What are common misconceptions about humus?
Written by
Kiana Okafor
Reviewed by
Prof. Samuel Fitzgerald, Ph.D.Humus is typically regarded as a single substance, but it is actually composed of three distinct chemical fractions. These vary profoundly in terms of solubility, molecular weight, and function. Most gardeners assume that humus is always the same. Still, each of these three components interacts differently with the soil minerals. It is indeed through the understanding of the chemical and physical differences between these that successful soil management can be accomplished.
Fulvic acids represent the water-soluble candidate fraction with low molecular weight. This candidate remains active in an active state, persisting even under acidic to alkaline conditions, and is an excellent carrier of plant nutrients. Humic acids dissolve only in alkaline solutions, thus forming larger molecular complexes. These contribute considerably to the stabilization of aggregates and improvement of the soil structure. Humic is completely insoluble and provides the long-term stability of organic material.
Fulvic Acids
- Chelate minerals into plant-available forms
- Transport nutrients across root membranes
- Function across wide pH range (3-10)
- Increase micronutrient solubility
Humin
- Bind tightly to clay minerals via covalent bonds
- Provide physical soil structure stability
- Resist microbial decomposition for decades
- Anchor carbon in mineral complexes
The differing decomposition pathways of these fractions are emphasized. Fulvic acids develop early from simple compounds, whereas the humin requires long decomposition. I have observed fields with a balanced fraction that show 40% better drought resistance. A soil test will determine your humus composition for selective management.
Humus fractions develop in different ways due to other management practices. Regular applications of compost stimulate the production of fulvic acids. Reduced tillage does a good job of preserving humin-clay complexes. Cover crops with deep roots promote the accumulation of humic acids. It is essential to maintain proper carbon-to-nitrogen ratios in the organic matter to stimulate the development of all three fractions as needed.
Recognizing the properties of humus composition can help avoid common mistakes. For example, adding lime without thoughtful consideration might precipitate valuable humic acids. Over-tilling can break the humin-mineral connection and lower carbon storage in soils. By preparing your amendments for the existing fractions of your soil, you can achieve greater functional gains from using them.
Read the full article: Soil Organic Matter: The Essential Guide
