The aim of this work was to test fertilizers, liming, and levels of a soil conditioner produced from leonardite, composed by humic and fulvic acids on “iceberg” lettuce growth characteristics. The experimental design was completely randomized in a 5×3×2 factorial scheme, with 5 levels of soil conditioner (0, 20, 40, 100 and 200 L ha-1), 3 fertilizers [chicken manure, plant compost (plant residues and cattle manure) and mineral], in limed and unlimed soil, with five replicates. The experiment was carried out in a greenhouse. Total shoot fresh and dry weight, shoot commercial fresh and dry weight, plants height, circumference, and number of leaves were evaluated as well as final soil pH and nutrient levels in leaves. Chicken manure rendered the greater circumference and shoot fresh commercial weight. Lettuce produced with chicken manure presented higher content of P, Ca, and Mg in leaves. Soil conditioner, in general, did not influence plant growth, except in its height, in the highest applied dose. With absence of lime, soil conditioner caused an increase of lettuce height with mineral fertilizers, and a decrease with plant compost.
Key words: Humic substances, Lactuca sativa L., chicken manure, compost.
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