Spatial Variability of Wheat Yield and Soil Properties on Complex Hills

Abstract Surface soil and wheat yield were sampled on a 400‐ by 250‐m grid to determine the relationship between wheat yield and soil properties influenced by erosion. The study, located in the Dunnigan Hills northwest of Sacramento, CA, began in October 1983. Five transects over complex slopes were made 50 m apart. Every 20 m, along each transect, depth to parent material, surface soil thickness, depth to carbonates, landscape position, and percent slope were recorded. In addition, soil samples were collected to determine particle size, percent organic C, water retention at 33 and 1500 kPa, and pH. One meter square plots of wheat were harvested at each of the 100 sites previously sampled. Subsoil samples were taken at select sites to determine organic C and particle size. Every meter along a 50‐m transect, surface soil thickness and percent sand in the surface soil were determined to assess the nugget effect of these variables. Standard regression analyses showed no correlation between percent slope and yield or soil properties. Semivariograms and cross‐semivariograms showed a strong spatial dependency between soil properties and wheat yield. Total above ground biomass increased from the knolls to the swales, while harvest index increased in the reverse direction. Particle size distribution varied by landscape position as a result of both mechanical and water erosion. Harvest index was shown to be positively correlated with percent clay <2 µm but not correlated with percent sand. Surface soil thickness was not found to be spatially correlated with crop growth.