Prof. Mike McLaughlin, from Adelaide University, has confirmed the important role that organic matter content plays within a soil.
The chemical breakdown of this fraction is very complex.
Speaking on the latest Tillage Edge podcast, McLaughlin highlighted the significance of two such components: Humic acid and fulvic acid.
“Humic and fulvic acids were terms used by chemists back in the day to describe the organic matter content of soils,” he explained.
“Fulvic acids are chemicals that are soluble at all acidities. In contrast, humic acids precipitate when soils are made acidic. This approach allowed chemists to differentiate soil organic matter into different pools.
“These days with new spectroscopic methods of analysis, we tend to move away from this classification scheme,” he added.
Soil organic matter
Prof. McLaughlin explained that no two soils will have the same organic structure.
“We are looking at a very complex collection of organic molecules that can change over time,” he said.
According to McLaughlin, the collective term ‘humic and fumic acids’ is a misnomer.
“The only way to produce these materials commercially is to treat brown coal with very strong alkalis. This is the only way to make them in a soluble form. So, in reality, these products are not acids at all,” he explained.
“A soil with a 2% carbon content contains approximately 20t of humic substances per hectare, measured to a depth of 6″ below the surface.
“Farmyard manure contains large quantities of complex organic carbon. A significant proportion of this will be soluble in the soil. This, in turn, encourages the production of those microbes which can actually manufacture humic substances.”
A small proportion of soil’s organic matter will be leached on a continuing basis.
“The drainage water from a field will always be coloured. This is caused by the soluble fraction of the humic component within the soil being leached away,” McLaughlin added.
“However, the vast proportion of the organic fraction within a soil is not soluble in water.”
The Adelaide-based soil scientist stressed that adding relatively small volumes of commercially manufactured humic substances to soil, possibly in conjunction with artificial fertilisers, will make little or no difference to its organic matter profiles.
“But adding large quantities of farmyard manures and composts will. Only by taking this approach is it possible to generate real change where soil structure, health and fertility care concerned,” he said.