How do I use my soil test results?

Soil analysis is a great tool to assess what amendments your soil might need to produce optimal crop growth and reach its yield potential. So how does soil testing work?  There are two steps, the first is collecting the soil sample and submitting it to a lab for analysis, and the second is to use the information it provides to make management decisions. Both parts are equally important; to get meaningful results back from the lab, we have to have confidence that our soil sample was representative of the field we are hoping to make a decision about. Similarly, if we are taking the time to collect a soil sample and spending the money to have it analyzed, we have to use this information to make a decision.

ISU has an extension publication called “Take a good soil sample to help make good decisions,” which is available free at: https://store.extension.iastate.edu/Product/Take-a-Good-Soil-Sample-to-Help-Make-Good-Decisions. This publication does a great job of walking you through the many things to think about, such as when to sample, how to divide fields into “sampling areas” based on soil maps and management zones, techniques for compositing samples, why it’s important to consistently sample to the same depth, and how often to soil sample. My goal here isn’t to repeat this publication, but rather to talk about it in terms of an example field.

When we soil sample one of the first things we have to do is determine some “sampling area” that we want our soil samples to represent. There are a couple ways to do this, one is to ignore variability within the field and try to treat it as a uniform area. This method simplifies how we use the information as we have decided to treat the entire area field the same. The disadvantage of this method is that we know there is variation in the field and we probably want to tailor our management decisions to be more specific for each area (this concept is one of the key ideas of precision agriculture)

Below I have a map of an approximately 40-acre field. This field has two soil types in it SyA and SwA, these are Symco loamy fine sand and Symco silt loam respectively. In general, the big difference between these two soils is the amount of organic matter in the soil with SyA estimated to have about 5% and SwA having only about 1.5 organic matter, which probably will make use want to treat the soils as different management zone for soil fertility purposes. Based on these differences in organic matter, we probably want to treat these soils as different management zones.

One more thing to note is that although the USDA Web Soil Survey (http://websoilsurvey.sc.egov.usda.gov/App/WebSoilSurvey.aspx) provides a great first approximation of how to divide your field, your firsthand knowledge can even more valuable. For example, in this field, in my experience the ridge of sandier soil (SwA) actually extends further across the field. Additionally, for the last several years this field has been divided into two as the wetter area (shown in the upper right corner) didn’t get planted to corn two years ago, but was instead planted to soybean, which altered the rotation slightly as it was planted to soybean two years in a row as result. In the upcoming year, the entire field is planned to be planted to corn.

Now that we have divided into management zones we have to determine how to soil sample. I personally like a soil step probe (similar to what is shown below). This probe can take up to a 12” deep core, but since I normally want to sample to 6” I make a nice dark line in permanent marker at the 6” depth so I know how deep to consistently push the probe into the soil. Within each management zone we want to get a good representative sample. We’d probably want to take between 10-20 cores from random spots within the field (I’d walk through the field in a zigzag pattern trying to space the patterns over the area. The collected cores are from each management zone can be placed in a bucket, mixed together, and then a subsample collected from this mixture to send for analysis. When you send the samples to the lab there are a few things to make sure of. 1. Make sure the sample bag is clearly labeled (and that you have a key that tells you what that soil represents), 2. The bag is securely closed, and 3. That you have indicated what parameters you want analyzed.

The next step is to get the results back from the lab and use them in making your fertilizer (either manure or other sources) application decisions. We typically use soil test results to aid in making phosphorus, potassium, and liming decisions. Your state should have some guidelines to help you interpreting how to use the results of your soil test. In Iowa, I use “A general guide for crop nutrient and limestone recommendations in Iowa” which is again available for free at https://store.extension.iastate.edu/Product/A-General-Guide-for-Crop-Nutrient-and-Limestone-Recommendations-in-Iowa. When I see soil test results I tend to think of them as giving me the probability of a yield response. If you are in the optimal, high, or very high range, then the expected crop response is minimal. But if you are in the low or very low range there is a high chance that applying phosphorus would impact our crop yield. In addition to the amount present in the soil though, when we determine the application rate we also want to consider how much will be removed when we harvest our crops. By incorporating information about both the current amount of phosphorus or potassium in the soil and the amount expected to be removed by the crop production, we can make estimates about how much phosphorus we should apply.Unfortunately, I don’t have I don’t have the soil test results yet for my example field but based on previous years tests I’m expecting most of the field to be in the option range or high range for phosphorus and the high range for potassium, so my anticipated fertilizer application is as shown in table 2.

Table 2. Anticipated soil test results and fertilizer application.

Management Zone	Area (acres)	P Test	K Test	Suggested P Application (lb P2O5/acre)	Suggested K Application (lb K2O/acre)
1	15	Optimal	High	98	55
2	5	High	High	50	55
3	8	Optimal	High	98	55
4	12	High	High	50	55

So, what does this mean? Well, if we hadn’t divided up are field into different management zone we might have gotten somewhat different fertilizer recommendations. If we got a result that soil all fields were in the optimal range, we would have applied 98 lbs P2O5 per acre everywhere in the field. This would have resulted in about an additional $400 worth of fertilizer being applied that would have had limited agronomic value. If we had got a result in the high range, we might be applying less phosphorus than we needed  and as a result thought our soil sample result weren't useful.

So here is to wishing you all a happy new year and hoping  one of your resolutions is to make the best use of the soil, crop production, and manure characteristics data you are all ready collecting that you can.

Dan