AGRONOMIC NEWS ITEMS
From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335
Spring 1997, No. 1
Soil sampling is a confusing matter because there are no standards. Questions center around what type of sampling method to use. There are several methods which a farmer might choose, including grid sampling, sampling by topography, and sampling by soil type. But which one is right for your situation? Here are a few things to consider when making the choice.
First, consider the reasons you sample the soil. The primary one is to determine the fertilizer rates appropriate for a field. Correct rates will produce maximum agronomic effectiveness and optimum economic returns to fertilizer applied. Agronomics and economics are not at opposite ends of the spectrum. Instead, they work together to form best management practices (BMPs). At the core of BMPs is a good understanding of the spatial variability of nutrient levels in the field.
Choosing an appropriate sampling scheme requires knowledge of the field you are sampling. If you are familiar with the field, you probably already have some knowledge about which areas are higher yielding and which are lower yielding. You may also know that one part of the field used to be a feedlot, or another area was once made up of smaller fields. Such knowledge is the starting point for selecting how you will sample the field. If you know parts of the field have different histories, then sample these areas separately. The key is not to gloss over known sources of nutrient variability, regardless of the soil sampling scheme you choose. Neglecting to sample variable areas will result in over or under fertilization, both of which reduce income and net returns and can have negative impact on the environment.
A few soil sampling 'rules of thumb' are beginning to emerge from research. The appropriate sampling scheme, as discussed above, is based upon knowledge of the field. If the field has not been part of a regular soil testing and fertilization program, there is a good chance that the variability in that field will be due to natural variables, such as topography, vegetation, parent material, and climate. When nature is the source of variability, sampling by soil type or topography may be more appropriate. If the field in question has variability that traces back to management practices, such as manure, lime, and fertilizer applications, sampling by grid may be more appropriate. In addition, if mobile nutrients are of interest, such as nitrate and chloride, sampling by topography may be the best way. If immobile nutrients such as phosphorus, potassium, calcium, and magnesium are the primary interest, grid sampling may be better.
Once you have selected a sampling scheme, don't feel 'locked-in'. If you have selected a grid sampling plan, that doesn't mean you can't augment those samples with some additional samples taken where known variability exists. The objective in sampling is to get the best idea of the nutrient variability in the field. To accomplish this, different sampling schemes may need to be mixed and matched.
The end result should be successfully identifying yield-limiting areas in the field. Unless more intensive, knowledge-driven sampling is used, field areas with lower nutrient levels may be missed. Overlooking these areas results in lower soil productivity, lower yields, and reduced profits.