I recently wrote a report on fertilizer technology for Miami-Dade County Agricultural and Rural Land Retention Study. I like to present it to our Newsletter readers and hope you will find it somewhat useful.

Dry granular fertilizer is still and will continue to be a major fertilizer source for crop production in Miami-Dade County. It is relatively inexpensive and always available from fertilizer distributors. Most large-scale growers have their own machinery for fertilizer application, while small-scale growers always get help from fertilizer distributors. Most growers have relatively more knowledge of this group of fertilizers than of any others. The changes of dry granular fertilizer technology will be 1) specific fertilizer for specific crops and 2) application technology. Special fertilizer such as "palm special", "lawn special", "tomato special", "lychee special", etc. will become progressively more available to growers. These special fertilizers are manufactured or mixed with distinct nutritional formulas to meet specific needs of the crop. However, variation between fields and between varieties should be considered also when selecting a special fertilizer. Another set of adjustments in the use of dry granular fertilizer involves application timing and rates. A recent fertilizer management survey for tropical fruit crops in south Florida showed that growers apply fertilizers 1-12 times per year for total amounts of 5 - 445 lb N/ac. These large variations indicate that it is necessary to optimize fertilizer application frequencies and rates.

Fertigation:

Fertigation is the application of liquid fertilizer through an irrigation system. The technology was developed in the 1970s and its adoption continues unabated to the present. Almost all tomato growers in Miami-Dade County use fertigation. Fertigation also has been used for other vegetable crops such as squash, eggplant, cucumber, etc. Our survey revealed that about 53% of tropical fruit growers are using either fertigation or soil drenches in their groves. Various innovations in fertigation equipment are being developed and marketed to growers. Research information is also more available though traditional publications and internet access. The advancement and progressive adoption of fertigation technology is certain to improve fertilizer use efficiency, overcome micronutrient deficiencies and reduce leaching of nutrients into groundwater.

Foliar fertilizers:

Foliar fertilizer technology came into use early in this century, but did not become common practice until the 1980s. This slow adoption was caused by the scarcity of fertilizer sources and appropriate application equipment. The application of foliar fertilizers is the quickest way to deliver nutrients to the tissues and organs of the crop. However, the plant leaf is structured in such a way that it naturally resists ready and reliable penetration of fertilizer salts. This is particularly true of N, P, and K. Research in Florida clearly shows that foliar application of N, P, and K does not result in consistent yield increases. Indeed some recent work has documented severe yield reductions with foliar nutrient sprays. On the other hand foliar application of micronutrients can be beneficial to correct certain nutrient deficiencies. However, a micronutrient formulation should be applied only when a specific deficiency has been clearly diagnosed. Based on over 30 years of experience with vegetable production in south Miami Dade County, Dr. Herbert H. Bryan concluded that only Fe and Mg foliar fertilizers effectively overcome corresponding micronutrient deficiencies of vegetable crops in this area.

Slow/controlled release fertilizers:

Slow release fertilizers are often interchangeably designated "delayed release", "controlled release" or "metered release" formulations. Such a formulation has a rate of dissolution (usually in water) much less than the rates for completely water-soluble compounds. Controlled/slow release fertilizer technology was developed in the 1960s, but became commercially available only in late 1970s and early 1980s. A recent fertilizer survey in Miami-Dade County showed that 18% tropical fruit growers use slow or controlled release fertilizer formulations. Price and availability of such formulations still restrict their use in traditional crop production. The quality of fertilizer is also of major concern. However, the development of new technologies for controlling nutrient release will lead to improvement in prices and quality of slow release fertilizer formulations.

Soil organic amendments:

Soil organic amendments usually consist of animal manures, cover crops, or composts. In many instances animal manures are the best source of organic matter. Indeed since the beginning of agriculture, animal manures have been used effectively for crop production. Animal manures supply significant quantities of essential plant nutrients and increase soil organic matter. However, availability, transportation costs, and regulations limit widespread use of animal wastes in crop production in Miami-Dade County. Cover crops, commonly referred as green manure, are also important soil organic amendments for sandy or gravelly soils in this area. Cover crops are used to improve soil physical properties, increase soil organic carbon, conserve soil water, reduce surface runoff, and recycle nutrients during the heavy summer rains. Sunn hemp (Crotalaria juncea L.), one of the new cover crops in south Florida, was found to produce 8.5 to 11.3 metric ton dry weight ha^-1 and to fix or retain up to 270 kg N ha^-1. A series of cover crop projects are currently in progress in Miami-Dade County. Composts have been increasing in popularity as soil organic amendments. Research has demonstrated that compost can serve as a soil amendment to increase organic matter, improve microbial activities in soils, provide nutrients, and ultimately improve plant growth and yield. However, composts are produced from various organic wastes and consequently, environmental concerns are always an issue in compost utilization. The potential often exists for heavy metals to accumulate in the soil and sometimes in the edible plant parts of vegetables and fruits. Yet, this is not as likely to occur in the County's calcareous soils as may occur in the acidic sandy soils further north. Nevertheless excessive nutrients or metals released from composts may be leached out of the root zone and into groundwater by irrigation or by rainfall. Other hazards such as human pathogens, viable weed seeds, and plant phytotoxicity should be considered when developing vegetable production systems with compost.

Mycorrhizae:

Mycorrhizae are certain soil fungi which for symbiotic associations with plant roots. These beneficial symbioses are ubiquitous in nature and almost all vascular plant species have some form of mycorrhizal association. Mycorrhizae were discovered in 1840, but were not studied extensively until the 1990s. Most research focused on the potential of mycorrhizal fungi to improve crop yields and to reduce the use of fertilizers. A recent experiment conducted in the Tropical Research and Education Center, University of Florida indicated that lychee plantlets derived from air-layers and inoculated with mycorrhizal fungi grow and develop much more rapidly than those which are not exposed to these beneficial symbionts. Many other scientists have reported significant functions of mycorrhizal fungi in crop production. However, such promising research results have not led to significant commercial utilization of mycorrhizae. Nevertheless, mycorrhizae are likely to play a very important role for crop nutrient management in the future.

Magical/mysterious products:

The 6 inner plants within each plot were harvested and average diameter and average head weight calculated (Table 1). Harvest dates were determined by observation.

Days to harvest were within expected values (Vegetable Production Guide for Florida - SP170) for all cultivars (70-95 days for iceberg, 60-80 days for all others). The transplant production period was longer than expected, especially for Redina (Fig. 2), Crispino and Lollo Rosa. Earlier than recommended planting dates and higher than optimum temperatures at planting may have slowed the initial growth of the transplants. Red colors are reported to fade under high temperature conditions but did not in this trial, particularly in the dark red Redina. Quality was high in all the cultivars although Terlana (Fig. 3) and Green Forest bolted quickly after reaching marketable size. Recommended cultivars from the quality evaluation by the class included Green Forest, Optima (Fig. 4), Royal Oak (Fig. 5), Valencia, Noisette and Redina.

Fig. 2. Redina

Fig. 3. Terlana

Fig. 4. Optima

Fig. 5. Royal Oak

(Lamb, Vegetarian 01-01)

(Stephens, Vegetarian 01-01)