Counter the loss of organic soil with straw and wood shavings

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Young spinach in soil amended with straw - Karolane Bourdon
Young spinach in soil amended with straw - Karolane Bourdon
Karolane Bourdon, a doctoral student at Université Laval’s Faculty of Agricultural and Food Sciences, shows that the application of straw or wood shavings could regenerate soils and restore their yield.

Half of the field vegetable sector’s sales come from production in organic soils, but if these are not protected from degradation, they could disappear within 50 years. Karolane Bourdon, a doctoral student at Laval University’s Faculty of Agricultural and Food Sciences, shows that the application of straw or wood shavings could regenerate these soils and restore their yield.

Many farmers in Montérégie have noticed a drop in production in the region’s peat soils due to their degradation. To ensure the sustainability of the organic soil production system, Karolane Bourdon, her thesis supervisor, Jean Caron, and thesis co-director, Josée Fortin, set out to develop and implement conservation practices.

Once organic soils are cultivated, they lose carbon in the form of CO2 emissions. This causes a progressive degradation of the soil, affecting microbial processes. "Comparing the least degraded soil and the most degraded soil, we found a loss of 7 tonnes of carbon per hectare per year for the former and 0.7 for the latter, a factor of 10 between the two. It’s impressive to see the extent to which the level of soil degradation affects microbial dynamics.

A solution adapted to reality

In the 80s, the recommendation was to apply copper to organic soils to reduce decomposition. However, some in the agricultural sector questioned the effectiveness of this measure. "The recommendation said it slowed decomposition by 70%, but when we tested it in the lab, the results were astonishingly low, with a maximum effectiveness of 37%, and an average between 4 and 7%, at most," reports Karolane Bourdon. "This is expensive for so little effectiveness, especially as copper can be carried by the wind to natural habitats and could contaminate the environment," she adds.

Karolane Bourdon therefore had to find another solution to keep these soils healthy. A regular supply of straw and wood chips showed great potential, given their high carbon content. "Eventually, we could sample the fields and, depending on the state of degradation and the nature of the organic soil, calculate the needs of agricultural enterprises. Some fields might need 2 tonnes of straw, and others 20 tonnes to compensate for carbon losses," explains the young researcher.

During her doctoral research, she also studied the effect of polyphenols on soil. These complex molecules can inhibit enzymatic activity in soils and slow down microbial degradation. "It’s a bit like slowing down the digestive system", she illustrates.

The remainder of the project will be used to optimize these solutions, since adding carbon to the soil means that microorganisms become more active and compete with vegetables for nutrients such as nitrogen, which can temporarily affect yields. "You could end up with a nitrogen deficiency, essential for crop growth. One option would be to pre-treat the straw, turning it into biochar for example," says Karolane Bourdon.

Future research will also enable us to test different agronomic itineraries. "If we plant spinach one year and carrots the next, when should we apply the straw? This information is important, as timing has a big influence on yield", continues the doctoral student. The growers taking part in the research have begun complementary trials on certain plots to answer this question.

The studies were published in the scientific journals Frontiers in soil science and Geoderma. The authors of these studies are Karolane Bourdon, Josée Fortin, Jacynthe Dessureault-Rompré, Christophe Libbrecht and Jean Caron.