A Systems Approach for Estimating Plant Available Nitrogen from Organic Materials and Fertilizers

View the project final report

• 2017 annual report

Commodities

• Vegetables: broccoli, tomatoes

Practices

• Crop Production: cover crops
• Education and Training: decision support system, extension, on-farm/ranch research, participatory research
• Soil Management: nutrient mineralization

Nutrient management on organic farms is aimed at feeding the soil and is reliant on the addition of organic materials to build the soil organic matter and fertility base. In the Southern Piedmont, composts, manures, and organic fertilizers (animal by-products and seed meals) are commonly used to add organic matter and provide plant-available nitrogen (PAN).  Organic nitrogen (N) must undergo mineralization before becoming plant available, and may be lost from the agroecosystem through immobilization, leaching, denitrification, or volatilization.  Due to N-cycling processes and the high variability in the nutrient content and composition of organic materials, estimating PAN is difficult for producers.  Further, the timing and method of application and environmental conditions determine the rate and amount of PAN released.  As a result, difficulties in synchronizing N release (mineralization) with plant-uptake can result in decreased yield, increased fertilizer costs, additional pest pressure, and potential environmental degradation.  Our group has developed and made available to producers a web-based calculator (aesl.ces.uga.edu/mineralization/) for prediction of PAN from cover crop residues, but more work is needed to develop a PAN calculator for additional organic materials. 

Previous work specific to the Northwestern U.S. has shown that a computer model can estimate PAN from organic materials if the optimum rates and potential amounts of N mineralization are determined through a long-term incubation (70+ days).  However, long-term incubations are impractical for producers, so a tool is currently needed for quick estimation of rates and potential amounts of mineralizable. Our preliminary work indicates that Visual+Near Infrared Spectroscopy (VNIRS) can be used to obtain a quick estimate of rates and potential amounts of mineralizable N. This project will work with producers to identify common organic materials, develop the information needed to estimate N release characteristics of the materials using a quick VNIRS scan, and create a web-based PAN calculator that will use the VNIRS data as input and take into account climatic data from one of the 80+ Georgia weather stations nearest to a producer’s field. 

A large sample (48) of organic materials will be incubated with soil for 100 days to determine amounts of mineralizable N and mineralization rates under laboratory conditions.   These amounts of mineralizable N and mineralization rates will then be used to calibrate VNIRS equipment to develop an affordable, rapid test for organic materials that will be made available to producers for input into the PAN calculator. Field data gathered at the University of Georgia and at farms from four different producer fields will then be used to develop and test the PAN calculator/model. Ultimately, this N management tool will aid in estimations of available N throughout the growing season, allowing producers to better synchronize N release with crop uptake.

Outreach to producers will be achieved through extension, N management workshops, and in conjunction with the Emory at Oxford Student Farm and another three cooperating farms.  The PAN calculator will be made available through the University of Georgia Agricultural and Environmental Services Laboratories website.  

1. Determine potentially mineralizable N and rates of N mineralization for 48 organic materials (manures, composts, other organic fertilizers), and determine the effect of soil texture on N mineralization rates of organic materials.

2. Calibrate NIRS/VNIRS to estimate mineralizable N and rates of N mineralization for 48 organic materials.

3. Develop a web-based PAN calculator that allows producers to calculate PAN over the growing season as a function of local weather and soil conditions.

4. Collect PAN data from plots at the UGA farm and at the four cooperating farmers, and use the data to calibrate and validate the PAN calculator.

5. Disseminate the N mineralization data, availability of the NIRS/VNIRS analysis for organic materials, and web-based PAN calculator to the public, farmers, and extension agents.