BMPs for Nitrogen Management

Idaho's Water
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 have and 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.

Federal and state drinking standards dictate that drinking water should not contain more than 10 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 plow down green manures.

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

1. Soil Sampling
2. Fertilizer Recommendations Based on Research
3. Timing of Fertilizer Application
4. Fertilizer Placement
5. Nutrient Credits for Legumes and Manures
6. Nitrification Inhibitors
7. Manure Management
8. Irrigation Systems Management
9. Slow-Release Nitrogen Fertilizers
10. Crop Rotation Selection
11. Variable Fertilizer Management

Specific N BMPs Include:
Soil Sampling
Soil sampling is a very important BMP that considers the amount of plant available N already in the soil profile. Soil sampling should be done three to four weeks before planting a crop. The soil samples should be representative of the field. Normal sampling depth is to 12 inches for phosphorus, potassium, sulfur, and micronutrients. Soil samples for nitrogen should be collected to the effective crop rooting depth.

You should take soil samples at least once during each crop rotation cycle. For best soil fertility management, especially for mobile nutrients, sample each year and fertilizer for nutrients, sample each year and fertilize for the potential yield of the intended crop. Having an analyses performed for every nutrient each year is not necessary. The need for analysis of a nutrient depends on such things as its mobility in the soil and the nutrient requirement of the crop to be grown. Maintain a record of soil test results on each field to evaluate long-term trends of nutrient levels.

Fertilizer Recommendations Based on Research
Nitrogen application rates for Idaho crops should be based on scientific information. Reliable fertilizer recommendations are developed by calibrating and correlating laboratory soil test values with plot research on crop response to fertilizer rates.

The University of Idaho has developed over 30 fertilizer guides for Idaho crops. The data base used to develop these fertilizer guides is extensive and has been collected for over 30 years. Fertilizer guides are based on years of field research and take into account the amount of residual N in the soil profile, the amount of nitrogen mineralized from organic matter during the growing season, the yield potential of the crop, and plant residue from the previous crop.

Timing of Fertilizer Application
The timing of nitrogen fertilizer applications is an important factor affecting crop yield, efficiency of nitrogen use, and a grower's economic return. The period between nitrogen application to the soil and actual crop uptake is critical. This is when high concentrations of nitrogen as nitrate can be lost through leaching. Where the water table is close to the soil surface groundwater quality is especially vulnerable.

Some BMPs for timing of fertilizer applications include: (1) applying nitrogen to a cool season crop in the spring instead of the previous fall, (2) applying only a portion of the needed nitrogen as a preplant treatment, (3) using split or multiple nitrogen applications, (4) using sidedress or topdress applications during the growing season, and (5) using a combination of tissue analysis for diagnosis and topdress nitrogen applications during the growing season.

Fertilizer Placement
Placement of fertilizers is an integral part of efficient crop management. Correct placement of fertilizers often improves the efficiency by which nutrients are taken up by plants and consequently encourages maximum yields of intensively managed agronomic crops. It is also apparent that correct fertilizer placement is more critical for maximum crop yields under reduced tillage operations than with conventional tillage management.

Nutrient Credits for Legumes and Manures
Effective use of nitrogen fertilizer requires the consideration of nitrogen supplied by previous legume crops in the rotation and from manure applications. Observations in other areas of the U.S.A. have shown that manures can supply crop nutrients effectively and may often meet the total N needs of the planted crops. In addition, a good clover or alfalfa stand may provide up to 200 pounds of nitrogen for subsequent crops in the rotation. Crediting N supplied from manures and legumes against crop nitrogen needs can substantially reduce nitrogen fertilizer application rates and the potential for overapplication of nitrogen.

Nitrification Inhibitors
Nitrification inhibitors prevent the conversion of relatively immobile ammonium to very mobile nitrate in agricultural soils. Research has shown nitrification inhibitors to be most promising in situations where nitrogen fertilizer is applied in the fall or early spring months of the year.

Manure Management
Manure is often viewed as a waste product for disposal rather than as a resource for supplying nutrients to the soil. Manure can supply sufficient quantities of nutrients to crops, add organic matter to soils, improve soil structure and tilth, and improve the soil's water holding capacity.

Irrigation Systems Management
Irrigation is practiced on more than 50 percent of Idaho's cropland. In many areas of Idaho the water table is shallow which makes irrigation management crucial. There is substantial evidence in Idaho that poor water management may be the primary factor in the introduction of nitrates into groundwater particularly in southwestern and southcentral Idaho and on the Fort Hall Reservation in southeastern Idaho. An irrigation manager should consider the following to protect groundwater: (1) irrigation scheduling to minimize leaching, (2) credits for nitrate in irrigation water, and (3) adequate precautions when practicing fertigation and chemigation.

Sound nitrogen management alone will not prevent groundwater contamination under irrigated conditions. Excess water from over-irrigation can cause nitrate to move below the crop rooting zone. Consequently, a best management practice utilizing proper irrigation scheduling should be implemented. A sound irrigation management scheduling program considers soil water-holding capacity, crop growth stage, evaporation, rainfall, and previous irrigation to determine the timing and amount of irrigation water to be applied.

Slow-Release Nitrogen Fertilizers
At present, the use of slow-release fertilizers is not economical for most crops grown in Idaho. This is because slow-release materials usually cost 30 to 40 percent more per pound of nitrogen than conventional nitrogen fertilizers. However, slow-release materials often improve nitrogen use efficiency in crops by up to 30 percent.

Crop Rotation Selection
The selection of crops in a rotation has an influence on the movement of nitrogen through soils. Legumes and other crops that don't require large additions of nitrogen fertilizers can often utilize or scavenge nitrogen remaining in the soil from the previous crop. In addition, crops with low nitrogen fertilizer requirements used in sequence with crops that require high nitrogen inputs or inefficiently recover nitrogen can reduce the amount of nitrogen inputs applied over a number of years.

Variable Fertilizer Management
Variable fertility management within a single field is a strategy that has the potential to improve nutrient use efficiency, improve economic crop return, and reduce environmental pollution. A variable fertilizer management strategy can be easily tailored for any field. The only knowledge a grower needs to implement this type of best management practice is to know how yield varies across a field. To use a variable management strategy follow these steps: (1) divide your field into different management units based on yield potential, (2) take separate soil samples for each management unit in your field, (3) apply nitrogen based on yield potential using fertilizer guides for each management unit, and (4) apply phosphorus, potassium, and sulfur based on soil sampling and analysis from each management unit.

Summary of Best Management Practices for Nitrogen

• Apply nitrogen at recommended rates for crop production in Idaho.
• Use preplant soil profile nitrate testing and soil and plant nitrate testing when appropriate.
• Base nitrogen application rates on realistic yield goals.
• Credit nitrogen contributions from legumes, manures, and other organic wastes.
• Plan nitrogen applications to correspond with crop demand (in season).
• Do not apply nitrogen fertilizer in the fall on coarse textured soils or shallow soil over fractured bedrock.
• Use nitrification inhibitors when soil conditions and nitrogen application timing may promote leaching.
• Apply manure uniformly in accordance with crop nutrient requirements.
• Schedule irrigation to minimize leaching.
• Manage fertigation systems carefully.
• Diversify crop rotations to include crops that utilize deep residual nitrogen.

The University of Idaho Cooperative Extension System has over 140 faculty strategically located throughout the state, including 84 agricultural agents and home economists stationed in 42 of 44 counties. In addition, faculty (specialists) are located on campus in Moscow and at research and extension centers in Aberdeen, Caldwell, Idaho Falls, Kimberly, Parma, Sandpoint, Tetonia, and Twin Falls.

This brochure, WQ-12, was prepared by R. L. Mahler, T. A. Tindall, and K. A. Mahler, Soil Science Division, University of Idaho, Moscow, Idaho 83844-2339.

3M 3-92

Comments to webmistress: karenl@uidaho.edu

All contents copyright © 1997-2003.
College of Agricultural and Life Sciences, University of Idaho.
All rights reserved.

Revised: January 3, 2003

URL: http://www.uidaho.edu/wq/wqbr/wqbr12.html