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Organic agriculture aims at closing on-farm nutrient cycles and thereby reducing the import of external inputs. Along with the further specialisation in the (organic) farming sector, however, nutrient gaps occur, especially in the case of farms with no or little livestock and inadequate access to cost-effective organic fertilizer. One of the most limiting nutrients for plant growth is nitrogen (N). Therefore, often fertilizers need to be applied to allow sufficient plant growth, and thus to provide sufficient crop yields. In conventional agriculture, mainly synthetically produced fertilizers are applied. Their production is energy intensive and thus produce large quantities of GHG emissions.
In organic agriculture organic fertilizers (manure, slurry, compost, biogas digest), plant residues (e.g., mulching) or N-fixing legumes in the crop rotation are utilized. These have the benefit that usually on-farm materials are used, and thus less GHG emissions for production and transportation are emitted compared to the synthetic fertilizers.
However, also organic fertilizers have implications for global warming (due to direct and indirect N2O emissions during storage and application), for air pollution/eutrophication (due to leaching/emissions of NH3) for and groundwater contamination (due to nutrient leaching). Furthermore, manure management causes CH4– and N2O- emissions.
What are the SOLMACC farmers doing?
To close nutrient cycles as much as possible, and to reduce GHG emissions, the SOLMACC farmers tested different practices:
1. Composting: the farmers composted different materials, such as farmyard manure, grass-clover and/or other materials, such as residues from wine and olive processing.
For this, the organic residues are built-up to a heap followed by regular turning which results into stabilised organic material (= compost) ready for field application. It is crucial to collect manure and/or plant residues with the right C:N ratio (not wider than 30:1). The compost heaps should have an optimised surface-area-to-volume ratio in order to facilitate microbiological transformation processes. The regular turning of manure piles either by using a specific turning device provided by a contractor (rented machinery) or using own farm machinery (e.g., tractor with fork lifter), and covering the compost piles with fleece blankets and using a solid underground for pile built-up help to optimise the process. By this, GHG emissions, mainly methane can be reduced efficiently compared to manure storage on an open dung heap or in a slurry pit. Further, indirect positive effects on the climate change adaptation potential derive from the compost for the farmer. Compared to the application of mineral fertilisers, the application of compost on the field improves the soil structure, and therefore, improves the resilience of the farm against extreme weather events (droughts, heavy rainfall). At the same time, compost bears less hygienic risks than fresh manure – which can be of importance in vegetable production or on grasslands.
2. Biogas production and utilization from liquid wastes: some of the SOLMACC farmers fermented their farmyard manure in their own biogas facility. By this, methane emissions can be reduced in comparison to keeping the manure as a manure pile. Additionally, emissions from fossil fuel house heating can be avoided as the biogas produced can be utilized. Last, the biogas slurry is used as an on-farm fertilizers.
3. Mobile livestock housing: here, livestock was held in mobile housing on the arable fields. The straw beddings and manure from cattle, pigs, sheep and chicken were used as a fertilizer for the arable fields.