Soil Sampling Depth - A Critical Piece of the Puzzle

When formulating a quality soil sampling plan, our focus is often directed toward parameters such as grid vs. zone, sample point locations, number of acres per sample, or number of cores per sample. However, a critical parameter that is often overlooked is sampling depth and the consistency of hitting the target depth every time. Cores that vary in collection depth by 1/2” – 1” or more can greatly impact the resulting data which translates straight to the bottom line when it is time to purchase and apply fertilizer inputs.

Nutrient concentration can vary greatly throughout the soil profile, especially under long term no-till conditions.Therefore reliable, consistent results can only be achieved when sampling depth is closely controlled. If you have employees or others helping in your sampling operation, make sure this topic is included and discussed in any training and instructions that you provide.

We often receive questions at the lab regarding sample collection in wet soil conditions.  Proper sampling can continue if good depth control can be achieved.  When the probe is placed into the soil, look in the top of the collection tube and ensure that the top of the soil core being collected is very near to the soil surface.  Under extremely saturated conditions the tip of the probe will not accurately cut through the profile and will push into the ground like a stake while compacting the soil around it resulting in inaccurate sampling depth.

Recommended sampling depth under varying cropping conditions are normally 4” in lawn and turf, 6” in no-till cropping and 8” in conventional tillage.  When choosing your desired sampling depth it is critical  to make your best choice for your conditions  and consistently sample at the same depth over time.  Most samples are collected every 2-4 years and the sampling depth must remain the same so that nutrient concentrations can be compared across collection dates and trends can be established.

Long term no-till. LOI, Reported as Bray P-1, ammonium acetate K. Source ALGL 2021

As seen in the data table above, phosphorus, potassium and soil organic matter tend to decrease as sampling depth increases.

If you would like assistance with any of your sample collection plans, please contact our agronomy staff.