Many current intensive systems of field vegetable production are not sustai
nable because they cause severe environmental damage. Often, large amounts
of nitrogen remain in the soil after harvest of the crop. This nitrogen inc
ludes residual soil mineral nitrogen and nitrogen present in crop residues.
Both sources of nitrogen may affect groundwater quality through nitrate le
aching and ail quality through nitrous oxide emission.
Residual soil mineral nitrogen levels after application of the recommended
rates of nitrogen fertiliser to Brussels sprouts, white cabbage and onions
are low to moderate (20-75 kg N ha(-1)). Application of the recommended rat
es to other field vegetables, however, may leave large amounts of residual
soil mineral nitrogen, especially after crops that are harvested before mat
uring, e.g., spinach, where residual soil mineral nitrogen may even exceed
a value of 200 kg N ha(-1) Obviously, large amounts of nitrate will then be
at risk of leaching and denitrification during the subsequent winter.
Crop residues of spinach and celeriac contain 25-60 kg N ha(-1), cauliflowe
r residues 80-120 kg N ha(-1), and white cabbage and Brussels sprout residu
es as much as 150-250 kg N ha(-1). If the residues are decomposed before wi
nter, nitrogen from the decomposed plant material may leach or denitrify du
ring the subsequent winter period.
Realistic estimates of nitrogen losses through leaching and denitrification
after harvest of field vegetables were generated with a simulation model.
It was calculated that leaching losses may exceed 200 kg N ha(-1) after spi
nach or leeks, but denitrification was low. Losses after Brussels sprouts a
nd cabbage were much lower.
Two kinds of measures can be taken to maximise nitrogen use efficiency whil
e minimising environmental losses. These are (1) matching nitrogen supply a
nd demand, and (2) reducing losses outside the growing season.