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Soil Nutrient Evolution during the First Rotation in Organic and Conventional Farming Systems

Since 2008, a 5-year crop rotation experiment (winter wheat, pea, potato, barley undersown with red clover, and red clover) has been run in Tartu, Estonia, to evaluate the changes in soil chemical parameters under four fertilizer managements: (1) unfertilized conventional plots (conventional I), (2) conventional plots with addition of mineral fertilizers (conventional II), (3) organic plots with cover crops during the winter period (organic I), and (4) organic plots with the same cover crops plus a yearly amendment of 40 t ha–1 of cattle manure (organic II). After the first rotation, results showed significant differences (P < 0.05) in soil acidity dependent on the system with mean values ranging between 5.67 (conventional II) and 6.10 (organic II). In the organic II system, manure had a significant effect on the system, increasing the organic carbon (C) content by 0.34%, but in both organic systems, both cover crops and cattle manure were insufficient for maintaining a constant level of plant-available phosphorus (P) or potassium (K) in the soil. In the conventional II system, mineral fertilizers provided a sufficient amount of nitrogen (N) to the system and increased the concentration of P to 8.7 mg per kg. The yearly mineral or organic amendments did not counteract the significant decrease in soil-available K after the first rotation. Lastly, calcium (Ca) and magnesium (Mg) availability, strongly influenced by the soil pH local conditions, decreased with time for all systems even though organic ones presented greater concentrations of both compounds. In conclusion, the four fertilization systems managed independently would not guarantee a constant soil nutrient concentration after the first rotation.