Heavy applications of nitrogen fertilizer can cause nitrates to accumulate in vegetables
Vegetables are a high-value crop. Farmers tend to apply large amounts of fertilizers, especially nitrogen. This is a reasonable insurance against yield losses from nutrient deficiencies, especially if fertilizers are fairly cheap. However, applying too much nitrogen fertilizer may be bad for human health.
Nitrates are nitrogen-oxygen chemical units which combine with various organic and inorganic compounds. Once taken into the body, nitrates may be converted into nitrites. Crops containing high levels of nitrates can be identified by laboratory tests. However, they appear normal to the eye. Nitrates in vegetables and fruit have no taste or smell.
Nitrates occur naturally in fruit and vegetables, but only in small quantities. They can rise to high levels in intensively grown crops. Organic vegetables are no safer than conventional crops, in this respect. As far as nitrates are concerned, it makes no difference whether the nitrogen comes from compost or from chemical fertilizers. Organic vegetables sometimes receive more than thirty tons of compost a year, and may contain high levels of nitrate.
There is a strong relationship between the amount of nitrogen applied to the crop, and the level of nitrates in the plant. Nitrate levels are also affected by the season, and even the time of the day. There is some evidence that nitrate levels in produce are very high in countries with a cold climate, where most vegetables are grown under structures.
High levels of nitrate in food or drinking water are known to be dangerous to babies in the first three months of life. They cause the blood to carry less oxygen, and the infant may suffocate. Older children and adults are not affected in this way. However, the prolonged intake of high levels of nitrate is linked to gastric problems, due to the formation of nitrosamines/.
Such compounds have been found to cause cancer in animals. The same may be true of human beings. Of particular concern is the possible link between fruit and vegetables with a high nitrate content, and cancer of the gullet. A current study in Scotland is examining the possibility that human saliva may be converting nitrates into carcinogens, which come into force at the gastro-oesphageal junction.
If nitrate levels in vegetables are too high, growers must reduce the amount of nitrogen fertilizer they apply. The problem from the grower’s point of view is that reducing nitrogen applications is likely to give lower yields. If growing healthy crops involves some sacrifice of yield, we are faced with the question of who will pay the cost of this.
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