The moist soil test for potassium, other nutrients
A new private soil-testing laboratory began operations this fall in Iowa (based in Ames), testing for most nutrients on non-dried soil samples. This has generated many questions concerning the procedure and interpretations of test results because the common lab procedure for most nutrients is to dry and grind the soil samples.
The idea for testing non-dried soil samples is not new. It has been known for decades that drying soil may affect the extraction and measurement of certain nutrients, especially potassium (K). However, drying soil is commonly done by labs because it used to be a more practical sample handling procedure, and it standardizes soil moisture across all conditions. Iowa State University (ISU) research during the 1960s and 1970s, mainly greenhouse trials but some field trials, had shown that testing non-dried (field-moist) soil samples provided a better estimate of K fertilizer needs than testing dried samples, but both procedures provided similar estimates for phosphorus (P). Therefore, testing field-moist samples was adopted by the ISU soil and plant analysis laboratory, and it was the standard procedure for P and K during the 1970s and 1980s.
Iowa State University discontinued field-moist soil testing in 1988 (I was a graduate student at the time); not because it was a bad procedure but because no other lab adopted it and only ISU soil test interpretations were based on moist testing. The ISU lab made that decision even though Iowa research had shown the moist test was better for K, and it was among the tests recommended for the North-Central region by the NCR-13 committee (North-Central Regional Committee for Soil Testing and Plant analysis). Since ISU discontinued moist soil testing in 1998, the NCR-13 committee dropped the procedure from its soil-test methods publication.
The general attitude about moist soil handling and testing changed considerably this year, when the private lab that began operations in Iowa developed a machine that easily handles moist samples and makes implementation of this procedure as practical as the common dry method (or even more practical because it avoids drying and grinding samples). This laboratory has been conducting soil testing research with Iowa soils since last year, some in collaboration with ISU.
Field research conducted during the 1990s showed much variability and uncertainty with K soil testing due to several reasons. So a portion of research my graduate students and I conducted since the early 2000s has focused on studying again if testing field-moist soil samples for K is better than testing dried samples.