Friday, 10 September 2021 12:00


Professor Michael H. B. Hayes Department of Chemical Sciences University of Limerick

Department of Chemical Sciences

University of Limerick



Soil Degradation – As Great an Immediate Threat as Global Warming


The most fertile soils of the world are under long-term cultivation (LTC), i.e. monoculture crops (for the most part maize (Zea maize) and soya beans (Glycine max)) are grown annually, or more frequently where climatic conditions and water availability allow, and crop residues and organic amendments are not returned to the soil. Such practices give rise to the microbial oxidation of the indigenous soil organic matter (SOM) and the CO2 released can be considered in the context of CO2 from fossil fuels. SOM reaches a steady state concentration in soil, and that depends on the management system that applies for each soil. Thus, soils in well-managed LT grassland will have a SOM content of 4-6%, but when put under LTC the SOM can decrease to a steady state of 1% or less, and at that level the soil aggregates are degraded and fertility is lost. Some estimates suggest that 50-100 crops from now the aggregate structures in the most fertile soils of the world will be degraded. That is of major concern in terms of support for a projected world population of 9.8 bn by 2050. It is obvious that we must seek ways to conserve soil aggregate structures.

In fertile soils the sand, silt, clay, and organic components associate in clumps or aggregates that give rise to what are generally known as soil crumb structures with pore spaces for air and water between the aggregates. SOM is the ‘glue’ that binds the inorganic components in aggregates, and we know that colloidal polysaccharide, polypeptide, humic and humin constituents are the essential components in the aggregate formation and stabilisation processes. Fifty years ago soils in the corn belt of the USA had 8-10% SOM in the plough layer (0-15 cm). Now the SOM in these soils is < 4%. That means that 7,5 – 8 x 104 L of ‘fossil’ CO2 has been released from each ha 15 cm. We have shown at UL that in LTC the SOM depletion takes place to the same extent throughout the soil profile. That means that the release to the atmosphere of fossil CO2 from such soils is very much greater. More than 90% of our Irish soils are under long-term grassland, forestry, or peat, and the SOM contents in these soils are in steady state and represent major sequesterers of CO2. We are not being given due credit for that.

The seminar will describe how it will be imperative to emphasize that, if LTC is to be continued, it will be essential to achieve conservation of aggregate structures, and of the OM in the aggregates. That will mean an in-depth awareness of the compositions and aspects of the structures of the organic components, how these arise from transformations of organic amendments, and how interactions or associations take place between these and with the soil inorganic components. Possibilities will be outlined for the macromolecularisation of structures in SOM to give materials with enhanced resistance to microbial transformations while maintaining soil fertility and crop yields. Emphasis will be given to work at UL in these areas.


Michael H. B. Hayes, MRIA, BAgr Sc University College Dublin, MS Cornell, PhD The Ohio State University. Employment: Chemistry at The University of Birmingham, 1960 till retirement in 1998. Honorary Appointment: Department of Chemical Sciences, University of Limerick since 1998. Honorary memberships: Soil Science Society of America; American Society of Agronomy and The International Humic Substances Society.

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