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

Received for publication February 23, 2006. 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 {approx}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