Humic and fulvic acids are very similar, the main difference being that fulvic acids are smaller, which means they're more soluble in water.
They’re both important in the soil, though, and I think about them as one. I mostly call them humates.
It’s extremely beneficial to build up humates in our soil. Compost and humus contain them, so if you’re composting, you’re probably in good shape.
It can also be useful to buy them as a product that is derived from leonardite, a soft brown coal. Sometimes they come from other kinds of shale or humate deposits. I found this surprising when I first learned about humates, but it makes sense - both coal and compost are, essentially, forms of carbon!
The quality varies drastically from product to product, so it’s a good idea to buy it from someone you trust and learn a little bit about where good sources come from and how they’re processed.
It’s nice to find a product that has been processed naturally instead of with harsh chemicals. The product I currently carry has been processed with potassium hydroxide, which is allowed under organic standards.
I use humates mostly in liquid foliar applications. The dark powder is messy and gets into everything, so it’s better to mix it outside on a day without wind, and not even bring it inside. I also recommend wearing a dust mask.
The main benefit with using it as a foliar spray is that the plant will be able to take up and utilize the other nutrients in the solution many times more effectively. That’s because of how the humic acids combine with the nutrients, a process called chelation.
They are mostly used in such tiny amounts that you can purchase a pound and it can last a long time in a typical residential garden. After much head scratching and calculating back and forth I realized that a good rule of thumb is one teaspoon of powder dissolved in one gallon of water.
When making compost tea, it helps to add one tablespoon of humic acid powder per five gallons of water.
And here’s a little expert tip: It’s a great idea to mix humates in with calcitic lime before you apply the lime, especially in a soil with low organic matter. The lime wants to sink out of the root zone and the humates want to rise to the surface, so they help keep the lime up where you want it. It’s a bit finicky, but I do this at two tablespoons of dry humates per ten pounds of lime.
Here's my humic acids product.