Phosphorus Fertilizer Management for Head Cabbage

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 . Head cabbage is an important vegetable crop in Hawai‘i, ranking third, behind cucumbers and tomatoes, with a farmgate value of $2.7 million in 2004 (Statistics of Hawaii Agriculture 2004). In many cases, cabbage farms have been in operation for several decades, and the farmers have typically applied relatively large amounts of so-called “complete” fertilizers, blends of fertilizers that contain nitrogen (N), phosphorus (P), and potassium (K), to each crop. Although nutrient inputs in the form of “complete” fertilizers are often required to ensure good crop yields, nutrients tend to accumulate in soils receiving these fertilizers over the course of many years, and this is especially true for phosphorus. Theoretically, when P concentrations are high in the soil, additions of more P fertilizer do not necessarily contribute to improved yields. In fact, adding P fertilizer to soils already high in P may be an unnecessary cost, reducing the farmer’s profit margin. Furthermore, soils high in P that leave the farm as runoff during rainstorms threaten the quality of fresh and coastal waters. A soil test for P is a useful tool farmers can use to determine whether their fields require P fertilizers or whether the soil contains sufficient P reserves for good crop growth. In this publication we summarize the results of several onfarm field trials testing the Phosphorus Fertilizer Management for Head Cabbage