant in improving stress resistance and germination. Biochar, however, only works as a soil amendment in improving plants’ uptake of water and nutrients and improving the soil structure.

Let’s take a look at the differences in appearance, their sources, and their uses for plants.

Table of Contents

1. Appearance

Humic acid products are liquid or solid with a dark brown color, but biochar is always solid with a deep black and sometimes glossy color.

Humic acid

Humic acid is derived from humus, peat, or leonardite, and it has the dark brown color of its parent materials. It is available in liquid, granular and powder forms all of which have the same dark brown appearance.

Liquid humic acid has the consistency of water, while the granular form is made up of small pellets with a rough texture. Finally, powdered humic acid has a texture that ranges from slightly coarse to very fine.

Biochar

Biochar, on the other hand, looks a lot like charcoal having a deep black, somewhat glossy appearance. Unlike humic acid, biochar exists only as solids and there are no liquid forms. It comes in chunks, powder, or flakes depending on the material it was made of.

The chunks and flakes have a rough texture and are brittle. Powdered biochar is made by grinding the biochar chunks or flakes, and can be coarse or very fine depending on the grinding process.

2. What is it made of?

Humic acid is derived from humus, peat, and leonardite. In contrast, biochar is obtained from the biomass of plants that’s been heated to extreme temperatures.

Humic acid

Humic acid is made of stable organic compounds found in highly decomposed plant organic matter that resists further decomposition. Humic acid is one of three humic substances in such materials (the other two being fulvic acid and humin).

The acid can be obtained from the humus on the surface of soils and in well-aged compost. It can also be extracted from leonardite and peat bogs.

Leonardite is a soft, waxy, black, or brown deposit of decomposed plant, insect, or animal matter mined from deep within soils. Leonardite is the chief source of humic acid as it contains up to 80% humic acid (Qian et al, 2015).

Peat is the next best source of humic acid, as it contains up to 40% humic substances (Jarukas et al, 2021)

Soil and compost humus contains the least amounts of humic acid, but are often the easiest to access. This is because the mining of Leonardite and the extraction of peat are complex processes, with considerable environmental impacts.

Biochar

Biochar differs significantly from humic acid, as there is no need for the decomposition of organic matter. Instead, it is made by a process called pyrolysis which involves subjecting plant biomass (like corn stover, hardwood, bagasse, switchgrass, rice hull, and fruit peels) to relatively high temperatures of around 572 to 1112 F (300 to 600 C) for short periods and in the absence of oxygen. As a result, biochar products that are black, porous, and carbon-stable materials have little to no oxygen content.

This differs from humic substances which contain significant amounts of oxygen due to the microbial decomposition of organic matter (HPTA, 2015)

The nature of the plant biomass used determines the form of biochar. For instance, bagasse and rice hulls will produce flaky biochar, while hardwood will produce chunks.

3. Uses for plants

Biochar and humic acid work differently to improve plant growth. Biochar is mostly a soil amendment, while humic acid is a biostimulant and also a soil amendment.

Biochar

Biochar is a soil amendment that improves water and nutrient uptake, increases pH levels, and improves aeration.

Biochar is mostly used as a soil amendment to modify the physical and chemical properties of soil for better plant growth and yield. The most prominent way it amends soil is by increasing water holding capacity, which it can do thanks to its high porosity that allows it to absorb and hold moisture for later use by plants.

Due to its high pH, biochar can increase the pH levels of acidic soils, and this improves soil fertility by encouraging microorganisms and lowering metal toxicity (Kookana et al, 2011).

Biochar is not a fertilizer itself, but due to its porous structure and charged surface, it can hold nutrients like nitrogen and phosphorus, preventing them from getting leached away. Research has shown that when used with compost, biochar can retain the nutrients released by compost, and can reduce the need for chemical fertilizers (Schulz and Glaser, 2012).

The ability of biochar to amend the soil by improving water and nutrient-holding capacity, and increasing pH levels, can enhance plant growth. One study found that the application of biochar to plants improved crop yield by an average of 10% (Jefferey et al, 2011)

Humic acid

Humic acid is a soil amendment and also a biostimulant. It improves soil properties like pH, and water capacity but also improves plant nutrient uptake and resistance to stresses.

Humic acid can serve as a soil amendment, modifying the physical and chemical properties of soil. Humic acid products can neutralize acidic and alkaline soils, as humic acid has shown buffer action between pH 5.5 to 8.0, which is a range of moderately acidic to moderately alkaline (Pertusatti and Prado, 2007)

Humic acid also improves nutrient uptake because its molecules have a net negative charge, attracting nutrients like magnesium, calcium, and iron, which are then transferred to the roots of plants for various plant processes. In a similar fashion, humic acid can reduce toxicity by capturing heavy metals like lead but without passing it on to the plant.

Humic acids can also enhance soil microbial activity, as it provides an energy source for microorganisms.

Unlike biochar, humic acid is also a biostimulant. It can affect the plant directly, and not through the soil. In addition to attracting and holding nutrients in the soil, humic acid can directly influence and increase plant uptake of nitrogen, phosphorus, and potassium (NPK) by altering root physiology and architecture (Canelas et al, 2014).

In foliar applications, humic acid enhances nutrient absorption by increasing cell wall permeability, leading to better crop health and yield.

As a biostimulant, humic acid can also make plants more resistant to temperature and water stresses. And it can influence the germination of seedlings (Turkmen et al, 2007)

The many benefits of humic acid as a soil amendment and as a biostimulant translates into improved plant growth and yield. And humic acid has been shown to increase plant yield by 38 to 62% (Ayuso et Al, 1996)

4. Are they necessary in the garden?

There are alternatives to biochar and humic acid. They include peat moss, coco peat, compost, and chemical fertilizers.

If for some reason, biochar and humic acid are unavailable to add to your garden soil, some other materials can be used as substitutes.

Peat moss is a great soil amendment, as it can increase the water and nutrient-holding capacity of soil by trapping moisture and nutrients within its fibrous structure. Peat is naturally acidic, and as such can be used to neutralize or lower the pH of alkaline soils.

Coco peat or coco coir has much the same benefits as peat. The major difference is that it is alkaline, so will neutralize or raise the pH of acidic soils.

Compost is an excellent soil amendment. It increases aeration by introducing soil organisms like worms and insects. It also improves water holding capacity, and it contains nutrients from the decomposed organic matter, and these nutrients are released into the soil for plant use. What’s more, compost will eventually become humus which contains humic acid.

Chemical fertilizers can be added to soils to supply both macronutrients like Nitrogen, Phosphorus, and Potassium as well as micronutrients like calcium, iron, and magnesium. The addition of these nutrients will improve plant growth in poor soils. But extended use of fertilizers isn’t advised, as it further degrades soils.

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References

Ayuso, M., Navarro, P., Hernández, T., García, C. M., & Pascual, J. L. (1996). A Comparative Study of the Effect on Barley Growth of Humic Substances Extracted from Municipal Wastes and from Traditional Organic Materials. Journal of the Science of Food and Agriculture, 72(4), 493–500. https://doi.org/10.1002/(sici)1097-0010(199612)72:4

Qian, S., Ding, W., Li, Y., Liu, G., Sun, J., & Ding, Q. (2015). Characterization of humic acids derived from Leonardite using a solid-state NMR spectroscopy and effects of humic acids on growth and nutrient uptake of snap bean. Chemical Speciation &Amp; Bioavailability, 27(4), 156–161. https://doi.org/10.1080/09542299.2015.1118361

Jarukas, L., Ivanauskas, L., Kasparaviciene, G., Baranauskaite, J., Marksa, M., & Bernatoniene, J. (2021). Determination of Organic Compounds, Fulvic Acid, Humic Acid, and Humin in Peat and Sapropel Alkaline Extracts. Molecules, 26(10), 2995. https://doi.org/10.3390/molecules26102995

Kookana, R., Sarmah, A., Van Zwieten, L., Krull, E., & Singh, B. (2011). Biochar Application to Soil. Advances in Agronomy, 103–143. https://doi.org/10.1016/b978-0-12-385538-1.00003-2

Schulz, H., & Glaser, B. (2012). Effects of biochar compared to organic and inorganic fertilizers on soil quality and plant growth in a greenhouse experiment. Journal of Plant Nutrition and Soil Science, 175(3), 410–422. https://doi.org/10.1002/jpln.201100143

Jeffery, S., Verheijen, F., Van Der Velde, M., & Bastos, A. (2011). A quantitative review of the effects of biochar application to soils on crop productivity using meta-analysis. Agriculture, Ecosystems &Amp; Environment, 144(1), 175–187. https://doi.org/10.1016/j.agee.2011.08.015

Canellas, L. P., & Olivares, F. L. (2014). Physiological responses to humic substances as plant growth promoter. Chemical and Biological Technologies in Agriculture, 1(1), 3. https://doi.org/10.1186/2196-5641-1-3

Pertusatti, J., & Prado, A. G. (2007). Buffer capacity of humic acid: Thermodynamic approach. Journal of Colloid and Interface Science, 314(2), 484–489. https://doi.org/10.1016/j.jcis.2007.06.006

Türkmen, N., Dursun, A., Turan, M., & Erdinç, E. (2004). Calcium and humic acid affect seed germination, growth, and nutrient content of tomato (Lycopersicon esculentum L.) seedlings under saline soil conditions. Acta Agriculturae Scandinavica, Section B – Soil &Amp; Plant Science, 54(3), 168–174. https://doi.org/10.1080/09064710310022014

HPTA. (2015). Biochar and humic substances: a comparison. Humic Trade. https://www.humictrade.org/wp-content/uploads/2013/07/Biochar-Report-HPTA-Science-Committee.pdf

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Carol Chung

Editor and Founder at AskGardening

Carol is a journalist, teacher, and plant lover. Over the past 20+ years, she has nurtured countless potted plants and orchids in small indoor spaces. Her expertise also extends to growing natural, organic vegetables in farms in South America. Passionate about sharing her knowledge, she writes to help others to create their beautiful gardens. Discover more about Carol.