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Too much nitrogen can depress yields so it is important to confirm rates following local trials. The figure above shows that trials in Turkey indicated a yield descrease when nitrogen was over-applied.
Over-use of ammonium N can cause problems. Trials in South Africa confirm significant yield losses of up to one third from use of ammonium in preference to nitrate.
In the same trials, where ammonium-N was the main N-source, ammonium was preferentially taken-up compared to potassium, calcium and magnesium, reducing levels of these important nutrients in the crop which will reduce tomato qualities such as taste. As a result of reduced uptake of calcium, the risk of BER increases.
When ammonium is used after fruit set and when crops are under moisture stress, BER incidence is even greater, as shown in studies in US.
In Switzerland, studies about tomato plant's susceptibility to fusarium crown and root rot found that nitrate inhibits disease development and decreases the effects of fusaric acid – a toxin released by the pathogen.
Crop quality and shelflife also suffer as a result of using excessive ammonium forms of nitrogen, as trial in West Indies shows. Fruit is firmer and more marketable where nitrate forms (calcium nitrate, potassium nitrate) are used.
Crops remove around 4.8 – 5.3lb for every ton of fruit produced. High rates of 223lb/ac or above are needed for average 44.6t/ac field crops. However, too much nitrogen can depress yields so it is important to confirm rates following local trials. Nitrogen form is particularly important in the tomato crop and it is critical to maintain a good balance between ammonium and nitrate forms to maintain fast growth and crop productivity.
|Nitrogen at tomato growth stages|
|Establishment||Promotes strong early growth|
|Vegetative growth||Ensures continued growth|
|Flowering - fruit set||Maintains plant growth and maximize flower numbers|
|Fruit ripening - maturity||In reduced amounts to maintain fruit fill|