August 2006

‘Tis the Season for Soil Sampling

Soil sampling is always an important component of all fertility programs – but even more so in years with low commodity prices and extreme weather conditions. Farmers cannot afford to ignore the contribution of soil nutrients by the soil. Areas that experienced drought with low crop yields undoubtedly left significant quantities of N in the soil. Conversely, areas that experienced excessive rainfall lost N due to denitrification and leaching.

Three questions regarding soil sampling often arise – timing, pattern and depth.

Sample Timing

Ideally, samples should be taken as close to seeding as possible. In reality, fall sampling is most practical, and this is when 80% of prairie soils are sampled.
Soil N levels increase slowly through the fall from breakdown of organic matter until biological activity slows (at <5 degrees C). Ideally, samples would be pulled between mid October and freeze-up, when microbial activity has ceased and N levels have stabilized.

But earlier sampling has become common because:

1. Samples are easier to take and depth are more reliable on untilled ground.
2. It allows more time to develop fertility programs for fall applications.
3. The uncertainty of when fall freeze-up may arrive may leave farmers without any samples taken.

As a general rule of thumb, soil temperatures do not decrease to 5 degrees until the middle of October. Earlier sampled soils may greatly underestimate the amount of soil N available for the subsequent crop. It has been estimated that nitrate-N accumulates at up to 0.5 to 1 lb per acre per day through early September. Fortunately, most soil labs now make adjustments for soil N accumulation between actual sampling time and soil cooling or freeze-up.

Sampling Patterns

With the introduction of precision farming and grid soil sampling, theories abound on which pattern to use for sampling fields. Most agronomists continue to use the traditional field composite test. In this method, 15-20 sample cores are collected in a selectively random manner from representative areas of the field. It is imperative to avoid including odd locations in the composite. Odd locations, such as eroded knolls and saline areas should be sampled – but submitted to the lab as separate samples. Including an oddball site will contaminate the composite and may lead to erroneous and costly fertility decisions.

For example:

1. Often eroded knolls have exposed subsoil. High pH and lower availability of micronutrients such as zinc and copper.
2. Depressional areas may have very high sulphate levels compared to well drained upper slopes.
3. Saline areas may have considerable buildup of N, P and K levels since crop growth and nutrient removal is reduced.

Sampling Depths

Use sample depth of 0-6” (0-15cm) for immobile nutrients such as phosphorus, potassium, pH and micronutrients. Deeper samples of 0 to 24” (0-60 cm) have traditionally been recommended for mobile nutrients such as N and S and for salinity. Manitoba research is indisputable – N uptake is best predicted by the 0-24” (0-60 cm) sample depth. In two separate findings, the correlation of soil nitrate-N to plant uptake was 0.64 and 0.64 for the 0-12” (0-30 cm) depth and 0.84 and 0.72 for the 0-24” (0-60 cm) depth, respectively. Sampling to these depths is not easy, and it is commonly suspected that core numbers per field suffers because of the chore.

Recent studies in other prairie provinces have found acceptable results with 0-12” (0-30 cm) sampling. Much of this success hinges on the hope that people doing the soil sampling will take more cores within each field and provide a better indication of the field average. So should your client choose to use the shallower 0-12" (0-30cm) sampling depth, increase the number of cores taken per field. The lab will then use an adjustment factor to estimate the total N and S availability.