Post-harvest nitrate testing

Updated 25 September 2019

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

Producers should complete a PHNT to a 30-cm depth after harvest but before fall precipitation leaches nitrate out of the soil sampling zone. Because nitrate leaches more easily in coarser-textured soil than finer-textured soil, the last date for 30-cm sampling is earlier in sandy soils than in other soils. See recommended ‘sample before’ dates below. The BC Soil Information Finder Tool can be used to help determine if an area has sandy soil (i.e. sand, loamy sand, sandy loam), in some regions of B.C.

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 to 60-cm must be completed within 15 days of the latest date shown for a given location in the above table, after which nitrate will be washed below the 60-cm depth (i.e. For Courtenay, a November 1 deadline shown in the table for a 30-cm sample becomes a November 15 deadline for a 60-cm sample). In other areas, sampling to 60-cm should be completed before spring planting.

How does harvest affect sampling?

In some instances, samples will have to be taken before a final harvest. In forages, a soil sample should be taken after the latest guaranteed harvest, providing a valid report on nitrogen use up to that point in the season. Waiting to sample after a later potential harvest may cause producers to sample to a 60-cm depth to have meaningful PHNT results.

Sampling and analysis for multiple tests

If samples were taken for a basic soil fertility test (0-15 cm), a general or basic soil analysis should be completed on the 0-15 cm sample only. Depending on the laboratory, nitrate (NO₃-N) may or may not be included. A nitrate analysis should be requested for samples taken to the 15-30 (or lower) depths.

Sample preparation and shipping

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.

• Download the soil test converter (Excel)

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.

• BC Soil Nutrient Studies

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.