Effects of cropping history and peat amendments on the quality of a silt soil cropped with strawberries

Long-term field experiments are invaluable sites for studying changes in soil quality due to management practices. We utilized a field experiment on silt soil with 18 years (1982–1999) of conventional and organic farming systems for studying long-term impacts of farming system on soil properties. During 2000–2002, strawberries were grown on the whole area. Some conventional and organic plots were amended with peat in 2000 prior to strawberry planting. Chemical, biological and physical soil properties were measured in 2000–2002 at the end of the growing season. Several chemical properties were lower in the organic than in the conventional farming system, including electrical conductivity (1.12 × 104 S cm−1 vs. 1.67 × 104 S cm−1) and amounts of extractable phosphorus (15.7 mg l−1 vs. 24.8 mg l−1 soil) and potassium (68.6 mg l−1 vs. 83.6 mg l−1 soil). In contrast, soil organic carbon was significantly higher in the organic than in the conventional system (2.5% vs. 3.0%) in 2001. Farming system had no impact on pH, extractable nitrate, calcium and magnesium. Microbial biomass carbon was 20–25% higher in the organic than in the conventional system each year and microbial biomass nitrogen 32 and 23% higher in 2000 and 2001, respectively. Farming system affected also earthworms positively by increasing their numbers from 64.9 earthworms m−2 soil in conventional to 134.9 in organic. The effectiveness of arbuscular mycorrhizal fungi was slightly elevated in organic as compared with conventional in 2001. In contrast, mycorrhizal spore numbers were lower in organic than in conventional (64.4 spores 100 ml−1 soil vs. 195.3 spores 100 ml−1 soil). Peat amendment increased soil organic carbon from 2.2 to 5.0% in 2000. In 2001, the corresponding values were 2.2 and 3.9%, indicating partial mineralization of added peat. Also water-holding capacity increased after peat amendment. In contrast, pH, electrical conductivity and amounts of extractable nitrate, phosphorus and calcium were lower in peat-amended plots. Most biological soil properties measured were also lower in peat-amended than in non-amended plots, including amounts of microbial biomass nitrogen (26.5 μg g−1 soil vs. 41.8 μg g−1 soil) and carbon (224.0 μg g−1 soil vs. 260.9 μg g−1 soil), AMF effectiveness (−18.5% vs. +4.2%) and arbuscular mycorrhizal spore numbers (59.9 spores 100 ml−1 soil vs. 168.6 spores 100 ml−1 soil) measured in 2000. Earthworms and nematodes were not affected by peat. It is concluded that management practice had a strong impact on soil quality, but more extensive studies are warranted for defining soil variables of major importance for crop yield.