The increasing interest in humic acid reflects growing concern that some soils are becoming less biologically active, and growing interest in what can be done to reactivate them.
Soils can contain less than one percent to over 10% organic matter. It is the soil’s organic matter, and humic acid in particular, that gives soil much of its water-holding capacity and ability to be penetrated by plant roots, by binding onto clay particles, thereby preventing compaction.
What is Humic Acid?
Humic acid is a complex but stable high molecular weight organic molecule that is the end result of the decomposition of plant and animal matter in the soil. It is quite soluble in water but resistant to leaching because of its ability to bind loosely onto clay particles. It can comprise up to 80% of the total organic matter in soils, and therefore the soil’s carbon content. It is quite homogeneous physically, and is what gives most topsoils their dark brown colour. It can also be formed as a relatively pure compound (up to 80% by weight) as a natural by-product of coal formation. During the compression and heating of organic matter that forms coal, liquid organic compounds can be forced out, forming humic acid as they react with one another, cool and solidify, often in very large quantities.
The Role of Humic Acid
Humic acid formed in the soil has many valuable properties on top of its ability to adsorb water in plant-available form and making the soil more porous. It has a very high cation exchange capacity (CEC), which enables it to store, in plant available form, far more cations by weight than do clays. It also helps to keep soil phosphate in plant-available form by complexing soil iron and aluminium that would otherwise result in more phosphate being ‘fixed’ in unavailable form by soil clay particles such as allophane.
It also acts as a sort of ‘facilitator’ for plant uptake by virtue of it being quite soluble in water. As water is taken up by the plant, the humic acid molecules, and the nutrients adsorbed onto them, are drawn closer to the roots. These cations are exchanged for hydrogen ions by the plant roots. Some lower molecular weight humic acid molecules are taken up directly in the roots, and can be taken up through the foliage if applied as a spray or suspension. Chelated trace elements are an example.
Good and Bad Conditions for Humic Acid Accumulation
The formation of humic acid in the soil requires the presence of soil bacteria, lignin from plant residues and animal excreta, and a source of nitrogen. This is why composting of garden waste is often more rapid if some fertiliser nitrogen is added. In agricultural soils, the buildup of humic acid will generally only happen in a grazed pasture situation, particularly those containing legumes such as clover or lucerne. With the application of ryegrass/clover seed, lime and sufficient inorganic nutrients to bring their content in the soil up to the levels required by the clover component, soil organic matter levels can triple over a period of 10-15 years before a new equilibrium is attained.
However, a variety of practices can start to reduce humic acid levels in the soil. Overstocking and the overuse of fertiliser N are probably the two most direct causes. Excess application of fertiliser N can stimulate soil bacteria to break down humic acid for food, because of the imbalance of the soil carbon to nitrogen ratio (C/N ratio). Resulting problems are increasing pugging, compaction, poor drainage, nutrient runoff, and increasing susceptibility of pastures to drought, plant pests and diseases.
Soil Amendment with Humic Acid
These problems have lead to the introduction of a variety humic-acid based commercial products, particularly for use on heavily cropped soils overseas, which have been effectively ‘mined’ of their humic acid content over decades of continuous cropping. Almost all of these commercial products are derived from natural organic deposits, generally associated with coal formation.
On New Zealand soils, where the decline in soil humic acid content is not yet as severe, the primary benefit of these type of products is likely to be in assisting nutrient retention and uptake, and in reducing phosphate fixation by soil clays. Some are modified to contain far more nutrients in various combinations than were present in the natural deposit. Much more research is required in this area, both in terms of actual effects and in the cost-effectiveness.