AGRONOMIC NEWS ITEMS
From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335
Fall 1997, No. 7
Think about why you are sampling. What variable factor of the system are you trying to measure with the information being collected? What are possible causes of that variability? What changes can be made to manage that variability? Answering these questions will help guide the decision of how to design the sampling pattern.
A uniform grid is the first cut for fields with histories of extensive fertilizer or manure application. For many factors it may be the best. It helps divide the field into smaller sections that can be studied in more detail and managed accordingly. Grid sampling --- and variable rate application for aglime --- were recommended by the University of Illinois as early as 1929, so this is not a new concept. The new twist is that sampling grids are getting smaller. And now we have new technology, such as global positioning satellites (GPS), geographic information systems (GIS), and the capability to vary rates of inputs on-the-go during application. Grid size should be determined on the basis of the amount of variability expected (or known) and the relative value of the production inputs being evaluated and the crop to be grown. Statistical analysis from sampling studies indicates a 2.5 acre grid is adequate for some fields, while for others a half-acre grid may not adequately describe the variability.
A uniform grid has a disadvantage in that results may be biased by pre-existing uniform patterns in the field due to fertilizer spread patterns, crop residue distribution, previous crop or tillage patterns, etc. An alternative is to use a systematic unaligned grid --- that is a grid pattern where sample sites are offset from the straight-line grid, so that no two samples line up either vertically or horizontally on the map. This will help avoid uniform patterns that exist in the field.
Where sources of variation are obvious, such as major topographic changes or known previous cropping differences or soil type changes, adjusting sampling patterns to avoid the transition areas may be advisable. This "smart sampling" approach helps to more clearly identify the impact of various factors. It is another case of being site-specific in the decision process, adjust sampling patterns to take advantage of what you already know about the field. So, whether to grid or not to grid is another site-specific question.
Fitting a sampling pattern to a field boils down to thinking about what you are measuring, how you plan to use the information being collected, and considering what you already know about the sources of variability. Then sample to take advantage of that information. With GPS and GIS technology, keeping track of a complicated sampling scheme is not a big challenge. However, you should strive to keep it as simple as you can.