Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Vegetable crops vary considerably in their needs for nitrogen with crops such as sweet potatoes falling on the low end and tomatoes on the high end. While a lack of nitrogen will definitely limit vegetable productivity, excess nitrogen can also cause production problems.
Excess nitrogen will often delay maturity in crops. This is a particular problem in fruiting vegetables and vegetables with harvested roots and tubers. Too much nitrogen will favor the growth of foliage over flowering and fruiting or formation of storage organs such as tubers and roots. In a crop such as pumpkins, this can result in delaying fruit set so long that the crop will not mature in time for sales. Excess nitrogen can also reduce yields by limiting storage organ formation. Sweet potatoes would be a good example of a crop that will have reduced yields with excess nitrogen.
Excess nitrogen can also cause reductions in the quality of fruits and storage organs both in flavor and physical characteristics. High nitrogen applications can result in lower sugar content, lower acidity, and reduced firmness in fruits and storage organs. It can cause reduction in nutritional content. In leafy green vegetables, it can result in the accumulation of nitrates in the plant tissue to unhealthy levels. High nitrogen can cause reduced volatile production and negatively impact flavor and aroma in vegetables and fruits. Excess nitrogen can increase disorders such as hollow stems of broccoli and reduce storage and keeping qualities of fruits and vegetables.
Excessive production of foliage from high nitrogen applications can also lead to an increase in disease pressure from having a higher proportion of young tissue that is more susceptible to infections, by creating a more humid microclimate favorable for disease development, and by making it more difficult to get good coverage with fungicides.
Excess nitrogen can cause reductions in the levels of other mineral nutrients in plants such as potassium, calcium, and magnesium, often resulting in the development of deficiencies and associated disorders.
Recommended nitrogen rates and timings for most vegetable crops grown in our region can be found in the Commercial Vegetable Production Recommendations (online at http://ag.udel.edu/extension/vegprogram/publications.htm). These recommended rates have been developed over many years of research by our universities. Applications in excess of these recommended rates is justified only under special circumstances (excess rainfall and leaching for example).