Digested livestock manure as organic n-fertilizer for greenhouse lettuce

In Italy, lettuce is extensively grown in greenhouse. Maintaining adequate nutrition is one of the most critical aspects of producing greenhouse crops: as an example, for greenhouse lettuce produced on soil substrate, an excessive mineral N-fertilization could lead to luxury consumption, with dangerous effect on plant healthiness and on environmental quality. The possibility to fertilize lettuce by applying organic fertilizers, as digested livestock manure, able to supply nitrogen and other nutrients by optimizing their use efficiency, represents a promising approach. In our work, we evaluated the agronomical performances of a solid fraction of pig livestock manure, previously digested under anaerobic conditions or not-digested, as organic fertilizers for lettuce production. In the greenhouse experiment, plantlets of Lactuca sativa L. (‘Cappuccina’) were transplanted in pots containing two soils with different textural, physical and chemical characteristics. A digested and a not-digested solid fraction of pig livestock manure, at two increasing N rates (200 and 400 kg ha-1) were applied as N-organic fertilizers, taking the granular urea as reference mineral fertilizer. Irrigation was managed in relation to plant water-demand. After 70 days, lettuce fresh and dry weight (g plant-1), total leaves’ area (cm2), number of leaves, residual soil N (mg pot-1) and related N-use efficiency (NUE %) were determined. Agronomic performance of the organic fertilizers was influenced by both the soil characteristics and the organic fertilizers. The biomass production and agronomical parameters related to lettuce showed that the digested livestock manure gave results similar to those obtained by applying the mineral fertilizer urea, even with lower N use efficiency. Moreover, the application of both the organic fertilizers determined a significant increase of residual N in the soil, especially in excess of N supply (highest rate).