EFFECT OF DIFFERENTIATED FERTILIZING SYSTEMS ON NITROGEN ACCUMULATION PATTERNS DURING THE GROWING SEASON - SUGAR BEET AS AN EXAMPLE

The rate of nitrogen uptake by sugar beet canopy during the growing season is a driving factor in both dry matter production and its distribution between leaves and the storage root. It has been hypothesized that nitrogen accumulation in both parts of the beet is significantly affected by the regime of P and K supply to plants. This assumption has been verified with the data obtained from a field static experiment, conducted in 2001-2003, with eight fertilizing variants: without nitrogen (absolute control, PK), without one of the main nutrients (KN, PN), with a reduced amount of phosphorus and potassium (N + 25% PK, N + 50% PK) and with the recommended amounts of basic nutrients (NPK, (NPK)-K-*, P-* - P in the form of PAPR). Amounts of in-season accumulated nitrogen in sugar beet parts were measured on eight consecutive sampling dates, in two- to three-week intervals. The general pattern of N accumulation in leaves is best described by a quadratic equation, but follows a linear function in storage roots. The maximum rate of nitrogen accumulation depended on years and fertilizing variants. Limited supply of nutrients to beet plants, caused by the course of the weather or the applied fertilizers (less than 50% of the recommended N rate and without K), was the main reason for a lower rate of nitrogen accumulation in storage roots in the first part of the growing season. The course of absolute and relative nitrogen uptake rates shows that in the second part of the season the sugar beet could compensate the uptake of N from its soil resources. However, the effect of compensatory N uptake on yield of storage roots was inconsistent. When water and nutrients were in ample supply, e.g. in 2001, the additionally absorbed nitrogen could be used as an indicator of the yield potential of sugar beet. Under other growth conditions, it is used mainly for restoration of leaf growth, with a different effect on the final yield of storage roots. The quadratic trend of N accumulation in beet canopy during the growing season reflects the crop's N saturation status, a prerequisite of high yields of storage roots, as in 2001. The linear model, manifesting itself in years with pronounced drought, represents a sub-optimal status of N management in sugar beet canopy, resulting in much lower yields.