The term hydroponics is generally used to describe any of several methods of growing plants without soil. A number of packaged hydroponic systems are available for use in commercial production or by hobbyists. These systems are variously referred to as water culture, gravel or sand culture, soilless culture, bag culture, solution or liquid culture, nutri-culture, the nutrient film technique (NFT), and float systems.

Today hydroponic systems are widely used throughout the world by gardeners, researchers, and commercial producers. In fact, we have about 850 acres of hydroponic vegetables produced in the US today, mostly in greenhouses. Florida has about 75-80 acres of vegetables being produced in greenhouse hydroponic systems. Primary crops in Florida include: colored bell pepper, tomato, European cucumber, and lettuce. Other specialty crops are also produced including: strawberry, herbs, specialty greens, and edible flowers.

The primary production system used by growers in Florida is lay-flat bag culture with perlite or rockwool as the soilless media. This system is popular for tomato, cucumber, and pepper. Nutrient film technique and floating systems are popular for bibb lettuce and herbs.

Vertical hydroponic systems such as Verti-Gro®, Verzontal®, or vertical bag culture may also be quite useful to increase plant populations in limited and valuable greenhouse space.

Trials have also been conducted at the NFREC-Suwannee Valley near Live Oak, FL to develop a hydroponic system that utilizes organic nutrient programs. This organic project has been successful in the vertical culture of hers and leafy green vegetables.

The Everglades Soil Testing Laboratory (ESTL) is located at the Everglades Research and Education Center in Belle Glade, FL. The ESTL is one of two University of Florida soil testing laboratories, and represents an important historic interface between the University of Florida/IFAS and the Everglades Agricultural Area (EAA) grower community. The primary mission of the ESTL is to quantify soil-test nutrients and provide growers crop-specific fertilizer recommendations based on crop response field research. Historically, the ESTL has offered calibrated soil-test P and K recommendations based on in-house developed extraction procedures for a variety of crops including sugarcane, leafy vegetables, celery, radish, and sweet corn.

Current P fertilizer recommendations for vegetables (Hochmuth et al., 1994, 1996) and sugarcane (Gascho and Kidder, 1979; Sanchez, 1990) are based on water extractable soil-test P levels. This P_w protocol was originally developed for vegetable crop commodities and has been in use since the mid-1940s. The procedure is detailed in Thomas (1965), who attributes the water extraction procedure to earlier soil chemistry work by Forsee (1945a). During the early 1940s, Forsee conducted studies that focused on the merits of three extractants, including carbonic acid, 0.5N acetic acid, and distilled water. Based on the assessment of the correlation between soil-test P levels with celery tissue P concentrations, acetic acid extractions were found unreliable, particularly across soils of varying pH. Conclusions favored the use of either carbonic acid or water since resulting soil-test P values correlated best with P uptake during celery growth (Forsee, 1945b). Ultimately, the P_w was adopted since extractions with water were cheaper to conduct than with carbonic acid (Forsee and Erwin, 1947).

In theory, water extraction provides a good estimate of quickly solubilized soil P (intensity factor; readily available for plant growth over the short term), which suggests that the P_w protocol is appropriate for assessing P requirements for short-season vegetable crops. Measuring only the intensity factor appears less appropriate for projecting nutrient needs for longer-term agronomic crops such as sugarcane. This issue has stimulated an interest in alternative soil extraction procedures. Sanchez and Burdine (1987) stressed the need to investigate new extractants that were less sensitive to soil pH and P-buffering capacities while providing improved estimates of soil Fe-, Al-, and Ca-P fractions (labile or quantity factor; readily available for plant growth over the longer term).

Using lettuce as a test crop, Sanchez and Hanlon (1990) evaluated six extractants including water (P_w), sodium bicarbonate, Mehlich-1 (M-1), Mehlich-3 (M-3), and two organic agents, ethylene-diamine-tetraaceticacid (EDTA) and ammonium bicarbonate-diethylene-triamine-pentaaceticacid (AB-DTPA). In fact, none produced superior correlations to lettuce responses relative to the water extraction. The M-1 extractant has been used by the University of Florida Analytical Research Laboratory (Gainesville) since the late-1970s (Hanlon et al., 1990). However, the M-1 did not perform well on EAA organic soils with a wide pH range (Sanchez and Hanlon, 1990). An important factor to consider is that the underlying limestone bedrock in the EAA contributes carbonates into the soil profile which are moved to the soil surface through subsurface irrigation practices. Acids that comprise the M-1 extractant (HCl and H₂SO₄) can be partially neutralized by soil carbonates, which may reduce the chemical effectiveness of the extractant. On acidic (pH=5) Florida organic soils, the M-1 accurately predicted lettuce responses (Diaz et al., 1988).

Although the sodium bicarbonate, EDTA, and AB-DTPA extractants generally correlate well with crop responses on calcareous soils in the western USA, they appeared unsuitable for EAA organic soils, despite the presence of carbonates (Sanchez and Hanlon, 1990). However, the authors suggested that procedural difficulties encountered during their study may have undermined the effectiveness of the organic extractants. The M-3 showed promise as an alternative extractant and additional research was recommended for all crops produced on Florida organic soils. The M-3 has several advantages over water. First, the M-3 is a "universal" extractant (routinely used to determine soil-test P, K, Ca, Mg, and micronutrient levels), and second, the M-3 generates a wider soil-test P range than that produced by water.

The soil-testing laboratory at the U.S. Sugar Corporation (Clewiston, FL) adopted the Bray extraction in 1958 to assess soil P levels for sugarcane grown in the EAA (Andreis and McCray, 1998). The Bray was selected over a water-based extraction since Bray soil-test P levels reflected a higher percentage of the acid-soluble and adsorbed P fractions present in the soil. The reasonable assumption is that over time, these P species will become available to the long-season sugarcane crop. Korndörfer et al. (1995) investigated several extractants for sugarcane, including M-1, acetic acid, and water. Soil-test P levels produced from acetic acid (P_a) and M-1 were 7.2 and 4.3 times greater than soil-test P_w levels. Another way to interpret this is that the P_w range (0-8 mg P/L) was compressed relative to the M-1 (1-24 mg P/L) and P_a (6-39 mg P/L) ranges. The authors favored continued investigations with the P_a extractant since its extraction behavior may be less affected by the presence of free carbonates and high organic soil pH buffering capacities than for P_w and M-1 extractants.

Based on a collective assessment of sugarcane fertility research from 1968 through 1990, the P_a extractant was more correlated to biomass and sugar yields than were P_w and M-1 (Korndörfer et al., 1995). Because acetic acid is a more aggressive extractant than water, resulting P_a soil-test levels should reflect an estimate of the quickly solubilized soil P (intensity factor) as well as an estimate of labile P that would presumably become available over time.

This brief history may help put the current situation into perspective. Since the early 1940s, the ESTL has used water to assess soil-test P levels in organic soils. It’s pretty safe to claim that very few soil-testing laboratories across the USA routinely employ water as an extractant. Currently, all P fertilizer recommendations for sugarcane and vegetable crops grown on organic soils are based on the P_w extraction. In response to grower interest in more aggressive soil P extractions, the ESTL began offering an acetic acid (P_a) extraction in the late 1980s. Although significant efforts have been undertaken to calibrate the P_a for sugarcane, the current status of this work is uncertain. None-the-less, many growers continue to request P_a and likely use it to some degree to determine P fertilizer application rates.

In recent years, the importance of the ESTL mission has increased markedly due to the EAA Regulatory Program which requires growers (by 1995) to adopt comprehensive soil testing as a BMP for determining P fertilizer application rates. The current situation finds growers compelled to provide regulators documented evidence of soil testing conducted prior to fertilizer application. Combined with today’s deadline-driven agricultural marketplace, a premium is now placed on rapid turnaround time for soil-test results. Slow sample turnaround time has been a limitation for the ESTL. The in-house developed P_w and P_a procedures include a 20-hour overnight soaking step followed by a 50-minute end-over-end tumbling event on a home-made spinning device. Clearly, efficient extraction procedures (M-1, M-3, or Bray) requiring a simple 5-minute shaking event are viewed favorably by growers needing quick results.

The current situation finds growers submitting soil samples with increasing frequency to private soil-testing labs. Consequently, many growers are receiving soil-test values produced by a variety of extraction procedures, the most popular including the M-1, M-3, Bray, and sodium bicarbonate (which uses a 30-minute shaking event). Additionally, the habit of splitting soil samples across different soil-testing labs (including the ESTL) as a "cross-reference" strategy can raise more questions than answers since soil-test P levels differ according to the extraction method and different labs typically return different P fertilizer recommendations. A philosophical problem is also raised in that private labs do not necessarily offer calibrated soil-test fertilizer recommendations based on rigorous crop response studies conducted on EAA organic soils.

To what extent soil-test P levels produced by different extraction procedures correlate with one another is not clear, particularly for the high-pH organic soils of the EAA. An investigation is currently underway to compare and contrast soil-test P values produced by 9 different extraction procedures (P_w, P_a, modified P_a, M-1, M-3, Bray, calcium chloride, sodium bicarbonate, and AB-DTPA). This work is being conducted on 13 soils collected from different geographic regions of the EAA. These representative soil samples are also being used in a second study which seeks to quantify the effect that shorter soaking and/or tumbling times have on soil-test P_w, P_a, and K levels. Results of these comparative studies will be reported in future Vegetarian articles.

Andreis, H.J., and J.M. McCray. 1998. Phosphorus soil test calibration for sugarcane grown on Everglades Histosols. Comm. Soil Sci. Plant Anal. 29:741-754.

Diaz, O.A., E.A. Hanlon, G.J. Hochmuth, and J.M. White. 1988. Phosphorus and potassium nutrition of lettuce on a Florida muck. Proc. Soil Crop Sci. Soc. Fla. 47:36-41.

Forsee, W.T., Jr. 1945a. Application of rapid methods of laboratory analysis to Everglades soils. Proc. Soil Sci. Soc. Fla. 7:75-81.

Forsee, W.T., Jr. 1945b. Soil investigations. Fla. Agric. Exp. Sta. Annual Report, 1945. 199-202.

Forsee, W.T., Jr. 1950. The place of soil and tissue testing in evaluating fertility levels under Everglades conditions. Proc. Soil Sci. Soc. Amer. 15:297-299.

Forsee, W.T., Jr. and T.C. Erwin. 1947. Soil investigations. Fla. Agric. Exp. Sta. Annual Report, 1947. 183-184.

Gascho, G.J., and G. Kidder. 1979. Responses to phosphorus and potassium and recommendations for sugarcane in south Florida. Fla. Agr. Exp. Sta. Bull. 809.

Hanlon, E.A., G. Kidder, and B.L. McNeal. 1990. Soil-test interpretations and recommendations. Fla. Coop. Ext. Serv. No. 817.

Hochmuth, G., E. Hanlon, R. Nagata, G. Snyder, and T. Schueneman. 1994. Fertilization recommendations for crisphead lettuce grown on organic soils in Florida. Univ. of Fla. Coop. Exp. Sta. Bull. SP-153.

Hochmuth, G., E. Hanlon, G. Snyder, R. Nagata, and T. Schueneman. 1996. Fertilization of sweet corn, celery, romaine, escarole, endive, and radish on organic soils in Florida. Univ. of Fla. Coop. Exp. Sta. Bull. 313.

Korndörfer, G.H., D.L. Anderson, K.M. Portier, and E.A. Hanlon. 1995. Phosphorus soil test correlation to sugarcane grown on Histosols in the Everglades. Soil Sci. Soc. Am. J. 59:1655-1661.

Sanchez, C.A. 1990. Soil testing and fertilization recommendations from crop production on organic soils in Florida. University of Florida Agr. Exp. Sta. Bull. 876.

Sanchez, C.A., and H.W. Burdine. 1987. Relationship between soil-test P and K levels and lettuce yield on Everglades Histosols. Proc. Soil Crop Sci. Soc. Fla. 47:52-56.

Sanchez, C.A., and E.A. Hanlon. 1990. Evaluation of selected phosphorus soil tests for lettuce on Histosols. Commun. Soil Sci. Plant Anal. 21:1199-1215.

Thomas, F.H. 1965. Sampling and methods used for analysis of soil in the Soil Testing Laboratory of the Everglades Experiment Stations. Everglades Station Mimeo Report EES 65-18.

Consideration 4. Obtaining and keeping pest control materials for use in strawberry production in Florida is a condition of grower control and function.

(Stall, Vegetarian 00-10)