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Assess the benefit of using management zones versus grids for sampling

Feb 19, 2014

Question: We have done 2.5 acre grid samples the last two years and spread fertilizer with the grids. Can I move out to 10-acre grids and get average and get the same results? We are in the southeast part of Georgia.

Answer: I would not let a grid establish how I pulled the samples.  I would look for any information I can get for how the field yields to start with.  However the field yields is how it should be sampled.  I like to lay soil types down first and then the yield maps and aerial imagery of fields over my soil maps to refine the zones.  I always respect my soil types and then subdivide those soil types down into zones based on water drainage, water availability and crop growth. Instead of looking at 2.5-acre or 10-acre grids, I would look at your yield maps.  If you don’t have yield maps in cotton and peanuts, I would look at aerial imagery (NDVI) as well as thermal imagery to help me define how the field yields. Take the high, medium and low yields in the field – that’s how it should be mapped out and sampled so you have a better picture.  Your yield map and soil test map should correlate. Granted, yield maps in corn are easy to get, while cotton and peanuts may be a lot harder to do. If that’s the case, I would rely on NDVI or thermal imagery to help me have a good map, and then I would sample it by management zones not grids.  A management zone can vary in size.  Some might be 10 acres in size and others as small as 2 acres.  Let soil type, topography, fertility and yield history separate the field out into management zones.  By that I mean, for example, with corn if I have one area making 220 bushel/acre consistently and another area making 150 bushel/acre, those are two separate management zones. If you know how that field yields, that’s how it should be tested.

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