Predicting overwinter nitrate-N changes at the subfield scale in leaching-susceptible, agricultural soils

View the project final report

Commodities

• Agronomic: wheat

Practices

• Crop Production: fertilizers, nutrient cycling, nutrient management, water management
• Education and Training: on-farm/ranch research, participatory research
• Farm Business Management: budgets/cost and returns
• Soil Management: nutrient mineralization, organic matter, soil analysis, soil chemistry, soil microbiology, soil physics, soil quality/health
• Sustainable Communities: infrastructure analysis

Nitrogen fertilizer is generally the largest input cost for producers in Montana and in much of the
western United States. Despite this fact, scientists, producers, and their advisors have not
established a consistent and well-tested system for determining areas of fields that are more or less
responsive to nitrogen fertilizer. This proposal attempts to develop such a system. We will employ
novel remote-sensing techniques to investigate subfield-scale changes in soil nitrate-nitrogen (N)
during winter months. Restricting the period of inquiry to the months between September and
April (henceforth ‘overwinter’) will eliminate or minimize impacts of confounding variables such
as crop uptake, fertilizer inputs, and denitrification, allowing for direct estimates of variables such
as leaching and mineralization. Temporally intensive overwinter sampling will facilitate
development of a model to predict overwinter N changes (ONCs), which are known to range from
+61 to -23 lb/ac in this region. Such a model will serve to minimize over- and under-fertilization
for the many farmers applying constant rates of fertilizer in spite of—or in lieu of—soil test results.
Complicating the situation further is the threat of warmer and wetter Montana winters, which could
feasibly exacerbate ONCs. This research will have implications for 1) grower profits through
enhanced N use efficiency, reduced N surpluses, minimization of suboptimal yields caused by
under-fertilization, and improved methods of variable rate fertilizer application.; 2) water quality
via minimization of N surpluses and N leaching; and 3) climate change impacts on modern farming
practices by strengthening awareness among farmers of how altered precipitation and temperature
patterns interact to affect N use efficiency, water quality, and soil health.

1. Quantify subfield-scale variability in ONCs toward improved precision ag methods such as
variable rate fertilizer application.
2. Develop a model to predict ONCs and to notify farmers when large ONCs are likely to have
occurred in order to minimize economic and environmental injuries resulting from uninformed
fertilizer management decisions.
3. Demonstrate to farmers the economic and environmental benefits of soil testing and the
implications of changing precipitation and temperature patterns for modern farming practices.