University of Idaho Extension


Over 90 percent of the drinking water consumed in Idaho is supplied by groundwater. Because this resource is so vital to Idahoans, best management practices (BMPs) for agricultural management are becoming more important. Nitrate is the most common groundwater pollutant in Idaho and in the United States. Nitrates in groundwater can originate from many sources, including agriculture, septic tanks, landfills, lawns and gardens, industry and municipalities.

Nitrogen is an element essential for all plant and animal life. The interlocking succession of nitrogen reactions occurring in the soil is known as the nitrogen cycle (fig. 1). Agriculture affects both nitrogen additions and subtractions to the soil. Additions include nitrogen fertilizers, crop residues, nitrogen fixation by legumes, and manures. Subtractions attributed to agriculture include crop removal (harvesting), plant uptake, and nitrogen leaching.


Federal and state standards dictate that drinking water should not contain more than 10 parts per million (ppm) NO3-N. In rural areas of Idaho, potentially significant sources of nitrogen for groundwater contamination include nitrogen fertilizers, private septic systems, livestock feedlots, barnyards, and legumes used as green manures.

Specific types of BMPs for nitrogen fertilizer management that should be employed in many areas of Idaho include:

  • Soil sampling
  • Fertilizer recommendations based on research
  • Timing of fertilizer application
  • Fertilizer placement
  • Nutrient credits for legumes and manures
  • Nitrification inhibitors
  • Manure management
  • Irrigation systems management
  • Slow-release nitrogen fertilizers
  • Crop rotation selection
  • Variable fertilizer management

Relevant Publications