Ignored for decades, moist sample analysis saves money and boosts yields. Soil labs and scientists alike abandoned field moist soil sampling decades ago. Common until the 1980s, field moist soil sampling was replaced with a faster and easier "dry and grind" procedure. The Iowa State University (ISU) soils lab discontinued the moist test in 1988, not because dry was better, but only because “drying soil was a more practical procedure and standardized soil moisture across all conditions," says Antonio Mallarino, ISU professor, soil fertility and nutrient management.

However, his 1990s field research showed much variability with potassium (K) soil testing when using the dry and grind procedure. “The dry test was better in some conditions than others; however it was difficult to predict the circumstances when it would be better. Soil conditions, drainage and texture all affected results."

Mallarino has confirmed that field moist test of K is more accurate, after 300 field corn and soybean trials conducted since the early 2000s.

Peter Scharf has found similar results, after testing field moist and dry sampling for nitrate and ammonia. The University of Missouri agronomy professor concludes, "Dry always comes out higher. There is even a difference with oven-dried versus air-dried samples: Oven-dried provides an additional bump."


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But is the "bump" available for crops? Scharf notes that ammonia and K have an identical charge and behave the same way. K attracts water, creating the same size particle as ammonia, and both structures get caught between layers of clay. He theorizes that when the clay particles dry during sampling, they release both K and nitrogen (N), a release that doesn't happen under field conditions.