Iron chlorosis commonly appears as patches in fields.
Iron chlorosis commonly appears as patches in fields.

Of the 16 elements known to be essential for plant growth, seven are used in very small amounts and, with the exception of iron, have an uptake of less than 1 pound per acre per year (Table I). These are classified as micronutrients and include zinc (Zn), iron (Fe), manganese (Mn), copper (Cu), boron (B), molybdenum (Mo), and chlorine (Cl). Interest in micronutrients has increased with increasing yield levels, increasing rates of nutrient removal in harvests, and availability of alternative micronutrient products.

Table I. Estimates of micronutrient uptake by crops.
Micronutrient
200 Bu Corn
60 Bu Soybean
6 Ton Alfalfa
lb/acre
lb/acre
lb/acre
Iron
Manganese
Zinc
Boron
Copper
Molybdenum
2.4
0.4
0.4
0.2
0.1
0.01
1.7
0.6
0.2
0.1
0.1
0.01
1.8
0.6
0.2
0.3
0.06
0.02
Adapted from: Role of Micronutrients in Efficient Crop Production, D.B. Mengel, Purdue University AY-239.

Micronutrient Availability

Some micronutrients are supplied to plants when weathering breaks down soil minerals over time, but the greatest supply usually derives from the breakdown (mineralization) of soil organic matter. Soils with low clay and organic matter content may be deficient in one or more micronutrients. Soils that receive regular manure applications seldom have micronutrient deficiencies.

Nutrient availability is affected by soil pH (Figure 1). The availability of iron and manganese increases with increased acidity. Soluble manganese concentrations can be toxic in soil with less than 5.0 pH. Copper, zinc, and boron availability increases with decreased pH until pH measures 5.0 to 5.5; below this level availability decreases. With the exception of molybdenum, micronutrient availability decreases as pH increases above 7.5.

Figure 1. Nutrient availability as affected by soil pH.

Soil organic matter and applied organic matter affect the availability of positively charged micronutrients through chelation. Chelation is the formation of bonds of varying strength between a metallic ion and an organic molecule. Chelation often increases the solubility and availability of a nutrient and delays reaction and tie-up of the nutrient with soil minerals. That’s why micronutrient fertilizer products often contain the nutrient in chelated form. Excessive chelation can occur in soil with greater than 10 percent organic matter, resulting in deficiencies of some micronutrients. Most Nebraska soils contain 1 to 4 percent organic matter.

High levels of one nutrient can affect the availability of some micronutrients. High rates of phosphorus application to calcareous soils or soils with low zinc levels can induce zinc deficiency. Iron uptake can be reduced by high bicarbonate concentration in the soil.

Climate conditions also can affect plant use. Under cool, wet conditions, uptake of zinc and manganese may be reduced due to slow root growth. Boron deficiency is more likely to occur with dry weather conditions.