Legume intercropping in chalky soil

Thierry Tétu, a teacher-researcher at the Jules Verne University of Picardie, has developed a method to double vetch cultivation in chalky soils where traditional crops like maize, sunflower, and soybean are not viable. His work focuses on replacing synthetic nitrogen fertilizers with biological nitrogen through high-density intercropping of cover crops with legumes.

Biomass growth through cover density

Tétu's key finding is that increasing the density of cover crops raises the concentration of CO2 in the soil, which enhances both the growth rate and the biomass yield of the crops. Selecting the right legume species for intercropping, based on soil type and previous crops, is crucial for success.

Practical farming near Côte d'Opale

Tétu manages a 35-hectare family farm near the Côte d'Opale, where the terrain is uneven and includes unproductive areas called "blanc." Since 1995, he has focused on intercropping with Alexandria clover and grows wheat, rape, and vetch, using the farm as a real-world testing ground for his research.

Challenges with ryegrass and legume inoculation

Ryegrass has become a significant problem in the "blanc" areas due to the difficulty of controlling it with herbicides in filtering soils. Tétu observed that legumes like soybeans fix nitrogen with the help of rhizobia bacteria, which can be naturally present in the soil if legumes have been grown frequently.

Testing legume cover strategies

Tétu experimented with double-density vetch on a legume cover platform and found that a previous legume crop significantly improved the growth of winter vetch. In 2025, he repeated the process with a second vetch crop, using a light harrowing to encourage ryegrass germination, allowing better chemical control later.

Nodule counts and crop rotations

Experiments showed that rotations including legume covers produced significantly more nodules on winter vetch plants compared to those without. Rotations with protein pea or vetch covers resulted in about 40 to 45 nodules per plant, nearly three times more than with mustard or no cover.

Future crop sequencing and benefits

Tétu suggests that sequencing peas and vetch with a large sorghum cover can help recharge the soil with carbon and test combinations like vetch-pea or vetch-pea-vetch. This approach can stabilize production, reduce pest risks, and improve soil coverage to limit infestations, while also producing seeds for future cover crops.