Crotalaria

• To meet the demand of the rapidly growing African population, there is widespread consensus that farmers need to boost cereal crop yields.
• This is because African farmers struggle to access affordable mineral fertilisers, while a lack of biomass also places organic fertilisers out of reach.

The precious nutrients in green manure

• Although they are crucial for improving cereal crop yields, they can nevertheless harm the environment if used in excess, and are often too expensive for farmers.
• The war in Ukraine has only worsened the region’s fertiliser shortage, though prices have stabilised since their peak in 2022.
• Through symbiotic bacteria, these plants have the unique ability to fix nitrogen present in the atmosphere to their tissues.

Fertilizers, climate and rice growth in Madagascar

• In a recent study, we looked at the possibility of boosting the yields of a rain-fed rice fields in Madagascar by replacing some of the mineral fertilizers with green manure.
• Both the saturation of the floodplains used for irrigated rice and the emergence of upland rice varieties have led many farmers to take up rain-fed rice cultivation in Madagascar’s highlands.

Mucuna and Crotalaria

• CIRAD, provided by the author In our experiment, the green manure grown was a combination of “Mucuna” and “Crotalaria” legumes.
• Thanks to their complementary growth and above-ground structures, these two plants can produce a large quantity of plant biomass and thereby fix atmospheric nitrogen.

A promising model

• We achieved this with 40 kg/ha of nitrogen provided by mineral fertilizers when green manure was added to the soil.
• By way of comparison, more than double this quantity or 100 kg/ha, was necessary without the use of green manure.

Trade-offs to be made

• First, and as previously noted, heavy rainfall will dampen the benefits of green manure on rice productivity.
• To tackle this, farmers will have to dynamically manage residues and fertilisers and tailor them to individual crops.
• Next, the gains linked to the use green manure do not entirely offset the losses resulting from its cultivation.

Dual purpose legumes

• The model that we have calibrated can help to make this type of assessment in silico.
• Likewise, farmers needn’t have to make the choice between food security and fertilisers.
• Certain pulse crops, such as groundnuts, can be both used as green manure and sustenance for farmers.