Nutrient and pH stratification with conventional and no-till management

This study investigated changes in soil test results associated with sampling depths in fields managed with conventional tillage, no-till for less than 3 years no-till for 3 to 6 years and no-till for more than 6 years. Soil samples from depths of 0-5, 0-10, 0-20 and 10-20 cm were collected from 59 fields with different tillage histories from several geologic regions, and analyzed by the North Carolina Department of Agriculture soil test laboratory. Some nutrient stratification was noted in all tillage categories. Soil test phosphorus (P), potassium (K), and zinc (Zn) concentrations were significantly higher in the 0-10 cm depth than in the 10-20 cm depth. Stratification probably results from the prevalence of tillage with disks, chisel plows or subsoilers, which do not mix the soil thoroughly. With the adoption of no-till methods, stratification becomes even more pronounced. Our data suggest that stratification in pH, calcium (Ca), manganese (Mn), and sulfur (S) is more likely for fields in the early stages of no-till (less than 6 years) than in longer-term no-till. In fields managed with no-till for less than 3 years or for 3 to 6 years, pH and Ca, Mn, and copper (Cu) concentrations were higher and the S concentration was lower in the surface 0-10 cm layer than in the underlying 10-20 cm soil layer. These differences were not significant for fields managed with no-till for more than 6 years or with conventional tillage. In fields with low P or K levels, higher nutrient concentrations near the surface result in lower fertilizer rate recommendations with shallower sampling depths. The overall pH effect across geologic regions was very small, generally 0.1 to 0.2 pH unit, so lime recommendation did not vary significantly with sampling depth in most cases. For monitoring changes in plant nutrient and heavy metal concentrations over time, sampling no-till fields requires even more attention to depth than for conventionally tilled fields.