Nitrogen Fertilization of Garden and Landscape

Oct. 25, 2020

All plants in your garden or landscape need a source of nitrogen in the soil. Nitrogen is used by plants for photosynthesis (the process of converting light energy into chemical energy), growth, reproduction. In native landscapes, plants are well adapted to their surroundings and nitrogen supplies are usually in balance plant needs. Since many plants we grow are exotics, they need supplemental nitrogen. Nitrogen is especially crucial to crop plants and fruit trees. The most common symptoms of nitrogen deficiency are: light green foliage, older leaves yellowing and/or dropping, and poor growth.

Supplemental nitrogen can be provided through chemical (synthetic) or organic fertilizers. Chemical fertilizers are produced in large batches by using chemicals such as anhydrous ammonia and nitric acid. Some common chemical fertilizers are ammonium nitrate, ammonium sulfate, urea, and potassium nitrate. These are inexpensive and give the fastest plant growth response. They are also subject to loss from the soil through downward leaching by water and volatilization to the atmosphere. The unexperienced gardener can also "burn" plants by applying excessive amounts. Organic fertilizers are manures or organic by-products from plant or animal sources. They are more expensive but tend to be released slowly and therefore are not subject to losses through leaching and volatilization. Common organic fertilizers are cow and chicken manure, bat guano, cottonseed meal, alfalfa meal, blood meal, hoof and horn meal, and fish emulsion.

Plants can take nitrogen from the soil in two forms: ammonium or nitrate. Chemical fertilizers provide these two forms directly to the plant. Organic fertilizers contain more complex compounds that must be broken down by organisms (bacteria, fungi, protozoans) in the soil to release ammonium and nitrate. Plants cannot differentiate between nitrogen that came from a chemical vs an organic source. Organic fertilizers have the added advantages of adding organic matter to the soil and increasing biological activity.

Check the analysis on the package to determine the percent nitrogen in the fertilizer. The nitrogen analysis is the first of the three numbers on the package. For example, ammonium sulfate analysis is 21-0-0. This means that 21% of the contents of the package is plant available nitrogen. The University of Arizona Cooperative Extension has recommended nitrogen applications for most crops and fruit trees as well as turf and ornamentals. These recommendations are usually in pounds of nitrogen so that the gardener can use the nitrogen fertilizer they prefer. For instance, the U of A recommends that apple trees should receive 0.1 lb of nitrogen per inch of trunk diameter per year. How much ammonium sulfate will it take for apple tree with five inch trunk diameter. Five times 0.1 equals 0.5 lbs of nitrogen for that tree. Using ammonium sulfate, one lb of fertilizer has 0.21 lbs of nitrogen (21%). This means that 2.5 lbs of fertilizer has roughly 0.5 lbs of nitrogen: the amount needed for the five inch tree. If the analysis of cow manure is 1-1-1, then it contains 1% nitrogen or 0.01 lbs nitrogen per pound of manure. In other words, 50 lbs of cow manure will apply the same amount of nitrogen as 2.5 lbs of ammonium sulfate. It may take a while to do the calculations, but once you master them, you will be able to supply your garden and landscape with the nitrogen it needs.

The University of Arizona Cooperative Extension has publications and information on gardening and horticulture. For questions, please email me at