The usual time to take soil samples for general fertility evaluation is in the fall or the spring, before the growing season has begun. These samples are analyzed for pH and lime requirement as well as phosphorus, potassium, calcium, and magnesium. Some labs also routinely analyze for organic matter and other selected nutrients, such as boron, zinc, sulfur, and manganese. Whether you sample a particular field in the fall or in the early spring, stay consistent and repeat samples at approximately the same time of the year and use the same laboratory for analysis. As you will see below, this allows you to make better year-to-year comparisons.
GUIDELINES FOR TAKING SOIL SAMPLES
- Don’t wait until the last minute. The best time to sample for a general soil test is usually in the fall. Spring samples should be taken early enough to have the results in time to properly plan nutrient management for the crop season.
- Take cores from at least fifteen to twenty spots randomly over the field to obtain a representative sample. One sample should not represent more than 10 to 20 acres.
- Sample between rows. Avoid old fence rows, dead furrows, and other spots that are not representative of the whole field.
- 4. Take separate samples from problem areas if they can be treated separately.
- Soils are not homogeneous—nutrient levels can vary widely with different crop histories or topographic settings. Sometimes different colors are a clue to different nutrient contents. Consider sampling some areas separately, even if yields are not noticeably different from the rest of the field.
- In cultivated fields, sample to plow depth.
- Take two samples from no-till fields: one to a 6-inch depth for lime and fertilizer recommendations, and one to a 2-inch depth to monitor surface acidity.
- Sample permanent pastures to a 3or 4-inch depth.
- Collect the samples in a clean container.
- Mix the core samplings, remove roots and stones, and allow mixed sample to air dry.
- Fill the soil-test mailing container.
- Complete the information sheet, giving all of the information requested. Remember, the recommendations are only as good as the information supplied.
- Sample fields at least every three years and at the same season of the year each time. On higher-value crops annual soil tests will allow you to fine-tune nutrient management and may allow you to cut down on fertilizer use.
Note: For a discussion of how to sample to assess the extent of nutrient variability across a large field, see the section “Managing Field Nutrient Variability”.
—MODIFIED FROM THE PENN STATE AGRONOMY GUIDE (2007–2008)