Research
Rice Response to Granular Zinc Sources Varying in Water-Soluble Zinc
This study evaluated four commercial zinc fertilizers with water-soluble Zn content ranging from 14% to 98%, applied to flood-irrigated rice at two locations over two years. Rates ranged from 2.3 to 18.0 kg Zn/ha. Researchers measured both immediate and residual effects on rice growth, tissue Zn concentration, and grain yield.
In year one, fertilizer source affected early-season seedling growth and tissue Zn concentration, with tissue Zn generally declining as the water-soluble Zn level of the source declined. Application rate had the greatest influence on grain yield, with near-maximum yield produced at roughly 9 kg Zn/ha. By year two, the residual effects on growth, tissue Zn, and yield were affected only by application rate, not by the original fertilizer source.
Conclusion: zinc fertilizer source matters most in the first crop year. After that, soil reactions equalize availability and soil testing controls the picture. Low water-soluble sources may need higher application rates to deliver equivalent immediately-available Zn.
Zinc Fertilization: A Review of the Scientific Literature
A broad overview of recent zinc fertility research covering the different product types currently available and their relative effectiveness. A useful starting point for understanding the field.
Dr. Westfall's Memo on Efficiency Ratios to AAPFCO
Dr. Dwayne Westfall writes this memo to AAPFCO (American Association of Plant Food Control Officials) to encourage them to require peer-reviewed research that their efficiency ratio products are effective. Dr. Westfall believes that if this cannot be done, the products are in violation of the AAPFCO labeling law.
University of Arkansas memo to AAPFCO
Associate Professor Nathan Slaton of the University of Arkansas tells AAPFCO (American Association of Plant Food Control Officials) that efficiency ratio products that they field tested are fine and as effective as zinc sulfate as long as they are used on a 1 to 1 ratio of what the soil test recommends. He is firmly against efficiency ratio claims and states that yields are reduced when products are applied based on an efficiency ratio.
Availability of Organic and Inorganic Zn Fertilizers - Colorado State University Technical Bulletin
This Colorado State greenhouse study evaluated dry matter production, total Zn uptake, and Zn concentration in corn fertilized with six commercial Zn products: three granular inorganic Zn sources, two granular organically complexed Zn sources, and liquid ZnEDTA. The soil was low in available Zn and limed to pH 7.2. Application rates ranged from 0 to 8 lb Zn/acre.
ZnLignosulfonate, ZnSO4, and ZnEDTA were always the most effective, with relative availability coefficients of 70 to 100% depending on the parameter measured. ZnOxysulfate at 55% water solubility performed reasonably well. The lower water-soluble materials (26% water-soluble ZnOxysulfate and 1% water-soluble ZnSucrate) were least effective.
The study's central conclusion: water solubility, not total Zn content or organic complexation, is the primary factor governing zinc fertilizer performance. A Zn fertilizer must be 40 to 50% water soluble to be an effective Zn source. Reacting a zinc ion with an organic complexing agent does not automatically make the resulting product behave like a true chelate; if the end product is not highly water-soluble, it will be inefficient.
True chelates such as ZnEDTA can be 2 to 5 times more effective at delivering Zn to the plant than other water-soluble sources. ZnSO4 and ZnLignosulfonate showed roughly a 1:1 effectiveness ratio in this work.
Validity of Zinc Fertilizer Efficiency Ratios - Colorado State University Technical Bulletin
This Colorado State study tested the "efficiency ratio" claims of two commercial Zn fertilizers (Wolftrax DDP, claimed 9:1 efficiency ratio, and Origin 10% LS, claimed 7:1) against ZnSO4 as the reference standard. The fertilizers were applied at their claimed efficiency ratios so that the comparison reflected real-world dealer recommendations.
ZnSO4 produced the highest plant Zn concentration and uptake levels and was the only source that produced a positive, consistent trend in Zn concentration and uptake as application rate increased. Origin LS10 showed no significant increase in Zn uptake or plant Zn concentration compared to the zero-Zn check, suggesting it provided little or no available Zn at the rates applied. Wolftrax did supply some Zn, in quantities between ZnSO4 and Origin LS10, but the values were not statistically different from the check, and no positive relationship between rate and plant Zn was detected.
The study concluded that water solubility remains the major factor controlling zinc availability to plants, and the efficiency ratio principle proposed by these manufacturers was not supported by the results.
ZINC: Are You Getting What You Paid For?
Efficiency Ratios and Zinc Fertilizer
"Based on my research there is no 'silver bullet' when it comes to zinc fertilizers. A pound of available zinc is a pound of available zinc, regardless of its source." — Dr. Dwayne G. Westfall, Professor of Soil Science, Colorado State University
Will the actual Zn rate applied equal your soil test recommendations?
Having the proper amount of available zinc from seed germination to harvest is essential for a crop to reach maximum economic yield (MEY). Years of university research have established the levels needed for individual crops. As a general rule, you need a minimum of 2 ppm of DTPA-extractable zinc in the soil and 25 to 70 ppm of zinc in a tissue test. Less than that, and you run a real risk of yield reduction.
Soil and tissue tests have been correlated with available zinc in various fertilizer products to develop application rate recommendations to achieve MEY. These recommendations are time-tested and based on peer-reviewed research, expressed as pounds of actual zinc per acre. A three-pound recommendation of zinc requires the application of 8.45 pounds of 35.5% zinc sulfate monohydrate.
Problems arise when a product is claimed to be more efficient and effective at reduced rates of actual zinc. There is no peer-reviewed research backing up these efficiency claims. These products may be profitable for the dealer but they short the grower of the necessary soil zinc to reach MEY. Foliars are effective but add expense that is unnecessary if proper soil zinc was already in place. Once visual zinc deficiency symptoms are visible, yield has already been reduced.
Does the application rate ensure adequate Zn through the end of the growing season?
If you use the soil-test recommended rates of actual zinc, you can be confident enough soil zinc is present to sustain growth through the season and reach yield goals. BLU-MIN zinc sulfate monohydrate is 100% water soluble and delivers the recommended amount.
Coating products such as Ele-Max Super Zinc FL and Wolf Trax 62% DDP are derived primarily from zinc oxide, which has very low water solubility. At their maximum allowed application rates, Ele-Max delivers a maximum of 18.2% of the soil-test recommended zinc, and Wolf Trax delivers a maximum of 11.2%. Ele-Max Super Zinc FL has a maximum application rate of 24 ounces per acre, or 0.6 pounds of actual zinc, beyond which the fertilizer blend becomes too wet to spread.
Wolf Trax does have a results guarantee, but to claim it you must apply within 35 days of application and have a check strip in the field. If a deficiency is proven, you receive more Wolf Trax product, not money back. Given that zinc is often applied in the fall, well before 35 days before the crop emerges, the practical value of this guarantee is limited.
Why would manufacturers direct dealers to apply 1/5th to 1/10th of the soil-test recommendation?
Coating product manufacturers cite various advantages: less micronutrient required per batch, less packaging to dispose of, less product to inventory, and higher dealer profitability. The unstated reality is that the grower is paying for a pound of zinc and only receiving 1.78 to 2.88 ounces of it.
Both Wolf Trax and Ele-Max Super Zinc are primarily zinc oxide. Zinc oxide has a very low coefficient of solubility. While it can be effective when finely divided and evenly spread, it is not five to nine times more effective than 99%+ water-soluble granular zinc sulfate monohydrate. The state of California has agreed there is a problem with efficiency ratio zinc fertilizers and worked with Wolf Trax to remove efficiency information from their website. See CDFA Letter May 7, 2009.
What is the cost per actual pound of Zn?
Coating products are very expensive per pound of actual zinc. For example, if a soil test recommends 3 lbs. of zinc per acre, you would receive these amounts of actual zinc at approximately the same per-acre cost:
BLU-MIN® 35.5% zinc sulfate monohydrate: 3 pounds
Ele-Max® Super Zinc FL: 0.55 pounds
Wolf Trax DDP® 62% Zinc: 0.33 pounds
Zinc doesn't leach. Will your application rate build or maintain Zn levels for next season's crops?
Zinc doesn't leach. Whatever isn't used in the current crop year remains available for the next. A balanced fertility program is like money in a savings account: it's there when you need it. Foliars work but are expensive, and if applied to correct an observed deficiency, yield damage has already occurred. An especially important time to have adequate zinc nutrition is at seed germination, since cold damp weather inhibits zinc availability and uptake. The most cost-effective way to deliver adequate zinc to annual crops throughout the growing season is soil application of BLU-MIN zinc sulfate monohydrate.