Soils - Part 7: Soil and Plant Considerations for Calcium, Magnesium, Sulfur, Zinc, and other Micronutrients
Zinc deficiency was first identified in Nebraska in 1955. Since that time, much research has been done with zinc.
Zinc in the Plant
Zinc is used by plants in the formation of chlorophyll and is apparently linked with iron and manganese in this process. Zinc deficiency is more likely to occur in corn or field beans than other crops commonly grown in Nebraska.
Typical zinc deficiency symptoms of field crops are chlorosis of the interveinal tissue and a shortening of internodes which makes the plant appear stunted. In corn, the typical zinc deficiency symptom is a chlorotic band on each side of the midrib of the lower leaves. The chlorotic band is found near the base of the leaf rather than near the tip. Where zinc is severely deficient, upper leaves may become chlorotic and lower leaves will turn purple to brown and die.
Zinc in the Soil
Most Nebraska soils contain adequate zinc for crop production; however, areas within fields may not contain enough zinc for normal plant growth. Zinc deficiency is more likely to occur in calcareous (high lime) soils of central and western Nebraska than in the neutral to acid soils of southeastern Nebraska. Zinc deficiency is often observed where the topsoil has been removed by erosion, land grading for irrigation, or during terrace construction. Also, zinc levels are low in many soils in the Sandhills. Zinc deficiency is favored by cold, wet weather.
Soil tests for zinc have been developed and should be used where the zinc status of a soil may be in doubt. Soils that have had topsoil removed, are low in organic matter, or contain excess lime are most likely to respond to zinc fertilizers. Careful field division and sampling are most important for an accurate zinc test.
Most soil testing laboratories use the DTPA extracting procedure to determine the zinc availability index. Normally there are three soil zinc availability levels as determined by DTPA. These are: Low (L) DTPA Zn (0 to 0.5); Medium (M) (0.51 to .8); and High (H) greater than 0.8.
Phosphorus-zinc interactions have received considerable attention the last 20-25 years. It has been observed that applying phosphorus without zinc on calcareous soils low in zinc can reduce corn yields drastically. If zinc is included with the phosphorus, yields may be improved. This reduction in corn yields by phosphorus occurs only on soils low or marginally adequate in zinc and usually containing excess lime.
Fertilizer Sources of Zinc
As shown in Table 7.3, zinc carriers can be classified into groups based on their chemical characteristics:
These groups are quite different from each other, and the fertilizers within a group differ chemically from others in the group. Mobility in the soil is quite similar for carriers in a group.
Zinc chelates are quite mobile in the soil and can move with soil water. Thorough mixing in the soil is not critical with chelates, even if granular formulations are used, provided water is available to distribute the zinc.
Organic non-chelate zinc carriers and soluble inorganic zinc carriers are soluble but not very mobile in the soil. They can be used in granular form but need to be well mixed in the soil to assure root-zinc contact.
Insoluble inorganic zinc carriers are not mobile in the soil, must be applied as small or finely divided particles, and placed in the soil where plant roots will contact the particles.
Applying Zinc Effectively
All zinc carriers, except the very insoluble compounds, are effective sources of zinc for crops, provided they are properly applied. Zinc materials may be broadcast on the soil and thoroughly incorporated or used in a band next to the seed at planting time.
Fertilizer materials are usually applied at a rate sufficient to grow the current crop; however, with zinc it may be more practical to increase the soil level of zinc and provide a multi-year supply. Zinc deficiency does not occur uniformly in an entire field, but rather in spots. These deficient spots are often associated with loss of top soil due to land grading, terrace construction, or erosion. Deficient spots may occur with a change in soil type or because of soil compaction. Where these spots can be isolated, zinc can be applied at rates sufficient to correct the problem for many years. Broadcast application of an inorganic zinc material and thorough incorporation are the best means of correcting the problem. Granular zinc sulfate or finely divided zinc oxide or carbonate are effective zinc carriers for broadcast treatment at minimal cost.
Zinc also can be applied in a band beside the seed at planting time. About 10 pounds of nitrogen per acre should be included with the zinc. Other nutrients, such as phosphorus, also may be needed. Where zinc is applied in a band, carrier selection is more critical, especially where zinc deficiency is great, because the material is not well distributed in the soil.
If dry, granular fertilizer is applied in a band in severely zinc-deficient soils; the organic zinc chelates may be more effective than the other zinc carriers. Where finely divided zinc carriers are used with liquid polyphosphate fertilizers, all carriers should give satisfactory results. Only under very extreme zinc deficiency conditions is there likely to be any difference in zinc carriers when applied in a band with liquid polyphosphate fertilizers. Under these extreme deficiency conditions, it is difficult to supply enough zinc in a band; thus, broadcast treatment with thorough incorporation is likely to be more effective.
Effectiveness ratios, such as 10:1 or 5:1, are often quoted in marketing promotions. A 10:1 effectiveness ratio means that a specific zinc source is 10 times more effective than another. These ratios are meaningless because it depends on the conditions where the carriers are used, how applied, degree of deficiency, carrier formulation, etc. Seldom is the effectiveness ratio greater than 5:1; and, when the materials are properly applied, it is usually near 1:1.
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