Soil organic matter management within the legal constraints of the fertilization laws – BOPACT field trial

Almost all soil and crop management practices have implications for soil organic matter (SOM) but the most obvious way for increasing the SOM content is by using organic fertilizers such as animal manures or compost. However, legislative restrictions related to the Nitrate Directive limit the use of fertilizers and consequently may constrain the built-up of stable SOM. Therefore, in the spring of 2010 a long term field experiment (BOPACT) was established at ILVO to investigate if the SOM can be increased within the legal constraints of the Manure Decree using slurry and the application of good agricultural practices (cover crops, straw incorporation) and, if not, if this goal can be reached with an extra dose of compost without increasing N leaching. The experiment has a strip split plot design with three factors and four replications. The factors are 1) slurry application (cattle vs pig slurry), 2) tillage practices (ploughing vs non-inversion tillage), and 3) compost application (0 vs 2 ton C.ha-1.year-1). The trial has a 4-year rotation with maize, potato, summer barley and leek, with cover crops during winter periods. After three years, the change in SOM content (0-30 cm) was significantly (p<0.05) higher for cattle slurry compared to pig slurry and for compost application compared to no compost amendment. Moreover, in 2012 the hot-water extractable carbon was significantly (p<0.01) higher in the compost plots than in the non-amended plots. An extra compost amendment did not increase the postharvest mineral N content in soil which could be leached over winter. As the experiment is still ongoing, we will continue to monitor the SOM evolution and nutrient dynamics.