Sampling No-till Fields
If the area has been in no-till corn management or long-term perennial grass for two years or more, it is advisable to use a reliable field kit to measure the pH of the surface soil. Surface applications of nitrogen fertilizers and manure may acidify this layer rapidly and decrease herbicide and nutrient effectiveness. Collect several cores less than 2 inches deep from the no-till area and mix thoroughly in a clean bucket. Remove a sample for pH measurement. Simple colorimetric pH kits normally are the most satisfactory for field use. If the pH of the surface soil is less than 6.2, take a standard soil sample for laboratory analysis. Apply the recommended lime as early as possible before corn plantings.
If this standard sample does not indicate a need for limestone and the surface pH is below 6.2, apply 2,000 lb/A of calcium carbonate equivalent. This amount should be adequate to neutralize the acidity created by the surface-applied nitrogen fertilizer.
Reference: The Agronomy Guide - Section 2: Soil Fertility Management