Abstract

In the past few years, organic farming has been proposed as a possible way of reducing N leaching from agricultural soils and improving the use efficiency of plant nutrients. This is, to a large extent, considered to be attributed to the fact that synthetic fertilisers are not allowed in such systems and the N inputs mainly originate in various types of organic manures. In this overview, results from a number of Swedish field studies are presented in which crop yields, nutrient-use efficiencies and leaching in organic and conventional systems are evaluated. Some studies were conducted in lysimeters and others in large tile-drained field plots. In two lysimeter experiments, leaching of N derived from either poultry manure or red clover (Trifolium pratense L.) green manure were compared with fertiliser N, all labeled with 15- N. In the lysimeters on which poultry manure was applied, 32% of N applied leached during three years, whereas only about 3% leached in ammonium nitrate fertilised lysimeters. In plots on a sandy soil, annual N leaching loads averaged over the whole 6-yr crop rotation reached 39 kg N ha-1 in the organic rotations and 25 kg N ha-1 in the conventional rotation. Phosphorusleaching loads were overall small in all systems, whereas K leaching was highest in the conventional rotation (i.e., on average, 27 kg ha-1 yr-1). In terms of crop yields, they were reduced by 20 to 80% in the organic rotations compared to the same crops in the conventional rotations. This was explained in terms of N deficiency, weed competition, and infestation of crop diseases in the organic systems. These results suggest that organic crop production uses agricultural soils less efficiently, with no benefit for water quality.