Optimizing nitrogen management on organic and biologically-intensive farms

View the project final report

• 2015 annual report
• 2016 annual report

Commodities

• Vegetables: broccoli

Practices

• Crop Production: organic fertilizers
• Production Systems: organic agriculture

The long-term goal of this project is to improve nitrogen fertilizer management on farms implementing biologically-intensive soil-building practices to grow diverse crops. Cover cropping and organic amendments contribute to the soil’s fertility beyond the year they were incorporated by building active organic matter. Through this participatory research we will be able to place dollar values on the importance of optimizing soil fertility. Growers will be able to more accurately estimate nitrogen mineralization from soil organic matter and adjust fertilizer applications up or down accordingly. Extensive outreach efforts will be used to directly reach over 200 growers in Washington and Oregon.

Problem to be addressed and producer need: Organic farmers face a difficult problem in managing nitrogen: the magnitude and timing of nitrogen mineralization from organic matter cannot be easily predicted. One organic farmer that helped conceive this study wrote, “We have a strong focus on soil fertility with our land in order to both produce healthy food and build healthier organic soils. We find projects of these kind to be instrumental for developing progressive organic soil practices. This information is very much needed in our agriculture world.”

Soil texture and management history both influence soil N-mineralization potential. Well-managed plant-soil systems are tightly coupled; they are balanced in terms of nitrogen availability and nitrogen uptake. Uncertainty about nitrogen fertility can lead to N deficient or N saturated systems caused by excessive and insufficient fertilizer applications. Too little fertilizer compromises yield and profit while over-application leaves nitrate in the soil that is prone to leaching during winter and poses a threat to water quality.

Project Innovation: Many direct-market farmers rely on organic nitrogen fertilizers that cost $5 to $9 per pound of nitrogen. This can cost $1,000 to $1,800 per acre for heavy-feeding crops. Soil building practices can reduce fertilizer need but it is difficult to quantify. This project will address this problem by evaluating a suite of soil tests designed to predict the magnitude of nitrogen mineralization from organic matter as well as new strategies to monitor in-season nitrogen mineralization.

This project will be unique because farmers and scientists will work together to interpret data and compare management strategies and soil parameters based on pooled N monitoring data. Decision making processes by farmers are complicated, and ag professionals have difficulty understanding how best to implement change (based on monitoring data). The proposed project fosters development of collaborative learning processes among farmers and ag professionals. This is an important sociological impact. 

By providing cooperating farmers similar transplants, using consistent soil and plant monitoring protocols, and installing environmental monitoring stations, this project is designed to overcome one of the chief barriers to understanding how nutrient management differs according to farm/field history. It is only when we control for differences among farm locations (like transplant quality, sampling methods, and microclimate) that we can hope to sort out the differences due to soil “quality.” Participating farmers are enthusiastic about this collaborative project because it will allow comparisons to a wider universe of farms/growing conditions.

Field Trials: An experiment to assess nitrogen contribution from organic matter and the economic benefit of five different rates of fertilizer application will be performed with five organic farms over two years for a total of 10 site-years. The same five fertilizer rates will be included at each farm site and the specific rates will be determined through consultation with farmer cooperators. Each treatment will be replicated three times in a randomized block design. In spring 2016 and 2017 broccoli will be transplanted following ground preparation and fertilizer application. Broccoli transplants will be prepared at a central location and will be transplanted mid-May in western Washington and mid-April in eastern Washington. Marketable broccoli yield will be taken from each plot. Additionally, three adjacent broccoli plants from each replication of each treatment will be destructively harvested and combined into a single sample and analyzed for biomass, total N, and total C. Weather stations will be set up at each site to record air and soil temperature for calculation of growing degree days. Soil moisture will also be continuously monitored through the growing season.

Soil Analyses: Soil will be sampled before planting broccoli and analyzed for texture, bulk density, water-holding capacity, organic matter, total C, active C (Haney Test), total N, nitrate-N, P, K, pH, and buffer pH. Soil nitrate-N will be more intensively sampled by sampling post-harvest and at 0, 28, and 56 days after transplant. In addition to laboratory analysis, soil nitrate will also be analyzed with a field nitrate testing kit (EM Quant™). Nitrogen mineralization predictive tests will be done for soils collected in spring before planting. Predictive tests will include 24-hr CO2 respiration, 72-hr CO2 respiration, 21-day nitrate + ammonium (NO3+NH4) mineralization, and 7-day NH4 mineralization.
 
Statistical Analysis and Interpretation: We will use a linear mixed model to separately regress broccoli yield and nitrogen accumulation against fertilizer treatment where fertilizer treatment is a fixed effect and site, rep, and year are random effects and predictive test data are covariates. We will also analyze the predictive test data for thresholds, which should prove useful for outreach and extension efforts. For example, at what value for a predictive test does the application of fertilizer cease to affect yield and N-uptake? Or, what is the potential value in crop yield from fertilizer application.

Outreach: Our outreach program will include nitrogen management workshops, a Farm Walk, a Webinar, and Extension publications.

1. Objective 1 Assist producers with fertilizer applications through pre-season soil tests that predict nitrogen mineralization. Though total organic matter can be provided by basic soil analysis, the academic, consulting, and practicing agricultural community is still uncertain about crediting organic matter when preparing N fertilizer recommendations. Lab tests that measure the amount of nitrogen mineralized or carbon dioxide generated in a relatively short period of time can estimate nitrogen release, but the predictive value of these tests has not been determined. Farmers could use knowledge of potential nitrogen release to adjust fertilizer applications and maximize productivity while protecting water quality.
   

   1. Activities and Timeline:
      

      1. Spring 2016, spring 2017: Identify sites for field trials on five cooperating farms; sample soil for N mineralization predictor tests (24-hr CO₂ respiration, 72-hr CO₂ respiration, 21-day nitrate + ammonium (NO₃+NH₄) mineralization, and 7-day NH₄ mineralization). Different sites on the same farms will be identified in each year.

2. Objective 2. Evaluate the net economic return of increasing amounts of organic fertilizer applied before planting across geographically dispersed farms with differing soil quality parameters.
   

   1. Activities and Timeline
      

      1. Spring 2016, spring 2017: Prepare organic broccoli transplants at a central location and ship to each participating farmer. Farmers apply organic fertilizers at five nitrogen rates, including 0, in a randomized block design. Rates are consistent across all five cooperating farmers.
      2. Summer 2016, summer 2017: Analyze broccoli yield from each plot in on-farm experiments. Determine net economic return of increasing organic fertilizer. Analyze three plants/plot for total above-ground biomass and nitrogen content.

3. Objective 3. Assist producers with in-season soil tests that allow adjustment to fertility during the growing season. These tests can be done by sending soil to a lab or by doing them in the field with quick-tests (e.g. EM Quant™). We will evaluate both methods.
   

   1. Activities and Timeline
      

      1. Summer 2016, summer 2017: Conduct in-season soil tests on all plots in on-farm experiments with lab tests and EM Quant™ quick tests. Evaluate how in-season soil nitrate changes with soil type and fertilizer application rate.

4. Objective 4. Assist producers in developing nitrogen management plans to increase profitability while protecting water quality.
   

   1. Activities and Timeline
      

      1. Fall 2015, fall 2016: Post-harvest nitrate tests will be used to evaluate nitrogen use efficiency at different fertilizer rates.
      2. Summer 2015 through summer 2018: Our 3-year outreach and education plan details the activities we will carry out to assist producers with developing nitrogen management plans, including: project website, two  nitrogen management workshops at regional conferences, a Farm Walk, two additional in-person local presentations, and a webinar. Written educational materials will complement these presentations and workshops.