Special Purpose Fertilizers

Special purpose fertilizers are primarily used in the small fruit and nursery industries. When shopping for fertilizer, you will find that some are packaged for very specific uses, such as blueberry food. The blueberry (sometimes called rhododendron or azalea fertilizer) food is one example of these specialty materials and belongs to an old, established group, the acid-plant foods. Some of the compounds (ammonium sulfate is usually the acidifying ingredient) used in these fertilizers are chosen because they have an acid reaction and may benefit acid-loving plants when the soil pH is too high. These materials are appropriate for use when small-fruit producers are trying to slowly lower the soil pH in an established blueberry orchard that is showing symptoms of iron deficiency due to the effects of a high soil pH. They are often used in combination with dilute foliar sprays of iron or soil applications of iron chelates. These materials are usually applied at a rate that will meet the nitrogen requirement of the orchard for that season. Once the soil pH is lowered to the desired level, the producer should switch to a less acidifying nitrogen source such as urea or ammonium nitrate.

Ideally, soil pH is adjusted to the correct range well in advance of trying establish a blueberry orchard. Elemental sulfur is the specialty fertilizer mo appropriately used for this purpose. It should be applied at least three month: advance of plant establishment. If the initial soil pH is very high, then a application of sulfur over a six-month period may be needed to reach the desired range.

In some agronomic cropping systems, special-purpose fertilizers are used when applying nutrients through irrigation systems (fertigation) or for foliar application. These are generally highly water-soluble materials and may also have a relatively low salt index to minimize the potential for burning of plant tissue.

It is difficult to make a blanket statement that a certain type of specialpurpose fertilizer is best for some plants. Be sure to perform a soil test before purchasing any special-purpose fertilizer materials to provide a more objective basis for application. Once the objective for using the special purpose fertili (i.e., soil pH lowered to correct range) is achieved, the producer should switch to more appropriate material. Continued use of a specialty fertilizer after this time can result in fertility problems.

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