Recent Studies have shown that water soluble Zn fertilizers are the most effective way to correct Zn deficiencies in soils used for crop production (Shaver et al., 2007, Gangloff et al., 2000, Amrani et al., 1999, 1997, Mortvedt et al., 1993, Mortvedt, 1992).Zinc fertilizer water solubility levels of 40-50% are needed to meet the Zn requirements for the current crop (Amrani et al., 1999, Mortvedt et al., 1993.), and high correlations have been found between Zn fertilizer water solubility and plant growth and Zn uptake (Amrani et al., 1999). Zinc sulfate (ZnSO4 . 2H2O) fertilizers have been found to be a very reliable because of their relatively high water solubility. However, there are new Zn fertilizer products continually being brought to the market. Some of these products claim an "efficiency ratio" meaning that one unit of their product is equal to several units of traditional Zn fertilizers, such as ZnSO4 when applied in the field. One new Zn fertilizer with a claimed efficiency ratio is Wolftrax DDP (dry dispersible powder), which claims an efficiency ratio of 9:1 because of a "micro-static adhesion" that allows the Zn fertilizer (in powder form) to stick to every granule of NPK fertilizer thereby increasing efficiency. Another Zn fertilizer with a claimed efficiency ratio is Origin 10% LS (lignosulfonate) which claims a 7:1 efficiency ratio due to natural organic agents that protect the metal ions from tie-up by soil particles and conversion to an insoluble form of Zn. The validity of these claims must be examined because some products simply rely on residual soil Zn to justify reduced application rates.

The objective of this study was to evaluate the advantages and limitations of the "efficiency ratio" claims of these Zn fertilizers using ZnSO4 as our reference standard by applying these fertilizers according to their efficiency ratios. By examining plant biomass production as well as plant Zn uptake and Zn concentration we can better understand what Zn fertilizers are effective in supplying Zn to the plant.


The ZnSO4 source produced the highest plant Zn concentration and uptake levels compared on the efficiency ratio basis with Origin LS10 or Wolftrax Zn fertilizers.The ZnSO4 was also the only source that produced a positive and consistent trend in Zn concentration and uptake in response to application rate. The Origin\u00ae LS10 source showed no significant increase in Zn uptake or plant Zn concentration as compared to the check treatment (0 Zn/ac) suggesting this source provided little or no Zn for crop uptake. The Wolftrax source did supply Zn to the crop in quantities intermediate to the amount supplied by the ZnSO4 and Origin LS10 sources, but these concentrations and uptake values were not significantly different than the check. However, the Wolftrax levels were in the same statistical array (not different) from the ZnSO4 material when averaged across all rates. However, no positive relationship between rates and plant Zn concentrations or uptake were detected with the Wolftrax material. Due to the relatively low Zn water solubility and extremely low actual Zn application rates of the efficiency ratio based sources, the plant roots have a low probability of encountering the Zn material, which greatly reduces the opportunity for plant uptake. The opposite is true of the highly water soluble ZnSO4 source where Zn is in the soil solution and\/or on the exchange sites. The ZnSO4 source outperformed the other two materials evaluated in this study. These results are consistent with our previous studies as well as other studies reported in the literature. Water solubility remains as the major factor controlling Zn availability of fertilizer Zn to plants. Our results do not confirm the "efficiency ratio" principle proposed by the fertilizer manufacturers.

For the complete Technical Bulletin, click here.

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 get maximum economic yield (MEY).  Years of university research have given us techniques to determine what these levels need to be 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 the very real chance of a reduction in yield. 

Over the years, university research and the soil lab industry have correlated soil and tissue tests with available zinc in various fertilizer products to develop recommendations for fertilizer applications rates to achieve MEY.  These recommendations are time tested and based on peer reviewed research.

These specific crop recommendations are normally expressed as pounds of actual zinc per acre of.  So, a three pound recommendation of zinc would require the application of 8.45 pounds of 35.5% zinc sulfate monohydrate (3 lbs./35.5%=8.4507Lbs.).  Problem arise when a product is said to be more efficient, and thus will perform equally well at reduced rates of actual zinc than the soil lab test recommends.  There is no peer reviewed research to back up these claims of increased efficiency.  These efficiency ratio products may be very profitable for the dealer but they short the grower of the necessary soil zinc to reach MEY.  While the use of foliars is effective, they add an expense that is unnecessary if the proper amount of soil zinc was already there.  Additionally, the yield has already been reduced if you have seen visual zinc deficiency symptoms.

Does the application rate assure adequate Zn will be available to your crop through the end of the growing season?

If you use the soil test recommended rates of actual zinc, you can be confident that there will be enough soil zinc to sustain growth through the growing season, and to reach your yield goals.  If you are using Blu-Min zinc sulfate monohydrate, you are getting a 100% water soluble zinc that will provide the amount of zinc recommended by your soil test.  If you use coating products such as Ele-Max Super Zinc FL or  Wolf Trax 62% DDP, you will be getting much less actual zinc than recommended.

Ele-Max will give you a maximum of 18.2% of the zinc recommended by your soil test and Wolf Trax will give you a maximum of only 11.2% of the recommended amount.  Both of these products are derived primarily from zinc oxide which have a very low water solubility.  While a finely divided powder may have advantages in getting a spread evenly over the field, it's very low solubility greatly reduces that advantage.  Also keep in mind there is a maximum amount at which either of these products can be applied.  Ele-Max Super Zinc FL has a maximum application rate of 24 ounces per acre, or 0.6 pounds of actual zinc.  Any more than that and the fertilizer blend becomes too wet to spread.  When asked what happens if that isn't enough zinc one dealer responded "we'll deal with it in the field."  The only way to deal with this shortage in the field is to use expensive foliars, after the yield damage has already occurred. 

Wolf Trax does have a results guarantee.  To collect on the guarantee, you must apply for it within 35 days of application and you must have a check strip in the field.  If a deficiency is proven, you get  more Wolf Trax product not any money back.  Considering how much zinc is applied in the fall, or well before 35 days before the crop breaks the ground,  is this guarantee of much value to you?  Link to Wolf Trax website

Remember Dr. Westfall, "A pound of available zinc is a pound of available zinc, regardless of it's source."

Do the manufacturer instructions direct dealers to apply 1/5th to 1/10th of the zinc the soil test recommends?  If so, why?

Manufacturers of coating products claim a number of "advantages" to sell their products.  All of them are used to rationalize why they are charging you for a pound of zinc and only selling you from 1.78 to 2.88 ounces. Let's look at few:
 1.  Less micronutrients required per batch;
 2.  Less packaging to dispose of;
 3.  Less product to inventory;
 4.  Higher profitability for the dealer.

What they don't tell you is that you are being short changed by your dealer.  Both Wolf Trax and Ele-Max Super Zinc are primarily derived from zinc oxide.  Zinc oxide has a very low coefficient of solubility.  While zinc oxide is an effective fertilizer when finely divided and evenly spread, it certainly isn't five to nine times more effective than 99%+ water soluble granular zinc sulfate monohydrate.  The state of California agrees that there is a problem with efficiency ratio zinc fertilizers:
"We have worked with personnel at Wolf Trax to address your concern regarding their zinc products and efficiency claims. Consequently, Wolf Trax Inc has removed the information regarding efficiency values from their website."  See CDFA Letter May 7, 2009

Will you get what you pay for?  What is the cost per actual pound of Zn?

Coating products are very expensive per pound of actual zinc.  For example, if your soil test recommends 3 lbs. of zinc per acre you will get these amounts of actual zinc at approximately the same acre cost:
  Blu-Min\u00ae35.5% zinc sulfate monohydrate 3 pounds
  Ele-Max\u00ae Super Zinc FL   0.55 pounds
  Wolf Trax DDP\u00ae 62% Zinc   0.33 pounds

Zinc doesn't leach.  Will your application rate build or maintain Zn levels in the soil for next season's crops?

Zinc doesn't leach.  What isn't used in this crop year is available for the next.  Keeping a balanced fertility program is like having money in a savings account; it's always there when you need it.  Foliars work, but are expensive.  If applied to correct an observed zinc deficiency in your crops,  some damage has already been done to your yield.  An important time to have adequate zinc nutrition is at seed germination.  Cold damp weather inhibits zinc availability and uptake.  Low soil zinc at this time will definitely lower yield.  The most cost effective way to get adequate zinc into your annual crops throughout the growing season is from soil application of Blu-Min zinc sulfate monohydrate.

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