| We measured soil quality indicators from three experiments: two involving long-term tillage and/or crop rotation systems and one involving on-farm site-specific sampling, in the Tennessee Valley region of northern Alabama. We also evaluated rapid in-field determination of electrical conductivity by a direct contact instrument (Veris®, apparent electrical conductivity - ECa) to estimate soil quality indicators and crop yield. In the first experiment, four conservation tillage cotton ( Gossypium hirsutum L.) systems with a rye (Secale cereale L.) winter cover crop [no-tillage (NT), NT with fall in-row subsoiling, NT with fall in-row paratilling, and spring strip tillage] and conventional tillage (CT) without a cover crop were evaluated after five years. Conservation systems resulted in storage of 3 to 6.9 Mg ha−1 more soil organic C (SOC) within the 0 to 24 cm depth than CT. Particulate organic matter (POMC) was 77 to 115% greater, and soil microbial biomass (SMB) was 36 to 160% greater, for conservation systems compared to CT within the 0–3 cm depth. Fall deep tillage extended SOC and SMB increases to the 6-cm depth. Seven treatments were evaluated in the second study: CT and NT cotton, with and without a wheat (Triticum aestivum L.) cover crop, and NT cotton rotated with corn (Zea mays L.), soybean [ Glycine max (L.) Merr.], and double-cropped wheat-soybean. Between 3.4 and 7.7 Mg ha−1 more C was sequestered (within the 0–24 cm depth) with NT and/or cover cropping compared to CT without a cover crop. Soil microbial biomass and POMc were also improved with these systems. Soil strength and bulk density increased with NT systems but infiltration was greater for these systems compared to CT without a cover crop. In both experiments, an atypical decrease in aggregate stability was associated with increased SOC in conservation systems. Soil quality improvements with conservation systems mirrored yield histories. The third experiment, a large variation in soil properties was observed for a 19.8 ha field subjected to grid sampling (237 grid points). Multiple regression using chemical properties explained 28 to 57% of crop yield variations (wheat and soybean in 1997; corn in 1998; wheat and soybean in 1999). Correlation coefficients between EC a (three sampling dates and two depths) and crop yields varied from - 0.07 to 0.22, 0.14 to 0.45, 0.24 to 0.48, 0.16 to 0.43, and 0.05 to 0.33 for the crops, respectively. However, when crop yields were categorized according to five ECa classes (very low through very high), relationship to yield was improved. The ECa values were linked to Ca, Mg, K, SOC, pH, and sand content. Our results demonstrated that conservation tillage with increased cropping intensity improved soil quality; however, rapid determination of soil quality indicators associated with crop yields using ECa is problematic. |
