Nitrogen can carryover from one growing season to the next, providing a vital resource for your next crop. This nitrogen can come from the mineralization of soil organic matter or fertilizer, usually in the form of nitrate, that has not been leached below the root zone. Residual nitrate levels in subsurface drip irrigated tomatoes in California have been found to range from 45-438 lbs/ac in the top 20” 1. Although these values vary widely, similar values of 23-107 lbs NO3--N /ac of were found in the top 48” in California wheat fields after harvest2. Levels for an individual field will vary according to texture and the amount of precipitation since harvest of the previous crop. A higher clay content or low rainfall will result in generally higher levels of residual nitrogen due to its impedance of leaching below the rooting zone. Winter cover crops can reduce nitrate leaching by utilizing this residual nitrate.
To find the residual nitrate content of a field, a soil sample should be taken prior to planting and/or application of any fertilizers. For any given field, around 10 individual samples should be taken from across the field in order to accurately capture the in-field variability. Samples should be taken to the depth of the crop rooting zone. If these samples are to be sent to a commercial soil test lab, they should first be air-dried to suppress microbial activity.
For subsurface drip-irrigated tomatoes: A recent study1 established a protocol showing that in 80" beds, two cores should be taken at either 15" and 30" from the drip tape, or 20" and 25" from the drip tape and when using 60" beds, three cores should be taken at 5", 10", and 20" or 5", 20" and 25" from the drip tape. These samples can then also be used to assess Olsen-P and exchangeable K, which would give you low error rates for both P (<10%) and K (<3%).
For other crops: Consult this FREP guide for soil nitrate sampling.
Soil Nitrate 'Quick Test'
To find nitrate content without sending samples to a test lab, a do-it-yourself nitrate “quick test” has been developed for California that does not require air-drying of soil samples and can usually be read within an hour.
Interpreting Soil Test Results
Results from both the nitrate quick test and many soil test labs are reported in the units of “parts per million” (ppm). To convert this value to lbs/ac, you must multiply the ppm (remember to divide by 1,000,000!) by the bulk density of your soil and the depth of sampling. The table below shows approximate conversion factors for varying soil textures and depths. To obtain a more accurate measure of your soil bulk density you can measure it yourself or get an estimate.
Potential Cost Reductions
Incorporating residual nitrogen into N fertilizer prescriptions has the potential to substantially reduce fertilizer costs per acre. The potential savings will vary according to the type of fertilizer being replaced by residual nitrate. Table 2 uses the maximum and minimum residual nitrate values from a survey of drip-irrigated tomatoes across California to illustrate the potential economic savings from reduced fertilization rates. These results are from a single season, but these savings can be found across multiple seasons. Visit this NRCS guide for more examples of potential cost reductions from incorporating residual N levels into nutrient budgets.
- Lazcano, C., Wade, J., Horwath, W., & Burger, M. (2015). Soil sampling protocol reliably estimates preplant NO 3- in SDI tomatoes. California Agriculture, 69(4), 222-229.
- Hutmacher, R., Orloff, S., Wright, S. (2013). Impact of Nitrogen Fertiliation Treatments on Residual Soil Nitrate Accumulation Patterns in California Wheat. California Wheat Commission Summary Report. (http://www.californiawheat.org/uploads/resources/646/hutmacher-etal_residualn-report_930_2013c.pdf)