THE SPATIAL AND TEMPORAL ROOTING PATTERN OF BRUSSELS-SPROUTS AND LEEKS

The vegetables Brussels sprouts (Brassica oleracea L. var gemmifera) a nd leeks (Allium porrum L.) differ widely in the degree of nitrogen ut ilisation under field conditions. Brussels sprouts usually take up nit rogen in a relative short period after planting and leaves a profile d epleted of nitrogen. Leeks, however, utilise in general only half of t he available nitrogen. This implies that for this crop large amounts o f nitrogen can be subject to losses to the environment during or after the growing period. In order to investigate the role of the root syst em in nitrogen utilisation, rooting intensity and distribution of the two crops were assessed in field experiments and in the Wageningen Rhi zolab, a rhizotron facility built in 1990. In field experiments rootin g was quantified in the various layers of the profile as the volumetri c Root Length Density (RLD, cm cm(-3)) and in the Rhizolab with horizo ntal glass minirhizotrons as the Number of Roots per cm(2) (NR). A reg ression procedure of RLD and NR on thermal time (accumulated average d aily temperature above ground) after planting revealed that the rootin g depth of Brussels sprouts increased faster with thermal time than ro oting depth of leeks, e.g. in one of the experiments in the Rhizolab 0 .13 cm (degrees C day)(-1) and 0.08 cm (degrees C day)(-1) respectivel y. Furthermore, leeks showed an unusual distribution of roots in the p rofile with maximal rooting intensity at depths of 10-20 cm. In all ex periments the proliferation of roots (calculated as the increase in NR or RLD with thermal time) was slower in leeks than in Brussels sprout s, especially in the deeper layers of the soil profile. Therefore leek s can be considered as a shallow rooting crop. Compared to the Rhizola b, the field experiments showed in general the same difference between the crops: for Brussels sprouts a higher root density, a deeper rooti ng and a faster proliferation of roots. For both crops, however. the p roliferation of roots at the deeper layers was in the field much slowe r, probably due to unfavourable conditions in the subsoil caused by hi gher bulk densities or temporary high water tables. The consequences o f different rooting patterns for nitrogen utilisation are discussed.