Characterisation of four sites in Denmark for long-term experiments on crop rotations in organic farming

A field experiment is conducted which focuses on different aspects of crop rotations for cereal production in organic farming. Three factors are included in the experiment in a factorial design: A) fraction of grass-clover and pulses in the rotation (crop rotation), B) catch crop (with or without catch crop or bi-cropped clover), and C) manure (with or without animal manure applied as slurry). The experiment is conducted at four locations (Jyndevad, Foulum, Flakkebjerg and Holeby), representing different soil types and climate regions in Denmark. There are 64 experimental plots at both Jyndevad and Foulum, 72 at Flakkebjerg and 12 at Holeby. In autumn 1996 soil samples were taken for soil chemical and physical characterisation. The soil horizons were described for 16 profiles in each plot. From these 16 profiles soil samples were bulked for 0-25, 25-50 cm, 50-75 cm and 75-100 cm depth. Soil texture, soil acidity (pH), soil content of total carbon, total nitrogen, labile phosphorus, exchangeable potassium, magnesium, extractable copper and cation exchange capacity (CEC) were determined in all or a subset of the plots. Soil water retention curves were determined in soil horizons in 2 to 4 profiles at each site. A few supplementary measurements were carried out in spring 1999 to determine soil water retention in the ploughing layer and penetration resistance. The elevation and slope of the areas were determined for Foulum and Flakkebjerg. The soil at Jyndevad is a coarse sandy soil characterised as Orthic Haplohumod, Foulum has a sandy loam classified as Typic Hapludult, Flakkebjerg has a sandy loam classified as Typic Agrudalf, and Holeby has a loam classified as Oxyaquic Hapludalf. There are no marked slopes or other elevation effects at Jyndevad and Holeby. The elevation varies about 3 m across the site at Foulum and about 8 m at Flakkebjerg. The maximum slopes are 4-5° at both these sites. The depth of the A-horizon varies between sites with average depths of 32 cm at Jyndevad, 44 cm at Foulum, 45 cm at Flakkebjerg and 35 cm at Holeby. The A-horizon is the upper soil horizon, which is influenced by tillage and which visibly has a larger humus content. The larger average depth of the A-horizon at Foulum and Flakkebjerg is largely caused by very deep A-horizons in parts of the experimental area. These two sites are also the only sites, where there is a consistent increase in clay content with increasing soil depth. The content of organic matter in the plough layer is about twice as high at Foulum compared with the other sites. The high organic matter content at Foulum causes the CEC here to equal that at Flakkebjerg in the upper soil layers, despite the differences in clay content. The soil pH was highest on the sandy loam and loam soils at Flakkebjerg and Holeby, but the content of plant-available P was lowest here, especially at Holeby. The content of exchangeable K was quite low at Jyndevad, indicating that potassium deficiency may become a problem on this coarse sandy soil. The soil capacity for plant available water in the depth 0-60 cm was on average 57 mm at Jyndevad, 127 mm at Foulum, 92 mm at Flakkebjerg and 120 mm at Holeby. The capacity for plant available water in the depth 0-100 cm was on average 76 mm at Jyndevad, 193 mm at Foulum, 142 mm at Flakkebjerg and 182 mm at Holeby. The larger contents of clay in the soil may, however, cause deeper rooting at Flakkebjerg and Holeby and thus larger total effective capacities for plant available water. The measurements of penetration resistance in the soil indicated a compacted layer at a depth of about 30 cm, especially at Foulum and Flakkebjerg. The dry bulk density increased with depth at all sites, but most markedly at Foulum and Holeby, where the dry bulk density approached 1.8 g cm-3 in the 75-100 cm layer. Both the compacted layer and the high bulk density may at least in places restrict root growth