Add inorganic fertilizer: freshwater marshes

How is the evidence assessed?
  • Effectiveness
  • Certainty
  • Harms

Study locations

Key messages

  • One study evaluated the effects, on vegetation, of adding inorganic fertilizer to restore or create freshwater marshes. The study was in Germany.


  • Community composition (1 study): One replicated, paired, controlled, before-and-after study in wet grasslands in Germany reported that the effect of annual fertilization (for 20 years) on the average moisture preference of the vegetation varied between sites.
  • Overall richness/diversity (1 study): The same study reported that the effect of annual fertilization (for 20 years) on total plant species richness varied between sites.


  • Overall abundance (1 study): One replicated, paired, controlled study in wet grasslands in Germany reported that plots fertilized every spring contained more vegetation biomass, after 4–18 years, than unfertilized plots.
  • Herb abundance (1 study): The same study reported that the effect of annual fertilization (for 20 years) on cover of herb groups (sedges, rushes, forbs, ferns, grasses and legumes) varied between sites.


  • Height (1 study): One replicated, paired, controlled, before-and-after study in wet grasslands in Germany reported that the effect of annual fertilization (for 20 years) on vegetation height varied between sites.

About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

  1. A replicated, paired, controlled, before-and-after study in 1987–2007 in two wet grasslands in northwest Germany (Poptcheva et al. 2009) reported that fertilized plots contained more plant biomass than unfertilized plots after 4–18 years, but that fertilizer had no consistent effect on vegetation cover, height or species richness. In the first year of the study, above-ground vegetation biomass was statistically similar in fertilized plots (540–590 g/m2) and unfertilized plots (480–510 g/m2). However, after 4–18 years of intervention, above-ground vegetation biomass was significantly greater in fertilized plots (520–820 g/m2) than unfertilized plots (240–390 g/m2). Over 20 years, other vegetation metrics did not respond clearly or consistently to fertilization across the two wet grasslands (data reported as graphical analyses; statistical significance of differences not assessed). These metrics included cover of plant groups (e.g. sedges, rushes, forbs), vegetation height, species richness and community moisture preference. Methods: In 1987, two plots (each 200–250 m2) were established in each of two wet grassland sites (with non-peaty soils, and maintained as fertilized pasture prior to the study). From 1987, all four plots were mown twice each year (June/July and September). From 1989, one plot in each meadow was also fertilized each year in early spring (60 kg/ha P2O5 and 120 kg/ha K2O). Vegetation was surveyed in mid-June. Cover/abundance of all plant species was recorded in four 4-m2 quadrats/plot, every one or two years between 1987 and 2007. Vegetation was cut from eight 0.25-m2 quadrats/plot in 1989, 1993, 1998 and 2007, then dried and weighed.

    Study and other actions tested
Please cite as:

Taylor N.G., Grillas P., Smith R.K. & Sutherland W.J. (2021) Marsh and Swamp Conservation: Global Evidence for the Effects of Interventions to Conserve Marsh and Swamp Vegetation. Conservation Evidence Series Synopses. University of Cambridge, Cambridge, UK.

Where has this evidence come from?

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Marsh and Swamp Conservation

This Action forms part of the Action Synopsis:

Marsh and Swamp Conservation
Marsh and Swamp Conservation

Marsh and Swamp Conservation - Published 2021

Marsh and Swamp Synopsis

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