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Doctoral thesis, 2018

Phosphorus leaching from Swedish arable organic soils

Riddle, Matthew


Organic soils account for 9% or ~225,000 ha of Sweden’s 2.5 million ha of agricultural land, with around half being intensively cultivated. This thesis examines the role of chemical and physical properties of arable organic soils in leaching of phosphorus (P) and whether these losses can be reduced by employing a metal-oxide coated biochar mitigation strategy. These issues were investigated by analysing two organic soils (organic 1 and organic 2) and two P-rich mineral soils (sand and loam) using laboratory and field-based methods. In a rainfall simulation study using short soil columns from four 20-cm soil layers to 80-cm depth, the main location of P release and the concentrations of P leached from the selected organic and mineral soils were identified. The highest concentrations of P were found to be released from the 0- to 20-cm layer in both organic and mineral soils, with 70-90% in phosphate-P form. The highest losses occurred from organic 2, followed by the sand. Soil test total-P correlated well with total-P and phosphate-P in leachate from the rainfall simulation study. A 17-month follow-up study using 90-cm long lysimeters with intact soil revealed that losses were highest from the organic soils, while the sand, known for its high P leaching, had relatively low leaching losses. The amount of total-P leached during the 17-month period decreased in the order: organic 2 (1.2 kg ha-1) > organic 1 (1.0 kg ha-1) > sand (0.3 kg ha-1) > loam (0.2 kg ha-1). Higher P losses from the organic soils were attributed to fewer sorption sites, humic matter competition with phosphate for those same sites and presence of preferential flow pathways (organic 1). Evaluation of ‘Skogens kol’, a wood biochar coated with iron (Fe3O4; magnetite) and ‘Ecoera’, an agricultural residue biochar coated with magnesium (MgO/Mg(OH)2; periclase/brucite) under laboratory conditions revealed a maximum sorption capacity of 3.38 and 65.4 mg P g-1, respectively. Results from the 90-cm lysimeter study also showed greater reductions in P leaching using Mg-coated biochar, with phosphate concentration in leachate being reduced by up to 74% in one organic soil. Magnesium-coated biochar performance was worse in the mineral soils, probably due to greater numbers of sorption sites already being present in those soils. Efficiency of P removal in relation to Fe applied on the biochar was shown to be good in laboratory studies, but poor at field scale. The two organic soils studied leached sufficiently high P concentrations and potential loads to contribute to eutrophication of surface waters. However, due to complex chemical and redox interactions with P, larger field-scale monitoring is required to identify whether the P losses measured in lysimeters are representative of those reaching surface drains.


phosphorus, organic soil, leaching, lysimeter, soil column, biochar, magnetite, periclase, brucite

Published in

Acta Universitatis Agriculturae Sueciae
2018, number: 2018:50
ISBN: 978-91-7760-240-8, eISBN: 978-91-7760-241-5
Publisher: Department of Soil and Environment, Swedish University of Agricultural Sciences

Authors' information

Riddle, Matthew
Swedish University of Agricultural Sciences, Department of Soil and Environment

UKÄ Subject classification

Environmental Sciences related to Agriculture and Land-use
Agricultural Science
Soil Science

URI (permanent link to this page)