iCropTrak makes a simple ipad tool for grid, free style, and zone soil sampling for $250/yr.

Price includes capability on the ipad to download USDA CLU boundaries around you to make get started faster.

Uses Dropbox to load boundaries, grids, and directed points and to unload results when done so you don't need a desktop.

Hope this helps, 

See more here:  www.icroptrak.com