Monitoring Nutrient Availability and Leaching Below the Root Zone in Organic Vegetable Production

Project Overview

Project Type: On-Farm Research
Funds awarded in 2008: $14,900.00
Projected End Date: 12/31/2010
Region: Southern
State: Florida
Principal Investigator:
Dr. Danielle Treadwell
University of Florida
Bee Ling Poh
University of Florida
Eric Simonne
University of Florida

Annual Reports


  • Fruits: melons
  • Vegetables: eggplant, peppers, tomatoes


  • Crop Production: irrigation, nutrient management


    The combined risk of leaching in Florida’s sandy soils and the poor predictability of nutrient release from plant and animal-based pre-plant organic fertilizers often results in a shortage of N and K during the season, thereby potentially reducing crop yield and quality. The goal of this project was to assist organic growers to improve best management practices by reducing N and K losses, thus reducing fertilizer input costs and imparting a positive impact to water quality in the Suwannee Valley area of Florida. Drainage lysimeters installed beneath the root zone of crops allow for measurement of nutrient concentration and provide an indication of the effectiveness of the farmer’s fertilizer management. Four drainage lysimeters were installed on a certified organic farm near Live Oak, FL in fall 2008 directly under beds for cropping in spring and fall 2009. Composted poultry litter (2-2-1.4 of N, K2O and P2O5; respectively) was broadcast and incorporated at a rate of 7.8 kg/ha both seasons. Soil moisture ranged between 8 and 12% during the season based on periodic measurements with time domain reflectronmetry. In March, cucumber (Cucumis sativus L.) cv. Cobra and zucchini (Cucurbita pepo L.) cv. Green Eclipse were planted over two lysimeters each. Cabbage (Brassica oleracea L.) cv. Bronco was planted over the four lysimeters in October. Irrigation was provided when soil moisture declined below 8% in the absence of precipitation. Rain gauges in the field corroborated with UF-IFAS weather station data located at the North Florida Research and Education Center – Suwannee Valley, approximately ten miles away. Two significant rainfall events occurred during the spring season (8 in and 12 in). Plant petiole sap K and NO3-N concentration and K, NO3-N and NH4-N concentration in leachate from drainage lysimeters were collected throughout both seasons as needed depending on irrigation and precipitation. Petiole K and NO3-N remained within sufficiency ranges for all crops. Leachate volume and nutrient content was variable among wells. Leachate K was less than 6.0 ppm on 15 of 17 occasions, 9.3 mg/L once and 27.4 mg/L once. Leachate NO3-N and NH4-N combined ranged from 25.7 – 37.7 mg/L on 3 of 17 occasions. Yields of cucumber and zucchini were 20% less than state averages for conventional crops, but due to a late freeze, nearly-mature cabbage was not marketable. This project increased the farmer’s ability to monitor nutrient shortages (poor N synchronization or excess rainfall) using a combination of leachate analysis and plant petiole sap analysis, thus avoiding many common pitfalls associated with nutrient and water management in Florida vegetable systems.


    Organic vegetable production in north central Florida typically includes plastic mulch and drip irrigation to assist in the retention of nutrients and water in the root zone of vegetables (FDACS, 2006; Hochmuth et al., 2006; Simonne et al., 2008). Soil texture is predominantly sand, and has low water holding capacity (< 10%) and low organic matter content (<1.5%), thus efficient nutrient and water management are critical to the overall success of the system. Typically, fertilization practices for these crops include a pre-bedding application of an approved compound fertilizer (often 8-5-5 Nature Safe), followed by fertigation events late in season using sodium nitrate (but the NOP clearly states that no more than to 20% of total N may come from this N source). The combined high risk of leaching in Florida’s sandy soil with the poor predictability of the nutrient release from the pre-plant fertilizer often results in a shortage of N and K in the middle or end of the season, thereby reducing yield and quality (Simonne et al., 2005). According to the organic growers in our area, increasing the in-bed rate of the organic fertilizer is currently not an economical option.

    Previous efforts coordinated by the University of Florida Extension Service and supported by Southern region SARE projects have (1) improved drip-irrigation scheduling practices by showing how fast water moves through the soil profile by using soluble dye, and (2) demonstrated the usefulness of weekly petiole sap testing analyses of nitrate and potassium to monitor plant nutritional status (Hochmuth et al., 2003, 2006; Simonne et al., 2005). Yet, these two techniques alone are insufficient for managing N and K in certified organic production because (1) the rate of organic fertilizer mineralization is not known and (2) the 20% sodium nitrate restriction leaves little flexibility for rescue fertilizer applications. Other projects conducted by this group have shown that nutrient movement below the root zone may be assessed with simple drainage lysimeters (Gazula, 2009). Hence, by measuring leachate parameters (volume, composition) and using sap testing results, the grower should better understand if nutrient shortage was due to leaching or inadequate release from the fertilizer. This approach will help organic growers increase efficiency in use of expensive organic fertilizers and at the same time improves best management practices by reducing N and K losses. Our hypotheses are that (1) costly certified organic fertilizer rate may be reduced without reducing productivity by improving irrigation management and (2) dual measurement of leachate electrical conductivity and plant petiole nutritional status allows for a more accurate determination of the cause of nutrient shortages.

    Prior to the start of this project, the Hoover family relocated from south Florida following Hurricane Andrew and purchased a farm near Live Oak, FL. Initially, they produced vegetables using conventional methods and utilized indirect market channels to distribute their products. Due to limited labor availability and low economic returns, the family approached UF-IFAS Extension to help them identify production alternatives two years prior to the start of this project. Prior to this project initiation, this team worked collaboratively with the Hoovers and with a local accredited USDA organic certifier, Quality Certification Services, to develop a farm plan to meet certification requirements. While the Hoovers had good irrigation practices as evidenced by this team’s early visits to the farm, they needed assistance developing an organic nutrient management plan, including sourcing raw materials for on-farm composting. Together, we identified an opportunity to test the hypotheses identified previously, and began working on a solution.

    Project objectives:

    1. Increase grower knowledge of the effect of irrigation and fertilizer management practices on soil N and K.
      Improve grower management of nutrients and irrigation to reduce nutrient losses due to leaching and/or suboptimal fertilizer management,
      Apply leachate EC and plant petiole sap data to provide a better understanding of when nutrient shortage occurs due to insufficient fertilizer nutrient release OR if irrigation is in excess.
    Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.