Untapped “Free” Nitrogen: An Adaptive Approach to Managing N from Soil and Irrigation Water
Most crops need significant nitrogen (N) to grow, but you may already have more N than you think—right in your soil or irrigation water. This “free N,” in the form of nitrate (NO₃⁻), can help you cut fertilizer costs, protect water quality, and meet regulatory requirements such as California’s Irrigated Lands Regulatory Program (ILRP). By measuring the nitrate that’s already present and crediting it in your fertilizer program, you can fine-tune your nitrogen use to save money without hurting yields.
This approach is rooted in Adaptive Nutrient Management, a process recommended by the USDA NRCS (Agronomy Technical Note 6). Rather than guessing or following old fertilizer rules of thumb, adaptive management uses data from your own fields—soil tests, water tests, tissue sampling, yield measurements—to make smarter decisions each season. Below is a straightforward, six-step guide to help you try it out.
Why Use “Free N”?
Lower Fertilizer Costs: Accounting for nitrate in your well water or soil means you buy less commercial N. Research from UC ANR shows that irrigation water alone can supply up to 20–60+ lbs N/acre-foot if nitrate levels are high. Residual nitrate in soil can be similarly valuable, as shown by field studies detailed by UC ANR’s Residual Nitrogen guide.
Protect Water Quality: Every pound of N you don’t over-apply is a pound that isn’t at risk of leaching into groundwater. This keeps you in better shape with regulations and reduces the risk of nitrates in drinking water.
Maintain or Improve Yields: “Free” N is still nitrogen—plants don’t know the difference. With careful management, you won’t see yield drops, and sometimes you’ll see improved quality or disease resistance by avoiding excessive fertilizer.
Meet Regulatory Requirements: Under programs like the ILRP, you must report total N applied. Crediting existing N sources makes your nutrient management plan more accurate and often leads to a better N-use efficiency ratio.
The Adaptive Nutrient Management Cycle
Plan – Decide how much N your crop typically needs and identify the sources you will measure (soil and/or irrigation water).
Implement – Apply the adjusted fertilizer rate, crediting the N already in your soil and water.
Monitor – Tissue tests, visual scouting, and mid-season checks help verify if the crop is receiving enough N.
Evaluate – Compare yields, fertilizer costs, and any quality parameters at harvest.
Adjust – If you see room for improvement, update your plan for the next season (less fertilizer if you still see excess N, or a bit more if there was a slight deficiency).
Repeat – Adaptive management is an ongoing cycle. Each season’s data helps refine decisions.
Step-by-Step: Using Free N in Your Fertilizer Program
If you’re unsure about cutting fertilizer, start with a Free-N trial. Pick a portion of a field (or a whole field) to credit N from soil and water, and compare it to an area where you don’t credit free N. Harvest them separately to see if yield differs. Replicating these plots gives you more reliable data.
Use guidelines from UC ANR or CDFA for your specific crop. Typically, you can start with the crop’s N removal (how many lbs of N per unit of yield) and add a small buffer if needed.
- Soil: Sample before applying fertilizer. A standard approach is to take 10–20 cores, combine them, and send a sub-sample to a lab for nitrate-N testing. For more info, see Residual Nitrogen in Soil (UC ANR).
- Irrigation Water: Take a water sample after the well runs a few minutes. Ask the lab for nitrate-N (if they provide only “nitrate,” convert by multiplying by ~0.225 to get nitrate-N). More info: Nitrogen in Irrigation Water (UC ANR).
Convert your lab results from ppm to lbs of N per acre using standard conversion factors. For water, 1 ppm nitrate-N ≈ 2.7 lbs N per acre-foot of water.
Once you know how many lbs/acre of nitrate are present in your soil and water, reduce your purchased fertilizer by that same amount. If you’re nervous, start by crediting 50–70% of the measured N the first year to build confidence.
Check tissue tests or visual indicators. If a deficiency starts to show, you can still add a “rescue” application of N. Many times, you’ll find the crop is perfectly satisfied.
At harvest, compare yields and quality from your Free-N area to your usual practice. Often, you’ll see no yield penalty and a clear fertilizer savings. Share results with neighbors, local grower coalitions, or advisors. Knowledge from your farm helps everyone improve.
Beyond Rate Cuts: Improving Efficiency
Better Timing & Placement: Split your applications or use fertigation to match the crop’s demand curve, ensuring N is in the root zone when the plant needs it.
Uniform Irrigation: Uneven irrigation can waste both water and fertilizer. By improving irrigation uniformity, you can further reduce nitrogen losses.
Adaptive Trials: Each season, use small trials to see if your new methods allow you to safely reduce N rates further or achieve bigger yields.
Getting Started
Talk to Your Advisor: Certified Crop Advisers (CCAs), UC Cooperative Extension farm advisors, and NRCS staff can help design trials and interpret data.
Leverage NRCS Programs: NRCS Adaptive Nutrient Management guidance can provide cost-share or technical support for improved nutrient management (Practice Standard 590).
Document & Refine: Record your soil/water test results, fertilizer rates, and yields. Over multiple seasons, you’ll see trends, which helps fine-tune your program and maximize both production and profit.
Embracing adaptive nutrient management means farming smarter, not harder. By recognizing the “free” nitrate your soil and irrigation water already contain, you can save money, protect local water quality, and maintain healthy yields. Why pay more for fertilizer when you’ve already got free nitrogen on-hand?