Dr. Dan Talks Agronomy

Time to Dig for Answers

The season may be busy, but this is the right time to think about pulling soil samples. (DTN file photo)

Soil sampling is a good way to repeatedly benchmark if your soils are providing enough nutrients to meet your yield goals.

Combines may be rolling and have most of our attention, but now is also a good time to gather those samples. Don't be tempted to skimp and skip the practice. It is still the best way to monitor pH and organic matter and available levels of phosphorus and potassium. Think about throwing in a few other soil health measurements too. Soil sampling basics haven't changed, but today there are additional tests available.


It's important to test your soil every few years to establish a baseline to identify trends, correct for pH issues and prevent soil test levels from dropping. Many growers have the routine of sampling every four years, but current thought is it may not be enough and should be done every second or third year.

Keep in mind that soil tests are only an index of the soil and what happens over time. If you measure a pH of 6.2, that doesn't represent every acre and there can be, and often is, considerable variability. Soil tests are at best a reliable prediction of what is happening to the soil over time, not an absolute value that applies to every acre. However, it is still the best tool we have available.

High yields are a good thing, but they take a toll on nutrient levels. You don't see that coming if you don't test. For example, a 200-bushel-per-acre corn crop pulls out 80 to 85 pounds of phosphorus and 60 to 65 pounds of potassium per acre. If you are not adding enough nutrients back, levels will be dropping.


Not much has changed in sampling patterns since the introduction of precision ag and grid and zone sampling. Choose the sampling pattern and number of points that suit you and your pocketbook. The more samples you pull, the more accurate and predictive the results. You can choose to grid sample every 2.5 acres or 4.5 acres or zone sample by soil type and landscape and get one composite sample or grid within the zone.


When it comes to sampling, the critical thing is to be consistent. Sample at 6- or 8-inch depth. When sampling, pull eight to 10 cores to create a representative sample, mix thoroughly, bag and label. Sample soil when it isn't too dry or too wet, maybe 70% to 80% of field capacity, so it is easy to push in the probe. Deliver or ship to a laboratory as soon as possible.


A typical soil test will include tests for: water and buffer pH, organic matter and nutrient levels. Nutrients usually tested include phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg). Additional tests can be run for micronutrients, sulfur, nitrate and total nitrogen. And some labs offer soil health testing as well.

Most soil test labs will use a standard Mehlich III test, unless otherwise requested. Many labs also offer Bray and Olsen to test phosphorus. Bray-1 and Olsen have been used extensively across the North-Central states. Bray underestimates plant-available P in calcareous soils. The Olsen test is run on high-pH, calcareous soils over 7.2. Mehlich III results are similar to Bray, except in high-pH soils. People have different preferences for the P test run and will stick with what they know.

IF you are interested in tracking soil health, request soil respiration testing and make it a routine part of your analysis package along with pH and organic matter. Soil respiration is a good indicator of overall soil health since so many environmental, management and soil factors affect it. Some laboratories offer the Solvita soil respiration test alone or as part of their package of soil health tests, which is based on the Haney test based on Rick Haney's work out of Texas A&M.


A soil test report can come with fertilizer recommendations, or your agronomist can use the data to make lime and fertilizer recommendations. Remember, soil testing is at best a predictive indicator of the soil, and the accuracy and value of the information grows with routine testing and development of a database over time.

I encourage growers not to set aside the results, but to closely review now and compare values to previous samplings. Look for trends that can reveal what changes are going on in the field, such as long-term changes in pH, organic matter and nutrient levels that have less to do with the last crop and more to do with long-term farming practices.

Routine and intense soil sampling is a significant and worthwhile investment. Take the time to review the data, and if you don't feel comfortable doing it yourself, reach out to an agronomist to do it for you. The results will pay dividends either in fertilizer savings, yield increases or recognizing and remediating a basic change in the soil that could have long-term effects on yield potential.

Dan Davidson can be reached at askdrdan@dtn.com