Changes in Soil Quality Indictors After 57 Years of Agriculture in a Central Texas Vertisol.

Soil quality assessment is a proactive process for understanding the long-term effects of soil and crop management practices.  Our objective was to assess the long-term impact of agriculture on soil quality indicators in fields within an agricultural watershed where the management history for nearly six decades was known. The fields were located near Riesel TX on the Houston Black soil series (Udic Haplustert), which has a high content of montmorillonitic clay. We sampled five sites representing an untilled native pasture, two areas previously tilled for row crops and then seeded to coastal Bermuda grass (Cynodon dactylon (L.) Pers.) 57 and 31 years ago, and two fields where crops have been generally grown for the last 57 years. Six to eight, 20-core composite (0 to 10 cm) soil samples were taken from each field.  Soil bulk density (BD), aggregate stability (AGG), texture, water-filled pore space (WFPS), pH, Mehlich III extractable P, K, Ca and Mg, KCl extractable NO₃ and NH₄, DTPA extractable Cu, Fe, Mn and Zn, electrical conductivity (EC), total soil organic carbon (SOC), total N, microbial biomass C (MBC), potentially mineralizable C (C_min) and N (N_min) were quantified.  In addition four enzyme activities were measured: 1,4 β- glucosidase (BG, a type of cellulase), N-acetyl β-D-glucosaminidase (a type of chitinase), acid phosphatase, arylamidase and fluorescein diacetate hydrolysis.  The Soil Management Assessment Framework (SMAF) was used to score 12 of the indicators (BD, AGG, WFPS, pH, EC, SOC, P, K, MBC, C_min, N_min, and BG) and to compute an overall soil quality index for each site.