REGISTER to the eventIn the second episode of the Radio Carbone podcast, we take a look at reducing tillage in organic farming.
While conversion to organic farming generally leads to an improvement in the carbon balance by eliminating the need for mineral nitrogen applications, some farmers fear that they will have to intensify tillage to manage soil contamination, which would have the effect of releasing some of the soil's carbon through mineralization of organic matter. This is the case of Heinrich, a farmer in Côte d'Or (21) who has been involved in the Soil Capital carbon program for ten years, and who has begun converting part of his farm to organic farming in 2020.
For Frédéric Thomas, farmer, founder of TCS magazine and trainer at Icosystème, ploughing is not the most effective solution, and carries risks. The challenge lies more in rotation planning, through the choice of crops, their sequence and soil cover.
In organic farming, weed control is a major concern in the absence of chemical solutions. According to Frédéric Thomas, ploughing is not the most effective way of reducing weed pressure. Although it eliminates the plants present, ploughing is only moderately effective on grasses, and very ineffective on broadleaf weeds". For example, Heinrich's mustard crop produces seeds that remain in the soil for years. As far as possible, Frédéric Thomas recommends leaving the seeds on the surface, where the depression is much better for both grasses and broadleaf weeds.
Fertility management is the second main challenge of AB. In the absence of mineral fertilisers, there is often a lack of available fertility, in other words, a lack of fertility flow. With 2.9% organic matter on average, Heinrich's soils have a certain reserve of fertility, called 'self-fertility' by Frédéric Thomas. However, ploughing leads to significant mineralising activity, and therefore the release of a large quantity of nutrients. "In late spring, these fertility flows can be used by the crops, but in late autumn vegetative growth is much weaker and we can see major leaks. This risk of losses is accentuated in a context where the plots have not been ploughed for many years, where we could almost be witnessing a grassland effect". Managing self-fertility is therefore central to organic farming, "we're going to have to learn how to use the self-fertility already present in the soil, recycle it and conserve it, or even increase it".
Frédéric Thomas explains that the dual objective of controlling soil degradation and managing fertility can be achieved through crop rotation. He advises reducing the proportion of winter crops (winter wheat, oilseed rape, winter mustard, etc.), which account for between 50 and 70% of Heinrich's crop rotation, but are ill-suited to organic farming. These crops are not well suited to organic farming, as they are long-lived, giving the soil time to become polluted, and their growth cycles are out of sync with natural mineralisation cycles.
Frédéric Thomas suggests switching the rotation to spring crops (spring wheat, spring barley, spring mustard, etc.) because they provide better weed control. Spring crops have vegetative cycles that coincide more closely with the natural mineralisation of organic matter, and they also make it possible to plant significant plant cover (summer or even winter cover until the crop is established) and to supply the system with carbon and fertility.
Frédéric Thomas also mentions the fact that spring crops are an opportunity to plant semi-perennial legumes (white clover, alfalfa, etc.), which can easily cover intercrops while recharging fertility.
At the end of a rotation or as a relay crop, it may also be a good idea to keep them for another year to restore structures, build up as much organic matter and fertility as possible and clean up the plots.
Finally, behind cereals, Frédéric Thomas suggests maximising cover by following a summer cover crop with a biomass-producing C4* plant such as sorghum, followed by a 'legumix' cover crop of faba bean, pea and vetch before a spring crop. In the event of significant fertility fluxes, it is possible that we might opt for a rye relay crop, which could allow us to consider planting a semi-direct soybean in the rolled rye cover. This latter sequence has been the subject of a great deal of work by ISARA in Lyon, and has proved effective in controlling weeds.
*A plant with a particular photosynthesis mechanism adapted to a hot climate, such as sorghum or maize.
By way of conclusion, Frédéric Thomas imagines the sequence shown schematically below, which would make it possible to "minimize tillage and maximize soil cover and biomass production, to constantly recharge fertility and return as much nitrogen as possible to the system".
In this podcast, Frédéric Thomas shows us that no-till farming with minimal tillage is possible, provided that the right crops and cover crops are chosen. In organic farming, weed control and fertility management must therefore be considered on a system-wide basis, using effective crop sequences and maximum soil cover.
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