Nitrogen Mineralization Potential in Important Agricultural Soils of Hawai‘i

Published by the College of Tropical Agriculture and Human Resources (CTAHR) and issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture. Andrew G. Hashimoto, Director/Dean, Cooperative Extension Service/CTAHR, University of Hawai‘i at Mänoa, Honolulu, Hawai‘i 96822. An equal opportunity/affirmative action institution providing programs and services to the people of Hawai‘i without regard to race, sex, age, religion, color, national origin, ancestry, disability, marital status, arrest and court record, sexual orientation, or status as a covered veteran. CTAHR publications can be found on the Web site . Most of the nitrogen (N) in the environment is in forms that are unavailable for plant uptake. Nitrogen in the plant root zone is either nitrogen gas (N 2 ), as a component of the air occupying the soil pore space, or organic N present in various forms, including plant and microbial proteins and amino acids, in the soil organic matter. In a process known as N mineralization, the organic N contained in soil organic matter is converted into plant-useable inorganic forms (ammonium, NH 4 , and nitrate, NO 3 ) as a result of the activities of soil microorganisms. In managing plant nutrients over continuing crop cycles, farmers face the challenge of estimating the amount of the soil’s organic N resource (“N pool”) that is made available for plant uptake (mineralized) over time. This publication discusses the soil N mineralization process, its primary controlling factors, and their plant nutrient management implications. It also shows how differences in N mineralization rates among Hawai‘i’s important agricultural soils can affect farm productivity and the need for nutrient inputs. For farmers, N mineralization is an important process to be aware of, particularly for the farmer who wishes to farm organically, without inorganic N fertilizers.