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

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

Summary

The demand for locally grown produce is strong in Florida and certified organic vegetable growers have a hard time keeping up with market needs. High-value vegetable crops such as tomato, bell pepper, eggplant, and watermelon are well suited for the growing conditions in the Southeastern United States, especially when plasticulture is used. 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. 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. Hence, by measuring leachate parameters (volume, composition) and using sap testing results, the grower should better understand the impact of fertilizer and irrigation practices on N and K release and losses. This approach will help organic growers increase efficiency in use of expensive organic fertilizers and at the same time improve best management practices by reducing N and K losses, thus making a positive contributing to improving water quality in the Suwannee Valley area of Florida. 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 better understanding of when nutrient shortage occurs due to insufficient fertilizer nutrient release or from nutrient leaching caused by excessive irrigation.

Objectives/Performance Targets

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, thus decreasing expenses for external inputs.
   Dual measurement of leachate EC and plant petiole sap will provide a better understanding of when nutrient shortage occurs due to insufficient fertilizer nutrient release OR if irrigation is in excess.

Accomplishments/Milestones

Field Preparation. The field was prepared in the fall of 2008 (October 16) by installing four drainage lysimeters. These lysimeters were installed in a manner to be directly under the beds for cropping in the spring of 2009. Soil in this field was left fallow from October, 2009 until March, 2009.
Lysimeter Construction. Each lysimeter system is composed of a collection container and a storage container. Collection containers were constructed of a 55 gallon drum cut in half lengthwise. Each container was 3 ft long, 2 ft wide, and 1 ft deep. Each a sealed five gallon bucket with clear polymer tubing (1 in diameter) threaded through a 2 in diameter PVC pipe that extended from the top of the soil to the bottom of the bucket to facilitate removal of the leachate.
Lysimeter Installation. Lysimeters were installed underground at a depth of 2 ft so they would be beneath the root zone and also avoid potential damage by tillage implements. Lysimeters were installed lengthwise in the row at a 2% slope to facilitate drainage and collection and to reduce the risk of a perched water table. The top soil to 6 in was carefully removed by hand and placed to one side. Subsoil was removed using a backhoe and placed in a separate location. The lysimeters were installed and soil was replaced accordingly.
Crop Planting. On March 12, 2009 soil was disked and composted poultry manure was applied. The compost was made on the farm using by volume 67% broiler litter and 33% miscellaneous wood chips both from a local source (Table 1). Compost was applied at a rate of 3.5 tons per acre by broadcasting over the entire field. The compost was immediately disked into the top 6 to 8 inches of the soil. Beds were formed on March 14 by pulling soil into two-foot wide raised beds and applying plastic mulch and drip irrigation. Beds were formed using soil with lower than preferred moisture.
Two beds with lysimeters were planted with cucumber seed on March 16. Two rows of mulched beds were seeded at 15 inch spacing in each row. Rows on the beds were approximately 8 inches apart. The same seeding method was used to establish zucchini squash on the other two lysimeter beds on March 17. Both crops were watered after seeding using a drenching method from a wagon. Soil in the beds remained dry on March 19, drip irrigation was initiated.

Impacts and Contributions/Outcomes

We are in the early stages of this work, and have no formal impacts to report.

Table 1. Nutrient content of compost applied prior to cucumber planting on March 16, 2008 on the Hoover’s organic farm in Live Oak, FL.z

Ingredient Content
Nitrogen (N) 63 lbs/ton
Phosphorus (P2O5) 74 /lbs/ton
Potassium (K2O) 59 lbs/ton
pH 8.3
Moisture 15.9%
Total Solids 84.1%
Total Ash 29.7%
zAnalysis conducted at University of Florida Livestock Waste Testing Laboratory, Gainesville, FL, and parameters are reported on a dry weight basis.

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