I marked off a 200 x 200 grid last fall. That is a little less than one acre. I pulled a sample about 20 feet inside each corner of this grid and one from the middle. 

My PH was close in all 5 samples but the P ranged from 10 to 21 ppm. The K ranged from 80 to 130 ppm. The sample from the center was not close to an average of the other 4 either. So, the question begs asking is how can sampling a 100,000 square foot area give you a definitive answer to what you have?

I've done grids in the past but prefer to spend that money elsewhere.