Getting nerdy about soil test results

Written by Janel Delage on Nov 19 2015

You ordered soil tests with good intentions and now you finally have some time to get your metaphorical hands dirty, so pull those results out of your desk and let’s make a fertility plan.

While you’ve been finishing up harvest and getting the machinery cleaned up and put away for winter, you may have noticed your soil test results coming back from the lab.  You ordered them with good intentions and now you finally have some time to get your metaphorical hands dirty, so pull those things out of your desk and let’s make a fertility plan.

One of the first things that I do when I get my soil test results is pull out my results from last year to compare them. Before you get into the nitty gritty, make sure the right field was sampled and that they didn’t hit an obscure area of the field like an old yard site or a manure pile. Then take a look at the soil attributes like pH, organic matter (OM), CEC and salinity. These attributes don’t generally change very much year over year but it’s nice to see the percent OM  increase over time.  Once you’ve looked at the absolute values and compared the soil attributes to last year, it’s time to start looking at the residual nutrients.

If you’ve switched labs you will want to understand the extraction method the lab uses to understand how to compare the values year to year.

 

When I look at the residual nutrient values in the soil I try to think of them as an index (low – medium – high).

  • I typically start with Nitrogen. When I looked at our results this year there were some dramatic differences between fields, which followed very closely with where we received rain, and the yields we pulled off. I like to do some quick calculations based on last year’s N residual, the N fertilizer we applied and yield we pulled off.  If you need any help with these calculations feel free to connect with your Cargill representative or agronomist and they can help you out. 
  • Phosphorous is the next nutrient to analyze.  P is always cycling in the soil; slowly moving from ‘available to the plant’ to ‘unavailable form’. It can vary from year to year, but as an index it should be relatively similar to years prior. If you’ve acquired new land recently you’ll find it interesting to compare its P levels to your old land where you’ve been diligently applying P for years. 
  • Unfortunately Sulfur values are hard to assess from a soil test. S is variable in a field, and if the core was taken from a hotspot it will spike the result. I don’t use the S results for much, and stick to the N:S ratio when determining S rates.
  • Lastly, I look at the micronutrients. These nutrient levels also typically change slowly over time. But I always keep a watchful eye on them because I feel it is important to be proactive to ensure we don’t get to a deficient state. 

Soil tests are one of those things where it is easy to get the test done, but interpreting the results and developing an actionable plan from the results can be difficult. I encourage you to be deliberate about reviewing the results, look for trends, compare yields with fertilizer application and residual nutrients. The more time you spend going through the results each year, the more comfortable you get and you’ll know what to expect. If you see anything that you’re unsure about, don’t hesitate to contact the lab or talk to an agronomist. Agronomists are generally pretty nerdy about soil tests so take advantage of it.

Tags: Fertilizer, soil test, crop nutrition

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