Monday, January 28, 2013

A Case of Too Much Nitrogen

Why do fruit trees have dark green leaves but bear little or no fruit, especially if they are well fertilized? 

Here is a brief lesson in plant nutrition: in general, nitrogen is for vegetative growth. Consequently, when fertilizing a lawn or any plant grown primarily for its leafy growth, the main nutrient to apply is nitrogen. When growing plants for fruit or flower production, as opposed to foliage, a balanced complement of phosphate and potassium needs to be applied along with the nitrogen. Applying large quantities of a high nitrogen fertilizer to fruiting trees like citrus and avocado will put them in a ‘foliar growth mode’, producing an abundance of dark green leaves but little or no fruit. Be careful how much high nitrogen fertilizer you apply to the plant.

Urea fertilizer contains 46% nitrogen, ammonium sulfate 21%, while manures in general contain less than 5% nitrogen, usually around 2%. For more precise instructions on fertilizing fruit trees, CTAHR has a short publication titled Fertilizers for Fruit Trees in the Home Garden #37. It can be accessed online or at UH CTAHR Office at 875 Komohana St, Hilo, Hawaii. Other state extension offices in the U.S will have similar publications.