Determination of the critical nitrogen concentration of white cabbage

A measure of a crop's nitrogen (N) status can be obtained by relating the actual N concentration of the crop to the critical plant nitrogen concentration (PNCc), the minimum N concentration required for maximum growth. In annual crops, PNCc declines as plant size increases. Describing this decline is one of the main challenges for the implementation of the PNCc concept in fertilizer management strategies. From two field experiments with repeated harvests of Dutch white cabbage and with N supply ranging from limitation to excess, the relation between PNCc and weight per unit ground area of plant dry matter exclusive of roots (W) was estimated as: PNCc = 5.1W−0.33% for the linear growth phase (W > 1.5 t ha−1; LAI > 1.4). From a third field experiment, a value of 4.5% N was estimated for PNCc for the pre-linear growth phase. Also a power function: LAR = 0.011W−0.33 described the relationship between leaf area ratio and weight. The exponent of the power function determines the rate of the decline. Therefore, having the same value of the exponent: −0.33, LAR and PNCc declined at the same rates. The proportional decline in LAR and PNCc corresponded to a constant nitrogen content on a leaf area basis of 4.7 g N m−2 until onset of head formation.

Research highlights▶ Leaf area ratio (LAR) and plant nitrogen concentration (PNC) declined with growth. ▶ The decline of LAR and PNC of cabbage was described by an allometric power model. ▶ The exponent of the power model was −0.33 both for LAR and PNC. ▶ Thus, LAR and PNC declined at the same rate.