Organic materials: sources of nitrogen in the organic production of lettuce


Abstract: This paper presents the results of 2 experiments: an incubation experiment and a subsequent field experiment. An incubation experiment was set up in order to determine the mineralization potential of different organic materials (OMs) (well-rotted farmyard manure [FTM], guano [G], soybean seed [S], and forage pea seed [P]), the kinetics of mineral nitrogen (N) release, and the correlation between OM content and the quantity of mineralized N. The results of the incubation experiment were checked under field conditions in which different OMs were used as N sources for lettuce (Lactuca sativa L.). The highest mineralization rate constant k (0.127 day^{-1}) and net mineralization rate (NM) (76.37% of total applied N) were obtained with G, and the lowest k (0.098 day^{-1}) and NM (30.12% of total applied N) were obtained with FYM. In the field experiment in 2007 and 2008 we applied different OM sources that contained the same amount of potentially mineralizable N (40 and 35 kg N ha^{-1}, respectively), calculated on the basis of k and NM. The use of OMs increased the concentration of mineral N in the soil and increased the yield of fresh lettuce. Nitrate content in the fresh lettuce was within acceptable limits both years (<2500 mg kg^{-1}). Total N in the above-ground parts of lettuce ranged from 44.38 kg N ha^{-1} (Ø, treatment without fertilization) to 67.45 kg N ha^{-1} (S) (2-year average). The results show that when determining the quantity of OM to apply it is necessary to take into account the quantity of potentially mineralizable N in order for the plants to use N as efficiently as possible, to regulate the nitrate content in fresh lettuce, and to control the quantity of residual mineral N in the soil at the end of the vegetation period.

Keywords: Lettuce, nitrogen, organic farming, potentially mineralizable N

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