Post-Harvest Nitrate Testing

A post-harvest nitrate test is a measure of the amount of unused nitrate remaining in a portion of the soil after a crop has been harvested. Samples should be taken at a time and to a depth that will represent the unused nitrate from the past growing season.  Complete a post-harvest nitrate test after harvest but before autumn rain leaches nitrate out of the upper portion of the soil sampling zone.

When and how deep to sample

To capture a representative sample of the unused nitrate at the end of a growing season soil samples should be taken to a 30-cm (12-inch) depth before rain washes it downwards through the soil. Fall climates vary throughout the province, creating shorter sampling windows in some regions like the South Coast and longer sampling windows in regions like the Peace River (Table 1).

Table 1. Latest sampling dates* on average for a 30-cm post-harvest nitrate sample in various B.C. locations
Location Sample before
South Coast October 1
Vancouver Island October 1
Cranbrook Ground freezes
Creston October 15
Fort Nelson Ground freezes
Fort St. John Ground freezes
Kamloops Ground freezes
Kelowna November 15
Prince George October 15
Salmon Arm October 15
Smithers October 15
Vanderhoof October 15
Vernon November 1
Williams Lake November 1

*based on climate normals, sampling prior to these dates will generally  ensure that rainfall will have not carried nitrate below the 30cm sampling zone

What if sampling is not completed in time?

If soils are not sampled before the dates noted above, samples should be taken to a 60-cm (24-in) depth to capture post-harvest nitrate which has moved lower into the soil profile.

In the South Coast and Vancouver Island, sampling must be completed prior to November 1, after which nitrate will be washed below the 60-cm depth.

Can sampling be done in the spring?

A pre-plant nitrate test can provide an assessment of residual nitrate from the previous season in regions where there is little autumn rain, and winter temperatures are consistently below zero.  These regions are generally noted in Table 1 where the sampling is recommended to be completed before the ground freezes.  A pre-plant test in these climates is considered an equivalent test to the post-harvest nitrate test as they both measure the amount of nitrate in the same portion of the soil. However, note that mineralization and nitrification of organic matter may cause nitrate values to be greater if soil samples are not taken early in spring.

Analyze samples immediately and keep refrigerated

Since soils that are kept warm and moist will continue to accumulate nitrate through microbial activity, samples should arrive at an analytical laboratory within 48 hours. Samples that need to be stored before being sent for analysis should be stored in a refrigerator. Samples should not be frozen as soil nitrogen can change forms while freezing and thawing and an accurate result cannot be obtained. If samples will not arrive at a laboratory within 48 hours, then they can be spread on paper to air dry before being shipped.

Regulatory requirements

Post-harvest nitrate testing is a component certain requirements in the Agricultural Environmental Management Code of Practice.

  • Farms that apply nutrient sources on a land base of 2 ha or more are required to take and maintain records of post harvest nitrate soil samples every three years
  • A gradual phase in of nutrient management planning for farms located over vulnerable aquifer recharge areas will link the results of post harvest nitrate tests to a requirement for developing a nutrient management plan.

If the post-harvest nitrate test result is 100 kg N/ha or more, the following requirements are triggered:

  • The field must be sampled again the following year;
  • If in a phased-in vulnerable aquifer recharge area, the farm may need a nutrient management plan depending on farm type and size.

Laboratories generally report results in parts per million (ppm) and may convert the results to lb/ac or kg/ha for comparison with the 100 kg N/ha threshold. Converting from ppm to kg/ha depends on the sampling depth and soil bulk density. To be below 100 kg N/ha the average soil nitrate-N values should be:

  • below 25 ppm at a 30-cm sampling depth
  • below 12.0 ppm at a 60-cm sampling depth

Soil sample nitrate results from the lab can be converted to kg N/ha with the soil test converter spreadsheet.

More information on Post-Harvest Nitrate

Several soil nutrient surveys and technical reports in British Columbia have monitored and investigated the use of post-harvest nitrate.

Much of the work on Post-Harvest Nitrate soil testing as tool to assess nitrogen management is based on research from Oregon State University. Their publication “Post-Harvest Soil Nitrate Testing for Manured Cropping Systems West of the Cascades” is a valuable resource for more information on the subject.