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The aim of the study was to compare the effect of two leonardites (Humitec® and Humic Gold®) and an organic compost (Nutrimais®) with an untreated control on lettuce. The pot experiment was carried out in NE Portugal, in autumn 2019 and spring 2020. Humitec, Humic Gold and Nutrimais were applied alone or as a supplement to nitrogen (N), phosphorus (P) and potassium (K). The experiment was arranged as a three-factor experimental design (organic amendment, NPK addition and growing season). The growing season significantly influenced lettuce dry matter yield (DMY), the spring cycle being the highest yielding (10.6 to 6.5 g plant −1 ). NPK fertilization also significantly increased lettuce DMY (11.2 to 5.9 g plant −1 ). The organic amendment significantly influenced the concentration of some nutrients in plant tissues and some soil properties, but it did not influence lettuce DMY. In respect of growing season, Nutrimais increased lettuce DMY in the first one. In relation to the absence or otherwise of NPK fertilization, Nutrimais increased lettuce DMY in the pots not receiving NPK. Leonardites did not influence soil properties, tissue nutrient concentration or lettuce DMY. The positive effect on the lettuce DMY of organic compost was probably due to the supply of N, apparently the plants’ most significant limiting factor. The non-positive results of leonardites were due to the fact that they neither provided N, nor enhanced its bioavailability from the soil. Thus, NPK fertilizer and Nutrimais, a compost of low C/N ratio, providing some N had the greatest effect on lettuce DMY.

journal of plant nutrition

Lettuce response to the application of two commercial leonardites and their effect on soil properties in a growing medium with nitrogen as the main limiting factor