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

Final Report for OS08-043

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
Co-Investigators:
Bee Ling Poh
University of Florida
Eric Simonne
University of Florida
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Project Information

Abstract:

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.

Introduction

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.

Cooperators

Click linked name(s) to expand
  • Robert Hochmuth
  • Brad and Bradley Jr. Hoover
  • Elena Toro

Research

Materials and methods:
Lysimeter Installation

The experimental area consisted of one five acre field divided into two equal sections. At the Hoover’s request, we installed lysimeters in locations that allowed them to maintain existing drive rows and plant beds and to avoid any potential damage to the lysimeters from vehicle traffic and tractor implements. A soils map from the U.S. Geological Survey and farmer consultation were used to determine the location of the lysimeters in the field, based on homogenous soil type, tractor spacing and row orientation. Four drainage lysimeter were installed on October 16, 2008, and there were two lysimeters installed in each section. Beds were formed directly above the lysimeters for cropping in the spring of 2009. Soil in this field was left fallow from October, 2009 until March, 2009. Lysimeters were precisely installed either 49 ft or 150 ft from the farm road, and at least 26 ft apart from each other 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 inside of the collection container. The top soil to 6 in was carefully with the backhoe and placed on a tarp off to one side. Subsoil was also removed using a backhoe and placed in a separate pile. The lysimeters were installed and soil was carefully replaced so as to not mix top soil with deep soil. The soil texture at the site was 90-93% sand, 0.8-2.8% clay, and 4-6% silt (USDA, 2006).

Lysimeter Construction

Each lysimeter is composed of a collection container and a storage container. Collection containers were constructed of a 55 gallon drum cut in half lengthwise. Each collection container was 3 ft long, 2 ft wide, and 1 ft deep. Leachate collected in the container passed through the soil, then a 6 in layer of gravel, then finally through a 1.5 in perforated PVC pipe covered in mesh screen prior to reaching the storage container. Each storage container consisted of a sealed five gallon bucket with clear polymer tubing (3/8 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 leachate collection. A permanent leachate access box was installed in the center of the drive row for each drainage lysimeter. A length of PVC pipe surrounding clear polymer tubing installed 18 in below the surface of the drive row and extended from the access box downward to the drainage lysimeter at a 90° angle. A peristaltic pump was used to recover water from the storage container through the clear polymer tubing to the surface. A general schematic of the drainage lysimeter is presented in Figure 1.

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 and was compliant with the USDA organic standards. Compost was broadcast applied at a rate of 3.5 tons per acre with a spinner spreader over the entire field and incorporated. A compost sample was collected the day of application, and total nutrient application was 220 lb/a N, 206 lb/a P and 259 lb/a K. 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 48 inch-wide raised beds (6 ft on center) and applying plastic mulch and drip irrigation. Beds were formed using soil with lower than preferred moisture.

Beds were seeded with cucumber (Cucumis sativus L.) cv. Cobra in over lysimeter numbers 1 and 2 and zucchini (Cucurbita pepo L.) cv. Green Eclipse over lysimeter numbers 3 and 4. Crops were seeded on March 16 and 17 with 15 in in-row spacing. Crops were planted to a double row, and rows on the beds were approximately 8 inches apart. Cucumber and zucchini were harvested every 2-3 days from May 8- May 26, and the site returned to a weedy fallow (primarily Bahia grass). The spring compost application rate was calculated based on lab analysis results on nutrient content and UF-IFAS fertility recommendations, but we found this rate to be insufficient. Yields in the spring were satisfactory to the grower, but could be improved. Therefore, in the fall, the compost application rate was increased to 5 tons/a and was applied with a drop spreader over tracked rows and lightly disked prior to bed formation. Compost application in the fall provided 314 lb/a N, 294 lb/a P and 370 lb/a K. Cabbage (Brassica oleracea L.) cv. Bronco was planted in a double row with 12 in in-row spacing and rows were 8 in apart. Cabbage was planted over both sections and thus all four wells in October and harvested in December. Spring and fall crops were watered after seeding using a drenching method from a wagon and drip irrigation was initiated within 48 hours of seeding. Irrigation was delivered at a flow rate of 1.9 L per minute to maintain soil moisture between 8% and 12% Soil moisture was measured with TDRs (soil tensiometers) at least once a week in the morning and afternoon to obtain data over a range of evapotranspiration rates (ETo). Irrigation scheduling was determined by estimated plant water use (ETo and soil water tension according to University of Florida recommendations (Simonne et al., 2010).

Data collection

Time domain reflectometry (TDR) was used to monitor soil moisture throughout the project (portable Hydrosense 20-cm probe) Leachate was collected as needed depending on precipitation and irrigation. Leachate was collected in the spring on April 10, April 16, and May 19, and in the fall on December 14 and January 25. Leachate volume was recorded, and electrical conductivity (EC) was measured from each sample using a hand-held EC meter (Omega Engineering). Two 20 ml vials were filled with leachate, acidified with 2 drops of hydrochloric acid, placed on ice and delivered directly to the University of Florida’s Agricultural Research Laboratory in Gainesville for NH4-N, NO3-N, and K determination using EPA methods 350.1 (modified), 353.2 and 258.1; respectively. Ion-specific electrodes (Cardy meters from Spectrum Technologies) were used to monitor crop N and K status. Petioles from twenty most recently mature leaves were collected for analysis using methods described by Hochmuth (2009). Spring petiole sap was collected from both cucumber and zucchini on April 16, April 27, May 8 and May 19. Figures 2, 3, and 4 highlight activities during the project.

Research results and discussion:

Leachate storage containers contained between 500 to 600 ounces of leachate, and were often different from one another at the same sampling date, perhaps due to differences in the percentage of clay over the wells.
In spring 2009, two leaching rain events contributed 8 in and 12 in of rain that coincided with flowering of cucumber and zucchini. Rainfall amounts were determined by rain gauges on the farm. According to the University of Florida’s Florida Agricultural Weather Network weather station in Live Oak, total precipitation during spring production from field preparation to final harvest was 9.61 in. The differences in recorded precipitation between the farm and the FAWN weather station in Live Oak could be due to the isolated nature of thunderstorms common in the spring in our area. The University of Florida defines a leaching rain as 3 inches in three days or 4 inches in seven days (Simonne and Hochmuth, 2005). Based on the University of Florida recommendations for nutrient management following a leaching rain, an additional 30 lb of nitrogen in the form of sodium nitrate (NaNO3) was injected through drip lines but based on subsequent petiole N sap analysis and observation this was insufficient to recover lost nitrogen. Spring leachate contained 0.1-0.5 mg/L NH4-N, 0.1-25.8 mg/L NO3-N, and 0.5-9.3 mg/L K, evidence of fertilizer nitrogen passing through the root zone and into the leachate storage containers (Table 1).
Petiole NO3-N sap ranged from 80-150 ppm in zucchini in early April and declined to 50 ppm in late April and early May right before harvest. These values are “insufficient” than the recommended range of 800-900 ppm NO3-N for the first fruit harvest reported by Hochmuth (2009). Although there are no established petiole K recommendations for zucchini, previous on-farm measurements from fields considered to be well-fertilized were considered sufficient within the 4000 to 6000 ppm range (unpublished data). Petiole K was excessive at the start of sampling (6000 ppm K) and maintained concentrations that ranged from 2200 to 2900 ppm for the rest of the season. Cucumber petiole sap was different from zucchini (Table 2). On April 27, cucumber petiole NO3-N sap was 80 mg/L, increased to 340 mg/L on May 8 then declined to 24 mg/L on May 19. All petiole NO3-N sap values were lower than recommended throughout the season. As for zucchini, there are no petiole sap recommendations for cucumber. Petiole K sap was 3000 mg/L on April 27 and then increased to 4500-4600 mg/L in May. Low petiole sap NO3-N for spring crops were attributed to an insufficient N content. Although the total nitrogen content in the compost was 220 lb/a, site conditions influenced mineralization in a rate or quantity that did not fully support crop production. While the leaching rains negatively impacted soil N content, likely an insufficient amount of compost N was added at the beginning of the season. Therefore, compost application was increased to 5 tons/a for fall crops.
In the fall, an abundant harvest of high quality cabbage was expected. In January, several nights of temperatures below 25 degrees F resulted in a crop loss, despite the use of row covers. Fall leachate contained 0- 0.5 mg/L NH4-N, 0.6-37.7 mg/L NO3-N, and 0.7- 14.8 mg/L K. Leachate N and K content was greater in December than January due to regular precipitation, cool nights and slow initial cabbage growth (Table 3). Despite an 0.8 in rainfall on November 10, leachate collection was negligible and we were unable to collect enough solution to submit for analysis. By December 15, 5.61 in of precipitation was recorded by FAWN at Live Oak, and we were able to obtain leachate samples for analysis. Petiole K sap consistently remained in the 4000 to 6000 mg/L range and was considered sufficient during the fall crop (data not shown).

Participation Summary

Educational & Outreach Activities

Participation Summary

Education/outreach description:

The results of this on-farm research project were disseminated throughout the state in a variety of county workshops and regional and state conferences including the Florida Society of Horticultural Sciences and Florida Small Farms Conference. The Hoover’s story has been one of great interest to many farmers, and they remain active collaborators with members of this team.

Project Outcomes

Project outcomes:

This team identified three important lessons as a result of this project: 1) we formed a collaborative partnership that involved learning and teaching among all members; 2) we increased our ability to monitor nutrient shortages (poor N synchronization or excess rainfall) using a combination of leachate analysis and plant petiole sap analysis; and 3) we improved nutrient and irrigation management by using and interpreting data from field instruments.

Economic Analysis

A monitoring program that incorporated both crop nutrient status and soil moisture content was beneficial and provided excellent information during periods of nutrient shortages and excesses. In organic systems where black plastic is used, 100% of granular fertilizer or compost must be added at the beginning of the season. Historically, the approach has been to add fertilizer or compost at a rate that will provide the most N possible without risk of leaching or burning the crop, but this practice is not ideal. This project provided the information needed to make informed decisions. Over the two year period cultural practices were modified to improve nutrient management. Previous crops did not produce enough yield to offset input costs, and we attributed this to a lack of sufficient nutrients. During this project, the rate of compost application was increased and the method of compost application was modified from broadcast over the entire field to broadcast on planting beds only. According to the Hoovers, adoption of these improved practices resulted in more acceptable economic yields and increased profits. Despite the economic loss of cabbage in the second season, biomass production was sufficient to utilize available soil N and K and thus leaching losses were not excessive. Based on the lesions we learned, our recommendations to other growers using compost, black plastic and drip irrigation are to utilize monitoring tools including ion specific electrodes and soil moisture monitoring devices throughout the season to reduce the risk of nutrient loss to leaching by responding to changes in soil moisture and plant nutrient status appropriately.

Farmer Adoption

The Hoover family gained knowledge on best nutrient and irrigation practices to respond to changing environmental condition and crop development. They learned that limiting the application of fertilizer in the bed row rather than broadcast over the entire field reduces the risk of nutrient leaching and reduces fertilizer expense, that soluble nutrients including N and K move in the soil solution downward through the soil profile with precipitation and irrigation, and that application rates of organic-compliant fertilizers must take into account anticipated mineralization rates based on cultural practices and environmental conditions. For example, application rates of compost were based on the assumption that 100% of the nitrogen would be available during the production season. This resulted in lower than expected yields. After the farmers increased their knowledge of biological mineralization, compost application rates were increased accordingly with improved results. The Hoover family also learned to use a variety tools to monitor soil moisture and crop nutrient status including EC meters, ion-specific electrodes, and TDRs. They now regularly use EC meters and ion-specific electrodes to assist in their fertility management program. These data inform them of when sidedressing is needed and when their irrigation scheduling needs to be adjusted. While they have not purchased TDRs, they work closely with local Extension faculty and obtain soil moisture readings when they need additional data to make informed management decisions.

Recommendations:

Areas needing additional study

More information is needed on the mineralization rate of compost and other granular organic fertilizers in hot and humid conditions. In addition, suitable liquid fertilizers compliant with the USDA organic standards are needed. Many organic farmers prefer to avoid sodium nitrate due to concerns of sodium accumulation in soil, and in recent years this material has been difficult to source. Current compliant liquid plant and animal-based fertilizers are expensive, so more information is needed to determine the best combination of pre-plant incorporated fertilizer and mid-season liquid fertilizer.

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.