The price of nitrogen fertilisers has increased during 2021 due to increasing energy prices, and the Russian attack on Ukraine by end of February 2022 has only worsened the situation. The result is that nitrogen fertilisers in April 2022 is about three times more expensive than at the beginning of 2021. Other mineral fertiliser nutrients, such as P and potassium (K) has increased dramatically in the same period due to the increasing energy prices. This situation raises the question whether we could utilise the nutrient in livestock manures better? Currently, around half of the nutrients in livestock manures are lost. A higher Nutrient Use Efficiency (NUE) of livestock manures would mean reduced environmental and climate impact of farming as well as less dependency on mineral fertilisers.
Agro Technology Atlas comprises scientifically validated descriptions of about 70 technologies for reducing the climate impact and environmental load of farming, and securing an optimal resource economy in primary food production. Among these are especially three groups of technologies that contributes to increased NUE in fertilisation with livestock manures*. These are:
- Slurry separation technologies, that are relatively cheap manure processing technologies with which slurry is separated into separation liquids with a high NUE of N and phosphorus (P), while the organic bound nutrients with low NUE are in the separation solids, that can be exported and for instance used as a valuable influent in an aerobic digestion process. Fertilisation with separation liquids rather than raw slurry would typically mean that the NUE of N is increased from 75% to 95%. In the case of P, the separation liquids contain a higher share of water-soluble P, and the total amount of P needed for fertilisation on a farm can be adjusted via the separation strength. Separation by use of a screw press will typically ensure that the amount of P in the separation liquids is complying with the needs for P fertilisation, and the P field and farm gate balances can thus in this way be eliminated or reduced to a minimum.
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Anaerobic digestion is due to the fermentation processes degrading a share of the organic matter in the influent biomass, depending on the digestibility of this as well as the retention time and the process temperature among other factors. Also, the pH would typically be increased with 0.5 - 1 units during the process, which also has some impact. Generally speaking, anaerobic digestion increases the NUE for N with about 15-20%, whereas the NUE for phosphorus increases with much more, typically 50-60%.
- Slurry acidification comprise three sub-technologies, namely in-field, in-tank and in-house acidification with each of them having characteristics that makes them most suited in specific situations. In-field and in-storage acidification will in general terms save the volatilisation of 15 kg N per ha via ammonia emissions, while this figure is about 30 kg per ha for in-house acidification because it also reduces ammonia volatilisation from stables and stores, besides from the field spreading. These figures are given under the assumption that fertilisation happen with a dose of 30 tonnes of slurry per ha. Furthermore, the adjustment of the pH results in a higher availability of P in the acidified slurry, wherefore acidified slurry is an effective fertiliser where a high effect of the P fertiliser is required, such as in case of start fertilisation of maize.
A proverb says that "An accident rarely comes alone.". It is in this case fair to turn this around and say that "A good agro-environmental technology has several advantages." and all three mentioned technologies provides more benefits than the basis for an increased NUE of livestock manures. For example, an aerobic digestion is also providing additional energy production, and slurry acidification, especially in-house acidification, contributes to reduce greenhouse gas emissions from livestock farming.
Please find the scientific substantiation for the above in https://www.agrotechnologyatlas.eu. For slurry acidification, see under "Other housing systems", "Storage systems" and "Spreading and outdoor handling".
* Whereas the NUE is not a standardised and fixed term, the used definition in this news post is that it is a percentage describing the share of the fertilised nutrient that is taken up by the crop. A NUE of 90% for nitrogen (N) is for example corresponding to a crop uptake of 90 kg N in case the crop is fertilised with 100 kg N.