Oklahoma State University Stillwater, OK
Oklahoma Alfalfa Oklahoma Cooperative Extension Service Oklahoma Agricultural Experiment Station

Sampling Soil in Alfalfa Fields
Monitor Potassium Depletion with August-September Soil Test  Soil samples are commonly taken in February or March from alfalfa fields as a guide to fertilizer management. This corresponds to the time of sampling used when the soil test was calibrated and is therefore, most reliable. However, for high producing fields that are borderline deficient in potassium (soil test ranging from 200 to 350) late winter samples may not identify potassium deficiency that occurs near the end of the growing season. Because wetting and drying, and freezing and thawing action in soils favors potassium release and increased availability, March soil tests may show significantly higher available potassium than was experienced by the crop late in the growing season (August-September).    When high yielding alfalfa fields show a potassium deficiency from mid to late season (July to September), yield of the last two cuttings has likely been reduced. This deficiency can be best corrected by applying potassium fertilizer after the second cutting. The rate to apply should be based on the soil sample from the previous year and be no less than 60 lb/ac of K2O.
Hailin Zhang, Nutrient Management Specialist  Department of Plant and Soil Sciences  Oklahoma State University

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