Regional P stocks in soil and in animal manure as compared to P requirement of plants in Finland
Phosphorus (P) is an essential plant nutrient and sufficient availability is sustained by soil reserves and fertilization. On the other hand, P leaching from agricultural fields causes eutrophication of the surface waters. Therefore, soil P levels and P fertilization should be adjusted according to plant need to minimize P leaching.
In this study we evaluated P requirement of plant production (cereals and grasses) at a municipality level to reach 95 % of the maximum yields. Soil test P (STP; acid ammonium acetate, pH 4.65) and content of P in manures were taken into account when estimating the regional P stocks and comparing them to the requirement for P fertilization. STP values originated from soil samples (total of 1 008 302 samples) taken by farmers from their own fields and analyzed at the soil testing laboratories in 2005-2009. These samples represent cultivated field area (92 %) that is participating in Finnish Agri-Environmental Program. Numbers of the production animals within the municipalities were obtained from the Information Centre of the Ministry of Agriculture and Forestry (TIKE), The Finnish trotting and breeding association, Saga Furs Oyj and Kopenhagen Fur. Information of the total amount of P excreted into the manure was estimated according to the feeding trials conducted at MTT Agrifood Research Finland.
Average STP value for the cultivated soils was 13.0 mg l-1 and for clay, coarser textured mineral and organic soils the average STP values were 12.3, 13.9 and 9.8 mg l-1, respectively. About half of the fields (49 %) had such a high STP value that P fertilization is unlikely to provide yield response. Least responsive soils for P fertilization were clay (69 % were nonresponsive), followed by coarse textured mineral soils (47 %) and organic soils (14 %). In the regional level, the shares of nonresponsive fields for P fertilization were highest in Southwest Finland (73 %) and in Åland (76 %).
Content of P in animal manures in Finland was 17.5 million kg in 2011, originating mainly from cattle (9.8 million kg), pigs (3.6), fur animals (1.8) and poultry (1.5). Most of the manure P was produced in the Ostrobothnia region (42 %). Manure P equaled to 8.8 kg ha-1, if spread evenly across all cultivated fields (excluding fallow) in Finland, whereas the P requirement of plants was 8.6 kg ha-1. However, in addition to manure P, 5.6 kg ha-1 of mineral P fertilizer was sold in 2011.
To depress the loading potential, P fertilization should be adjusted according to plant requirement, which would lower high STP values. At the moment manure P content would be enough for plant P requirement, with no need for mineral P fertilizer, if manure could be spread to areas with actual need for P. However, this will require novel methods for manure processing to make P transportation economically viable. Moreover, current Finnish legislation allows P use that is not optimal from the view of neither plant production nor the environment.
The work described in the report was performed within Work Package 4 (WP4) “Standardisation of manure types with focus on Phosphorus” within Baltic Manure.
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