2 AGRONOMY NEWS

Micronutrients In Crop Production

An understanding of micronutrient deficiency symptoms, common conditions leading to deficiencies, and sources of micronutrients is essential to understanding the role of micronutrients in agronomy.

Questions are often raised about the relative importance of micronutrients in crop production. It has been well established that optimum yields generally are not possible without N, P and K fertilizers, and the secondary nutrients to a lesser extent. Most growers and dealers follow the recommended rates and methods of application to achieve top yields, but they may not consider that one or more micronutrients also may be limiting their yields.

Micronutrients are those elements which are essential for plant growth, but are required in much smaller amounts than those of N, P and K. The micronutrients are boron (B), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), zinc (Zn), and chloride (Cl). Because Cl deficiencies rarely occur, this report will discuss the other six micronutrients.

Micronutrient deficiencies have been verified in many soils through increased use of soil testing and plant analyses. Soil tests should be included in all micronutrient fertilization programs, first to assess the level of available micronutrients and later to determine possible residual effects (buildup). Plant analyses give an indication of the micronutrient status of crops during the growing season.

Boron

Boron deficiency symptoms first appear at the growing points. This results in a stunted appearance (rosetting), barren ears due to poor pollination, hollow stems and fruit (hollow heart), brittle discolored leaves, and loss of fruits and nuts.

Boron deficiencies are mainly found in acid soils, on sandy soils in regions of high rainfall or under irrigation, and those soils with low soil organic matter. Borate ions are mobile in soil and can be leached from the root zone. Boron deficiencies are more pronounced during drought periods when root activity is restricted. Crops that are susceptible to B deficiency are alfalfa, sugar beets, clovers, and some vegetable crops. There have been few reported crop responses to applied B in Colorado.

Copper

Deficiency symptoms of Cu are dieback of stems and twigs, yellowing of leaves, stunted growth, and pale green leaves that wither easily. Cereal crops are especially susceptible to low Cu levels, with curled leaves at tillering, head and stem bending, shriveled grain, and delayed maturity.

Copper deficiencies are mainly reported on organic soils (peats and mucks), and on sandy soils which are low in organic matter. Copper uptake decreases with increases in soil pH and increased P and Fe availability in soils. Some crops that are sensitive to Cu deficiency are alfalfa, barley, corn, oats, wheat and some vegetable crops. Copper deficiencies have not been observed in Colorado crops.

Iron

Iron deficiency is expressed as yellow leaves due to low levels of chlorophyll (chlorosis), which first appears on the younger upper leaves in interveinal tissues. Severe Fe deficiencies may cause leaves to turn completely yellow or almost white, and then brown as leaves die.

Iron deficiencies are found mainly on calcareous (high pH) soils. Cool, wet weather enhances Fe deficiencies, especially on soils with marginal levels of available Fe. Poorly aerated or compacted soils also reduce Fe uptake by plants. Uptake of Fe decreases with increased soil pH, and is adversely affected by high levels of available P, Mn and Zn in soils. Some crops which are sensitive to Fe deficiency are corn, sorghum, wheat and ornamentals. Iron chlorosis has been widely observed in grain sorghum and on ornamentals in Colorado.