Phosphorus: P-Balance and P-Content in Soils
An indispensable nutrient for plants and animals, phosphorus (P) circulates in fairly large quantities in the plant and animal production sectors. Animals excrete excess phosphorus in their faeces and urine, which is later spread on fields as fertiliser. In plant production, excess phosphorus from fertilisers (farmyard manure and mineral fertilisers) accumulates in the soil.Phosphorus from the soil can, however, end up in surface waters if it is washed away with the topsoil or washed out of the soil into the groundwater or tile drains.
In small and medium-sized lakes this can lead to environmental problems (so-called eutrophication), since phosphorus strongly fosters algal blooms on the water surface. After the algae die, they are broken down on the lake bed by oxygen-consuming aerobic bacteria. Once sufficient oxygen is no longer available, anaerobic bacteria become involved in the decomposition process. These produce reduced and in some cases toxic substances such as hydrogen sulphide, ammonia and methane. The lack of oxygen and the toxic products can cause fish die-off. Prudent handling of phosphorus is not only important because of eutrophication, however: unlike nitrogen, phosphorus is a non-renewable resource. Furthermore, mineral phosphorus fertilisers are often contaminated with harmful substances such as uranium or cadmium.
P-Content in the Soil
The P-balance is calculated with the SAEDN farm data, using the OECD method. A negative P-balance (input < output) can inhibit plant growth, while a positive balance (input > output) can lead to eutrophication. Consequently, it is aimed to achieve a P-balance that is as even as possible. In these circumstances, the plants are supplied with sufficient phosphorus and environmental pollution is limited.
In general, the growing year, which begins after the harvest of the previous main crop and ends with the harvest of the current main crop, is taken as the reference period. Farmyard manures (slurry, dung), mineral fertilisers, recycling fertilisers (compost, digestate) seed and atmospheric deposition are considered inputs for the P-balance. For atmospheric deposition, a value that is representative for Switzerland is assumed. The plant foods (e.g. bread grains, table potatoes) and animal feed (e.g. grass, hay, feed grains) produced, as well as the straw removed from the field, are considered outputs of the system.
The plant-available phosphorus content in the soil is estimated analytically with three extraction methods: The H2O-CO2 and H2O10 methods depict the phosphorus fraction in the soil readily available to plants, whilst the AAE10 method represents the reserve fraction. The H2O-CO2 method is typically used on field crops and grassland, the H2O10 method in vegetable and fruit production and viticulture. As part of the Proof of Ecological Performance (PEP), the soil phosphorus content is determined on each plot every ten years. To increase representativeness for agricultural practice as a whole, all data collected throughout Switzerland are used to calculate the indicator, rather than just the figures of the approx. 300 farms taking part in the SAEDN.
The H2O-CO2, H2O10 and AAE10 figures provide information on the quantity of extractable phosphorus, and are averaged over different areas or categories (e.g. the whole of Switzerland, cantons, landscape regions, types of use) in order to track their long-term development. In addition, the figures are interpreted according to the Principles of Agricultural Crop Fertilisation in Switzerland (PRIF 2017) and classified into five phosphorus supply categories ranging from A (poor) to E (enriched). The interpretation depends not only on phosphorus content (mg/kg), but also on soil properties such as texture, pH and soil organic carbon. The percentage of plots per supply class is also calculated for various aggregation classes.
One can assume a low environmental risk from phosphorus in the soil if as many plots as possible have no more than a sufficient supply of phosphorus (supply classes A, B and C). A soil with an adequate supply of phosphorus can cover the phosphorus requirements of crops with medium phosphorus sensitivity in the short term, even without fertilisation. To ensure the production performance of these soils in the medium term, maintenance fertilisation is necessary. The available phosphorus in the soil is therefore largely absorbed by the plants, whereby the risk of phosphorus release into other environmental compartments can be considered low. By contrast, a phosphorus supply to the soil that significantly exceeds the phosphorus requirements of most crops (supply classes D and E) indicates an increased risk of phosphorus transfer from the agricultural system to groundwater and surface water.
Bodeneigenschaften und Bodenanalysen.
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