Nitrogen and field production of vegetable cropsGetting the amount of nitrogen fertiliser right for vegetable crops involves careful analysis of sources of supply as well as demand . Failure to match supply and demand can lead to reductions in yield , or in the case of oversupply lead to reductions in quality , and increased risk of nitrogen loss by leaching . For maximum utilisation of nitrogen from fertilisers they should be applied in a form that becomes available in synchrony with crop demand . Fertilisers should also be placed in the soil within the range of active crop roots . This section highlights the main factors affecting the nitrogen requirements of field vegetable crops and how they can be met from soil reserves and from timely applications of fertiliser nitrogen . A few general comments The amounts of fertiliser nitrogen typically recommended in the UK for field vegetable crops vary widely . For example , the recommended amounts of N following cereals are 300 kg ha - 1 N for sprouts , 250 for cauliflowers , 200 for lettuce , 150 for leeks , 90 for bulb onions , and 60 kg ha - 1 N for carrots ( MAFF 1994 ) . Where vegetable crops are grown there are often long periods of time without soil cover , frequent and intensive soil cultivations , and large quantities of N returned to the soil in crop residues . These factors all contribute to larger amounts of nitrate leached from soils growing horticultural crops than from arable cropped soils ( Jürgens - Gschwind 1989 ) . In order to reduce the risk of leaching the amounts of N to be supplied as fertiliser need careful assessment . Getting the amount and timing of fertiliser right for vegetable crops involves careful analysis of sources of supply as well as crop demand . It also depends on what is required by the market , for example for a given crop spacing of onions the amounts of N required for small grade onions would be less than for larger grades . The economic consequences for failing to meet such targets for yield and quality are large particularly where produce is destined for supermarkets . Sub - optimal supply can lead to delays in maturity , and cause reductions in yield and quality , e . g . for cauliflower crops ( Rahn et al . 1993 , Rahn et al . 1998 ) . As the economic consequences for failing to meet target yields are so large there is always a temptation to apply more nitrogen than is necessary . By contrast over supply can lead to problems with establishment on dry sandy soils and an increased incidence of disease later , as for example with stem rot in broccoli ( Everaats 1994 ) and rot in onions , ( Batal et al . 1994 ) . Excessive N has also been implicated in several physiological disorders , such as hollow stem of cauliflowers ( Scaife and Wurr 1990 ) . Large amounts of N can increase whole plant yields of broccoli but may not increase marketable yield ( Van den Boogaard & Thorup - Kristensen , 1997 ) and may lead to the accumulation of nitrate content ( Blom - Zandstra 1989 ) . Taste may be affected , topdressings of N increased the bitterness of sprouts ( Scaife and Turner 1985 ) . Storage quality may be affected by N ; Berard ( 1990 ) suggested that large amounts of N increased the incidence of black midrib in stored cabbage . Oversupply can also lead to large amounts of potentially leachable nitrogen at harvest ( Rahn et al . 1992 ) . Nitrogen requirement of field vegetable crops A good starting point for any estimate of nitrogen requirement is the assessment of total plant yield and its nitrogen content for crops providing marketable produce . The ‘ KNS ’ system ( Lorenz et al . 1989 ) does allow such calculations to be made from a series of look up tables . Table 1 shows some examples of N requirement which are related to typical yields of marketable produce and total plant yield at harvest . Table 1 - Nitrogen requirements of a selection of field vegetable crops based on KNS ( Lorenz et al . 1989 ) system . CropMarketable Yield t / ha FreshTotal Yield t / ha FreshTotal N uptake + kg ha - 1 Brussels sprouts 1590270 Cauliflower 3580220 Carrots 6070135 Leeks 4565195 Lettuce 4060100 Onions 4065120 Radish 302560 White cabbage ( Fresh market ) 60110260