Drying Soil Samples - Still a Standard?

Wet soil sampling is not a new concept.  No, this does not mean sampling a wet field or placing mud into a sampling bag.  This is in reference to analyzing nutrient values from a certain amount of soil to represent a much larger area.  A well-known fact about soil is the variability it can hold.  Whether this be different types of clay, topographies, climates or soil moisture.  One way to eliminate some variability, and inconsistencies, is to eliminate one variable in the sample.

Once a sample is received at the lab, it must go through a preparation period before analysis.  In short, for a standard soil sample analysis, the soil sample bag is referenced and logged in then transferred to a breathable container on drying racks.  At A&L Great Lakes Laboratories, these racks are placed in a custom drying room.  These drying conditions never exceed 104 degrees Fahrenheit.  This ensures certain extractable figures such as potassium and pH are not affected.

The sample is cross referenced for sample ID etc. then ready for the grinder.  To get a homogenized sample after the grinder, it must be uniformly dried.  Then it is processed through a sieve to remove larger materials such as rocks or other debris.  The result before analysis is a representative soil sample with no physical inconsistencies.  This is paramount to the next step of “scooping”.  By taking a volumetric scoop of the soil, consistency is needed.

This seems like a lot of work for the lab when some new technologies are providing nutrient analysis with wet soil samples.  There are several key factors that warrant a dry sample before the extractions can be conducted.  A standard moisture content creates a repeatable process for samples.  Wet soil sampling can be done for certain extractions, but without a standard moisture content, it can be inconsistent depending on environmental conditions, soil types and sampling depths. 

Drying creates a stable sample.  With a wet sample some biological and chemical reactions are still occurring.  Soil is alive, and this does not change once it is cored from the field and is placed in a bag.  Soil microbial processes require moisture to continue.  By drying the sample, these natural conversions are halted especially for nitrogen analysis. 

A solid foundation of agronomy is understanding patterns.  Patterns require consistency, and sound agronomic advice starts at the beginning, with the soil.  To better understand soils and application techniques, an average is calculated.  Then a trend can be set from a larger data set to make the best economic and environmental fertility decisions.