Zinc Application to Wheat – Is it Needed?

We have had a question about whether wheat would benefit from zinc (Zn) fertilizer. Zinc is one of the essential plant nutrients. An optimum amount of Zn is needed for the synthesis of carbohydrates, proteins, and chlorophyll in plants. It also plays a critical role in various enzymatic activities. Therefore, Zn should be available for crops in adequate amounts to avoid yield reductions.

Would we expect to see a deficiency or the need for Zn in wheat in Ohio? – probably not. Table 30 in the Tri-State Fertilizer Recommendations only shows corn and soybean as crops where a deficiency may occur on high pH soils and soils with low soil test Zn level, not wheat.  Keep in mind, field crops in Ohio have a very infrequent response to micronutrient fertilization (https://ohioline.osu.edu/factsheet/agf-519). For example, this year, winter wheat yields were extremely high, which can be primarily attributed to good environmental conditions during grain fill. Cool temperatures resulted in a long grain fill period and low disease. Although it was dry (also contributing to low disease), there was adequate soil moisture, and thus, high yielding conditions. No micronutrients or biostimulants were needed for the extra yield.

Past Ohio research has not shown any benefit from adding Zn to wheat. We compared a phosphorus-zinc fertilizer to other phosphorus fertilizers, such as MAP and DAP, at the OARDC Northwest and Western Agriculture Research Stations in 2009-2010 and only at the Northwest station in 2011. Soils at Northwest were not responsive to any phosphorus fertilizer, there was no yield increase either from the additional Zn. Soils at Western were responsive to phosphorus fertilizer but there was not an increase in yields from the Zn product compared to the other phosphorus fertilizer products without Zn. Thus, these studies showed no benefit to adding Zn to wheat at two sites. In addition, we have measured flag leaf Zn concentrations at flowering from wheat studies for the past 20 years and have not detected sub-sufficiency levels. We have not seen any environmental change to suggest that we may now need Zn fertilizer on wheat compared to old research studies. However, Zn is an essential nutrient for plant growth, so we want to give some general background even though we would not expect a response on wheat.

Where to expect Zn deficiency?

Soil type and conditions: Zinc deficiency can be prominent in peat, muck, and mineral soils with pH more than 6.5. However, we do not recommend planting wheat on organic soils because of the high risk for lodging. Plants may also show Zn deficiency under cool soil temperatures as the reduced mineralization rate of organic matter slows down the release of Zn into the soil solution. Additionally, sandy soils with low organic matter could be deficient in Zn. Some studies also show that the high level of phosphorus may cause Zn deficiency, especially when Zn soil test levels are in low range.

Crop type: Wheat is unlikely to be deficient in Zn whereas crops such as corn and beans are highly sensitive and can respond to Zn application.

How to diagnose Zn deficiency?

The deficiency symptoms appear as interveinal chlorosis (yellowing) on the young to middle leaves (Fig. 1). The plants are usually stunted, and maturity is delayed. However, keep in mind, environmental stress and deficiency of other nutrients may also cause yellowing.

Fig. 1. Zinc deficiency symptom in wheat (photo courtesy of IPNI)

It is recommended to do soil and plant analysis to diagnose Zn deficiency. Most labs in the region use Mehlich-3 as an extractant to determine Zn in soils, however there are uncertainties about this test. Therefore, supplementing soil test levels with plant analysis is crucial for Zn diagnosis. For plant analysis, upper leaves should be collected prior to initial bloom. If possible, collect soil and plant samples from ‘good’ and ‘bad’ areas of the field to compare results.

When to apply Zn fertilizer?

Zinc can be applied if soil test levels are low, we do not have a recommendation based on tissue tests. Since the availability of Zn is regulated by soil pH level, the fertilizer recommendations are as:

Pounds Zn/acre = (5.0 x soil pH) - (0.4 x 0.1 N HCl extractable Zn in ppm) – 32

An important consideration is that most of the research indicates low probability of yield increment with Zn application. It could be beneficial to test it before large scale application by applying it on a strip to identify if Zn response is economical under your field conditions.

How to apply Zn fertilizers?

Zinc banded with the starter fertilizer is the most economical method to ensure Zn availability throughout the growing season in row crops. Incorporating before planting would be desirable if unable to band. The 1995 Tri-State Fertilizer Recommendations has a table for the Zn rate based on soil test Zn (based on 0.1 N HCL extractant) and soil pH (Table 1).

Table 1. Zinc fertilizer recommendations for responsive crops (1995 Tri-state fertilizer recommendations; Vitosh et al. 1995)

Broadcast application of 5-10 lb/ac of Zn is recommended to correct Zn deficiency or build Zn levels in Zn deficient soils. Foliar application of Zn sulfate (36% Zn) at 0.3-0.7 lb of Zn per acre mixed with 20-30 gallon of water is another option. Caution should be taken if the source of Zn is in chelated form as over application can cause foliar injury and under-application is not effective. Other commonly used Zn fertilizers include Zn-ammonia complex (10% Zn), Zn oxysulfates (variable % Zn), Zn oxide (50 to 80% Zn) and Zn chelate (9 to 14% Zn).

Key points/summary

• Zinc is an essential plant nutrient required for optimum plant growth and yields.
• Wheat is unlikely to be deficient in Zn, so less likely to respond to Zn application.
• Zinc can be banded, broadcasted, or applied as foliar application, however, banding it with starter fertilizer is the most economical method.

References:

Vitosh, M.L., J.W. Johnson, and D.B. Mengel. 1995. Tri-State Fertilizer Recommendations for Corn, Soybean, Wheat, and Alfalfa. Extension Bulletin E-2567. agcrops.osu.edu/publications/tri-state-fertility-guide-corn-soybean-wheat-and-alfalfa