One Acre Grids - Real Accuracy or Perceived Accuracy

The latest issue of Prairie Farmer Contains  an Article about sampling with one acre grids. I see problems with this technique because it is still a grid sample.  Grid samples are by definition a spot sample.  The idea of a soil sample no matter technique is to represent a particular part of the landscape by sampling and testing a very small part of that landscape.  We then try to relate soil test results to the potential for the crop to respond to additions of fertility.  The hope is that we can maximize yield potential by increasing fertility to a level where the crop is not likely to be stressed by nutrient deficiency in that sample area.  By sampling on smaller grids we hope to have  more accurate picture of what is going on in a field.  In 1976 and 1977 Dr. Ted Peck was trying to teach me everything he knew about soil fertility and soil sampling.  He had a University owned field that had been sampled on a very intense grid in 1960.  256 samples had been pulled in that 40 acre field representing 0.15625 acre in each sample.  This was grid sampling at its most intense.  Results of Bray P1 testing are shown below.  You would think that by pulling samples so close together we would begin to pick up a pattern in the field, but it seemed that the closer together we got the more random the result in relation to other results.  This pattern held for the most part on pH, and Potassium tests as well.  The only place we saw a little bit of a pattern was in the calcareous soils.  Dr. Peck used these results to recommend that soil samples be pulled as close together as possible.  Thus we went from puling 11 samples in a 40 acre field to pulling 16 samples and the 2.5 acre grid was born. 

My opinion is that the results below make a good case for trying to map areas with similar soil properties and then pull samples from the whole management zone.  Our zones average about 10 acres in size, but they can be much smaller if needed to properly characterize a problem area or potential problem area.  We also use yield maps or yield potential to confirm the zones that we define by making our own maps that define soil type by landscape units and drainage class as well as topography.  We sample across the whole area that we are trying to represent instead of trying to represent a small segment of the landscape with an even smaller sample of that landscape.  We also sample frequently, most often yearly, in order to quickly correct any quirks that we find in any given year.  Quirks are most often caused by differences in soil moisture and temperature from year to year.  By sampling frequently and at the same time of year each year we can pick up patterns in fertility change pretty quickly.  We believe that zone sampling is an accurate reflection of what we are trying to accomplish in a given field and takes out the randomness of grid sampling that is caused by spread patterns and blindly sampling a given area whether it accurately reflects the land or not. Here is another blog on our sampling philosophy.