The St. Johns River has been identified by the state of Florida as a priority water body in need of restoration under the auspices of the Surface Water Improvement and Management Act implemented by the Florida legislature in 1987.  Personnel from the St. Johns River Water Management District (SJRWMD), University of Florida, multiple state government agencies, and the North Florida Grower’s Exchange have developed “Best Management Practices” (BMP) for potato production in the Tri-County (St. Johns, Putnam, and Flagler Counties) Agricultural Area (TCAA).  The purpose of BMP implementation is to reduce the potential nitrate run-off from approximately 20,000 acres of land in potato production in the St. Johns River watershed.  The SJRWMD manages the BMP program through the TCAA Water Quality Protection Cost Share Program.  The program was developed to provide potato growers in the TCAA with an economic incentive to voluntarily implement verified BMPs that may incur a greater cost and/or risk by area growers.

The average amount of nitrogen applied to potato acreage in the TCAA is 255 lb N/A.  This rate is falls between a high of 350 lb N/A on some chip potato acreage to a low of 175 lb N/A on fresh market potato acreage.  The IFAS recommended nitrogen rate (200 lb N/A) has been adopted as the BMP nitrogen rate for the TCAA.   Grower opinion is that the BMP nitrogen rate is not sufficient to maintain historical potato yields during all years.  In production years with heavy rainfall, nitrogen can be leached from potato beds making it unavailable to the potato plant.  Provisions have been made in the BMP program to allow for additional nitrogen fertilization during seasons with leaching rains (30 lb N/A).  However, depending on when leaching rains occur, growers are concerned that they may not be able to side-dress the crop during the critical bulking period resulting in reduced yields.

Fertilizer technology currently exists, if developed for the region, which could provide a long-term solution to the problem of nitrate leaching on sandy soils and the need for supplemental nitrogen applications during the season.  Controlled release fertilizer (CRF) technology could overcome the concerns of both growers and regulatory agencies by supplying nutrients to the crop while reducing the potential for off-site movement of nutrients.  Relatively recent improvements with formulation and prill coating technology have made it possible to release nutrients based on soil temperature independent of soil moisture.  This insures that even under heavy rainfall, nutrients will be left in the prill for release later in the season.

Two challenges need to be overcome before a CRF can be used in commercial potato production.  First, a product needs to be identified or developed that releases nutrients at a rate required by the potato plant.  That is, the product should produce a potato crop with yields and quality characteristics equal to or better than conventional soluble fertilizers.  Secondly, the cost of CRF products needs to fit the economics of potato production.   Cost of CRFs for growers will ultimately be determined by the rate of material used, pricing by the manufacturer based on large scale production (economics of scale), and whether CRFs are adopted as a reimbursable BMP in the SJRWMD Cost Share Program.

Initial research on CRFs conducted by IFAS personnel at the Hastings REC has had a two fold approach.  First, CRFs have been evaluated for their ability to produce a marketable potato crop compared to conventional soluble fertilizers.  Second, nutrient release curves for nitrogen, phosphorus, and potassium have been constructed for multiple CRFs and soluble fertilizers under field conditions.

The research has demonstrated to this point that CRFs can be used successfully for potato production.  Tuber production and quality using CRFs have been equal to or greater than soluble fertilizers at equal nitrogen rates (Table 1).  Initial results have verified that approximately 50 lb/A of nitrogen can be saved by using a CRF in a dry season instead of a conventional CRF without a loss in quality or yield.  Nitrogen savings would be greater in wet years when supplemental nitrogen is applied to replace leached nitrogen from soluble fertilizers.  All fertilizers in studies to this point have been incorporated at planting and the crops produced using standard practices common to seepage irrigated production in the Hastings area.

Initial in-field nutrient release studies have demonstrated that under heavy leaching conditions, CRFs leach much less nitrate than conventional ammonium nitrate.  Two CRFs have been identified which have release curves that compliment the nutrient uptake of the potato plant.

Three projects are currently underway this season to further evaluate the influence of CRFs in potato production.  CRFs from four companies are being evaluated in potato production and release curve studies.  Also, the influence of leaching irrigation events on potato production and nitrate movement is being evaluated in plots fertilized with either conventional or controlled release fertilizers. In addition, two large-scale, on-farm studies are being conducted comparing potato production using CRFs to the grower’s standard fertilizer practices in the Hastings area.

Development of a successful controlled release fertilizer program for potato production will be a “win-win” situation for growers and regulatory agencies.  Florida’s farmers will be able to continue to farm with the knowledge that Florida’s natural resources are protected.

(Chad Hutchinson and Eric Simonne - Vegetarian 02-03)

Calcareous soils usually contain from 3% to 94% calcium carbonate (CaCO₃). The pH values of calcareous soils are greater than 7, and commonly in the range of 7.4-8.4.  Iron chlorosis is the most frequent nutritional disorder encountered in crops grown on calcareous soils. Inorganic forms of Fe in calcareous soils are largely or almost totally unavailable for plant uptake. High concentrations of bicarbonate in the soil solution can prevent Fe uptake by the plant, as well as its transport within the plant.

Iron is an essential nutrient for plant growth, which includes the formation of chlorophyll. When the amount of iron available to plants is not enough for normal growth, plant leaves become pale green, yellow or white, particularly between the veins. The symptoms begin with young leaves first.   Severely affected plants fail to flower or set fruits and may even die from lack of iron. Iron deficient plants are more susceptible to wind damage during windy winters in south Florida.  The visual symptoms are often clear for plants grown on calcareous soils, but they can be confused with other deficiencies such as magnesium, manganese, zinc or boron.  Tissue analysis will be helpful to confirm iron deficiency.

There are many approaches to deal with iron deficiency of vegetable crops.  Most vegetable crops commonly grown on calcareous soils in Florida have been selected for good adaptation to high pH soils. Thus, vegetable crops generally do not suffer from Fe deficiency.  Some iron-efficient crops release organic acids from their roots to neutralize the bicarbonate and to mobilize soil Fe.  Other iron-efficient crops possess high Fe-reductase activity, or other superior physiological and biochemical characteristics.  However, many new crops and varieties are introduced in to south Florida and many of them are native to acid soils and iron-inefficient.  It is important to test these new crops on a small scale before a large acreage is planted. 

Growers often ask whether they should use soil acidulents such as elemental sulfur (S), sulfuric acid, triosulfate salts, etc. to acidify the calcareous soil. To date, no research data have been generated to establish a beneficial effect of applications of any acidic products on calcareous soils in Florida.

Both soil and foliage application of inorganic sources of Fe such as ferrous sulfate (FeSO₄) or ferric sulfate [Fe₂(SO₄)₃], are ineffective and should not be used on calcareous soils with high concentrations of calcium carbonate such as soils in Miami-Dade County.

Many chelated iron are available in various formulations. The most popular synthetic organically chelated forms of Fe include Fe-EDTA, Fe-HEDTA, Fe-DTPA, and Fe-EDDHA.  These chelated irons can be used as foliar fertilizer and often mixed with other micronutrients in a fertilizer product.  Foliar application of iron fertilizer cannot effectively correct severe iron deficiency.  Fe-EDDHA is only an effective source if iron is applied through soil for calcareous soils.  Soil drench (water plus iron) or fertigation (through the microirrigation system) are more effective and responses of plants to iron fertilizer are much more rapid.  

Other factors may also cause iron deficiency and iron fertilizer may not be needed.  Extreme high or low temperatures can affect Fe uptake by plants and cause chlorotic symptoms.  Plants will grow normal after the weather condition changes.  Over-watering, poor drainage or high water tables also stress plants and affect iron nutrition in soils and plants.  Root diseases such as Fusarium and Rhizoctonia are often associated with wet soils and cause iron deficiency. Poor drainage is quite common in south Florida.  Growing crops on raised beds probably will avoid root diseases. 

(Yuncong Li - Vegetarian 02-03)

There is a great deal of concern among members of the agricultural community in south Florida about the potential impact of regional water management decisions on crop production.  The primary concern is the potential for crops to be flooded as a result of elevated canal levels.  Currently, regional water management decisions in south Florida are generally based on large-scale hydrological (2 x 2 mile) grids or larger.  These regional scales are generally too large to make predictions at the field (farm)-scale level.

In Miami-Dade County, the hydrological and soil conditions are unique and currently not well understood.  This can result in the inability to predict accurately the effects on individual fields of different canal management scenarios adopted at the regional level.  Work has been initiated by Rafael Muņoz-Carpena, Bruce Schaffer and others at the Tropical Research and Education Center (TREC) of the University of Florida Institute of Food and Agricultural Sciences (IFAS), and colleagues at the U.S. Department of Agriculture, to gather more detailed information about the interaction between the canal and field hydrological conditions. This work requires quantification of the small-scale variability of the hydrological properties of the soil and aquifer and their effects on soil and ground water flow and water table depth changes. The effort will lead to development and testing of new (and existing) smaller scale hydrological models that will allow the prediction of flooding events in individual fields (or specific areas within a field) in response to a given canal management scenario. Water quality issues (nutrients and pesticides) linked to these dynamic conditions are also being researched both at the surface water (canal) and groundwater. A challenging problem under study is how the canals and the shallow Byscane aquifer in this area interact and exchange chemicals at the field scale.

A critical issue that needs to be resolved for south Miami-Dade County is the lack of detailed information on surface elevations for the agricultural area.  This is extremely important for successful development and application of field-scale models for predicting flooding in agricultural fields as a response to canal levels under specific water management schemes.

Plant Responses to Flooding

Hydrological conditions need to be linked to plant responses to minimize the potential effects of high water levels on crop production.  Work has been underway by researchers at IFAS and other institutions to determine the effects of flooding on crops and to identify, develop, and recommend flood tolerant crops for areas that may be affected by elevated water tables in the future.

Vegetable Crops

Flooding is the major risk to fresh vegetable production in south Florida especially in the south Dade area.  Although most soils are normally well drained, low-lying areas are often prone to flooding during periods of high rainfall.  In Miami-Dade County, agriculture loss estimates from flooding as a result of rainfall (13.9") in December 2000 were 13 million dollars. In October 1999, vegetable crop losses due to Hurricane Irene were estimated to be about 77 million dollars with nearly 19,000 acres of agricultural production damaged by floods.  A project is currently being conducted to develop effective management techniques to prevent or reduce flooding damage to vegetable crops.  Yuncong Li at TREC and Stewart Reed at the USDA in Miami are currently studying flood tolerance of vegetable crops and developing effective management techniques to prevent or reduce flooding damage to these crops. 

Ornamental Crops

Jorge Peņa of TREC has been working on testing woody ornamental crops for flood tolerance in the “Frog Pond” area adjacent to Everglades National Park.  He has found that some native ornamental species [Conocarpus spp., Quercus virginiana, Sabal palmetto] can survive flooding very well and even require fewer pesticides under flooded conditions compared to non-flooded conditions.  Plants have been grown under “organic” and “chemical” systems.  Those plants grown with minimum to no insecticides and herbicides have similar market quality to those grown with the use of agrichemicals (agrichemicals).  An economic analysis for both systems will be done at the end of the study to provide growers with alternative systems for growing native plants under conditions in the Florida Everglades.

Tropical Fruit Crops

For the past 15 years, Bruce Schaffer and others at the TREC have been studying flood-tolerance mechanisms of tropical fruit crops and trying to develop flood-tolerant rootstocks.  Much of this work is published, but some of the highlights are listed below.

1.  Scions of commercial Annona trees such as 'Gefner' atemoya have been successfully grafted (with the help of Gary Zill of Zill’s High Performance Plants Nursery) on rootstocks of the non-commercial species Annona glabra (pond apple), which is native to the Florida Everglades.  The commercial crops grafted on their traditional Annona squamosa rootstocks can only survive about 3 days of continuous flooding, whereas plants grafted on Annona glabra rootstocks have survived and grew for up to 17 months in continuously flooded conditions.  These Annona cultivars are currently being evaluating for horticultural and fruit quality characteristics and in the future they will be field tested in conjunction with Jorge Peņa on his flood-prone experimental site in the “Frog Pond” near the Everglades National Park.
2. For avocado, it has been determined that a strong synergistic effect exists between Phytophthora root rot (PRR) (caused by Phytophthora cinnamomi) and flooding.  The disease generally does not cause mortality under non-flooded conditions.  By preventing Phytophthora infection, one can greatly improve flood-tolerance of avocado trees.  Flood tolerance of avocado was improved by preventing Phytophthora infection with the use fungicides applied to the soil or injected directly into the tree.  However, this approach is impractical since fungicides are expensive and growers do not want to apply them if they are not sure that their fields will be flooded.  The best solution for improved avocado flood tolerance is Phytophthora-resistant rootstocks; however, there are no truly resistant rootstocks (some cultivars are said to have resistance, but this is only because the roots outgrow the pathogen).  Although the avocado tree has little or no resistance to PRR, other Persea species in the subgenus Eriodaphne are highly resistant to the disease.  Unfortunately, these related species are sexually and graft-incompatible with avocado.  A promising but long-term solution is the work being done in Richard Litz’s laboratory at UF TREC.  Through somatic hybridization and other genetic manipulation approaches, artificial hybrids between avocado and the PRR resistant species are being synthesized.  One of the PRR resistant species is Persea borbonia, which is native to the Florida Everglades.  Plants that develop in-vitro from fused protoplasts of resistant species and avocado are still in the test tube stage and first need to be acclimated to pots, than the field tested for horticultural performance.  Developing flood-tolerant avocado through molecular techniques is very promising, but a long-range goal.
3. Work with carambola trees has indicated that short-term flooding (a couple of days) actually can stimulate flowering of carambola, whereas longer-term flooding can harm the plants.  Trees will recover from flooding stress after the water recedes, unless the flooding goes too long (several weeks) or there are repeated flooding cycles for a few days each.
4. Anatomical and morphological features that confer flood-tolerance to some mango trees, have been identified by Bruce Schaffer and colleagues at TREC.  These features have been utilized as markers for screening and selection of flood-tolerant rootstocks.  The mangoes that are grown in south Florida are of the monoembryonic type and do not come "true to type" from seed, unlike polyembryonic mangoes. Thus, in order to assess the genetic diversity within mango germplasm for flooding tolerance, it is essential to develop clonal material of monoembryonic mango accessions to successfully screen and select for flood tolerance.  This work has been tried in Richard Litz's lab using tissue culture techniques. The study was successful in the laboratory, but long-term survival following transplantation from test tubes to the field is a problem.   

The Bottom Line

Several conclusions can be made concerning flooding in agricultural fields in South Florida:

• A better understanding of the hydrological system by experimental fieldwork is critical for developing and testing much needed field-scale models.  These small-scale tools could then be linked to more general regional models currently in use to assess the effects of different canal management scenarios on the field in the south Florida agricultural area. Initial work by researchers at TREC and the USDA is under way in this direction, but will need sustained funding by the water and soil management institutions in the area.

• Detailed surface elevation data are critically needed for accurate predictions of flooding at the field (farm)-scale.  Public and private institutions in the area should join efforts to obtain this information for the agricultural land in South Florida.

• Research has been conducted to understand crop responses to flooding and identify or develop flood-tolerant crops and rootstocks.  There are some promising results with native woody ornamentals.  For tropical fruit crops, we may soon be successful in developing flood-tolerant rootstocks for Annona, but whether it is economical is uncertain.  There also may be other horticultural problems associated with this new material so it is being tested.  Flood-tolerant rootstocks for some tropical fruit species (i.e., avocado) can be developed using molecular and tissue culture techniques, as a long range, environmentally sensitive solution to the problem and to overcome the current reliance upon agrichemicals for disease control.

(B. Schaffer and R. Muņoz-Carpena- Vegetarian 02-03)