Soil Testing Considerations
Fall and Spring are usually the time when farmers or fertilizer dealers take most soil tests to make recommendations for fertilizer applications. In the last several years, most dealers now do that service for their farmer clients. A few years ago, looking at soil test collected over several years, there was a big jump of .5 to 1.0% increase in soil organic matter (SOM) on all this no-till farmers fields. I started asking questions.
He was already a no-tiller and using cover crops. Had he done anything different? No really. Then I asked who took the sample? He said the dealer. Who took them before? He said he had. Is there that much sampling difference between dealers and farmers? The biggest difference is that dealers use a machine (a soil sampler on a four-wheeler) while most farmers use a shovel or a probe. The standard procedure for collecting a soil sample 10 years ago was to wipe the soil surface clean of residue, then probe the soil, taking at least 10-15 samples in a randomized pattern across the field. Often now dealers do zone sampling by soil type or topography. They also use a machine that does not wipe the soil. By not clearing the soil, soil samples for SOM can easily be .5 to 1.0% higher than before.
Another issue is timing of a soil sample. To compare results, it is best to take the samples the same time of year. Usually either in the fall of spring. Just as important, is ideally the soil should be warm and moist. About 50% moisture is ideal. The soil probes easier though it may be a little harder to get sticky soil out of the probe. However, most years its either drier or wetter. Does it matter. The answer is Yes. For example, potassium or potash (K) is almost always higher in a dry year (2-3X) higher in dry soil than in wet or compacted soil. Due to K induction, under wet soil conditions or in compacted soils, the K+ ions get locked up and tied up by the clay particles. This K+ is in the soil, but not readily available to plant roots nor does it show up on the soil test. This can cause a problem. You may have plenty of K in the soil, but it’s not readily available. Comparing dry years to wet years, there may be a 2-3X difference in K soil
test numbers.
A good practice is to write down the weather conditions when taking a soil sample. Dry and wet weather has an effect on soil samples and nutrient availability but also soil temperature along with moisture. Most microbes process the soil nutrients and make them plant available. My favorite saying is “Each soil microbe is just a soluble bag of fertilizer!” If it is too hot or too cold, too dry or too wet, it changes the availability of nutrients. Most nutrients are released by anaerobic bacteria so warm moisture conditions favor nutrient release. However, without adequate SOM, many of these soluble nutrients can flow with the excess water and flush out of the soil. It’s a balancing act between having just enough water and good soil temperatures to make nutrients available, but just as important is having a way to tie up ose nutrients so they do not leave the soil. That’s where live roots from a cover crop, wheat crop, or hay crop can keep
nutrients recycling. Or having high SOM to keep nutrients in place.
When soil sampling, take representative samples and sample to the same depth. Usually that is 6.67 inches or 8 inches. Just be consistent so you can track soil test over several years to identify trends and to solve agronomic problems and to make informed decisions about what inputs to apply to improve your bottom line. This also improves water quality because you not guessing and putting on too much expensive fertilizer or not enough when needed.
As dry as it has been in Northwest Ohio, taking soil samples may be difficult. The soil is hard and the soil temperature is probably warmer than normal. The dry warm conditions will impact your soil tests. The best time to take a sample may be after a good rain, at least 1 inch but it may take more. The soil is so hard and dry down to a deep level. Make sure you at least record the weather conditions so if anomalies show up, you have a possible explanation. When comparing soil test over time, it is easy to forget what the weather and the year was like years ago.