Comparison of Long‐Term Organic and Conventional Crop–Livestock Systems on a Previously Nutrient‐Depleted Soil in Sweden

Abstract

An 18‐yr field study was performed to compare organic and conventional cropping on a highly P and K depleted soil in southern Sweden that had not received any inorganic fertilizers (or pesticides) since the mid‐1940s. The major management differences between the systems were (i) growth of legumes every second year and use of legumes as cover crops in the organic rotation; (ii) application of P in the organic system at higher rates than for the conventional system; (iii) exclusion of oilseed rape (Brassica napus L.) from the organic system but inclusion of potato (Solanum tuberosum L.); (iv) frequent mechanical weeding in the organic system; and (v) use of solid manure in the organic and liquid manure in the conventional system. Concentrations of soil‐exchangeable P increased more after application of large amounts of basic slag and apatite in the organic system than after application of P fertilizers in the conventional system. Organic systems, which rely mainly on legumes for their N supply, will acidify soils faster than systems with fewer legumes in rotation. Crop yields were, on average, 50% less and weed biomass was greater (1–3 Mg dry matter ha−1) in the organic system than in the conventional system. Nitrogen was identified as the main yield‐limiting nutrient for organically grown crops. Despite this, and even with use of cover crops, N leaching was not reduced by organic farming. Soil carbon (C) concentrations decreased in both systems, but less so in the organic system due to higher C inputs and lower soil pH values. Still, organic farming seems not be an option for sequestering C in soil in Sweden. After adjusting the two systems to the same boundary conditions for an unbiased modeling comparison, the C input is ≈60% higher in the conventional system than the organic system. The agronomic efficiency of N was 9 to 10 kg grain yield kg−1 N in the organic system compared with 16–18 kg grain yield kg−1 in the conventional system. The long‐term use efficiency of P was lower in the organic system (7%) than in the conventional system (36%). These results show that yield and soil fertility are superior in conventional cropping systems under cold‐temperate conditions.