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Micronutrient Management

Plant nutrition is extremely important in the production of foliage, flowering and bedding plants. Generally speaking, most growers use some complete fertilizer to supply nitrogen (N), phosphorus (P), and potassium (K). These elements are referred to as macronutrients because they are required in relatively large quantities for plant growth and development. Boron (B), chlorine (Cl), copper(Cu), iron (Fe), manganese (Mn), molybdenum (Mo) and zinc (Zn) are referred to as the micronutrients because they are needed by the plant in much smaller quantities than the macro's.

Providing optimum micronutrient levels can be challenging. Although many pre-mixed growing medium have a "micronutrient charge", most are only designed to last for 2 - 3 irrigations. This means that some additional micronutrient source must be supplied to meet the nutritional needs of the plant. The primary considerations for micronutrient fertilization include: source used, amount provided, and availability or form.

There are a variety of sources that can be used to supply micronutrients. Several "complete packages" are available that supply all 7 elements. These can either be incorporated in to the growing medium or applied through the irrigation system. However, there is a risk of putting out too much or too little of a specific nutrient using this shotgun approach.

Many soluble N,P,K fertilizers also provide some level of micronutrients. If properly selected this approach is ideal for providing a constant, low-level source of micro's for plant growth. Study the label carefully to determine the amounts supplied and be sure the product provides a gauranteed analysis.

Growing medium components can also contribute some micronutrients to the system. As these materials breakdown their mineral content becomes available for plant uptake. However, these levels are so low additional sources are generally required.
Growers frequently overlook their water supply as a micronutrient source. Much of the water in Texas, and throughout the Southwest, contains appreciable amounts of B, Cl, Cu, Fe, Mn, etc. A current water analysis is very important in designing a micronutrient fertility regime (see page 2 for details on how to get your water tested).

Using results from the water analysis and based on the type of N,P,K fertilizer used, growers can determine what micronutrient sources, and the amount that will be most effective in providing optimum nutritional levels. Table 1 presents some of the more common micronutrient sources and recommended levels.

Alkalinity and pH influence the availability of micronutrients in the growing medium. As acidity/pH goes down the availability of B, Cl, Cu, Fe, Mn and Zn increases. Although not all growers and researchers agree, a pH range of 5.5 - 6.5 generally provides optimum micronutrient availability.

The pH of a nutrient solution is also important. Since fertilizers are usually concentrated 100 to 200 times in the stock tank, excessively high pH/alkalinity can cause precipitation. If you notice large amounts of sludge developing on the bottom of the tank you may have a serious pH problem. Figure 1 provides some more specific information on the effect of pH on nutrient availability.

Most growers that experience chronic micronutrient deficiencies are usually dealing with a pH/alkalinity problem. Be sure to address the problem (high pH/alkalinity) and not just the symptom (chlorosis, stunted growth, etc.).