Optimizing Nutrient and Water Management for Organic Mixed Vegetable Production Systems

Progress report for LS20-334

Project Type: Research and Education
Funds awarded in 2020: $299,116.00
Projected End Date: 03/31/2023
Grant Recipient: University of Florida
Region: Southern
State: Florida
Principal Investigator:
Gabriel Maltais-Landry
University of Florida
Kevin Athearn
University of Florida
Eban Bean
Agricultural and Biological Engineering, UF/IFAS
Dr. Carlene Chase
University of Florida
Tatiana Sanchez
UF/IFAS Extension Alachua County
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Project Information


Optimal nutrient and water management are critical to sustainable farming and its goals of maximizing yield, food quality and profitability while maintaining soil fertility and soil health and minimizing environmental impacts. Certified organic growers must rely on non-synthetic amendments (e.g., compost, manure) and nitrogen fixation from legumes to meet crop nutrient demand. However, relying on these fertility sources poses several challenges in terms of balancing inputs and outputs for different nutrients (e.g., nitrogen, phosphorus) and synchronizing nutrient release in sufficient quantity to satisfy crop demand. Resulting organic amendments inputs are typically high to meet crop nitrogen demand and build soil organic matter but can result in phosphorus over-fertilization. Increasing nitrogen inputs via nitrogen fixation can also increase soil fertility, but without proper synchronization of nutrient release, nitrogen transfer to subsequent crops can be low (20-30%), which may increase losses via leaching, especially for sandy soils. Water management will also affect nutrient cycling, with potential impacts on nutrient inputs, yields, crop nutrient uptake, nutrient-use efficiency, and ultimately losses to the environment. Conversely, nutrient and water management affect weed and plant-parasitic nematode abundance, affecting the overall benefits of different nutrient and water management approaches. Ultimately, potential synergies and trade-offs among different nutrient and water management approaches will be site-specific and driven by practical considerations and constraints, affecting economic returns and the likelihood of grower adoption.

The proposed research will combine on-farm trials and a trial on a research station to better understand the linkages between nutrient and water management in organic mixed vegetable production systems in Florida. The ultimate goal is to determine how nutrient and water management can be improved in these systems. Our first objective is to evaluate current nutrient and water management practices of three farms to compute nutrient and water budgets and identify how nutrient and water management can be improved in these systems. In collaboration with these growers, we will develop on-farm field experiments to evaluate the effects of changes in management practices, such as using rainfed cover crop polycultures, different processed organic fertilizers, and/or sensor-controlled irrigation systems. Growers will also provide input as we design the replicated four-system experiment that will compare different nutrient and water management approaches at a research station site, which has similar soil and climatic conditions as participating farms. Finally, we will quantify economic costs and returns for all systems under study to determine their economic viability.

Both sets of experiments will be conducted for two years, focusing on crop productivity, nutrient cycling and soil fertility, soil health, water use, weed and nematode abundance, and economic costs and returns. Research results will be shared with individual growers to identify the most beneficial practices for their operation. We will develop extension materials and hold field days to share our results with the broader farming community. Ultimately, this project will identify how organic vegetable production in Florida and the Southeastern US can become more sustainable by identifying the most beneficial and profitable nutrient and water management approaches.

Project Objectives:

Objective 1: Build nutrient and water budgets with grower participation, using ancillary data and limited sampling, to identify leverage points to improve on-farm nutrient and water management;

Objective 2: Conduct on-farm trials to determine how different systems aimed at improving nutrient and water management affect yields, soil fertility and soil health, water application, and the abundance of weeds and plant-pathogenic nematodes;

Objective 3: Conduct a parallel trial in controlled conditions at UF’s Plant Science Research and Education Unit (PSREU) comparing how different systems developed with grower participation affect the impact of nutrient and water management on the same indicators as objective 2;

Objective 4: Compare the economic costs and returns of alternative nutrient and water management strategies;

Objective 5: Synthesize information and communicate key outcomes to cooperating growers, Extension agents and the farming community via field days, Extension documents, and professional development workshops.


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Materials and methods:

Objective 1

Completed this reporting period: Before objective 1 was initiated, we submitted our project to the Institutional Review Board (IRB), including the survey we will use to collect information from growers, and we were approved as exempt. We ensured every member of the team completed the necessary trainings before starting the work. We initiated contact with growers during the summer to discuss the project, especially the details of on-farm trials (e.g., factors/treatments to compare, timeline, crops to grow - see objective 2 for details), and two were very responsive (Charley Andrews, John Bitter), participating in many phone calls and email conversations. The third grower (Mike Stephens) was not responsive, and after several emails and phone calls placed between August and November, the decision was made to look for a new cooperator (Cody Galligan, from Siembra Farm) who agreed to join the project. After delays due first to COVID (which prevented face-to-face interactions), then the non-responsive grower, and finally finalizing contracts between growers and UF, in-person interviews are now scheduled to happen in April 2021. While this has delayed our timeline relative to original plans, it allowed us to hire the postdoctoral fellow, who will now participate to all activities.

To be completed in the next two years: We will collect the information necessary to compute nutrient, water and financial budgets, and we will identify leverage points to improve nutrient and water management. The data collected will include inputs of fertilizers and organic amendments, use of cover crops, yields,  and irrigation practices. Other management information collected will include crop rotation, tillage, mulching, and weed and pest management. If the information collected during the initial phase is insufficient, we will conduct targeted sampling to refine estimates of different fluxes (e.g., cover crop biomass and nutrient concentration). Current irrigation practices and efficiency will also be evaluated, including taking subsamples for irrigation water quality, along with a limited assessment of current weed and nematode pressure. This will help reduce the uncertainty for key factors that are central to the proposed research.

The last stage of this initial objective will be to finalize the design of the different farming systems that we will compare on individual farms and at UF's Plant Science Research and Education Unit (PSREU). For on-farm trials, meetings with individual growers at their farm will allow us to finalize design. Individual farmers will also provide their input for the four systems to be compared at PSREU.

 Objective 2

Completed this reporting period: The original goal of this objective was to compare three systems per farm (two plots per system), on a 0.25-acre field. After discussing with growers and realizing that the systems compared would vary among farms, the research team and growers agreed that it would be more relevant to compare two systems using four plots per farm, in order to get the adequate replication on each farm to conduct statistical analyses. In addition, because one cooperator changed, the systems to be compared for that farm changed as well. In collaboration with growers, we established the following treatments to compare.

1. Hammock Hollow Farms

  • System 1 will combine a sunn hemp (Crotalaria juncea) summer cover crop, manure-based compost applied at planting, in-season fertilization with processed organic fertilizers (e.g., fish emulsion), and overhead irrigation with sprinklers.
  • System 2 will be similar to system 1 but low-P fertilizers (e.g., blood meal) will be used to reduce N and P imbalance between nutrient inputs and crop removal.
  • Both systems will be used to grow one crop of cauliflower (Brassica oleracea var. botrytis) from mid-fall to early winter.

2. Frog Song Organics

  • System 1 will use a combination of sorghum sudangrass (Sorghum bicolor x S. bicolor var. sudanese) and millet (Pennisetum glaucum) terminated by mowing and disking, fertilization with pelletized poultry manure and overhead irrigation with a water gun.
  • System 2 will be similar to system 1 but the cover crop will be grazed by chickens instead of mowing and disking, to measure the effects of grazing these cover crops.
  • Both systems will be used to grow one crop of cabbage (Brassica oleracea var. capitata) from mid-fall to early winter.

3. Siembra Farm

  • System 1 will combine a sunn hemp (Crotalaria juncea) summer cover crop, fertilization with pelletized poultry manure and irrigation with sprinklers.
  • System 2 will be similar to system 1 but clay from a local sand mine will be added to improve water and nutrient retention in these very sandy soils. Unlike for other practices, we expect to add the clay only in the first year of the experiment, and monitor its effects through two years.
  • Both systems will be used to grow one crop of cabbage (Brassica oleracea var. capitata) from mid-fall to early winter.

To be completed in the next two years: For each plot in each system, initial soil conditions will be analyzed in the tilled layer. We will measure soil inorganic N and potential net mineralization (PNM) using short-term incubations to estimate N release. We will also measure resin- and hexanol-extractable P (an indicator of P found in microbial biomass), several soil health indicators (“active” carbon via permanganate-oxidizable C, soil organic matter via loss on ignition, total soil C and N via combustion, C mineralization via short-term incubations), general soil fertility (soil pH and conductivity, macronutrient and micronutrient concentrations, and cation-exchange capacity via an external laboratory) and physical properties (bulk density, texture, water-holding capacity, infiltration rate). We will also determine the specific composition of nematode populations using molecular tools. Weather station, data loggers, and meters for soil water use will be installed prior to cover crop planting.

We will install soil moisture sensors (for interested growers) after cover crop planting, in addition to anion-exchange resins installed below the tilled layer and deep soil profiles to estimate nutrient leaching during the cover-cropping phase. Cover crop canopy closure will be recorded throughout the cover cropping season and weed density and biomass will be assessed midway through and at the end of the cover crop season. At termination, cover crops will be sampled for biomass by species using 0.5 m2 quadrats and nutrient concentrations will be measured (total C and N via combustion, P and other elements via digestion and ICP). Originals plans of measuring 15N in cover crops to determine the amount of N derived from biological N-fixation will be dropped, as preliminary tests indicated that this was not going to be successful. Throughout the cover cropping season, weather data, irrigation use, and soil moisture will be monitored using the infrastructure installed on each farm.

Soil samples for inorganic N, PNM, P, and nematode abundance will be taken before cauliflower or cabbage planting. Anion-exchange resins will be removed from under the tilled layer and extracted for N and P concentrations; new resins will be installed to estimate leaching during the cash crop growing season. Pre-plant fertilizers and organic amendments will be applied, and another round of soil sampling for inorganic N and PNM will occur after planting. Soil inorganic N and PNM will be measured midway through the cash crop growing season, in addition to weed and nematode abundance. Marketable yields and total aboveground biomass will be measured at harvest, in addition to nutrient concentrations in both marketable and total aboveground biomass. Nematode damage on roots will also be evaluated at harvest. After cash crop residues are disked into the soil, soil inorganic N, PNM, P and nematodes will be measured, and anion-exchange resins will be analyzed to estimate nutrient leaching; an additional deep soil profile will be analyzed to quantify nutrients found below the tilled layer. Throughout the cash cropping season, we will monitor weather data, irrigation use, water quality of irrigation (nutrients and conductivity) and soil moisture.

After this first harvest, another round of cash crops will be grown according to grower preference. Soil inorganic N, PNM, P and nematodes will be measured after harvest, along with yields and nutrients exported via marketable biomass. These soil samples will also be used to determine conditions prior to the summer cover cropping phase in 2022. The relaxation in field sampling will allow for key personnel to process samples in the laboratory, compile experimental data, and conduct a preliminary analysis of experimental results.

Cover crops will be planted again in early summer 2022, followed by a squash cash crop, using the same protocols as in 2021 for resin deployment, soil collection and analysis, weed monitoring, and biomass harvest and analysis. After the 2022 harvest, we will sample soils for soil inorganic N, PNM, P, nematode abundance, soil health indicators, soil general fertility, and soil physical properties. We will also repeat our deep soil sampling approach (one deep soil sample before cover cropping and after fall cash crop harvest).

Over the course of these on-farm trials we will sample soils:

  • Once for molecular assessments of nematode specific composition;
  • Twice for soil health indicators, physical properties and general fertility;
  • Four times for ion-exchange resins and deep soil profiles estimating nutrient leaching;
  • Six times for resin- and hexanol-extractable P;
  • Seven times for nematode abundance;
  • Ten times for inorganic N and PNM.

Sample processing, data compiling, statistical and economic analyses, and manuscript writing will be completed during winter 2023, before submission to scientific journals in spring 2023.

Objective 3 (to be completed in the next two years) 

We will establish a four-system comparison at the PSREU in early summer 2021. Each system will be replicated four times in a randomized complete block design. There will be a business as usual (BAU) system compared to three alternative systems. The BAU system will use sunn hemp as a summer cover crop followed by cabbage (2021) or squash (2022) production in the fall. Fertilization will be made with poultry manure and NatureSafeTM 10-2-8 at planting, based on typical grower practice. Drip irrigation will be used on a fixed schedule under plastic mulch. The plots will be managed according to organic standards.

The three alternative systems will consist of a “nutrient-only” improvement, a “water-only” improvement, and a “water and nutrient” improvement. The exact design of these systems will be finalized based on grower input, but they should be consistent with the following guidelines:

  • “Nutrient-only” improvement: apply chicken manure at planting based on expected P export from the crop and add enough N to meet crop demand using fertigation and/or low-P fertilizers (e.g., blood meal). Other management practices would be identical to BAU.
  • “Water-only” improvement: apply drip irrigation based on a soil moisture sensor. Other management practices would be identical to BAU.
  • “Water and nutrient” improvement: apply chicken manure at planting based on expected P export from the crop and add enough N to meet crop demand using fertigation and low-P fertilizers (e.g., blood meal), apply irrigation based on soil moisture sensor, and modify sunn hemp monoculture to a four-species cover crop mixture of millet, sorghum, cowpea (Vigna unguiculata), and sunn hemp.

Sample collection/processing and data analysis will follow a very similar schedule as the one detailed in objective 2. This includes a similar experimental timeline of starting trials in summer 2021 with cover cropping, end in late fall 2022 after squash harvest, and complete data analysis and manuscript preparation by the end of spring 2023.

Objective 4 (to be completed in the next two years) 

We will establish a recordkeeping system to track material costs, labor hours, use of equipment and durable supplies, and marketable yields for the the different systems. This information will be collected for both the on-farm trials and PSREU experiments for the whole experimental period (spring 2021 – fall 2022). The information will be used to create partial budgets comparing the costs and returns for the different production systems, with most data analysis occurring in fall 2022 and winter 2023. Financial comparisons will be shown under varying input and output price scenarios for the different management systems.

Objective 5 (to be completed in the next two years) 

Detailed activities for Objective 5 are detailed below. Briefly, we will share results with participating growers in spring 2022 and 2023, we will organize two field days in spring 2023, and we will produce extension fact sheets, blogs and videos in Spring 2023.

Participating growers and their staff will be heavily involved in the design, implementation, and completion of the research. We will share preliminary results after the first year of the experiment (i.e., in Spring 2022) to get their feedback and also determine if changes should be made to the different systems. After the completion of experiments in early 2023, we will hold meetings with individual farmers to discuss their experience with these systems. These semi-structured interviews will ask growers and key staff members for feedback regarding the systems, first without research results accessible, and we will also ask growers if they would likely adopt the different systems under study. We will then share the main research results of the experiment with them and discuss whether their assessment differ, and why. We will also use these interviews to seek their inputs to determine which results will be most relevant to include in extension documents, field days and professional development workshops.

Two field days will be organized in the third year of the project, and planning meetings for field days will start in Summer 2022 to determine the best format to use and the most relevant information to share with growers. During the field days, we will present research results and provide a tour of the field sites, including the technology involved, and we we will ensure there is ample time for discussion among participants. We will provide a pre-test survey of participants’ knowledge and perceptions about what constitutes sustainable water and nutrient management, using Qualtric or paper versions, with questions focusing on nutrient and water management knowledge as well as practical implications for growers. At the end of the field day, we will assess knowledge gain, aspirations to adopt the recommended practices, level of confidence in practicing a new skill, and changes in attitudes toward the research subject. Finally, we will follow-up with another survey 6-12 months after field days to determine the extent of adoption of new practices from participants and the level of knowledge transfer achieved during field days. We would also invite growers to share results of trials conducted on their farm and whether their experience is consistent with what was presented during field days.

As many of the management practices are applicable to non-organic vegetable systems of Northern Florida, we will reach this broader audience by producing different extension documents, primarily factsheets shared on blogs from the Institute of Food and Agricultural Sciences (IFAS) including the participating county (Alachua) and other counties of Northern Florida. Key research outcomes will also be shared on the commercial horticulture Facebook page in addition to the county’s Facebook page (> 73,000 followers), as many research outcomes apply to water quality and conservation beyond vegetable production. The production of these documents is expected to take place mostly in Fall 2022 and Winter 2013, after data have been collected and analyzed. Finally, the main research results will be summarized in a short presentation that can be shared during other field days and/or extension events. This presentation will also be delivered at the annual Organic Food and Farming Summit, which gathers about 100 diversified growers from around Florida. This short presentation will evolve through time based on feedback from attendees as well as growers that implemented the management practices presented.

Furthermore, we will produce two short videos that summarize key research outcomes, with one focusing on nutrient cycling and interactions with water management, and the other video focusing on the broad implications of our research (e.g., impacts on weed/nematode management, economic impacts of the different systems). Both of these videos would analyze the different practices and systems from different perspectives, including impacts on farm management as well as profitability. These videos would be produced in winter/spring 2023 and will be posted on social media (e.g., YouTube) for broader dissemination to growers in Florida and the Southeastern US. Key research outcomes will be summarized in an article submitted to the Florida State Horticultural Newsletter to get a statewide distribution. Teaching faculty will share research outcomes with students in a formal classroom setting during the classes they teach, helping disseminate the information collected throughout this project given that many students enrolled in these teaching programs have professional work aspirations (e.g., crop consultants, water management districts).

Research results and discussion:

The original proposal had limited data collection during the first year of the project, and unexpected delays caused by COVID and a non-responsive grower pushed back the first data collection event to the next reporting period.

Participation Summary
3 Farmers participating in research


Educational approach:

The educational approach and timeline is described in objective 5 of the research. Most educational and outreach activities will take place in years 2 and 3 of the project.

Educational & Outreach Activities

3 Consultations

Participation Summary:

Education/outreach description:

As most educational and outreach activities will take place in years 2 and 3 of the project, there is little to report for this year.

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.