Soil Quality Assessment of Corn Fields with Areas of Poor Canopy Development.

Soil quality assessment is a critical component in understanding the long-term effects of soil and crop management practices within agricultural watersheds.  In the South Fork of the Iowa River Watershed, an aerial survey was conducted during the summer of 2006, and fields that were planted to corn and appeared to have sections with underdeveloped canopy were marked.  Our objective was to determine if a soil quality assessment could suggest the reasons for the poor canopy development.  Fifty-one marked fields were assessed in autumn of 2006.  Four samples were taken from each field, three from the dominant soil types and the fourth from the area of poor canopy development.  Each soil sample consisted of a composite of 20 cores taken at the 0-10 cm depth.  Soil bulk density, aggregate stability, texture, 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 (TN), microbial biomass C (MCB), potentially mineralizable C (C_min)and N (N_min), and β- glucosidase (BG) activity were measured.  The Soil Management Assessment Framework (SMAF) was the assessment tool that was used. There was no single cause for underdeveloped canopy across  all fields.  Overall, the SOC, MCB, C_min, N_min, and BG activity were lower in the areas with poor canopy development.  SMAF indicator scores for carbon, which compensate for differing soil types, were significantly lower for  the poor canopy areas.

Although there was little difference in the mean SMAF Soil Quality Index (SQI), which included eleven indicator scores, of the normal canopy areas vs. the poor development areas, the individual indicator scores could pinpoint specific problems in individual fields. Some of the specific problems included lower soil organic matter and other indications of poor nutrient cycling, low extractable P, high bulk density, and low water-filled pore space at time of sampling.  Using SMAF to determine specific problems will help land managers develop management schemes ameliorate the poor performing areas of the fields.