Leonardite is a natural mineral of plant origin, rich in organic matter, that sits in an intermediate stage between peat and lignite. It formed from plant material buried over millions of years and, unlike ordinary lignite, it has undergone greater oxidation, so it holds a far higher proportion of humic acids. It is not a bulk material for growing media like perlite, but a soil conditioner and biostimulant, the richest available source of humic and fulvic acids for soil and plants.
What humic substances are
Humic substances come from humification, the long decomposition and oxidation of plant residues, and they split into three groups with different roles. Their main difference is molecular size and solubility.
| Group | Characteristics | Role in the soil |
|---|---|---|
| Humic acids | Large molecules, insoluble in acidic conditions | Improve structure and cation exchange capacity |
| Fulvic acids | Small molecules, soluble at any pH | Carry nutrients and are taken up by roots and leaves |
| Humins | Very large, insoluble molecules | Stabilise the soil and store carbon long term |
Leonardite stands out because it has the highest humic acid content of any natural source, often in the range of 60 to 70%, well above peat or compost.
How it improves soil and plants
Leonardite works on three levels at once. On soil structure it loosens heavy, clay soils, raises porosity and improves moisture retention. On nutrition it raises cation exchange capacity, so the soil holds more nutrients, while humic and fulvic acids act as chelating agents that keep micronutrients such as iron, zinc and manganese available, reducing fertiliser losses to leaching. On soil biology it feeds beneficial microorganisms and supports root development and plant resilience to stress. It also has an acidic pH, around 5.4, so it helps to lower the pH of alkaline soils gradually.
Where it delivers most
The greatest benefit shows up in soils and crops with specific problems. In alkaline and calcareous soils the chelating action keeps iron available and limits chlorosis, a problem that affects crops such as kiwifruit, citrus, grapevine and strawberry. In light, sandy soils with low cation exchange capacity it raises nutrient retention, so nutrients are not lost with irrigation. In intensive crops with heavy fertilisation it reduces fertiliser losses and improves how efficiently they are used.
The forms it comes in and how much to apply
Leonardite is sold as a powder, as granules for spreader application, and in a liquid form produced by alkaline extraction, while it is often combined with zeolite as a single soil conditioner. Solid forms usually come in 25 or 40 kilogram sacks. Because it is a concentrated source of humic substances, the rates are small compared with manure or compost. As a guide, trees receive a few kilograms each, for example around 3 to 6 kilograms for an olive tree, while broadacre crops use a few tens of kilograms per thousand square metres. In the garden and in pots it is worked into the soil at planting or added to the compost heap, while the liquid form is applied with irrigation. The exact rates are always on the packaging, depending on the humic content. It is non-toxic and allowed in organic farming.
Leonardite for alkaline and for acidic soils
Different types are sold according to soil pH. Types for alkaline and calcareous soils, which often contain humic iron, aim to lower the pH gradually and to address iron chlorosis, a common problem in kiwifruit, citrus, grapevine and orchards. There are also products with added calcium, suited to acidic soils, that supply the organic matter and humic substances without lowering the pH further. A soil test before choosing helps match the type to the soil.
How it combines with other materials
Leonardite does not replace bulk materials, it complements them. In a potting mix or a soil blend it is added at a small proportion to raise nutrient retention, while aeration and drainage are handled by materials such as perlite. It is often combined with zeolite, since both raise cation exchange capacity, or used together with manure and compost as a source of stable organic matter. Compared with peat, leonardite offers no bulk but a much higher concentration of humic substances.
What to watch
The humic content varies significantly between products and deposits, so it is worth checking the humic and fulvic acid content stated on the packaging. Leonardite improves how nutrients are used, but it is not a fertiliser and does not replace normal feeding with nitrogen, phosphorus and potassium. Its effect is gradual and builds through the season and from year to year, as soil structure and life improve, so dramatic results should not be expected from a single application.
Frequently asked questions
What is leonardite
It is a natural mineral, an oxidised form of lignite, and the richest source of humic and fulvic acids. It is used as a soil conditioner and biostimulant, not as a bulk material for growing media.
What is the difference between humic and fulvic acids
Humic acids are large molecules that mainly improve soil structure and cation exchange capacity. Fulvic acids are smaller, soluble at any pH, and carry nutrients that are easily taken up by roots and leaves.
How much leonardite do I apply
Because it is a concentrated source of humic substances, the rates are small. Trees receive a few kilograms each and broadacre crops a few tens of kilograms per thousand square metres, while the liquid form goes in with irrigation, always following the product instructions.
Leonardite for alkaline or for acidic soils
For alkaline and calcareous soils, types that lower the pH gradually and often contain humic iron are used for chlorosis. For acidic soils, products with added calcium are preferred, supplying the humic substances without lowering the pH further.
Leonardite or manure
Manure adds bulk organic matter and nutrients but can carry weed seeds and salts. Leonardite provides stable, concentrated organic matter and a high dose of humic substances in a small amount, without weeds or salts, and the two are often used together.
Does leonardite acidify the soil
Yes, it has an acidic pH around 5.4, so it helps lower the pH of alkaline soils gradually, without the abrupt action of chemical acidifiers.
