Fall Soil Sampling: To Grid or Not to Grid

When planning for fall soil sampling there are a number of things to consider. Like a lot of things, it used to be pretty simple. You took your shovel, dug some soil from a couple spots in the field and called it a day. Over the past decade or more, however, we’ve become much more aware of how variable the soil environment can be. We’ve also developed some really powerful tools and technology that can use our soil test data to help us dial in on factors that are driving yield and quality. So it’s not so simple anymore, but it’s still not rocket science.The first thing to remember is that you need to soil sample. Yes, it’s worth it. Without a soil test, there is no baseline for improvement. We need to replace the fertility we are removing when we harvest our crops and we can’t do that without knowing where we stand. That soil test is not only a prescription for improving yields, but also for maintaining them. And it’s not just yield, a healthy, high-quality crop is dependent on having a well-balanced, mineralized soil. Plus, given the technology we have available, there is so much we can learn from soil sampling regarding our limiting factors and the interactions that are taking place between soil properties. The next thing to remember about soil sampling is to do it at the same time of year every year. Most of us soil sample in the fall when the crops are off, which is great. However, sometimes producers run out of time and decide they want to soil sample in the spring. The problem is, if you usually fall soil test, a spring soil test will look a lot different. Soil tests give an idea of what’s going on in our soil, but it’s just a snapshot, and consistency is the key. You can get a good idea of where you need to go with the soil test, and to best monitor your progress taking soil samples at the same time of year is important.The last thing to consider is how to soil sample. Do you want to do a simple composite sample, a grid sample, or sample management zones?

 

“Without a soil test, there is no baseline for improvement.”

 

A composite sample is the simplest way to soil sample, and it’s the way many growers have soil sampled in the past. It involves walking the field in a “W” pattern and taking a series of cores across the field, mixing them together and sending them to the lab as one sample. The number of cores taken depends on the field size, and you may end up taking a subsample of your sample if you have a large field. Since you are only sending in one sample, you get a general overview of the field, but no information about variability within the field. Therefore, composite sampling is most useful on fields that are very uniform in topography and soil type.

If you have a lot of variability in your topography or soil types, or if the field has been managed in a way that may produce variability (for instance, bales were fed to livestock in one part of the field for many years) then grid sampling may be a good idea. Grid sampling involves laying a grid of a pre-determined size over the field and taking a series of 5-10 cores around each grid point (where the lines intersect). The size of the grid is often set at 2-2.5 acres; however, if you desire greater accuracy grids of 1 acre can be used. Remember that the smaller the grid the more samples and the higher the cost. Grid sampling is a good way to get a picture of what’s going on in the soil. It allows you to see the variation in soil properties across the field and plan accordingly. If you lay a yield map over your grid sampling data it can also give an indication of what soil properties are the most closely correlated with yield.

Sampling by management zone is another option. If you know your field has several different soil types and that yield generally varies with these soil types, then it makes sense to take a series of cores within each zone. Contour strips are another example of when sampling by management zone makes the most sense. Sampling this way gives you the ability to tailor your fertilizer applications to the needs of each zone and therefore make better and more practical decisions.