Cropping System The crops in a crop rotation will draw more on the soil zinc supply; a rotation of corn and edible beans with higher Zn demands may require a more frequent application. Zn is a nutrient that has limited mobility and could be soil applied ahead to meet crop demands in future years. A corn, soybean and wheat rotation may only require Zn supplementation for the corn. Applying Zn in a starter band is likely the most efficient way to position Zn for uptake. With limited mobility in both the soil and the plant, placing Zn in the root zone will optimize uptake. Crop Growth Stage The demand for micronutrients is generally important in the early vegetative growth stages although recent research has demonstrated a season long uptake pattern. Soil Test or Tissue Test There is an accredited test for Ontario using the DTPA extraction method and reporting the value as an index that is modified by soil pH. The critical index is 15; any values below would indicate a need for zinc on high demand crops. Table 4.12 Zinc Soil Index Interpretation Zinc Soil Index1 Greater than 200
Suggested Treatments Contamination of the sample or of the field is likely.
25 to 200
Soil zinc availability is adequate for most field-grown crops.
15 to 25
Zinc availability is adequate for most field crops. If the field sampled is uneven in soil texture, pH, or erosion, some areas may respond to zinc applications.
Less than 15
Zinc is likely to be deficient and should be applied in the fertilizer.
Zinc Index = 203 + 4.5(DTPA extractable zinc in mg/L soil) - 50.7(soil pH) + 3.33(soil pH)2 1. These values are indices of zinc availability based on extractable soil zinc and soil pH. Source: Micronutrients – Soil Diagnostics, Ontario Crop IPM, http://www.omafra.gov.on.ca/IPM/english/soil-diagnostics/micronutrients.html, accessed April 4, 2016.
When zinc is required, it may be applied to the soil, mixed in the fertilizer at rates supplying 4-14 kg/ha (3.5-12.5 lb/acre). The higher rate should be sufficient for up to three years. Not more than 4 kg/ha (3.5 lb/acre) should be banded at planting. Zinc may be applied as a foliar spray at rates supplying 60 g/100 L (0.6 lb/100 gal). A wetting agent should be added. Spray to leaf wetness. Tissue testing can be used to determine the zinc status of plants. Assuming critical levels are available for interpretation or sample good and bad areas separately for direct comparison of values. Often a foliar application is used if the values indicate a need to supplement. Since micronutrients are needed in small amounts, foliar applications can often deliver a sufficient quantity to the right plant part (the newest growth) where deficiencies are most pronounced. Timing of Application Timing of zinc is thought to be better in the early season as often the deficiencies can be seen as early as three to five leaf corn. Placing Zn in a starter band close to the developing roots will aid in early season uptake. Zn uptake is lower in high pH soils. Placing Zn in an acidifying band with P will enhance Zn availability.
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PROFICIENCY AREA IV - Potassium, Secondary Macronutrients and Micronutrients