Don’t guess, soil test. It has taken on new meaning in the past year as commodity and fertilizer prices have fluctuated wildly.
Soil testing is more important than ever because of changes in crop and fertilizer prices. Over the last two years the cost of soil sampling and analysis has increased little, while fertilizer prices have increased twofold to fourfold.
To get the most accurate fertilizer recommendations and provide for increased efficiency, you’ll need:
- good soil samples and well-based soil sample information; and
- soil test calibration relationships that reflect both crop response and profit response.
The goal of soil testing is to provide an accurate assessment of the soil’s fertility to make fertilizer recommendations. With the increasing awareness of fertilizer effects on environmental quality, soil tests also can be used to determine where fertilizers or manure should not be applied.
Collecting Accurate Soil Samples
- Sample soils every three to four years for pH, organic matter, phosphorus, potassium and zinc.
- Sample soils to a 3- or 4-foot depth for residual nitrate before corn, wheat, sunflower and sugar beets and to a 30-inch depth for dry beans and millet. Many producers, however, sample every year as a means of spotting outliers and to establish long-term trends.
Today, producers fit soil sampling into one of two management approaches:
- traditional, whole-field and
- site-specific precision.
Currently, most soil sampling is done on a whole field basis, annually collecting 10 to 20 cores from areas of 20 to 50 acres. Because of soil variability, producers should actually sample smaller areas. Soil sampling instructions 30 years ago suggested sampling different areas of the field to determine variability.
Knowing variability in pH and phosphorus can provide information for variable rate fertilizer application that may not save fertilizer costs, but does allow the farmer to put the right amount in the right place. Farmers can still average soil test levels from those smaller areas and apply an average fertilizer rate. Precision approaches (grid or zone sampling) provide additional detail, but are more expensive.
Applying Test Results
Crediting Other Nutrient Sources. An important factor to consider for fertilizer efficiency is giving proper credit to other nutrient sources, such as legumes, manure, and irrigation water. The cost to collect and analyze water samples for nitrate, or manure samples for nutrient content, has increased little while the value of nutrients has increased substantially. Until retail fertilizer prices drop to match the recent downturn in grain prices, the importance of accurate soil sampling information cannot be overemphasized. It is critical that investments of this size be made with the best information possible, based on careful soil testing and a deliberate accounting of all available nutrient sources.
Accurate Interpretation of Results. Different soil testing laboratories develop fertilizer recommendations based on different philosophies such as deficiency correction versus maintenance or maintenance plus build-up. Research by the University of Nebraska-Lincoln (Soil Testing Lab Comparison Study) highlighted the problem of how interpretations and recommendations for a given soil test level differ among various soil test labs and the university. The science of soil analysis is not in question, but rather how soil test results are interpreted and recommendations are made.
Soil sampling has always been an important management tool to help producers determine nutrient status and need. The basic principles of what constitutes a good soil sample have not changed, but its economic importance as a risk management tool to maintain your soil’s productivity while still making a profit is greater than ever.