Agriculture Reference
In-Depth Information
must be sufficient to supply a crop with the expected total yield and N uptake.
For example, in southern Sweden, bulb yields of 60 t/ha absorbing 120 kg/ha
are aimed for (Gertsson and Bjorklund, 2002). The amount of mineral N in soil
is difficult to predict and varies from year to year, since rates of mineralization
and leaching are affected by weather conditions. By analysing the soil as onion
crops develop, the quantities of N fertilizer applied at each stage can be
adjusted, so that the sum of all mineralized plus fertilizer N equals the target
value at each stage (see Fig. 6.4). A simplified system involving just two target
N values in the top 30 cm of soil - 40 kg/ha at sowing and 120 kg/ha when the
crop mass is about 1 t/ha - is more easily manageable for growers and almost
as effective. To achieve maximum potential yields using these N balance
techniques a certain quantity of residual N, termed the 'N buffer', must be left
in the soil after harvest (Stone, 2000b).
Environmental concerns have prompted research on rotations in which the
high residual N remaining in soil after onion crops is exploited by a deeper-
rooting successor crop like sugarbeet or maize. In experiments in Colorado,
USA, furrow-irrigated onions recovered only 15% of up to 224 kg/ha N applied,
a level typical of commercial practice. Maize, planted the following year without
additional fertilizer, recovered 24% of the applied N, giving a total recovery of
39% of fertilizer N from the two-crop system (Halvorson
et al.
, 2002).
Fig. 6.4.
The amounts of mineral nitrogen (N
MIN
) found in the top 30 cm in a light soil
in southern Sweden and the quantities of applied N fertilizer needed to make up the N
content to this depth to target values required at sowing, 1 June, 15 June, 1 July and
20 July in five successive seasons. The target values were considered sufficient to
produce bulb yields of 60 t/ha without leaving a large quantity of residual mineral N in
the soil after harvest. The amounts of mineralized N varied considerably from year to
year, so that the quantities of applied N needed to meet the objectives also varied from
year to year (from Gertsson and Bjorklund, 2002. Courtesy of
Acta Horticulturae
).
