What professionals should know about soil organic matter

Post date:

Sunday 20 January 2019

For many years, having more organic matter in soils was considered to be better. The first lecture within the masterclass 'Living Soils' gave, however, quite a different perspective.

More soil organic matter is often seen as a double win for agriculture. The more organic matter there is in soils, the more CO2 is sequestered. More soil organic matter also leads to a higher nutrient availability to crops.

Ratios

Paul Bodelier, scientist at the Netherlands Institute of Ecology (NIOO-KNAW) presented his perspective on the topic. He showed that soil organic matter is not that simple: ratios among nutrients matter a lot.

The more phosphorous a soil contains, the faster the turnover and thus the decomposition of soil organic matter. Also the ratio between nitrogen and phosphorous may have an influence on it. To put it simple: more fertile soils also release more carbon, meaning that there is, by definition, no win-win correlation between nutrient availability and carbon storage.

Bodelier showed his work to a broad audience of professionals working in fields related to soil ecology. Attendees, included policy makers working at a government, but also a fertilizer product manager and some commercial advisers.

Masterclass

They all attended the masterclass 'Living Soils' organized by the Centre for Soil Ecology (CSE) and Wageningen Academy. It is the first time CSE is organizing such a series of events. During four lectures, given by scientists from Wageningen University, Wageningen Research and NIOO-KNAW they discussed various aspects of soil ecology.

The first course day, which took a whole afternoon and evening, was about soil organic matter, currently a hot topic in the management of (agricultural) fields.

Models

Marjoleine Hanegraaf, scientist at Wageningen Research showed how carbon turnover can be predicted using mathematical models. Historically, assumptions were made that roughly 2% of the organic matter decomposes in one year, but Hanegraaf included information such as nitrogen, clay content and current crop into her models. By doing so, she could predict the turnover rate of soil organic matter very accurately.

Mechanisms

Ellis Hoffland, professor at the Soil Biology Group of Wageningen University explained more about the mechanisms behind the formation and turnover of soil organic matter. She explained that most of the soil organic matter derives from plants. Living roots turned out to be the most important input for soil organic matter. They are even more important than above-ground litter, which is already well known for enhancing soil organic matter.

Janna Barrel set up rotation experiments, with oat crops followed by wild plants. She showed that the most common grass in the Netherlands, Lolium perenne, has a negative effect on oat growth. On the contrary, she showed that some other species enhance the soil organic matter content, as well as the development of an oat-supporting soil community. This soil community turned out to be more important for oat growth than the soil organic matter content itself.

Next weeks

Together, the presentations tell that simply enhancing soil organic matter not necessarily improves the fertility of soils. For instance, enhancing soil organic matter with cover crops or with compost can have a completely different outcome.