Quality response of crisphead lettuce and kohlrabi to mineral and organic fertilization in different soils

Two levels of fertilization (medium and high), using mineral fertilizer (MF) alone, farmyard manure (FyM) alone, or a combination of MF and FyM, together with non-fertilized controls, were tested in northeastern Italy in three different soils (clay, peaty-clay and sandy). The clay and peaty-clay soils demonstrated to be the most suitable for the quality of both lettuce and kohlrabi, showing high incidence of heavier heads and larger-sized stems, respectively, for the two crops, low dry matter and high raw protein content. In lettuce, a higher nitrate content was observed when plants were grown in peaty-clay soil (spring- and autumn-grown lettuce) and in clay soil (autumn-grown lettuce). Fertilization showed different effectiveness on crop quality in relation to soil type. For spring-grown lettuce in sandy soil, the distribution of 200,100 and 280 kg ha-1 of N, P2 O5 and K2O as mixed fertilization gave qualitative performance (head weight, uniformity, shape, dry matter and raw protein content) similar to those of more fertile soils. Fertilization also improved quality in clay and peatyclay soils, but the effects were less evident than in sandy soil. Autumn-grown lettuce benefited by the residual effect of fertilization. Kohlrabi gave the best stem size and homogeneity in peaty-clay soil, with 100, 50 and 140 kg ha-1 of N, P2O5 and K2O, as mineral fertilizer. Clay soil also proved to be suitable for this crop, especially when mixed fertilization was used. In sandy soil, the quality of the product was lower even when large amounts of nutrients were applied. In both lettuce and kohlrabi, the nitrate content of the edible product was only slightly influenced by fertilization.