Nutrient use efficiencies and leaching of organic and conventional cropping systems in Sweden

Organic farming has been proposed as a means of reducing leaching and improving the use efficiency of plant nutrients in agriculture. In such systems, nutrient inputs originate from various organic sources or from naturally occurring minerals with low solubility. In this study, measurements of leaching and crop uptake of N, P, and K and determinations of mineral N in soil were conducted in tile-drained plots during a 6-yr period in two organic crop rotations, one with and one without addition of animal manures. In the latter, N was provided by green manures. For comparison, two conventional systems in which mineral fertilizers and pesticides were used (one with cover crops) were also included. Leaching loads of N were smallest in the conventional system with cover crops, on average 25 kg N ha(-1) yr(-1) over the 6-yr period. The corresponding amounts in the other systems were 39 (organic with animal manure), 34 (organic without animal manure), and 38 (conventional) kg N ha-1. Phosphorus-leaching loads were small overall in all systems (< 0.25 kg ha(-1) yr(-1)). Potassium-leaching loads reached on average 27 kg ha(-1) yr(-1) over the 6 yr in the conventional systems and 16 kg ha(-1) yr(-1) in the organic systems. When N leaching was expressed as a percentage of total N removal during the 6-yr period (leaching plus harvested N with crops), it represented 59% in the organic system without animal manure, 33% in the conventional system, and 22% in the conventional system with cover crops. These results clearly suggest that N use efficiency is improved if inorganic N fertilizers are used rather than green manures, especially in combination with cover crops. The superior system from all considerations was the conventional system with a cover crop.