Soil Sampling and Testing
Because the fertility status of a soil cannot be determined visually, a good soil test is essential. Soil testing is available from the Penn State Agricultural Analytical Services Laboratory. The Penn State report is divided into four parts:
- Soil Test Report
- Soil Nutrient Levels
- Additional Results
Taking Soil Samples
A soil test is no better than the care given to taking samples. Follow these guidelines for taking soil samples. Not following these instructions or the instructions from your soil testing lab can invalidate your soil test results and interpretations.
Guidelines for Taking Soil Samples
- Do not wait until the last minute. The best time to sample is in the summer of fall.
- Take cores from at least 15 to 20 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.
- Take separate samples from problem areas if they can be treated separately.
- 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 3- to 4-inch depth.
- Collect the samples in a clean container.
- Mix the core samplings, allow to air-dry, and remove roots and stones.
- Fill the soil test mailing container.
- Complete the information sheet, giving all of the information requested. Be sure to include the soil name. Remember, the recommendations can be only as good as the information supplied.
It is very important to completely and accurately fill out the soil test information sheet that goes to your testing lab. Mailing kits for submitting samples to the Agricultural Analytical Services Laboratory are available from all Penn State Extension offices.
Sampling No-till Fields
If the area has been in no-till corn management or long-term perennial grass for two years or more, it is advisable to use a reliable field kit to measure the pH of the surface soil. Surface applications of nitrogen fertilizers and manure may acidify this layer rapidly and decrease herbicide and nutrient effectiveness. Collect several cores less than 2 inches deep from the no-till area and mix thoroughly in a clean bucket. Remove a sample for pH measurement. Simple colorimetric pH kits normally are the most satisfactory for field use. If the pH of the surface soil is less than 6.2, take a standard soil sample for laboratory analysis. Apply the recommended lime as early as possible before corn plantings.
If this standard sample does not indicate a need for limestone and the surface pH is below 6.2, apply 2,000 lb/A of calcium carbonate equivalent. This amount should be adequate to neutralize the acidity created by the surface-applied nitrogen fertilizer.
Penn State Soil Testing Program
This information is useful if you compare analytical results from different labs. Direct comparisons can be made only between labs using exactly the same procedures. There are many different methods in use around the country. Which test will be used in a given area is based on research to determine how well the test works under local soil, crop, and climatic conditions.
Selecting a Soil Testing Lab
Be careful in selecting a soil testing lab, especially one in a different region. It is very important to send samples to a lab that uses soil tests that are appropriate for your region's conditions. A lab in a different part of the country may provide excellent quality results, but if the tests are not appropriate for our conditions, these tests may not result in correct interpretations and recommendations. The tests used by the Agricultural Analytical Services Lab at Penn State have been determined through extensive research to work best for Pennsylvania conditions.
Recommended Testing Procedures
Note that some state regulations, such as the Pennsylvania Nutrient Management Law, require soil tests to follow the recommended procedures in the list above. While most soil testing laboratories doing business in Pennsylvania use the recommended test methods listed above, it is important to determine if the lab you use is following these recommended procedures for your soil test results to be valid.
Converting the Units of Your Test Results
Soil nutrient levels on the Penn State soil test report are given as parts per million (ppm) elemental P, K, and Mg. However, while other labs doing soil testing in Pennsylvania may use the recommended methods above, they may report the results in different units.
Below are simple mathematical conversions that can be
used to change soil test results from labs using the recommended
procedures, but reporting results in different units than units used on
the Penn State soil test report.
- lbs K/A ÷ 2 = ppm K
- lbs MgO/A ÷ 3.2 = ppm Mg
- ppm MgO ÷ 1.6 = ppm Mg
- lbs Mg/A ÷ 2 = ppm Mg
For example if a soil test reports the Mehlich 3 P level as 140 lb P2O5 /A you would need to divide by 4.6 to convert this to 30 ppm P. If the result were reported as 60 lb P/A you would simply divide by 2 to convert this to 30 ppm P. Finally, if the result were reported as 70 ppm P2O5 you would divide by 2.3 to convert this to 30 ppm P.
Note that in this example, 140 lb P2O5/A, 60 lb P/A, and 70 ppm P2O5 are all the same as 30 ppm P, just reported in different units. This conversion is important if you want to compare results from different labs because they must all be in the same units. Also, if you want to look up a Penn State recommendation using a result from another lab, the results must be in ppm P, K, and Mg.