RELATIVE AVAILABILITY COEFFICIENTS OF ORGANIC AND INORGANIC Zn FERTILIZERS

Abstract

Zinc sulfate (ZnSO₄) has traditionally been the “reliable” source of Zn fertilizer but other sources of Zn are also available. Some are derived from industrial by-products, varying from flue dust reacted with sulfuric acid to organic compounds derived from the paper industry. The degree of Zn availability in Zn sources derived from these various by-products is related to the manufacturing process, the source of complexing or chelating agents (organic sources), and the original product used as the Zn source. Many claims are made regarding the relative efficiency of traditional inorganic Zn fertilizers and complexed Zn sources. The objective of this greenhouse study was to determine the availability coefficients of several commercial Zn fertilizer materials (organic and inorganic) that are commonly used to correct Zn deficiencies in soils. We evaluated the dry matter production, total Zn uptake, and Zn concentration in corn (Zea mays L. cv. P3752) plants fertilized with six different commercial Zn fertilizers. The sources included three granular inorganic Zn sources, two granular organically complexed Zn sources, and liquid ZnEDTA. The soil was low in available Zn (AB-DTPA Zn = 0.48 mg kg⁻¹) and limed to a pH of 7.2. The Zn fertilizers were added to 5 kg pots at rates equivalent to 0, 0.21, 0.42, 0.84, 1.68, and 3.36 mg Zn kg⁻¹ of soil. A relative availability coefficient (RAC) was used to determine the relative availability of all sources. The RAC is determined by comparing the slopes of the regression lines of each measured parameter with the slope of the reference material. The ZnLignosulfonate, ZnSO₄, and ZnEDTA were always the most effective material in supplying the plant's needs. The RAC of these three materials ranged from 70 to 100%, depending on plant parameter measured. The ZnOxysulfate, with 55% water-solubility, also performed well with a RAC from 48 to 69%. The lower water-soluble materials (ZnOxysulfate, 26% water-soluble, and ZnSucrate, 1% water-soluble) were least effective with RAC values ranging from −12 to 25%. When comparing all sources, water-solubility was the primary factor governing the performance of Zn fertilizers. High water-solubility is required if a Zn fertilizer is going to be effective in meeting the plant's Zn needs. Zinc ions that are reacted with an organic complexing agent do not guarantee the resulting fertilizer will perform like a true chelate and have high plant availability. If the end product is not highly water-soluble, it will be very inefficient in supplying Zn to the plant. These results confirm our previous research where we concluded that a Zn fertilizer must be from 50% water-soluble to be an effective Zn source.

ACKNOWLEDGMENTS

This research was supported by the Colorado State University Agricultural Experiment Station and Tetra Micronutrients, Inc. Appreciation is expressed to the sponsors of this project. Appreciation is also expressed to Mr. Michael Fredrickson for his assistance in laboratory and greenhouse procedures.