Sample timing and depth

The timing and depth of a soil sample is dependent on the type of soil test being done. For those looking to complete both a basic soil fertility and a post-harvest nitrate soil test, sample timing will be based on the guidelines for post-harvest nitrate testing as it is the more time-sensitive soil analysis.

Basic soil fertility test

A basic soil fertility test should be completed every one to three years before planting or before the growing season begins in spring. In regions of B.C. where soil conditions prevent timely sampling and analysis of soil before planting, then sampling and analysis may be completed in the fall. In such a case, producers should note that pH, P, and K will increase over time from fall to spring, potentially leading to larger fertilizer recommendations. If soils are intensively managed, complete a basic soil fertility test at the same time every year to monitor changes in pH and nutrient concentrations.

Soil samples should be taken from the surface to 15-cm depth for basic soil fertility testing.

Post-Harvest Nitrate Test (PHNT)

Producers should complete a PHNT to a 30-cm depth after harvest but before fall precipitation leaches nitrate out of the soil sampling zone. Coarser soils (sand, loamy sand, and sandy loam) should be sampled before 75 mm of cumulative precipitation, while finer soils (loams, clay loam, and clay) should be sampled before 125 mm of cumulative precipitation. September 1 is the starting point for calculating cumulative fall precipitation. The BC Soil Information Finder Tool can be used to determine an area’s soil texture if it is unknown.

What if sampling is not completed in time?

If soils are not sampled before the dates noted in the tables 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.

On the South Coast and Vancouver Island, sampling to 60-cm must be completed within 14 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 sample timing?

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 require producers to sample to a 60-cm depth to have meaningful PHNT results.

Alternative to fall sampling for PHNT based on cumulative precipitation

In areas where there is a high likelihood of receiving less than 75 mm/125 mm of cumulative precipitation between September 1 and spring planting of the following year (such as Fort Nelson and Fort St. John), a spring pre-plant nitrate test can be completed instead of a post-harvest nitrate test. This 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. It should be noted that the processes of mineralization and nitrification of organic matter may cause nitrate values to be greater if soil samples are not taken in a timely manner in the spring.

Alternative sampling depth

In some instances, shallow soils cannot be sampled to the recommended depth. If this is the case, soils should be sampled as deeply as possible. Additionally, these samples should be taken before October 1 to ensure soil nitrate is not lost before sampling. Be sure to record final sampling depths for accurate reporting.

More soil sampling topics:

• Soil Nutrient Testing
• Soil sampling methods & tools​
• Sample handling, laboratory submission, and analysis