Long-term effect of organic fertilizers on crop yield and nitrogen fertilization

Consequences of the use of different organic fertilizers (green manure, cereal straw, manure at 35 and 70 t ha-1 every 3 years and cattle slurry at 60 m3 ha-1 every 3 years) and mineral fertilizer (four doses nitrogen) are tested in Changins since 1976. This study analyses the long-term effect (34 years) on crop yield, the need for nitrogen fertilizer and the stock of mineral nitrogen (N) in the soil. When N is not limiting, organic fertilizers have different effects on grain yield. The year of organic input and the subsequent years, manure and slurry increase yields compared to the control without organic fertilizer, while green manure and systematic restitution of the cereal straw decrease it. However, on average over the past 34 years, these effects remain weak. On the contrary, when nitrogen is limiting, all forms of organic fertilizers have a positive long term effect on crop yields. The direct effect of organic fertilizer (first year of field application) may be positive or negative. The non fertilized green manure increases the need for nitrogen fertilizer during the year of its destruction but reduces it the following year. When fertilized with 60 kg N ha-1, it decreases the need for nitrogen fertilizer the year of its destruction as well as the following year. The fertilizing value of the cereal straw is negligible. Manure and slurry reduce significantly the need for N fertilizer on the three years following the application. When the fertilizer value of manure is not taken into account, the stock of mineral N in the soil present at harvest was higher in treatments with manure than in the control without organic fertilizer.